Practical Assessment Techniques for the Impact of Acid Mine Drainage on Riverine Systems

Indian Journal of Engineering & Materials Sciences Vol. 5, August \998, pp. 147-161

Practical assessment techniques for the impact of acid mine drainage on riverine systems

N F Gray Department of Civil, Structural and Environmental Engineering, Trinity College, University of Dublin, Dublin 2, Ireland Received 10 June 1997

New technical procedures and protocols have been developed to assess the environmental impact of acid mine drainage (AMD) on riverine (lotic) systems. The impact of AMD is characterised at community level to identify and prioritise key mechanisms. Conductivity is used as a rapid field method to assess the strength of AMD and the degree of contamination of both surface and ground waters. Toxicity is assessed using both the Activated Sludge Inhibition Test and the Microtox Bioassay procedure. Evaluation and calibration of test methods should be done using artificial AMD. Fish toxicity testing is also examined, and in-situ toxicity assessment using macro-invertebrates is also evaluated for riverine conditions. Biological surveillance and sediment contamination assessment procedures are reported. A substrate-classification index provides a rapid visual assessment of AMD impact on rivers; while an objective water quality index allows sensitive classification of both AMD and contaminated waters, regardless of relative variation of key parameters. To be effective, these new techniques should be used within the framework of a remediation or management strategy. An example of such a strategy is given for Avoca mines in Ireland.

Acid mine drainage (AMD) is a multi-factor The major effects in each category are reviewed in 2 pollutant'. It affects aquatic ecosystems via a detail by Gray and Sullivan • number of direct and indirect pathways. Major The impact of AMD on rivers is very difficult to impact areas are coastal waters, rivers, lakes and predict due to the variability of discharge rate from estuaries, although AMD affects different aquatic adits, variation in adit strength and composition ecosystems in different ways. Due to its complexity, which often varies seasonally, the .effect of surface the impact of AMD is difficult to quantity and runoff from exposed areas of the mines during predict in lotic systems. It is simplest to categorise heavy rainfall, and the effect of the catchment AMD into a number of pollutional categories. These discharge characteristics affecting dilution, and the are (a) metal toxicity which affects the biota both concentration of organic matter in the water directly and indirectly through bioaccumulation and chelating soluble metals present. Assessment is biomagnification, (b) sedimentation processes which difficult due to the complexity of the impacts, can be further separated into turbidity and deposition although diversity and abundance are key variables of iron (III) hydroxides, (c) acidity and the for biotic evaluation. Fish movement and migration destruction of the bicarbonate buffering system, and is also a useful indicator. However, there has to be a (d) salinization. The effects of AMD are complex balance/compromise drawn between simplicity and but can be categorised as physical, chemical, actual interactions. Actual systems may be so biological and ecological (Fig. 1). In. order to complex that no useful information can be obtained evaluate the impact the interactions of each from attempting to model them, while a simpler pollutional category must be considered in detail at approach, concentrating on the major interactions the community level. This is most easily done by (e.g. toxicity of key metals or the degree of substrate using flow diagrams of the major interactions, modification caused by iron precipitation which is allowing the assessment of the impact to be directly linked to pH), may prove to be more useful conceptualised in a step-wise approach (Figs 2-5). in understanding AMD impacts and predicting them.

/ \ 148 INDIAN J. ENG. MATER. scr., AUGUST 1998

Increased acidity Substrate modification Behavioural Habitat modification Reduction in pH Increase in stream Respiratory velocity Destruction of Reproduction Niche loss bicarbonate Turbidity buffering system Osmoregulation Bioaccumulation within food chain Sedimentation Acute and chronic Increase in toxicity soluble metal Adsorption of metals Loss of food Death of sensitive concen trations onto sediment source or prey species Increase in Reduction in turbulance Elimination of particulate metals due to sedimentation Acid-base balance sensitive species increasing laminar flow failure in organisms Reduction in Decrease in light Migration or primary penetration avoidance productivity

Food chain modification Fig. I-Summary of the major effects of AMD on a lotic system

Between June 1993 and June 1995 Trinity sulphate ions. As sulphate is a difficult anion to College co-ordinated a European Union sponsored measure directly in the field then conductivity, for project (Contract: EV5V-CT93-0248) entitled which accurate and robust electrodes and meters are biorehabilitation of the acid mine drainage available, is ideal for routine field screening of water phenomenon by accelerated bioleaching of mine samples for AMD contamination. Sulphate analysis waste. The role of Trinity College was to look at the is normally carried out by Ion Chromatography impact of acid mine drainage on riverine systems which requires sample dilutions. The use of and to develop new assessment procedures and conductivity ensures accurate sulphate analysis by protocols where they were deemed necessary. selecting ideal dilutions. Below a number of new assessment techniques for Conductivity can be used to predict sulphate evaluating the effect of AMD on riverine systems concentration in both AMD and contaminated are described. surface waters using regression analysis. Most accurate predictions are achieved by using equations Techniques given for specific conductivity ranges or AMD Field assessment of AMD contamination using conductivity sources. However, for general use with AMD Both sulphate and conductivity are excellent (including raw AMD, surface runoff from spoil and indicators of AMD contamination. This is due to workings, and leachate streams or adits) then sulphate being an end product of pyrite oxidation. sulphate (y) can be predicted from' the conductivity Unlike pH, both parameters are extremely sensitive (IlS/cm) (x) using Eq. (\): to AMD even where large dilutions have occurred. y=-1974+1.67x ... (1) The advantage of using sulphate to trace AMD is FOr impacted surface waters the general equation that unlike other ions it is not removed to any great (2) should be used: extent by sorption or precipitation processes, being unaffected by fluctuations in pH. These two y = -69.5+0.77x ... (2) parameters are also closely associated, as would be Fytas and Hadjigeorgiou' have shown that expected, as conductivity is especially sensitive to intermittent manual sampling does not adequately

