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CR/02/006N MINDEC user guide.

1. 4 5 19 Risk assessment results unne oeneeneneeerenenneereenenvenvenensensennnnenreen 50 4 5 20 Source Term Composition nnen nennen eennnnennnns vens eereneeener venen even 51 4 5 21 Source Term Module nanne nennen ennn eeeene eenen veren vensensennnnnnnnn enn 52 4 5 22 Summary of Water Chemistry Data nnee 53 4 5 23 Surface Transport flow to receptor nnn enennnnnnnnerere neren ennen 54 4 5 24 Transport Module sassen sent werdt ina ltiaedeenee den vertekend 55 4 5 25 Water Quality StandardS eiii di enten dae dien daneen 56 4 6 On screen help unne oeeneneeeeennnenseeerenenenreennnnnnnneneneneeneervennnnnnnneenen 58 4 7 Troubleshooting and Technical support nennen en eeneneennnen eenen eennenn 58 5 GLOSSAR EE 59 A heehee eae ous ln ever Leeann ates 62 MINDEC manual version 1 0 April 2002 1 INTRODUCTION 1 1 Background Large volumes of freshly broken and crushed sulphide bearing rock are discarded as waste at metalliferous mine sites This material commonly gives rise to contamination of surface and groundwater by high concentrations of trace metals and other potentially harmful elements MINDEC provides a decision support tool for environmental planners and regulators concerned with the management of mine wastes and with the contamination of waters arising from those wastes MINDEC has been developed with funding from the UK Department for International Development DFID under the KAR Know
2. screen resolution can be checked from the Windows Control Panel if required On 25 MINDEC manual version 1 0 April 2002 start up MINDEC interrogates the system hardware of the computer that it is running on and will display a warning message and then abort the program if the available screen area is insufficiently sized 4 4 Installation MINDEC can be installed by running the executable installation file mindec setup exe MINDEC consists of the following files Mindec xls MINDEC Excel VBA application file Refdata xls Water quality standard reference data file Mdechlp hlp MINDEC on line help file Example mdc An example scenario file readme txt Text files Conditions txt To run the program open Mindec xls and then wait while MINDEC loads This may take some time If a message referring to running macros appears choose to accept macros 4 5 MINDEC program screens 4 5 1 Introduction MINDEC presents a series of graphical screens to the user The screens have been designed to focus on different parts of the source pathway receptor linkage and to prompt the user for required input data in a logical sequence see Figure 4 1 The screens are described below in alphabetical order 26 April 2002 MINDEC manual version 1 0 MINDEC screen map Fig 4 1 SS SS A ll 27 MINDEC manual version 1 0 April 2002 4 5 2 Alternative water source concentrations Enter contaminant concentrations
3. 2 2 Pyrite oxidation and acid mine drainag e nennen rene eeenereene nennen 3 2 3 ASSUMPTIONS seriinin vannenerkannener E e renee 7 DIMAS TEEM Aartsen Eert dee dente OE 7 239 2 EUN EN tii 8 2392S ROCCO lis an van A dee area wanne 9 2 4 Comparisons with real data oooooooooooncconicccccccononcnnnccnoncnnnnnnnnnnncnnncnnnnnnnnnnnnnnnnnns 9 3 THEORETICAL BACKGROUND TO MINDEC nnen eeeeeeee eenn 10 3 1 Conceptual mMmode wesc rincon daa ae aA aa O EAA 10 3 2 Source term composition an naannnennnnneeeeneneneeeenennennnnnneeren veren eee nenen tennene 10 3 3 Leachate MUX id 12 3 4 Duration of acid generation nnee eneeneeneeeneneenenenenenenennnnnnnenee 13 3 5 Surface water transport nnn ann oeeeeeeneneeneereeenensennnnnennnsenneerenseeenenveen eenen 14 3 6 Groundwater transport eonenni a a a a a ia 15 3 7 Contaminant attenuation and retardation nnee nnen eeeenereenenenn 18 3 9 Water USAGC siata a dani 20 3 9 RISAS OSMA iia A A AA es a A AA IA el 21 S29 1 TOXIC RS Kata aatd 22 3 9 2 Carcinogenic RI a 23 4 USING MINDE C cc ccoo da ado 25 4 1 Software requirements nnn annnnnnnnn eenen eeen eeeeennnennnnnnseerenenee een eennennnnnnneeen 25 4 2 International ISSUES anneer entente rin 25 4 3 Hardware ISSUeS rninnt nenten adri 25 4 3 1 Minimum requirements occccccconcccnccnnnnnccnncnnnnnncnnnnnnncnnnnconnnccnnnnnnnnnrincnnnnnneos 25 4 3 2 Screen IZ A ete belde 25 AA Install learn rinda ea e aa pi
4. dose In MINDEC the Hazard Quotient is used as a measure of toxic risk Transport the movement of contaminants away from the source The transport processes considered in MINDEC are surface water flow and groundwater flow Unsaturated Zone the zone between the surface and the water table in which the water pressure is less than atmospheric i e the zone is in suction Water Quality Standard WQS a set of maximum allowable concentrations of contaminants in water WQS Exceedance Index a numerical value used in MINDEC as a measure of the departure of water quality from the WQS 61 MINDEC manual version 1 0 April 2002 6 References Breward N and M Williams 1994 Preliminary appraisal of the environmental geochemistry of the Bau mining area Sarawak Malaysia Nottingham British Geological Suirvey Technical Report WC 94 67R Domenico P A 1987 An anlytical model for multidimensional transport of a decaying contaminant species Journal of Hydrology 91 49 58 Domenico P A and F W Schwartz 1998 Physical and Chemical Hydrogeology New York John Wiley amp Sons Freeze R A and J A Cherry 1979 Groundwater Englewoods Cilff N J Prentice Hall Inc Kobayashi J 1971 Relationship between the Itai Itai disease and the pollution of a river water by cadmium from a mine Proceedings of the Fifth International Water Pollution Resources Conference San Fransico Lallemand Barres A and P Peaudecerf
5. leachate contaminant concentrations The same set of standards will be used to compare the calculated concentrations at the receptor at a later stage in the program MINDEC is initially supplied with 3 sets of water quality standard data 1 World Health Organization WHO drinking water guidelines and concentrations likely to give rise to complaints from consumers 2 European Union EU drinking water standards 3 United Kingdom UK drinking water standards Additional water quality standards can be added edited or deleted by the user by clicking on the Add Edit button A data form will appear allowing existing data sets to be edited or deleted and new data sets to be added The WHO EU and UK data sets can also be updated in the same way if required A Reset button is also provided to allow the user to re set the WHO EU and UK standards to their default values if these have been edited in error Pressing this button will not affect any user defined data sets One of the water quality standard data sets can be selected as the default data set by clicking on the Set as default check box The selected standard data set will then appear as the default standard whenever MINDEC is run subsequently The water quality standards data are held in an Excel file called Refdata x s If MINDEC cannot find this file it may prompt the user to locate it If the file is still not available then the WHO drinking water guidelines will be us
6. the incorporation of a chemical into a solid or liquid Adsorption the incorporation of a chemical onto the surface of a solid or liquid Advection Dispersion Equation ADE the differential equation that describes the transport of a contaminant in a porous medium aquifer It is a statement of the law of conservation of mass Freeze and Cherry 1979 provide a derivation of this equation for the interested reader Aquifer a geological unit that is sufficiently transmissive to yield significant quantities of water Aquifer Properties the range of physical variables particular to a rock unit that affect the movement of water and contaminants through that unit Aquifer properties include hydraulic conductivity porosity and dispersivity Attenuation the reduction in contaminant concentration due to physical chemical or biological processes as it passes through a medium Averaging Time in risk assessment calculations the length of time over which the dose is averaged For carcinogenic risk it is the average life expectancy For average daily dose calculations it is 365 days i e one year Buffering Potential the potential of a mineral on dissolution to control the pH of a water and thereby prevent the build up of acidity Calcite a carbonate mineral of calcium with the formula CaCOs Carcinogenic Risk is the increased risk of getting cancer due to exposure to a carcinogen It is usually expressed as a probability Contaminant Linear Vel
7. 1978 Recherche des relations entre la valeur de la dispesivite macroscopique d un milieu aquifere ses autres caracteristiques et les conditions de mesure etude bibliographique Bulletin du Bureau de Recherche Geologiques et Minieres 2 1 4 1978 277 284 Lee J S B Klinck and Moore Y A 2000 Dispersal risk assessment modelling and bioavailability of arsenic and other toxic heavy metals in the vicinity of two abandoned mine sites in Korea Nottingham British Geological Survey Technical Report WE 00 1 Lee J S B Klinck Moore Y A and Chon Hyo Taek 1999 Environmental Contamination and Bioavailability of Toxic Heavy Metals in the Vicinity of Abandoned Mines Korea 2nd Asia Pacific Symposium on Environmental Geochemistry Seoul National University Seoul Korea Rawlins B G T M Williams N Breward L Ferpozzi B Figueiredo and R Borba 1997 Preliminary investigation of mining related arsenic contamination in the provinces of Mendoza and San Juan Argentina and Minas Gerais State Brazil Nottingham British Geological Survey Technical Report WC 97 60 Smedley P L 1996 Arsenic in rural groundwater in Ghana Journal of African Earth Sciences 22 4 459 470 Smedley P L W M Edmunds and K B Pelig Ba 1996 Mobility of arsenic in groundwater in the Obuasi gold moning area of Ghana some implcations for human health Environmental Geochemsitry and Health 113 163 181 Smedley P L
8. 2 Zn 15100 e1 1PH 11 MINDEC manual version 1 0 April 2002 3 3 Leachate flux The annual flux of leachate over the total area of the waste pile is calculated as follows L R P A T where L Leachate flow rate m per year R Recharge as a fraction P Precipitation rate m per year Aw Area of waste pile m T Tailings process water volume m per year The recharge rate will depend critically on the geometry and permeability of the waste material and on climatic factors which affect the evaporation rate from the surface of the mine waste Precipitation infiltrating a mine waste pile will percolate downwards towards the water table which may lie either within or below the mine waste at a rate which depends principally on the unsaturated zone permeability the water filled effective porosity and the depth to the water table The movement of water within the unsaturated zone may also be complicated by the presence of perched water tables developed over low permeability horizons within the waste In general there will be insufficient site specific hydrogeological information to calculate travel times through the unsaturated zone The processes of sulphide oxidation and heavy metal leaching may depend in a quite site specific way on the availability of oxygen within the waste pile seasonal fluctuations in this availability and the presence of minerals principally carbonates which act to neutralise t
9. 3 0E 03 30 Coarse sand 7 3E 05 35 Medium sand 2 1E 05 35 Fine sand 6 3E 06 40 Silt 1 4E 07 45 Till 1 4E 09 50 Clay 2 2E 10 50 Karstic limestone 1 4E 04 30 Limestone 7 7E 08 10 Sandstone 4 2E 08 20 Siltstone 3 7E 10 10 Shale 1 4E 11 10 Permeable basalt 8 9E 05 25 Basalt 2 9E 09 5 Fractured igneous and 1 5E 06 10 metamorphic rocks Unfractured igneous and 2 4E 12 1 metamorphic rocks Weathered granite 1 3E 05 5 Weathered gabbro 1 4E 06 5 Data from Domenico and Schwartz 1998 and Freeze and Cherry 1979 30 MINDEC manual version 1 0 April 2002 4 5 4 Background Groundwater Concentrations Enter contaminant concentrations as measured in groundwater upstream from the contamination source under consideration Background concentrations can only be input for elements for which leachate concentrations have been entered or calculated by MINDEC Note If a concentration value is not entered for a particular element then the concentration for that element will be assumed to be zero Enter all concentrations in mg l 31 MINDEC manual version 1 0 April 2002 4 5 5 Background Surface Water Concentrations Enter contaminant concentrations as measured in surface water upstream from the site of mixing with leachate Background concentrations can only be input for elements for which leachate concentrations have been entered or calculated by MINDEC Note If a concentration value is not entered for a
10. H B Nicolli A J Barros and J O Tullio 1998 Origin and mobility of arsenic in groundwater from the Pampean Plain Argentina Water Rock Interaction Arehart and Hulston Rotterdam 275 278 USEPA 1999 Exposure Factors Handbook EFH Washington DC United States Environmental Protection Agency 62 MINDEC manual version 1 0 April 2002 Williams T M 1995 Hydrochemistry of mine waters associated with porphry Cu Au amp epithermal gold mining operations North Luzon Philippines Nottingham British Geological Survey Technical Report WC 95 15R Williams M F Fordyce and A Paijitprapapon 1996 Arsenic contamination in surface drainage and groundwater in part of the southeast Asian tin belt Nakhon Si Thammarat Province southern Thailand Environmental Geology 27 16 33 Williams T M N Breward A G Gunn and C Cummins 1994 Environmental Geochemistry of the Penjom Mine area Kuala Lipis Pahang Malalysia Preliminary Results Nottingham British Geological Survey Technical Report WC 94 20R 63
11. calculated concentrations for waters from different sources at the receptor site together with leachate contaminant concentrations The Combined water at receptor concentrations represent the average concentrations in water ingested by the consumer receptor taking into account the proportions of water from the different sources that are consumed This set of concentration values is then used as the basis for the human health risk assessment calculations These concentration results should be interpreted bearing in mind the large number of assumptions that are inherent within the modelling process It is likely that they will not equate to actual concentrations at the receptor site but can be used as best estimate values for the purposes of site prioritisation 53 MINDEC manual version 1 0 April 2002 4 5 23 Surface Transport flow to receptor MINDEC requires a flow rate for the surface watercourse that transports leachate to the receptor as measured upstream of the point at which the leachate enters the watercourse This flow rate is used to calculate the extent of contaminant dilution within the watercourse Input options for the surface water flow rate are as follows 1 Enter a flow rate value directly and select appropriate flow rate units or 2 enter channel dimensions width and depth and water velocity and select appropriate velocity units Then press the Calculate flow rate button to calculate a fl
12. can occur over a prolonged period without ill effect Risk estimates are based on a comparison of actual exposure to this reference dose Rfd for the particular chemical involved and a Hazard Quotient dimensionless is calculated as follows Ho APD R A where ADD is the average daily dose The units of the ADD and Rfd are mg kg of body weight per day A toxic risk exists for HQ gt 1 The ADD is calculated as follows C IR EF BW 365 ADD 22 MINDEC manual version 1 0 April 2002 where C is the contaminant concentration mg l IR is the ingestion rate of water day BW is body mass kg EF is the exposure frequency days year The MINDEC model uses the Rfd values given in Table 3 1 The Rfd is compound specific and updates of these values may be obtained from the US EPA IRIS database Table 3 1 Reference doses used in MINDEC Metal RfD As 0 0003 Cd 0 0005 Cr 0 005 Hg 0 0001 Ni 0 02 Zn 0 3 Mn 0 014 3 9 2 Carcinogenic Risk Carcinogenic compounds differ from systemic toxic compounds in that there is no lower limit for the existence of cancer risk Carcinogenic risk is calculated as follows risk 1 exp SF LADD where SF is the slope factor that is derived from laboratory dose response experiments assumed to be linear in the low dose range It should be remembered that this equation calculates the increased risk of getting cancer and not of dying
13. from cancer The US EPA considers an increased risk of cancer of 1 in 1000000 as significant The LADD is the lifetime average daily dose and is calculated from the ADD as follows LADD ADD a LE where ED is the exposure duration in years and LE is the average life expectancy 23 MINDEC manual version 1 0 April 2002 Although risk assessment involves the application of seemingly trivial mathematical equations problems arise in their parameterisation and a detailed knowledge of exposure factors is required in order to make the calculations useful The following table Table 3 2 indicates the information required in order to parameterise the risk assessment module of MINDEC Table 3 2 Data requirements for the risk assessment model Data Type Data Requirement Demographic Data Life expectancy body mass Exposure Data Exposure duration frequency of exposure water ingestion rate The US EPA have recently published a revised issue of the Exposure Factors Handbook now available on CD USEPA 1999 and also on the internet at http www epa gov nceawww1 exposfac htm and reference should be made to this for further background information 24 MINDEC manual version 1 0 April 2002 4 USING MINDEC 4 1 Software requirements MINDEC is programmed in Visual Basic for Applications VBA and runs under Microsoft Excel 97 Excel 2000 but will not run under pre 97 versions of Excel Either Microsoft Excel 97 or Excel 2
14. insufficient time has elapsed for the contaminated groundwater to reach the receptor or background groundwater contaminant concentrations exceed leachate contaminant concentrations or 35 MINDEC manual version 1 0 April 2002 e contaminant present in leachate has been massively diluted by groundwater flow and therefore no significant contaminant plume exists at the location of the receptor WARNING the concentrations shown are based on calculations that assume flow through a porous medium f fracture flow is significant then the contaminant may arrive at the receptor at an earlier time and at higher concentrations than those indicated 36 MINDEC manual version 1 0 April 2002 4 5 9 Contaminant Transport Calculation Data If required the user can input all of the data required for groundwater contaminant transport calculations rather than allow MINDEC to adopt default values The parameters that can be set by the user are as follows Age years age of source term Porosity aquifer effective porosity Hydraulic conductivity mls aquifer hydraulic conductivity Hydraulic head gradient the hydraulic head gradient along the groundwater pathway Contaminant average linear velocity mls the velocity is calculated automatically from the porosity hydraulic conductivity and hydraulic head gradient values Porosity and velocity data must be entered in the appropriate boxes before continuing to the next scre
15. of pyrite and calcite in the waste pile can be calculated as follows Voy 1 p Vw Coy Veal 1 p i Vw Coal where Vw Volume of mine waste pile including pore space m 13 MINDEC manual version 1 0 April 2002 Voy Volume of pyrite m Vea Volume of calcite m Cpy Pyrite content of waste by volume Ceai Calcite content of waste by volume p Porosity of mine waste taken as 0 2 by default and these volumes can be converted into weights in moles as follows Woy 5 10 Vpy 126 Weat 2 71 10 Vea 100 where Wpy Total weight of pyrite moles Wea Total weight of calcite moles Hence Loy Woy Roy Leal Weal Real where Lpy Pyrite lifetime years Loa Calcite lifetime years The lifetime calculations involve a number of assumptions see 2 3 Assumptions which must be fully understood before the results can be interpreted The calculated lifetime values should be taken as crude approximations They may however provide general guidance when prioritising sites 3 5 Surface water transport Leachate from the waste pile may remain within the groundwater system or may issue at the ground surface typically at or near the base of the steep sides of the waste pile and then enter surface drainage The program user is required to specify the percentages of the leachate that are transported via the surface drainage and groundwater systems The configurati