\ GRAY: ACID MINE DRAINAGE ASSESSMENT 149

METAL I TOXICITY I I t t I Direct I Indirect

I bioaccumulationl I biomagnification

~ loss of loss of loss of tolerance modified heterotrophs sensitive f+- sensitive behaviour, and periphytoil develops plant animal reproduction species species

+ loss of decline elimination elimination herbivores in 1° of I-- of and grazers production consumers predators

accumulation, exclusion, behavioural chanzes L..J 10&8 of habitats I

Modification of food chain, I ./ , ..••. ~ especially higher trophic levels I

reduction in species ./ r-, ./ r-, diversity.

Fig. 2-1mpact of metals arising from AMD on lotic systems describe the variability of AMD, and recommend ideal parameter for sampling and monitoring acid the use of continuous monitoring. Conductivity is mine waters. an extremely reliable, accurate, simple and cheap There is also potential to use conductivity to parameter to monitor continuously. In contrast, predict approximate concentrations of key metals sulphate is a difficult ion to monitor in the field, when the pH of the water is within their respective and especially continuously, as there is no ion- solubility ranges (Table It. specific electrode available. Automated calorimetry Toxicity assessment of AMD can be used, however, due to the presence of iron oxides in AMD and natural humic acids in rivers Toxicity assessment selection such as the Avoca River, as well as other In order to identify rapid, sensitive and repeatable influences, automated sulphate analysis in the field toxicity tests for assessing the environmental impact is currently unreliable, lacks precision and is very of acid mine drainage, a literature review was expensive. Therefore conductivity appears to be the undertaken'. Macroscopic test organisms such as

I

\ VI -o SEDIMENTATION I I Sediment I , I Turbidity, , decrease clog filters, gills Toxic modification! inhibition plant tissue loss in 1° I light and feeding sediment destruction production of vision z I I smothered penetration mechanisms e of substrate ~ ~ ~ ~ • reduction Direct/indirect reduction in ~ I I habi tat loss : plants I I reduction in I in feeding p toxicity I eliminated productivity photosynthesis <, efficiency , ~ reduction in CIl reduction in species diversity herbivores! plant growth species move ! I grazers suppressed out of area or P [ are eliminated > loss of § CIl I . I I decrease in I herbivores! .., modification of food cham I I olant species I grazers :g 1 00 / reduction in habitat diversity

t ~ reduction in ~ modification species diversity of food chain

Fig. 3-Impact of sedimentation processes arising from AMD on lotic systems GRAY: ACID MINE DRAINAGE ASSESSMENT 151

BUFFERING SYSTEM

t t Alkalinity! pH Hardness I Acidity I I I I I I

ACID MINE DRAINAGE

I reduction I Destruction of inpH bicarbonate I I buffering capacity

organism acid-base balance affected

increasing ionic imbalance decrease in spp metabolic ..- develops across using bicarbonate geochemical malfunction cell membranes as inorg. source I effects I

decrease in elimination 0 1 producers of sensitive decomposition mainly of minerals species macrophytes I I

I metal release I ! (see metal toxicity) Decrease in habitat I ! diversity reduction in species diversity

Modification of t • food chain

Fig. 4--lmpact of acidity arising from AMD on lotic system

Daphnia sp '., macro-invertebrates and fish were not heterotrophic micro-organisms which an: considered due to a lack of repeatability, sensitivity comparable to the basic trophic level in most lotic at low levels of contamination, and cost. Only systems. For this reason treatability and microbial and biochemical test methods were biodegradability test methods were also evaluated. examined, with particular attention paid to those While there is a wide variety of microbial and methods employing mixed populations of enzymatic methods for the assessment of toxicity,

/ \ 152 INDIAN J. ENG. MATER. sct., AUGUST 1998 treatability and biodegradability, no reference was emitted by a lyophilised luminescent bacterium found in relation to their use with acid mine Photo bacterium phosphoreum. On exposure to drainage. The review examined biochemical tests, toxicants the light output is reduced and this both enzymatic and ATP assays; bacterial tests, reduction is proportional to the toxicity of the test including assays based on the measurement of substance. The rate of oxygen uptake is a useful growth inhibition and viability of bacterial cells, parameter for assessing whether bacteria are in a inhibition of oxygen uptake, substrate utilisation, normal, healthy and active state. The respiration rate and carbon dioxide production; microcalorimetry; of a bacterial culture -vill respond rapidly to the microcosm toxicity tests; and continuous simulation presence of inhibitors. This is the principle on which tests. The review identified three tests which could the other two tests selected are based. (b) The potentially be used for the routine assessment of acid Biochemical Oxygen Demand (BOD) Inhibition Test mine drainage toxicity. (a) The Microtox Bioassay measures the effect of a toxicant on the oxygen Test is based on the measurement of the light uptake by micro-organisms grown on a known, readily degraded substrate as measured by the 6 standard five day BOD test ,7 (c) The Activated Sludge Respiration Inhibition Test uses a mixed population of micro-organisms collected from the activated sludge sewage treatment process. Activated sludge, in the presence of synthetic sewage will respire rapidly, and the addition of a toxic concentration of a chemical will result in a decrease in the respiration rate proportional to the toxicity of the toxicant",

Artificial acid mine drainage Increase conductiVIty Increase in so.. utilisation by Acid mine drainage (AMD) collected in the field micro-organisms is often highly variable and deteriorates rapidly on storage. This makes laboratory based studies on

Osmoregulating problems toxicity, treatment, buffering and dilution of AMD difficult and expensive. To overcome this problem AMD was simulated in the laboratory for routine use", Simulated AMD was found to be very similar elimination of chemically to real AMD collected directly from the sensitive species mine adits, yet it ensured a consistent material in Fig. 5--Impact of salinization arising from AMD on lotic terms of strength and composition. The toxicity of systems simulated (ECso 4.10%, sd 0.68) and real AMD

Table l=-Correlation between conductivity (~S/cm) and pH, zinc (mg/L), iron (rng/L), copper (mg/L), cadmium (ug/L) and

sulphate (mg/L). LeveI s 0f sigrusi iftcance are p<0.05* ,p< 0.01 ** , an dp

Acid mine drainage

RawAMD 21 -0.628** 0.581·* 0.784*** 0.976*** 0.976**· 0.983··· Surface runoff 16 -0.703·· 0.518· 0.985·** 0.956··· 0.926··· 0.985··· Leachate 51 -0.768*·· 0.732**· 0.900··· 0.891··· 0.799*" 0.862···

Rivers and streams

Upstream mines 31 0.218 -0.343 -0.210 0.018 0.430· Downstream mines 11 -0.085 0.850*** 0.699* 0.790· 0.337 0.743·* Contaminated stream 11 -0.567 0.813·* 0.701* 0.702* 0.694* 0.998*·* tn is the number of samples

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(ECso 4.69%, sd 0.76) from Avoca mines, as metabolism. From the data simple linear models measured by the Microtox bioassay procedure, is have been derived to estimate inhibition within similar (p>0.05) using a 5 min test interval. Acidity surface waters so long as the pH of the river water for hydrolysed samples was also similar at 1,320 after mixing and the dilution of the AMD is

mgIL as CaC03. The simulated AMD is flexible known 10.Most accurate predictions are made using with the final material easily adjusted to give any equations for specific pH ranges (Table 2), while a strength of AMD in terms of metal and sulphate less accurate estimation can be obtained using the concentrations or pH, or any combination of cations general equation (3): and anions. Simulated AMD is convenient for Inhibition (%) = assessing the effects of buffering, dilution, treatment -2.34 pH + 6.41 AMD dilution + 22.1 ... (3) process and toxicity in the laboratory, although it should be used in conjunction with real AMD to The Microtox Bioassay (using the 15 min confirm results obtained. The use of non-analytical interval) proved to be a rapid but expensive toxicity grade salts in the preparation of simulated AMD screening test. Work on the Avoca River has shown ensures a wide spectrum of trace contaminants as that the leachate from the main Adit draining the found in real AMD. The simulated AMD should be mines at Avoca (i.e. the Deep Adit) had an ECso of made up fresh daily and stored at 4°C. Metals do 2.20% (sd 0.37), while of the river sites examined precipitate out of solution with time, especially Fe only the site immediately below the mixing zone and Cu, although not as readily as real AMD9. was found to be toxic with an ECso of 67.1 % (sd 13.5). Simulated AMD was used for comparative Toxicity assessment of AMD work on Microtox bioassay and was found to be The BOD Inhibition Test was found to be slightly more toxic with an EC50 of 1.43% (sd 0.55). unreliable in practice with extensive precipitation Simulated AMD was also used to examine the effect and bacterial development within the BOD bottles of alkalinity on toxicity. Toxicity was found to over the incubation period. It was found that decrease when AMD was diluted with water with significant proportions of the metals were present increasing alkalinity. For example the EC50 values were complexing out of solution over the period. were 1.30 (sd 0.13) for distilled deionized water Low dilutions of AMD necessary in order to achieve (alkalinity 0 mg/L as CaC03), 1.39 (sd 0.22) for soft stable oxygen readings after the 5 day incubation well water (alkalinity 30 mg/L as CaC03), and 2.63 period resulting in variable ECso values being (sd 0.10) for hard well water (alkalinity 300 rng/L as achieved. It appears that the BOD Inhibition Test is CaC03). Avoca River water had an alkalinity of 40 unsuitable for such a complex inhibitor as AMD. mg/l. as CaC03 resulting in an EC50 of 1.43 (sd In contrast, the Activated Sludge Inhibition Test 0.55)10. was found suitable for toxicity assessment of AMD. The immediate toxicity test procedure was used Fish toxicity initially to define the concentration ranges necessary Atlantic salmon (Salmo salar L.), an indigenous for definitive testing. Concentrations of 50% fish species to the Avoca River, were subjected to simulated AMD caused complete inhibition of the the major cations (Cu and Zn) emanating from the activated sludge. Toxicity fell with increasing pH Avoca mines in combination with ammonia, a major with ECso values of 4.6,5.1,6.5, 10.0, and 16.7% pollutant in the river some 7 km downstream of the simulated AMD at pH 4, 5, 6, 7, and 8 respectively. first adit discharge point. Static renewal bioassays The variability in toxicity was found to be due to the indicated that Cu (96 hr LC50 of 0.036 mg/L) was 13 availability of metals as well as the affect of pH on times more toxic than Zn (96 hr LC50 of 0.479 mglL)II. The survival time usinga lethal mixture of Cu and Zn (in equitoxic proportions) was greatly Table 2--Percentage inhibition of surface waters calculated reduced and produced a 96 hr LC50 of 0.933 toxic for specific pH ranges units, indicating that the metals strongly effect each pH3 Inhibition (%) = 7.62 AMD dilution + 8.2 other's lethal action in the restricted sense of the pH 4 Inhibition (%) = 8.91 AMD dilution + 8.4 pH 5 Inhibition (%) = 8.19 AMD dilution + 8.8 rapidity of death of the fish. A clearly defined pH6 Inhibition (%) = 6.65 AMD dilution + 6.9 incipient LCso of 0.019 mg unionised ammoniaIL pH7 Inhibition (%) = 4.39 AMD dilution + 6.3 was obtained after only a short exposure period. The • pH 8 Inhibition (%) = 2.72 AMD dilution + 3.8 combined effect of Zn, Cu and ammonia in