16. particular element then the concentration for that element will be assumed to be zero Enter all concentrations in mg l 32 MINDEC manual version 1 0 April 2002 4 5 6 Comparison of Leachate Chemistry and Water Quality Standard Leachate concentration data are compared with the water quality standard WQS data set previously selected Concentrations that exceed the water quality standard data are displayed on screen 33 MINDEC manual version 1 0 April 2002 4 5 7 Comparison of Water Chemistry at Receptor Site and Water Quality Standard Calculated combined source contaminant concentrations at the receptor are compared with the water quality standard WQS data set previously selected by calculating contaminant concentration standard ratios for each element for which suitable data are available The highest of these calculated ratios is quoted as the WQS exceedance index for the modelled scenario Calculated concentration data can be viewed by clicking on the Summary data button Calculated relative doses as of total dose from different water sources for each element of interest can be viewed by clicking on the Relative doses button Click on Risk assessment to carry out a human health risk assessment based on the calculated concentrations 34 MINDEC manual version 1 0 April 2002 4 5 8 Concentrations vs Time in Groundwater at the Receptor Site Contaminant concentrations are calcula
17. 000 is therefore required in order to run MINDEC MINDEC is compatible with Windows 95 Windows 98 Windows NT version 4 and Windows 2000 4 2 International issues MINDEC has been tested with English and Spanish language versions of Excel 97 under English and Spanish Windows Regional Settings It has not been tested with other language settings and will display an advisory message on starting up for the first time if it detects a non English language environment The user should be aware that although MINDEC on screen text is displayed in the English language default VBA message boxes which display during program execution contain buttons with captions which will be automatically displayed in the language appropriate to the user s version of Windows 4 3 Hardware issues 4 3 1 Minimum requirements Recommended minimum requirements are for a 150MHz Pentium PC with 32Mb RAM Using a faster PC will speed up screen loading and unloading Operating the program with inadequate video RAM may cause Out of memory errors The program requires 6Mb of hard disk space for installation 4 3 2 Screen size MINDEC screens are modal which means that they cannot be minimised maximised or otherwise re sized A usable screen area sufficiently large to display the fixed size screens is therefore required In order to achieve this a screen resolution of at least 800 x 600 should be available with a nominal screen size of at least 15 inches The
18. 1 x distance from source Retardation a retardation factor describing the retardation of a contaminant relative to groundwater movement most commonly due to sorption the ratio of the average linear groundwater velocity to the average velocity of the contaminant Note that the retardation factor will be applied equally to all the elements included in the scenario x distance from source m the distance between the source and the receptor measured in the horizontal plane parallel to the main direction of groundwater flow y distance from source m the distance between the source and the receptor measured in the horizontal plane perpendicular to the main direction of groundwater flow z distance from source m the vertical distance between the source and the receptor Half life days half life describing decay of the element of interest Note that the half life will be applied equally to all the elements included in the scenario It may therefore be best to model elements one by one if decay is to be included in the calculations To enter a half life check the Specify a half life box and enter a half life in days 38 MINDEC manual version 1 0 April 2002 4 5 10 Graph Options To display a graph of changes in contaminant concentration with time at the receptor site select the appropriate element and enter maximum time and concentration values or accept the default values The default time value is set at twice the a
19. Source term composition Contaminant concentrations in leachate draining from a mine waste source may be input directly to MINDEC if these concentrations are known Alternatively concentrations may be estimated by MINDEC based on the leachate pH which may itself be input directly or estimated based on a knowledge of the ore deposit host rock composition Maximum likely concentrations of Al Cd Cr Cu Fe Mn Ni Pb and Zn in leachate for acid conditions are determined in MINDEC on the basis of simple metal concentration pH empirical relationships Table 3 1 These relationships have been established by collating analyses of metalliferous mine waters reported by Williams et al 1996 Williams 1995 Williams et al 1994 Breward and Williams 1994 Smedley et al 1996 Smedley 1996 Smedley et al 1998 and Rawlins et al 1997 10 MINDEC manual version 1 0 April 2002 MINDEC CONCEPTUAL MODEL Pracionallon face cerime a murs pin Tr Diiin Advection r orient in Eger on Wad erie DE las ae A ria maden rpp 4 TERT a m RECEPTOR Figure 3 1 MINDEC conceptual model showing contaminant source surface and groundwater pathways and receptors Table 3 1 Equations to determine the maximum likely concentrations of metals in mine waste leachate Metal Equation Al 15800 e 2PH Cd 102 6 Cr 28 90H Cu 4660 e 29PH Fe 281000 e 1 27 Mn 3320 e 79PH Ni ade Sn Pb 0 17pH
20. USER GUIDE Version 1 0 April 2002 BGS report CR 02 006N MINDEC was developed by the British Geological Survey with funding from the UK Department for International Development DFID under the KAR Knowledge and Research programme project R7118 Cost Effective Evaluation of Hazards from Mine Waste British Geological Survey MATURAL ENVIRONMENT RESEARCH COUNCIL ee B6s Department tor International Development Bibliographic reference Klinck B A Hawkins M P and Moore Y A 2002 MINDEC user guide British Geological Survey Report CR 02 006N MINDEC manual version 1 0 April 2002 Conditions of use Software downloaded from the BGS web site is provided as is without warranty of any kind either express or implied including but not limited to the implied warranties of fitness for a purpose or the warranty of non infringement Without limiting the foregoing the BGS makes no warranty that i the software will meet your requirements ii the software will be uninterrupted timely secure or error free iii the results that may be obtained from the use of the software will be effective accurate or reliable iv the quality of the software will meet your expectations v any errors in the software obtained from the BGS web site will be corrected Software and its documentation made available on the BGS web site vi could include technical or other mistakes inaccuracies or typographical error
21. achate composition data are available or can be estimated Where actual data are not available MINDEC can estimate typical leachate contaminant concentrations based on a measured or estimated pH value and simple empirical concentration pH relationships These concentrations may not be accurate but they are likely to be conservative especially where the movement of water through the waste piles is rapid and water residence time in the waste is insufficient to allow equilibrium concentrations to develop The lifetime of pyrite in the waste pile is based on calcite mass balance considerations and is calculated on the basis that e the sulphate concentration value entered equals the annual average sulphate concentration of the leachate e all of the sulphate in the leachate originates from sulphide oxidation e the mine waste pile has a porosity of 20 e the current rate of sulphide oxidation will continue until all the pyrite has been consumed MINDEC manual version 1 0 April 2002 The lifetime of calcite in the waste pile is calculated on the basis that e the calcium concentration value entered equals the annual average calcium concentration of the leachate e all of the calcium in the leachate originates from calcite dissolution e the mine waste pile has a porosity of 20 e the current rate of calcite dissolution will continue until all the calcite has been consumed 2 3 2 Pathway MINDEC employs a solution to the advection di
22. al sites will require more site specific models and extensive site characterisation and data collection Any interpretation of results obtained using MINDEC will require a thorough understanding of the assumptions that underlie the model see sections 2 3 and 2 4 MINDEC manual version 1 0 April 2002 2 BACKGROUND 2 1 Mine waste and mine waters Ore deposits often contain high contents of sulphide minerals Mining and mineral processing activities give rise to large volumes of sulphide rich wastes and also create void space Sulphide oxidation due to exposure of waste rock and process tailings to the atmosphere gives rise to acid mine and acid rock drainage which is a major environmental problem The oxidation and dissolution processes also release other potentially toxic heavy metals e g Cd Zn Cu and Pb which are highly soluble in acidic solutions The acid and high heavy metal load can represent a serious hazard for communities living downstream from a mine Watercourses and groundwater contaminated by mine water may be used for irrigation or drinking water purposes and this can lead to ill health For example inhabitants of the Jintso River Valley in Japan suffered Itai Itai disease Kobayashi 1971 as a result of Cd poisoning Local farmers used water with high concentrations of Zn Cd and Pb from a nearby zinc mine to irrigate rice paddies The rice contained Cd concentrations of around 1ppm which led to Itai Itai disease Ou