\ 154 INDIAN 1. ENG. MATER. SCI., AUGUST 1998 equitoxic proportions was found to be 0.952 toxic A significant toxicity effect was only recorded units, i.e., just slightly more than additive. The within the mixing zone of the river, with the position of this toxicity curve was distinctly close to completely mixed water immediately below the the Cu-Zn curve indicating that all the metals were mixing zone showing no significant toxicity. contributing to the overall toxicity of water, as However, while the field toxicity tests indicated that opposed to ammonia. The metal concentrations only the mixing zone was impacted, routine entering the river, expressed as toxic units, indicated biological surveillance has shown the impact to be that the Deep Adit alone imposed on average 247 on a more extensive scale with the entire river below (range 73-359) toxic units per litre on the clean the mines (12km) severely damaged. These results upstream waters. Despite continuous dilution by suggest that factors other than water toxicity are clean water from upstream, the first post-mine responsible for the elimination of species in the sampling site (3km downstream of the mixing zone) lower Avoca, although the water quality may result has a persistent annual toxicity of over 3 toxicity in long term chronic toxicity':'. units, with the mean Cu and Zn concentrations over Biological surveillance 8.9 and 19.4 times higher than that permitted by the Routine biological monitoring of the Avoca River EU Freshwater Fish Directive (78/659/EEC). Seven downstream of the input of acid mine drainage km downstream, the toxicity imposed by the mine (AMD) consistently shows a significant impact on on the river is reduced to less than two toxic units. both diversity and productivity. A number of sites At this point, however, a fertiliser factory were studied to identify if there was a significant discharging ammonia waste water increases the difference in productivity and diversity between overall river toxicity to in excess of 130 toxic units. habitats at individual sites, and to establish the most The use of toxicity assessments allows direct effective sampling methodology to assess the impact comparison of various pollutional inputs into the of AMD. Only three discernible habitats could be river, allowing pollution control strategies to be identified in the Avoca River. These were classed as prioritised in terms of impact!" riffle, glide and pool, although the latter was restricted due to the erosional nature of the river. Six Field toxicity site locations were selected to examine the Field toxicity experiments were conducted in the uncontaminated river, the mixing and recovery Avoca River during August and September 1994 in zones. Five replicate samples were taken as an order to evaluate the toxicity of acid mine drainage even Iy spaced transect across the river at each site 14. (AMD) to a number of selected macro-invertebrate Forty-two taxa were identified during this study, species. The main objective of the experiment was indicating that collection from a wider selection of to design and evaluate a field toxicity method for sub-sites -at each location did not significantly rivers receiving AMD. increase either the diversity or abundance compared A simple experimental flow-through chamber to the routine sampling approach currently adopted was designed and tested in the field. It was found to (i.e. three min kick and stone wash sample at a produce reliable and reproducible results". Three single selected site). However, selection of areas at test macro-invertebrates were evaluated, Gammarus each sample site is shown to be critical, due to the duebeni, Ephemerella ignita and Baetis rhodani. wide variation in diversity and abundance across the Only the former species proved both robust and river. Therefore, it is desirable to select a range of sensitive enough for toxicity assessment work using different sites at each sample location. At all habitat the chambers. Three replicates were used at each test types there was a clear decrease in the number of site comprising twenty individuals per replicate (five taxa and faunal abundance after contamination by per chamber). Large specimens of a small size range AMD, with lowest levels in the mixing zone, and a were used to eliminate size/age effects. Mortality at slow improvement downstream but not full the control site was acceptable at <10%. It was recovery. The percentage representation of found that the difficulty encountered in using the Chironomidae showed an increase in response to more sensitive insects may be overcome by AMD rising to almost 100% within the mixing zone collecting animals in early spring. However, their and then decreased. In general, there was no taxa smaller size may make them more suitable for unique to sites impacted by AMD. As noted above, laboratory-based experiments. the response of the faunal community to AMD was

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\ GRAY: ACID MINE DRAINAGE ASSESSMENT 155 a significant drop in the number of taxa desorption in sediments, showing a significant accompanied by a sharp drop in the abundance of correlation (p<0.00 1). The Zn concentration in the taxa remaining. sediment falls below background concentrations Collection of five replicate pond-net kick samples obtained from the upstream reference site. However, (20s duration) is considered adequate for monitoring the metal came out of solution when the pH the impact of AMD on the Avoca River. While all increased downstream of a fertiliser factory quantitative indices (Taxa numbers, Faunal (pH>8.0), some 7 km below the mine. In contrast Cu abundance and Percentage representation of and Fe concentrations significantly increased Chironomidae in the fauna) demonstrated a (p<0.001) both in the subsurface sediment (0-30 statistically significant impact of AMD on all mm depth) and the surface ochre deposit habitats (Riffle, Glide and Pool), the use of taxa immediately below the mixing zone. Iron numbers in riffle areas was found to have the concentration decreased with distance from the greatest discriminatory power. It is therefore AMD source, whereas Cu showed a second peak in recommended as a simple and effective index". concentration below the fertiliser factory. Copper removal appears to be primarily by co-precipitation Sediment contamination Usually the concentration of most trace elements with iron showing a significant relationship. Organic is much larger in solid or surface phases than in the matter content in sediment was highest at the water phase. In aquatic environments four abiotic sampling site closest to the mine discharge'", reservoirs for metals are distinguished: suspended Higher sediment enrichment factors for all metals matter, bottom sediments, free water phase and pore were obtained in the surface sediment layer (ochre) waters. These four reservoirs strongly interact with deposited on larger stones and in floc material each other (Fig. 6)15. collected in sediment traps, compared to the The discharge of acid mine drainage (AMD) from subsurface sediment. Copper showed particularly sulphur bearing mineral deposits causes iron to strong enrichment in the surface layer and floc. precipitate as hydroxides and the formation .of Cadmium was not recorded in any of the sediment ochreous deposits on the substrate of the receiving collected at the detection limit used (0.01 ug g-I). river system. The sediments in the Avoca River Metal deposition in the sediments was found to be were studied during a low flow period to establish spatially variable, so sub-sampling is required, the degree of contamination and to identify the although replicates show less variation. Results major processes affecting sediment metal indicate that short term variation in metal inputs can concentrations. It was found that the pH plays a be identified by sampling the surface layer or by major role in regulating Zn adsorption and collection of floc material in sediment traps, whereas

dissolved AMD metals River I I .1 1 phase

dissolution precipitation

particulate ...• phase

resuspensionl settling/ desorption adsorption .....---- ....• bottom ...• sediment

Fig. 6-Flow diagram showing interaction of metals with the various compartments of the river system 156 INDIAN 1. ENG. MATER. SCI., AUGUST 1998 sampling of the subsurface layer is more suitable for protocol for AMD impacted riverine sediments identifying long-term trends in sediment quality", which includes sediment sampling, Fe-hydroxide Water and suspended particulate matter (SPM) floc sampling, chemical analysis, interstitial (pore) were collected at weekly intervals during low flow water collection, sediment elutriates, sediment conditions from the Avoca "River (May and August, fractionation and physical analysis. The importance 1994). The total concentration of Zn, Cu, Cd and Fe of bioassay testing as well as quality assurance and was measured in both the dissolved and particulate assessment approaches to define sediment quality phases. Flow measurements of the river and the criteria are also discussed. main leachate streams were taken, to model metal Substrate classification index fluxes. It was shown that the acid mine drainage The overall impact of acid mine drainage (AMD) (AMD) discharged into the Avoca River from the on lotic receiving waters is complex and difficult to two leachate streams (the Deep and Ballymurtagh assess. While low pH, elevated sulphate and metals Adits) decreased concurrently with a decreasing all characterise AMD and these vary both spatially river flow. Concentrations of dissolved Zn, Cu and and temporally, with high river dilutions resulting in Fe generally decreased downstream from the AMD low absolute concentrations which are difficult to input showing maximum metal concentrations at the measure. From field observations the best site closest to the AMD discharge points. No Cd was indications of AMD contamination of lotic systems detected «0.001 mgIL) in either the water or appear to be the brownish-yellow precipitates of iron particulate matter in river samples. (III) hydroxide, high densities of chironomids, and Zinc adsorption onto suspended particulate matter the absence of macrophytes'". Using some of these is clearly influenced by pH, with concentrations of observations, a simple index, based on the degree Fe and Cu higher in the particulate phase compared of deposition of ochre on the river substrate, and to the dissolved phase, suggesting that the main the level of floc formation, has been devised'". The transport mechanism of metals is through index is purely a descriptive one and is calculated association with the particulate phase. Temporal rapidly using a visual inspection of individual sites. fluctuation showed that metal concentrations, The derived index values are related to the particularly Fe and Zn, are strongly effected by biological impact that can be expected (Table 3). hydrological factors in the river and leachate The index value is calculated using Table 4 and streams. Metal fluxes were shown to differ can range from 0 (unaffected by AMD) to 10 (being substantially under various flow regimes, with Zn very severely affected). So the index is directly not removed from the dissolved phase during low related to AMD pollution. Using Table 5 the river is river flow. Using sulphate as a conservative ion enables quantitative predictions to be made with regard to percentage metal contribution from the Table 4---Index for the visual assessment of the impact of acid mine drainage on lotic systems. main adits. A significant relationship between Category No floc Rank Floc collect- Floc collect- percentage contribution of sulphate and the ratio of Floc in ing ing river/AMD discharge was apparent". water between large in areas of low The important factors affecting sampling of stones flow only riverine sediments include sample site location, field A 8 9 10 9 observations, representative sampling, sample B 6 7 8 7 collection techniques and sample preservation. Herr C 4 5 6 6 D 2 3 4 4 and Gray" have prepared a sampling and processing E o I 2 2