23. ant attenuation and retardation MINDEC has provision for inputting attenuation biodegradation decay and retardation factors due to sorption for groundwater transport but the program does not otherwise consider contaminant attenuation processes other than dilution Sorption is determined experimentally by measuring the amount of solute retained by a solid in batch experiments The sorption may be described as a linear relationship between the sorbed concentration C and the solute concentration C see Figure 3 2 such that 18 MINDEC manual version 1 0 April 2002 C Kg C where Kg is the distribution coefficient Figure 3 2 The linear isotherm The retarded advective velocity v is then calculated using the following equations Ps 1 Pa Ka n vV T Where ris the retardation factor 7 is the effective porosity Pa is the dry bulk density v is the advective velocity 19 MINDEC manual version 1 0 April 2002 3 8 Water usage In order to evaluate the risks to the health of water consumers at the receptor site MINDEC considers three possible sources for drinking water e Groundwater potentially contaminated by mine waste leachate e Surface water potentially contaminated by mine waste leachate e Water from some other source such as rain water or water not potentially contaminated by mine waste Some careful interpretation of transport pathways may be required in order to establish appropria
24. as measured in the alternative source water Alternative water concentrations can only be entered for elements for which leachate concentrations have been entered or calculated by MINDEC Note If a concentration value is not entered for a particular element then the concentration for that element will be assumed to be zero Enter all concentrations in mg l 28 MINDEC manual version 1 0 April 2002 4 5 3 Aquifer Geology Select the lithology that best describes the groundwater pathway between the source and the receptor from the drop down menu Only one lithology can be selected Appropriate values for the hydraulic conductivity and porosity of the chosen lithology are then applied from MINDEC s database Table 4 1 To enter values for the hydraulic conductivity and porosity directly press the Advanced button Other parameters required for contaminant transport calculations are given default values To view and or change any of these values press the Advanced button Use the option buttons to indicate the relative importance of matrix fracture flow If fracture flow is significant or predominant then the user is warned that the groundwater transport calculations may be invalid 29 MINDEC manual version 1 0 April 2002 Table 4 1 Representative values of porosity and hydraulic conductivity Lithology Hydraulic conductivity mls Porosity Gravel
25. ater quality standards and or combined with water consumption data to calculate hazard quotients for potentially toxic elements and a carcinogenic risk for arsenic 3 9 Risk assessment Measured or estimated contaminant concentrations in leachate at the source and at the receptor site can be compared with water quality standard data The World Health Organization European Union and United Kingdom drinking water standards are provided as default data sets in MINDEC Alternatively the user can input any alternative water quality standard data sets A Water Quality Standard WQS Exceedance Index is calculated as the highest of the ratios of the actual or estimated contaminant concentrations to the respective WQS values for each of the elements under consideration The combined concentration Com for any element is calculated as follows Coom Cor Y Cs S Cat a 100 where Cy Concentration in groundwater mg l Cs Concentration in groundwater mg l Cat Concentration in groundwater mg l g groundwater consumed s surface water consumed a alternative source water consumed If water quality criteria are not met then a deterministic risk assessment can be carried out Risk assessment is a formalised framework enabling determination of the relationship between the predicted exposure concentration of a given substance or substances and predicted adverse effects on human health Hazard and risk are frequen
26. ch Ouch characterised by severe bone pain In Korea work by Lee et al 1999 Lee et al 2000 has demonstrated that both Chinese cabbage and rice are accumulators of mining contamination derived arsenic and constitute a major pathway of exposure to this metalloid for the local farming communities A range of pH values is observed in waters draining from metalliferous mine sites Very low pH values may be found indicating the presence of acid mine drainage produced by the oxidation of sulphide minerals releasing acid and heavy metals into the environment A more neutral pH may signify that buffering has occurred due to reaction with minerals which consume acidity e g calcite while a high pH may indicate that process water rich in hydroxides has entered the system 2 2 Pyrite oxidation and acid mine drainage Acid mine drainage AMD and acid rock drainage ARD commonly arise from the reaction of iron sulphides notably pyrite pyrrhotite and arsenopyrite with oxygen from the atmosphere The following equations describe the processes that lead to the formation of AMD ARD from pyrite oxidation MINDEC manual version 1 0 April 2002 2FeS 2H20 70 2Fe 4S0 4H 1 Aqueous oxidation of pyrite FeS2 leads to the formation of ferrous iron and sulphuric acid The oxidation reaction of pyrite described by Equation 1 can be subdivided as follows 4Fe 4H Oz 4Fe 2H20 2 Partial oxidation of
27. concentrations in surface waters where there may be large changes in pH Eh conditions MINDEC adopts the conservative assumption that concentrations in surface waters are reduced only by dilution 3 6 Groundwater transport Once leachate reaches the water table the concentrations of dissolved trace elements will be modified by dilution with groundwater passing beneath the mine waste site and by dispersion during flow away from the site The extent of dilution beneath the mine waste will depend on the groundwater flow rate and the 15 MINDEC manual version 1 0 April 2002 effectiveness with which mixing between the groundwater and the contaminants occurs MINDEC calculates the diluted contaminant concentrations beneath the mine site as Co C G L Cog Ro l G L Rg where Cy Diluted contaminant concentration in groundwater mg l C Contaminant concentration in leachate mg l G Proportion of leachate entering groundwater system L Flux of leachate from waste pile total I s Rg Flow rate of groundwater upstream from mixing site l s k i A 1000 where k Hydraulic conductivity m s i Hydraulic head gradient A Area of inflow m m d where m mixing zone depth m d width of contaminant source perpendicular to groundwater flow m MINDEC calculates the width d as the diameter of a circular mine waste pile using the value of the mine waste surface area as input by the user If th
28. e hydraulic head gradient is unknown then MINDEC provides an opportunity to enter the topographic gradient since this may be much easier to measure in the absence of water level data from monitoring boreholes MINDEC then calculates a nominal head gradient as a proportion 0 9 of the topographic gradient on the basis that head gradients in unconfined aquifers frequently reflect the surface topography However in rocks with a high permeability such as karstic limestones and or where recharge rates are low this approach may significantly over estimate the head gradient This may result in an over estimation of contaminant dilution as a consequence of the unrealistically high flow rates predicted 16 MINDEC manual version 1 0 April 2002 Mixing and dilution of the leachate beneath the mine waste site are considered for the purposes of the model to occur essentially instantaneously The diluted contaminant concentrations form the source term values for groundwater transport calculations Dissolved contaminants will migrate down gradient within the moving groundwater producing a plume of contamination emanating from the source When a contaminant travels through a porous homogeneous aquifer medium its movement is governed by the advection dispersion equation ADE The equation may be written as follows Freeze and Cherry 1979 eID DSD at x yr oo y g oC aC aC aC aC aC aC v y v where C is contaminant concentra
29. e of protons The precipitation of Fe OH z ochre results in an orange deposit commonly seen coating stream beds in mining contaminated areas The coating smothers and kills benthic life but does provide sites for sorption see below which can act to reduce the metal loading of the water Aluminosilicate minerals can also act as buffers though the kinetics of dissolution are slower than for carbonate minerals The acidic dissolution of albite releases 1 mole of aluminium and removes 4 moles of protons Equation 9 Subsequent dissolution of gibbsite often observed as a white precipitate in streams contaminated by AMD ARD re releases 3 moles of protons and hence 1 mole of protons is removed overall Equation 10 NaAlSizOg 4H 4H20 Na AI 3H4SiO4 9 Albite dissolution Al 3H20 Al OH 3 3H 10 Gibbsite precipitation By raising the pH of the system the metal loading of the water can also be reduced via e Precipitation of carbonates e g ZnCO3 and hydroxides e g Zn OH e Sorption Under acidic conditions the surface charge of ochre is positive This attracts anions in solution e g arsenate and molybdenate ions As the pH rises the charge on the ochre surface becomes negative and thereby attracts cations in solution e g Cu The cations bind to the ochre surface and become immobilised in the solid phase However should conditions change the metals may be released The composition of
30. e oxidation of ferrous to ferric iron Ferric iron which has a higher solubility in water than oxygen reacts with pyrite as an oxidant to produce more ferrous and sulphate ions This is MINDEC manual version 1 0 April 2002 known as the propagation cycle Figure 2 1 and is the root cause of acid mine drainage FeS 3 590 H 0__________ Fe t 250 7 2H Initiator reaction Fe2 0 250 H gt _ Fe3 0 5HO FeS 14F BHO pp 15Fe 250 2 16H Figure 2 1 The oxidation of pyrite to form acid mine drainage Buffering minerals such as calcite and dolomite may reduce acidity and the heavy metal loading of mine waters These carbonates rapidly undergo dissolution in acidic solutions Equations 5 and 6 consuming protons and thereby raising the pH CaCO 2H Ca H COs 5 Calcite dissolution CaMg CO3 4H Ca Mg 2H CO3 6 Dolomite dissolution However certain carbonate minerals e g siderite have no buffering effect Ferrous iron released during siderite dissolution oxidises to ferric iron and precipitates as iron oxy hydroxide The number of protons released during precipitation is equal to that consumed by the initial dissolution of siderite Equations 4 7 and 8 FeCO3 2H Fe HCO 7 Siderite dissolution consumes 2 moles of protons MINDEC manual version 1 0 April 2002 Fe 0 2502 H Fe 0 5H20 8 Ferrous iron oxidation consumes 1 mol