Table 3--Severity of AMD impact as assessed by visual index

Biota Index Impact Damage to Macroinvertebrate Substrate Abundance Diversity Fish Macrophytes 8-10 Very severe ++++ 6-7 Severe +++ +/- +/- 4-5 Impacted ++ ++ ++ 1-3 Affected + +++ +++ + ++ 0 Unaffected ++++ ++++ ++++ ++++ / GRAY: ACID MINE DRAINAGE ASSESSMENT 157

Table ~ubstrate categories used in the calculation ofthe substrate classification index. Category A: The substrate is welded or cemented together with a thick smooth layer of iron (III) hy- droxide. The material coats the entire bed so that the substrate is left as a smoothly irregular surface'with- out any exposed areas of the substrate. Dull to bright brownish-yellow in colour, the crust can be prised off, although often very strong, to reveal a thick crust of iron made up of many thin deposited layers. These tend to occur close to adit discharge points. Floc material can be deposited very rapidly at times of low flow, with a thin soft layer of ochre on the surface. Category B: A thick crust only forms on top of the large stones comprising the substrate and is not found on the smaller material between. The smaller substrate may be coloured brownish-yellow or covered with deposited floc but is still loose. Category C: The larger stones have a thin coating of material resulting in the surface being densely coloured brownish-yellow. The stones however do not have a thick or discernible crust formation. The smaller material between the larger stones mayor may not be coloured to the same degree, and is still loose. Floc deposition may be occurring. Category D: The larger stones have a very thin coating of material resulting in the surface being only lightly coloured either fully or partially. The stones are distinctly different to either category B or C. The smaller material between the larger stones mayor may not be co loured to the same degree, and is loose. Floc deposition may be occurring. Category E: The larger stones have no brownish-yellow coloration at all with all the material comprising the substrate free from coloration and loose. There may on occasions be deposited floc material either between stones or in areas of low flow. placed into a category (A to E). This is done by Table 6-Parameters and weighings used in the calculation of visually examining the substrate either from the the AfV1DI Parameter Unit Weighing bank or preferably from within the river. Once a w' category has been selected the effect of the floc I I pH 0.20 formed within the river is used to help differentiate 2 Sulphate mglL 0.25 sites within a single category. This is done by 3 Iron mgIL 0.15 selecting one of four options. If there is no floc 4 Zinc mgIL 0.12 visible either suspended in the water or settled on 5 Aluminium mgIL 0.10 6 Copper mglL 0.08 the substrate, then the lower value is selected. This 7 Cadmium IlgIL 0.\0 base value increases if floes are visible suspended in Total weighing 1.00 the water, and increases further if floc can be seen collecting on the surface of the substrate between larger stones within the river. If floc is seen considered to be of greatest indicator value as they collecting in areas of low flow only, for example in were unaffected by sorption processes, while bank side puddles or areas where there is little or no sulphate was also unaffected by natural flow, then this is considered to be of less neutralisation processes. A weighted water quality significance due to other factors causing flocculation rating table (Table 7) is used to calculate the water of iron (III) hydroxides. quality score for each of the seven variables from Objective index of water quality which the AMDI is calculated using Eq. (4) below. = [L water quality scores]' (4 Temporal and spatial comparisons of acid mine AMDI . .. ) drainage (AMD) contaminated waters are difficult 100 due to the complex physico-chemical nature of the The AMDI as proposed is designed to detect and pollutant. An objective index has been developed quantify contamination from AMD and to help and fully evaluated for the assessment of such categorise samples, quantify impact to receiving waters". The acid mine drainage index (AMDI) is waters and to monitor recovery. A worked example calculated using a modified arithmetic weighted for a sample of water rising as a spring at the base of index using seven parameters most indicative of a worked spoil heap is given in Table 8. AMD contamination. These are pH sulphate, iron, In order to make the index as robust as possible zinc, aluminium, copper and cadmium. Weighting then a correction factor can be used if parameters are was used to express the relative indicator value of not measured for technical or cost reasons. The each parameter (Table 6). The pH and sulphate were correction factor for missing parameters is [1/ (new