31. ed as the default standard No other data sets are available in this case 56 MINDEC manual version 1 0 April 2002 As MINDEC finds the end of the database by looking for the first empty record in the data set title field it is important not to introduce blank records into the middle of the database If these are introduced during editing they must be removed by clicking on the Edit button or by editing the Refdata xls spreadsheet independently of MINDEC 57 MINDEC manual version 1 0 April 2002 4 6 On screen help Help information is available using the Help buttons on the program screens The Help Contents page provides a structured list of available topics under the headings e How to use MINDEC help MINDEC background MINDEC program screens Click on a topic to view the appropriate page of information For help with a particular program screen look for the appropriate screen title in the list under MINDEC program screens The same help pages are also available from the Index that provides an alphabetical list of all available help topics A search facility is also available by clicking on Index followed by Find Use Find to search for any word in the help pages text 4 7 Troubleshooting and Technical support For further information and technical support contact the following e mail address mindec enquiries bgs ac uk 58 MINDEC manual version 1 0 April 2002 5 GLOSSARY Absorption
32. en Height of source area m this refers to the height of the notional cell of groundwater beneath the contaminant source mine waste within which leachate groundwater mixing occurs Mixing within this cell is assumed to be complete and effectively instantaneous The cell acts as the source term for groundwater contaminant transport calculations By default the height is taken to be 10m Width of source area m this refers to the height of the notional cell of groundwater beneath the contaminant source mine waste within which leachate groundwater mixing 37 MINDEC manual version 1 0 April 2002 occurs A default value for the width is calculated from the area of the waste as entered by the user and an assumed circular shape for the waste pile Dispersivity m in x direction a measure of the extent of dispersion mixing of the contaminant in the horizontal plane and parallel to the direction of flow By default the dispersivity in the x direction is set to 0 1 x distance from source Dispersivity m in y direction a measure of the extent of dispersion mixing of the contaminant in the horizontal plane and perpendicular to the direction of flow By default the dispersivity in the y direction is set to 0 01 x distance from source Dispersivity m in z direction a measure of the extent of dispersion mixing of the contaminant in the vertical direction By default the dispersivity in the z direction is set to 0 00
33. ernas 26 45 MINDEC program STRESS NE EEE 26 4 5 1 IntroductlOn 25 arresten esa 26 4 5 2 Alternative water source concentrations nnn nennen senen eenenennenen 28 4 5 SAquier GOOG iia a Satta deh adden see adrenerge 29 MINDEC manual version 1 0 April 2002 4 5 4 Background Groundwater Concentrations nnen nennen 31 4 5 5 Background Surface Water Concentrations nnn nannnnen enn eneen 32 4 5 6 Comparison of Leachate Chemistry and Water Quality Standard 33 4 5 7 Comparison of Water Chemistry at Receptor Site and Water Quality Standards ns eenheden ee enden eneen 34 4 5 8 Concentrations vs Time in Groundwater at the Receptor Site 35 4 5 9 Contaminant Transport Calculation Data ooooccccnccnnnonccccccccnnncccnnnnaninnnnns 37 4 5 1 0 Graph Options nternet Gil nnen rme veenendaal 39 4 5 11 Groundwater Transport nnn ennennnnnnsnnneeenen een seeensnnnnennnnseeneneeenen eenen 40 4 5 12 Human Health Risk Assessment nnn anssen eerenneneerrenensenrenneneerenn 42 4 5 13 Leachate Concentrations nnen eeenennenenennennnseren senen eenn serene enen 43 4 5 14 Leachate FlUX nnie nine a vaneen den 44 4 5 15 Mine waste composition and weathering rate nnn eneen 45 4 5 16 Receptor Module irrien aa a aa ERA ded aiken 47 4 5 17 Relative doses at receptor from different water SOUrCES annen 48 4 5 18 Risk assessment direct input of concentrations in water at receptor site 49
34. f flow It is a dimensionless quantity Host Rock a body of rock serving as a host for a mineral deposit Hydraulic Conductivity the constant of proportionality in Darcy s Law The ratio between the flow of water through a rock and the hydraulic gradient across it Ingestion Rate for the purposes of MINDEC the ingestion rate is the contaminant concentration multiplied by the number of litres of contaminated water ingested Units are mg l day Leachate the heavily mineralised solution formed by the percolation of rainfall and other surface waters through waste Lithology the physical description of a rock based on such characteristics as grain size and mineralogy Matrix Flow groundwater flow through the interconnected porosity of a rock matrix Pathway the route taken by contaminants from the source to the receptor In MINDEC the pathways considered are surface water and groundwater Permeability the capacity of a porous rock to transmit water pH a measure of the activity of hydrogen ions in solution In the field it is determined with an electrode a pH lt 7 is acidic and a pH gt 7 is alkaline Porosity the percentage of the volume of a rock or soil occupied by void space The term effective porosity refers to the percentage volume of interconnected void spaces through which flow can occur Porous Medium a lithology through which the principal flow mechanism is through connected pores Process Water the water used on a mine
35. ferrous to ferric iron and consumption of protons Ferric iron may then accept electrons allowing further oxidation of pyrite 3 or may undergo hydrolysis 4 FeS 14Fe 8H20 15Fe 25047 16H 3 Fe 3H20 Fe OH 3H 4 Micro organisms play a key role in the formation of AMD ARD The bacterium Thiobacillus thiooxidans oxidises sulphide minerals whilst Thiobacillus ferrooxidans oxidises ferrous iron Both T ferrooxidans and T thiooxidans are chemolithotrophs that obtain energy by the oxidation of iron and sulphur respectively The bacteria oxidise ferrous to ferric iron by a series of enzymatic reactions that occur across the cell wall leading ultimately to the reduction of oxygen to produce water The conversion of Fe to Fe yields energy for the bacteria however this is quantitatively small compared to the contribution of many other inorganic bacterial substrates and in order for the bacteria to achieve an adequate supply of energy from the oxidation of iron they must utilise large amounts It has been suggested that the action of bacteria increase the rate of AMD ARD production by a factor of 1 million Chemical oxidation of ferrous iron occurs rapidly at near neutral pH When pyrite is first exposed to oxygen and water the reaction described in Equation 1 proceeds only via a chemical pathway The reaction produces acidic conditions under which ferrous iron is kinetically stable 7 ferrooxidans then catalyses th
36. for mineral processing Pyrite a sulphide mineral of iron It is a common mineral in many ore deposits 60 MINDEC manual version 1 0 April 2002 Receptor living beings or resources that may be exposed to and affected by contamination Recharge Rate the rate at which effective precipitation rainfall moves through the unsaturated zone to the water table Reference Dose an estimate of the exposure to a toxic contaminant that can occur for a prolonged period without adverse health effects Retardation a measure of the reduction in solute velocity relative to the flowing groundwater due to processes such as adsorption Risk Assessment the process of estimating the risks of adverse health effects or environmental impacts due to exposure to a contaminant Scenario a conceptual model of a site that considers a specific set of parameter inputs Scenario analysis can be used to check model sensitivity to changes in a particular parameter Slope Factor a mathematical description of the dose of a contaminant received and the incidence of tumour development The function is usually considered to be linear in the low dose range Sorption a term including both adsorption and absorption Source Term the quantity and concentration of a pollutant discharged at the pollution source Sulphide Oxidation the natural process in which sulphide is converted to sulphate Toxic Risk the relationship between the average daily dose and the reference
37. ge of the source term The default concentration value is set to display all of the available data Note only elements for which data are available are selectable from the drop down menu To proceed without plotting a graph press the Cancel button 39 MINDEC manual version 1 0 April 2002 4 5 11 Groundwater Transport Distance Enter the straight line source to receptor distance in metres Age of Leachate Source Enter the age of the leachate source in years Gradient Enter a value for the hydraulic gradient x y if this is known or can be estimated If the hydraulic head gradient is unknown then MINDEC provides an opportunity to enter the topographic gradient since this may be much easier to measure in the absence of water level data from monitoring boreholes MINDEC then calculates a nominal head gradient as a proportion 0 9 of the topographic gradient on the basis that head gradients in unconfined aquifers frequently reflect the surface topography However in rocks with a high permeability such as karstic limestones and or where recharge rates are low this approach may significantly over estimate the head gradient This would cause both the extent of dilution of contaminants beneath the waste site and contaminant velocities to be over estimated As a consequence contaminants might reach the receptor at a higher concentration than predicted although actual travel times might be longer than those estimated Therefore