\ 158 INDIAN J. ENG. MATER. SCI., AUGUST 1998

Table 7-Water quality rating table for acid mine drainage and contaminated surface and ground waters Score pH Sulphate Iron Zinc Aluminium Copper Cadmium (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) 25 <10 24 10-14 23 15-29 22 30-49 21 50-99 20 ::;;6.5 100-199 19 6.2-6.4 200-299 18 5.9-6.1 300-399 17 5.6-5.8 400-499 16 5.2-5.5 500-599 15 4.9-5.1 600-799 <0.05 14 4.5-4.8 800-999 0.05-0.99 13 4.1-4.4 1000-1499 1.00-4.99 12 3.9-4.0 1500-1999 5.00-9.99 <0.05 II 3.7-3.8 2000-3999 10-24 0.05-0.49 10 3.5-3.6 4000-5999 25-49 0.5-0.9 <1.0 <10 9 3.3-3.4 6000-7999 50-99 1.0-4.9 1.0-4.9 10-24 8 3.1-3.2 8000-9999 100-149 5.0-9.9 5.0-9.9 <0.05 25-49 7 2.9-3.0 10000-11999 150-199 10-24 10-24 0.05-0.99 50-99 6 2.7-2.8 12000-13999 200-249 25-49 25-49 1.0-4.9 100-249 5 2.5-2.6 14000-15999 250-499 50-74 50-99 5.0-9.9 250-499 4 2.3-2.4 16000-17999 500-749 75-99 100-299 10-24 500-749 3 2.1-2.2 18000-19999 750-999 100-249 300-799 25-49 750-999 2 1.8-2.0 20000-21999 1000-1999 250-499 800-1199 50-99 1000-1499 I 1.5-1. 7 22000-24999 2000-2999 500-749 1200-1999 100-249 1500-1999 o <1.4 ~25000 ~3000 ~750 ~2000 ~250 ~2000

Table &--Example of the calculation ofthe AMDI for some Table 9--Classification of contaminated and uncontaminated leachate from spoil at Avoca mines waters by AMDI at Avoca mines. Parameter Value Water quality score AMDI Type of water Qiwi 0-20 Raw AMD with little or no dilution, mainly I pH 2.7 6 seepage from spoil collecting in surface ponds. 2 Sulphate 16750 mg/L 4 0-15 Surface runoff directly from spoil. 3 Iron 820 mg/L 3 15-35 Surface runoff after prolonged rainfall or due to 4 Z~ %mg/L 6 excessive dilution by water from 5 Aluminium 359 mg/L 3 uncontaminated areas. 6 Copper 16 mg/L 4 20-35 Adit discharge. AMD subject to dilution from 7 Cadmium 794 ug/L 3 groundwater. Also interflow entering river at (Summation of scores)? = 292 = 841 bankside and all water collected within flooded Divided by 100 to give an AMDI of 8.4 area of mine. 25-90 Mixing zone of river. AMDI dependent on total 1100)]. For example if aluminium analysis was degree of mixing. Also contaminated surface unavailable then the correction factor would be streams and tributaries not within mining area. 70-98 Impacted river downstream of the mines (1/0.90). So the calculation of AMDI would be: including recovery zone. 90-100 Surface and groundwater uncontaminated by AMDI water quality scores x (1/0.90) = II f AMD. 100 . (5) making up the AMD. However, problems arise in Using the example in Table 8 the new total water discriminating between sites where the degree of quality score excluding aluminium is now 26. Using contamination is very low results in high water Eq. (5) then the revised AMDI is calculated as: quality scores (e.g. impacted rivers and lakes). In (26 x 1I0.90i = 8.4 contrast, low total water quality scores all indicate 100 strongly contaminated water. Therefore, by using The total water quality score is in itself a measure the modified version (Eq. (4» of the arithmetic of the combined effects of the selected parameters weighted index, the AMDI value becomes more Stalre I: StapV: Protocol Fa

c

~ > Q <, o ~ Stal·IV: ~ Remediation Sta,. VI: Sta,e II: options Implementation Data collection Mild development ~ ••rrudlitio" z o> l'I'l

CIl> CIl l'I'l CIl CIl -: ~ No ~ -I JU'IMeIlU!ntAtion not o~..,

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Fig. 7--Schematic layout of the proposed protocol for the' remediation of AMD impacted surface waters VI \0 160 INDIAN 1. ENG. MATER. sci., AUGUST 1998 sensmve at higher total water quality scores levels of reduction of specific metals or pH ensuring a higher discriminatory power. So a total in order to achieve objectives). water quality score of 10 has an AMDI value of 1, (e) Remediation. If remediation is required the 30 an AMDI value of 9, 50 an AMDI value of 25, alternatives are: 90 an AMDI value of 81, and 95 an AMDI value of (i) Treat all AMD from site. 90. Approximate AMDI values for different types of (ii) Try to reduce AMD generation to AMD and contaminated waters are given in Table 9. acceptable levels for safe discharge Conclusions (iii) Try to reduce AMD generation to a level While a wide range of new techniques to help in which can be treated prior to discharge. the evaluation of the environmental impact of AMD (iv) Do nothing and accept permanent on surface waters, in particular rivers, have been reduction in environmental quality. developed, a working protocol is required to help The nature (strength) and volume of AMD can be develop cost-effective and environmentally sound altered by controlling generation and dilution on strategies for the management of AMD discharges site. The optimum strategy for AMD for abandoned and active mines. Such a protocol has reduction/modification by site remediation and the been devised for use at Avoca mines, although it is level (and type) of treatment required must be primarily concerned with assessing the impact of identified through cost-benefit analysis. Some AMD on surface waters making use of the new remediation strategies may have adverse effects on procedures and tools developed described above. the environment and this should be assessed using The protocol must achieve the following: standard risk-benefit analysis. Figure 7 shows the major steps in a working (a) Define the area causing the problem protocol to remediate AMD impacted surface (b) Define the impact. It is critical to establish waters". These are: natural background levels. (c) Identify potential remediation goals. This is Step 1. Site characterisation done by using various criteria for water use. Step 2. Development of an AMD water quality The critical question is whether or not the management protocol which should be AMD discharge needs to be controlled. If the integrated into the existing catchment answer is yes then the reasons why need to be water quality management plan clearly specified. From this, specific usage of Step 3. Remediation strategy the surface water must be specified, e.g., Step 4. Implementation strategy abstraction, amenity, recreation, salmonid- A procedure for developing an AMD water cyprinid fisheries, etc. Each water usage has its quality management protocol, assessing the own set of water quality criteria from which discharges from the mines, and the prediction of the standards can be formulated. The critical and impact of AMD in the receiving water has been physico-chemical indicators of AMD pollution presented by Grall. The AMD water quality must be identified (e.g. pH, acidity, S04, Fe, protocol comprises five stages: (i) The establishment Zn, Cu, Cd, AI, As etc.). These are the of water quality criteria and standards for the parameters actually modified by AMD and so receiving water; (ii) Calculation of the natural are the ones of primary importance in terms of assimilation capacity of AMD by the receiving regulation. What are the I and G values for water; (iii) Impact assessment including projections these key parameters under the various EU over a'20 year period based on future development directives or other legislation? These will be of the site, long term AMD generation assessment the same for adopted standards. If it is the biota and the effect of any site remediation work; (iv) only that is to be preserved (excluding fish) AMD control; (v) Compliance monitoring and then criteria must be established using toxicity review of discharge standards. testing. (d) Management. The key areas here are: (i) Data collection, storage and manipulation. References (ii) Modelling data to simulate impact and 1 Kelly MG, Mining and the freshwater environment establish remediation guidelines (e.g. (Elsevier Applied Science, London), 1988.

\ GRAY: ACID MINE DRAINAGE ASSESSMENT 161

2 Gray N F & Sullivan M, The environmental impact of acid II Gray N F & Sullivan M, Fresenius Environ Bull. (In' mine drainage. Technical Report: 27, Water Technology Press). Research, Trinity College, University of Dublin, Dublin, 12 Gray N F, Procedure for the estimation of the impact of 1997. acid mine drainage to surface waters. Technical Report: 3 Fytas K & Hadjigeorgiou J, Environ os«, 25, (1995) 36- 29, Water Technology Research, Trinity College, 42. University of Dublin, Dublin, 1995. 4 Gray N F, Environ Geol. 27 (1996) 358-361. 13 Byrne C & Gray N F, Fresenius Environ Bull. 4(1995) 5 Kilroy A & Gray N F, Bioi Rev Cambridge Philos Soc, 70 583-588. (2) (1995) 243-275. 14 Byrne C & Gray N F, Fresenius Environ Bull. 4(1995) 6 Methods for assessing the treatability of chemicals and 589-596. industrial waste waters and their toxicity to sewage 15 Herr C & Gray N F, Environmental impact of acid mine treatment processes. Methods for the examination of drainage on the Avoca River: Metal fluxes in water and waters and associated materials, Department of the sediment Part I. Introduction to metal accumulation in Environment HMSO, London, 1982. riverine sediments. Technical Report: 19, Water 7 5-Day BOD (BODs) Test and dissolved oxygen in water Technology Research, Trinity College, University of (2nd ed. ). Methods for the examination of waters and Dublin, Dublin, 1995. associated materials, Department of the Environment 16 Herr C & Gray N F, Water Sci Technol. 33 (1996) 255- HMSO, London, 1988. 261. 8 OECD, Guidelines for the testing of chemicals: Activated Sludge Respiration Inhibition Test. Method 209 17 Herr C & Gray N F, Environ Geol, 29 (1997) 37-45. Organisation for the Economic Co-operation and 18 Herr C & Gray N F, Environ Geochem Health, 19 (1997) Development, Paris, 1987. 73-82. 9 Gray N F & O'Neill C, Fresenius Environ Bull. 4 (1995) 19 GrayNF, Water Res. 30(1996) 1551-1554. 481-484. 20 Gray N F, J Chartered lnst Water Environ Manage. 10 10 Gray N F & O'Neill C, Environ Geochem Health, 19 (1996) 332-340. (1997), 165-171. 21 Gray N F, Environ Geol. 30 (1997) 62-71.

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