38. h does not flow to the receptor site and e groundwater These values will be difficult to establish for most sites and it may be necessary to run the model with a number of different estimates for these values Some careful interpretation of transport pathways may be required in order to establish appropriate percentage values for these various sources since some water may arrive at the receptor via a mixed surface groundwater path For example water abstracted from a well immediately adjacent to a river might contain contaminants that have been transported largely within surface water before being drawn into the well It would therefore be more appropriate for MINDEC to consider this water as surface water rather than groundwater since groundwater transport calculations would over estimate travel times and mis calculate the extent of any dilution and dispersion Similarly careful consideration should be given to the possibility that contaminants might reach the receptor after migrating first through groundwater and then discharging into surface water Click on the navigation images in the top right corner of the window to move directly backwards to the MINDEC source term or leachate flux screens if required Note that it is not possible to jump forwards to the Receptor screen 55 MINDEC manual version 1 0 April 2002 4 5 25 Water Quality Standards Select a water quality standard from the drop down menu with which to compare the
39. he acidity produced Bearing these uncertainties in mind MINDEC takes a conservative approach to conditions in the unsaturated zone by assuming that the source term leachate arrives at the water table instantaneously and without any solute attenuation Essentially a saturated piston flow recharge process is assumed 12 MINDEC manual version 1 0 April 2002 3 4 Duration of acid generation The mine waste module provides a very approximate indication of the potential lifetime of acid generation at the mine waste site by calculating the time required to oxidise all the pyrite in the waste pile The long term buffering potential of calcite in the waste pile is also assessed by calculating the time required to dissolve all the calcite in the waste pile During pyrite oxidation one mole of pyrite produces two moles of sulphate 1 mole FeS _ 2 moles SO and during calcite dissolution one mole of calcite produces one mole of calcium 1 mole CaCO _ 1 mole Ca By making a number of assumptions see 2 3 Assumptions the weathering rate of pyrite and calcite can therefore be related to leachate sulphate and calcium molar concentrations as follows Roy 0 5 SO F Rea Ca F where Rpy Weathering rate of pyrite moles a Rea Weathering rate of calcite moles a Ca Concentration of calcium in leachate moles SO4 Concentration of sulphate in leachate moles l F Leachate flux l a The volumes
40. if the nominal head gradient based on the topographic gradient is considered to be unrealistic the user should enter a better estimate of the head gradient even if accurate data are not available Aquifer Property Data Select either the BASIC option in which case porosity and hydraulic conductivity values are taken from MINDEC s internal database of values based on a lithology specified by the user in the 40 MINDEC manual version 1 0 April 2002 following screen Other parameters for groundwater transport calculations are set to default values or the ADVANCED option in which case all parameters required for groundwater transport calculations can be set by the user Select the appropriate option to take into account background groundwater contaminant concentrations or to assume that the groundwater is uncontaminated upstream of the source being modelled 41 MINDEC manual version 1 0 April 2002 4 5 12 Human Health Risk Assessment Data entry 1 Enter the following water consumer receptor data in the appropriate boxes Water intake litres per day Average body weight kg 2 For the toxic hazard calculations click on the grey Toxic hazard tab and enter Exposure frequency that part of the averaging time 365 days during which the receptor has been exposed to contaminants 3 For the carcinogenic risk calculation click on the grey Carcinogenic risk tab and enter Lifetime expectancy the time
41. ined by running the model using a range of input values i e by conducting a sensitivity analysis MINDEC only considers contaminant transport via surface water and groundwater pathways At some sites where wind velocities are significant and dry finely ground tailings are inadequately stabilised the dispersal of potentially toxic elements through the atmosphere may be an important pathway MINDEC manual version 1 0 April 2002 2 3 3 Receptor The human health risk calculations carried out by MINDEC consider only water consumption and do not take into account possible trace element intake from foodstuffs e g derived from eating fish caught from contaminated waters and vegetables irrigated with contaminated water or grown on contaminated soil The use of published health related factors reference doses slope factor and water quality standards involve a number of simplifications and assumptions The reader is referred to the relevant source documentation for discussions of these issues see for example the US EPA Iris database at http www epa gov iriswebp iris index html 2 4 Comparisons with real data It is important to recognise that MINDEC only calculates annual average contaminant concentrations at the receptor site At any particular point in time actual contaminant concentrations may differ from the MINDEC calculated values due to inaccuracies arising from the assumptions outlined above or due to seasonal or short term fluc
42. leachate draining from mine waste may be affected by a number of site specific factors including the mineralogy and grain size of the minerals present the mining and processing methods employed the hydrogeological setting MINDEC manual version 1 0 April 2002 and the prevailing climate Mines that exploit fine grained sulphide rich ore deposits and that crush the ore to a very small size fraction are more likely to experience acid mine drainage problems The smaller the grain size the greater the surface area exposed to oxidation and the greater the rate of reaction The duration of the residence time of water in contact with ore minerals determines whether or not chemical equilibrium is achieved or if oxidation products are flushed into surface and groundwater before n situ supersaturation is achieved 2 3 Assumptions Key assumptions upon which MINDEC is based include the following 2 3 1 Source Term MINDEC considers only a single contaminant source although there is provision to input background groundwater and surface water contaminant concentrations If multiple contaminant sources are present these will need to be modelled separately or taken into account when setting background concentration values The possibility that there is additional contaminant input along the transport pathway for example from metal rich alluvial sediments downstream from the mine site is not considered It is also assumed that representative le
43. ledge and Research programme project R7118 Cost Effective Evaluation of Hazards from Mine Waste It has been developed primarily for use in situations where the resources available for on site data collection may be insufficient for a full assessment of a large number of sites The program can be used e to prioritise mine waste sites for further study monitoring and or remediation work on the basis of the threat that these sites pose to ground and surface water quality and to the health of water consumers e to provide guidance concerning the relative risks associated with using different water sources at a receptor site e to predict whether groundwater at the receptor site is likely to deteriorate in quality in the future e to aid in the development of more complete conceptual models of contaminant migration at individual sites e as a predictive tool to provide some indication of water quality at a hypothetical receptor site such as a planned water supply borehole that has not yet been constructed or e to carry out human health risk assessments of the water pathway using actual contaminant concentrations at a receptor site where these data are available MINDEC considers mine waste contaminant migration in terms of e a source i e the mine waste e a pathway i e a surface waterway or a groundwater body e a receptor which might be for example a surface water body a water well or a water consumer MINDEC
44. manual version 1 0 April 2002 MINDEC provides a best estimate of the threat to ground and surface water quality and to the health of water consumers at a distant receptor site by calculating e expected concentrations at the receptor site of a suite of potentially harmful trace elements in waters originating from the mine waste site under study e human health risk indices toxic hazard quotients and for arsenic a carcinogenic risk factor for consumers of the contaminated waters e a water quality standard WQS exceedance index which provides an indication of the extent to which the expected concentrations at the receptor exceed water quality standards e relative contaminant element doses as percentages of the total dose from surface water groundwater and other water sources The program also gives some indication of e contaminant travel times from the mine waste source to the receptor through the groundwater pathway e the expected duration of contaminant generation at the mine waste site 1 2 Limitations In order to accommodate the wide variety of conditions at different mine sites MINDEC takes a necessarily simplistic approach to site assessment It should be emphasised that due to the assumptions involved in the calculations see 2 3 Assumptions the results presented by MINDEC are suitable as a guide to site prioritisation only and should not be interpreted as accurate predictions Detailed assessments of individu
45. ocity also known as the advective velocity It is derived by dividing the Darcy velocity by the effective porosity Dispersivity a characteristic property of a porous medium with dimension L a measure of the tendency of a solute to disperse within groundwater moving through the medium Eh the potential of a half cell measured against a standard hydrogen half cell It is measured in the field using a portable electrode and gives an indication of the reduction oxidation potential of water Exposure Frequency the number of days per year a receptor is exposed to a contaminant Needed to calculate the average daily dose Exposure Duration the number of years a receptor is exposed to a carcinogen Needed to calculate the lifetime average daily dose 59 MINDEC manual version 1 0 April 2002 Flux the rate of mass transport of a contaminant Fracture Flow groundwater flow through fractures Groundwater sub surface water that occupies the saturated zone Groundwater Abstraction the withdrawal of groundwater as a water supply Half life the time taken for one half of a substance to decay Hazard Quotient HQ the ratio between the average daily dose and the reference dose An HQ gt 1 indicates the existence of a toxic risk Head Gradient also known as the hydraulic gradient the ratio between the difference in water level at two points divided by the distance between those two points or the rate in change of head per unit distance o
46. on of the surface water system between the mine waste site and the receptor is difficult to generalise MINDEC accommodates the possibility that only a 14 MINDEC manual version 1 0 April 2002 proportion of the leachate entering the surface water drainage system will flow towards the receptor site being considered The estimation of a surface water flow rate for the purposes of the dilution calculations should be carried out bearing in mind the need to include additional tributary waterways which contribute water flow between the leachate source and the receptor Within the surface drainage system dilution and attenuation processes will modify contaminant concentrations The diluted contaminant concentration is calculated as follows Cs C1 S L Cos Rs 1 S L Rs where Cs Diluted contaminant concentration in surface water mg l C Contaminant concentration in leachate mg l Cys Contaminant concentration in surface water upstream from mixing site mg l L Flux of leachate from waste pile total l s S Proportion of leachate entering surface water system and flowing towards the receptor R Flow rate of surface water upstream from mixing site l s A flow rate value in a variety of units can be input or can be calculated from flow channel dimensions depth and width and water velocity Attenuation processes such as sorption and precipitation may be important in reducing contaminant
47. ow rate The Reset depth width velocity button can be pressed if necessary to adjust the channel dimension and or water velocity data The calculated flow rate can be viewed in a number of different units by selecting the appropriate option button Selecting a different unit will not affect the dilution calculations carried out by MINDEC Use the Background chemistry option buttons to input contaminant concentrations for the surface water upstream from the leachate entry point or allow the program to assume that this water is uncontaminated The estimation of a surface water flow rate for the purposes of the dilution calculations should be carried out bearing in mind the need to include any additional tributary waterways which contribute water between the leachate source and the receptor Where Eh pH conditions are suitable precipitation and or sorption especially onto precipitated ochre deposits may substantially reduce contaminant concentrations in surface waters As these processes are difficult to quantify MINDEC takes a conservative approach by only considering the effect of dilution in surface waters 54 MINDEC manual version 1 0 April 2002 4 5 24 Transport Module Use the option buttons to choose whether to consider surface and or groundwater transport of leachate Then enter percentage values for the distribution of leachate between e surface water which flows to the receptor site e surface water whic
48. pril 2002 4 5 19 Risk assessment results Calculated hazard quotients and a carcinogenic risk factor for arsenic are displayed Press Back to return to the main Risk Assessment screen 50 MINDEC manual version 1 0 April 2002 4 5 20 Source Term Composition Elements metals likely to be present in the mine waste leachate should be selected using this screen Either enter a measured leachate pH or if this is not available select a mineralisation host rock type from the drop down menu to allow MINDEC to estimate the pH In the latter case the estimated pH is shown in a message box and the user then has an opportunity to accept or reject this value A measured or estimated pH value is required for MINDEC to estimate a leachate composition 51 MINDEC manual version 1 0 April 2002 4 5 21 Source Term Module Select an appropriate option button to either Open a previously saved MINDEC scenario mdc file or e Create a new scenario with leachate contaminant concentration data to be entered by the user or e Create a new scenario with leachate contaminant concentration data to be estimated by MINDEC or e Carry out a human health risk assessment using contaminant concentrations in water at the receptor site i e with no consideration of source term or transport pathways 52 MINDEC manual version 1 0 April 2002 4 5 22 Summary of Water Chemistry Data This screen presents a summary of
49. s The BGS may make changes to the software or documentation made available on its web site vii may be out of date and the BGS makes no commitment to update such materials The BGS assumes no responsibility for errors or omissions in the software or documentation available from its web site In no event shall the BGS be liable to you or any third parties for any special punitive incidental indirect or consequential damages of any kind or any damages whatsoever including without limitation those resulting from loss of use data or profits whether or not the BGS has been advised of the possibility of such damages and on any theory of liability arising out of or in connection with the use of this software The use of the software downloaded through the BGS site is done at your own discretion and risk and with agreement that you will be solely responsible for any damage to your computer system or loss of data that results from such activities No advice or information whether oral or written obtained by you from the BGS or from the BGS web site shall create any warranty for the software MINDEC manual version 1 0 April 2002 Table of Contents INTRODUCTION cuca ais 1 11 Backers 1 A reren teert eere an et ae Do eee ae 2 2 BACKGROUND cuco lai aaora lcheeaydgeiaelalenesgalselealaitinnayetguaeabaldiancaalseddalaldusmeenoudets 3 2 1 Mine waste and mine waters oocccccccnoncccccccnnnnnccnncnconnncnnnnnnnnnnnccncnnnnnnacncnncnnnnnnos 3
50. span over which an average daily dose of each contaminant of interest is to be calculated Exposure time that part of the lifetime during which the receptor has been exposed to the carcinogen Note that the check boxes in the Calculate frame are for information only and cannot be re set by the user The carcinogenic risk calculation is not available if data for arsenic have not been entered The toxic hazard averaging time is set to 1 year and cannot be adjusted by the user 42 MINDEC manual version 1 0 April 2002 4 5 13 Leachate Concentrations Enter the leachate concentrations in mg l for elements for which data are available by pressing the Data option button to activate the appropriate data input box Then type the concentration value into the box or use the up down spin buttons to scroll to the required value 43 MINDEC manual version 1 0 April 2002 4 5 14 Leachate Flux The leachate flux from a mine waste source is calculated over the total area of the waste pile The user must provide values for precipitation rate cm yr and area of the waste pile m The recharge rate will depend critically on the permeability of the waste material and on climatic factors which affect the evaporation rate from the surface of the mine waste The Mine waste module button allows the user to calculate the potential lifetime of acid generation at the site Click on the navigation images in the top right corner of
51. spersion equation to calculate the extent of contaminant dilution during groundwater flow This approach assumes that flow occurs through a uniform aquifer that can be modelled as a porous medium with known aquifer properties The possibility that fractures or highly permeable rock units can provide faster pathways for the migration of contaminants may significantly affect the accuracy of MINDEC s predictions by allowing contaminants to arrive earlier than anticipated and at higher concentrations than expected In general MINDEC does not consider changes in chemistry during transport in any detail other than those that can be approximated using a simple decay factor as these changes are likely to be quite site specific In reality this approach is likely to prove conservative since in many cases a number of natural attenuation and retardation processes such as sorption precipitation volatilisation may act to reduce contaminant concentrations and or slow contaminant movement MINDEC does not consider transport through an unsaturated zone that may exist between the mine waste and the water table In effect this movement is assumed to be instantaneous and to involve no changes in contaminant concentrations A number of the parameters which are required as input to the model may be difficult to estimate In general the extent of any uncertainty in the parameter values chosen should be assessed and the likely effect on MINDEC output exam
52. te percentage values for these various sources since some water may arrive at the receptor via a mixed surface groundwater path For example water abstracted from a well immediately adjacent to a river might contain contaminants that have been transported largely within surface water before being drawn into the well It would therefore be more appropriate for MINDEC to consider this water as surface water rather than groundwater since groundwater transport calculations would over estimate travel times and miscalculate the extent of any dilution and dispersion Similarly careful consideration should be given to the possibility that contaminants might reach the receptor after migrating first through groundwater and then discharging into surface water An effective combined concentration at the receptor site is calculated by MINDEC based on the relative volumes of water consumed from these three sources as follows Cecom Dg Cg Ds Cs Da Ca where Cecom Effective combined concentration mg l Cy Concentration in groundwater mg l D Volume of groundwater consumed as proportion of total Cs Concentration in surface water mg l D Volume of surface water consumed as proportion of total Ca Concentration in alternative source water mg l 20 MINDEC manual version 1 0 April 2002 Da Volume of alternative source water consumed as proportion of total The combined concentrations can be compared with w
53. ted using a 3 dimensional solution to the advection dispersion equation and the source term and aquifer property data entered into the model It is assumed that the concentrations cannot fall below the groundwater background concentrations that have been entered into the model By default the time scale x axis extends from zero to twice the age of the source term as input by the user Zero represents the time at which the leachate first entered the groundwater system No account is taken of the time taken by the leachate to migrate through the unsaturated zone between the source term and the groundwater The indicated time therefore provides a conservative estimate of groundwater travel times To adjust the time and concentration ranges shown or to select a different element to plot press the Re plot button When comparing re plotted graphs attention should be paid to the concentration scale y axis as this may adjust automatically to display most effectively the values to be plotted If an appropriate water quality standard WQS concentration is available from the WQS data set selected previously this value is plotted as a red line so that this concentration can be compared with the predicted groundwater concentrations The displayed graph may show concentrations rising with time as a plume of contaminated groundwater arrives at the receptor or concentrations which are unchanged with time Constant concentrations may indicate that e
54. the window to move directly backwards to the MINDEC Source Term screen if required Note that it is not possible to jump forwards to the Transport or Receptor screen 44 MINDEC manual version 1 0 April 2002 4 5 15 Mine waste composition and weathering rate The mine waste module provides a very approximate indication of the potential lifetime of acid generation at the site by calculating the time required to oxidise all the pyrite in the waste pile The long term buffering potential of calcite in the waste pile is also assessed by calculating the time required to dissolve all the calcite in the waste pile Enter the appropriate data where available after activating the corresponding data input boxes by un checking the Not known boxes Calculation of a pyrite lifetime requires a leachate flux a mine waste volume a waste pyrite content and a leachate sulphate concentration Calculation of a calcite lifetime requires a leachate flux a mine waste volume a waste calcite content and a leachate calcium concentration The leachate flux is displayed on this screen but the value can only be adjusted by returning to the Leachate flux screen The lifetime calculations involve a number of assumptions that must be fully understood before the results can be interpreted Pyrite lifetime The lifetime of pyrite in the waste pile is calculated on the basis that 45 MINDEC manual version 1 0 April 2002 e the sulphate concentra
55. tion v is advective velocity and D is the dispersion coefficient the subscripts x y and z referring to the cartesian co ordinate direction The dispersion coefficient is defined as follows D av D Here _ is the dispersivity and D is the diffusion coefficient normally ignored in calculations because of its small size compared to the product of _ and v The advective velocity v is calculated from Darcy s Law k dh n dx where k hydraulic conductivity dh dx hydraulic gradient _ effective porosity Based on the work of Lallemand Barres and Peaudecerf 1978 dispersivity is usually estimated to be 10 of the flow path length and is essentially independent of aquifer type 17 MINDEC manual version 1 0 April 2002 The ADE can also be used to estimate the transport and attenuation of contaminants that are reversibly adsorbed and result in retardation in contaminant transport rate The ADE assumes that the porous medium is homogeneous isotropic and saturated with fluid An analytical solution suitable for the situation of a mine site releasing leachate into an aquifer is given by Domenico 1987 as C 4 C x y z t exp A i 1 ae 8 2a v Aha x vtl 5 P eds NY 505 re 2 2 Ea e aa E gt afl zig where Co is the initial concentration and _ is the decay constant This equation is readily manipulated in a spreadsheet and has been implemented within MINDEC 3 7 Contamin
56. tion value entered equals the annual average sulphate concentration of the leachate e all of the sulphate in the leachate originates from sulphide oxidation e the current rate of sulphide oxidation will continue until all the pyrite has been consumed Calcite lifetime The lifetime of calcite in the waste pile is calculated on the basis that e the calcium concentration value entered equals the annual average calcium concentration of the leachate e all of the calcium in the leachate originates from calcite dissolution e the current rate of calcite dissolution will continue until all the calcite has been consumed The calculated lifetime values should be taken as crude approximations They may however provide general guidance when comparing sites 46 MINDEC manual version 1 0 April 2002 4 5 16 Receptor Module Leachate from the waste pile may remain within the groundwater system or may issue at the ground surface typically at or near the base of the steep sides of the waste pile and then enter surface drainage The program user is therefore required to specify the percentages of the leachate which are transported via the surface drainage and groundwater systems MINDEC considers the possibility that a water consumer the receptor ingests water from a variety of water sources e Surface water potentially contaminated by the source under consideration e Groundwater abstracted from a well borehole potentially contaminated b
57. tly confused they are not synonymous At the most basic level hazard equates to danger and in the risk assessment context a hazard exists if a potential exists to cause harm Conversely risk is the likelihood of an adverse event occurring in response to a hazardous situation 21 MINDEC manual version 1 0 April 2002 A risk assessment is subdivided into three stages essentially following the classical source pathway receptor model They are 1 Hazard identification i e the contaminated groundwater and the metals present 2 Exposure assessment i e determination of contaminant concentrations at the receptor 3 Dose response assessment based on contaminant ingestion by the receptor The preliminary step in exposure assessment is the construction of a conceptual model that represents the exposure pathways In MINDEC the exposure pathways are via the consumption of surface and contaminated groundwater The dose assessment is achieved by estimating total environmental exposure to a particular hazardous compound identified in the source Elements derived from mine waste either constitute a toxic hazard or in the case of arsenic both a toxic and a carcinogenic hazard 3 9 1 Toxic Risk The general practice is to assume that a toxic chemical has a threshold below which toxic effects do not occur Toxic hazard estimates are expressed relative to a reference dose concentration The reference dose is an exposure that
58. tuations in concentrations which occur during the year as a result of a number of time variant climatically induced factors including e variations in infiltration rates through the waste pile e variations in surface water flow rates e variations in groundwater flow rates and e changes in chemical conditions within the waste pile and along contaminant pathways For example rising groundwater levels within the waste pile and or rises in river levels may cause sudden increases in metal concentrations as highly soluble secondary minerals are dissolved and re mobilised from the unsaturated zone This is a particularly important mechanism in Chile for example where jarosite hydrolysis is an important mechanism of AMD ARD generation in dried up stream beds at the beginning of the rainy season MINDEC manual version 1 0 April 2002 3 THEORETICAL BACKGROUND TO M NDEC 3 1 Conceptual model Site evaluation using MINDEC is achieved by considering the contaminant source term migration pathways and potential receptors in turn Figure 3 1 Contaminant concentrations are estimated at a chosen receptor site by estimating leachate flux and composition dilution during surface water transport and advection and dispersion during groundwater transport Calculated concentration values can be compared with water quality standards and combined with water consumption data from the receptor site to calculate the risk associated with water intake 3 2
59. y the source under consideration and e Water from some alternative source possibly uncontaminated Enter appropriate values into the boxes and then press OK Click on the navigation images in the top right corner of the window to move directly backwards to the MINDEC source term leachate flux or transport screens if required 47 MINDEC manual version 1 0 April 2002 4 5 17 Relative doses at receptor from different water sources This screen presents a summary of calculated relative doses to which the receptor is exposed through consumption of surface water groundwater and water from an alternative source Doses are calculated from estimated concentrations ion the different waters and the relative volumes of these waters consumed by the receptor The relative doses should be interpreted bearing in mind the large number of assumptions that are inherent within the modelling process 48 MINDEC manual version 1 0 April 2002 4 5 18 Risk assessment direct input of concentrations in water at receptor site If analytical data are available for the water at the receptor site then enter the data in the appropriate boxes and press the OK button to carry out a human health risk assessment Data can only be entered for those elements for which reference doses have been established As Cd Cr Hg Mn Ni Se Zn since these values are required for toxic hazard quotient calculations 49 MINDEC manual version 1 0 A

CR/02/006N MINDEC user guide.

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