Impact of western coal mining—I. Chemical investigations of a surface coal mine sedimentation pond

Water Research Vol 13 pp 1023 to 1031 O Pergamon Press Ltd 1979 Printed m Great Britain

0043d354/79/110|-102350200/0

IMPACT OF WESTERN COAL MINING--I. CHEMICAL INVESTIGATIONS OF A SURFACE COAL MINE SEDIMENTATION P O N D S C TURBAK,G J OLSON and G A MCFETOtS Department of Mlcrobtology, Montana State Unlverstty, Bozeman, Montana 59717, U S A

(Received 3 Aprd 1979) AMtraet--Chemacal mvesngaUons of the sedlmentatmn pond system at the West Decker Mine m southeastern Montana, U S A , were undertaken during an 18 month period from November 1975 to April 1977 Sedimentation pond water, essentmlly "altered" groundwater, differed chemically from the average groundwater m coai-beanng aquifers The mcrease in certain chemtcal constRuents (sodmm, sulfate, btcarbonate and inorganic mtrogen) and the decrease m dissolved oxygen m pond water during autumn 1976 and winter 1977 was the result of erratic pond operatton, dredging and mcreased nearby manmg actlwty Ehscharge from the pond comphed wRh the gmdehnes advanced for effluents from surface coal manes Levels of mercury m water samples collected from the sedlmentatmn pond system were htgher than those recommended for the protecUon of aquatm hfe and wfldhfe The smtabflRy of pond water for selected purposes and recommendations for tmprovement m pond design are discussed

tzatton of western sods so that dust production and the sedmaentatlon of watersheds m mamng-|mpacted areas may prove problematic (U S Envtronmental Protection Agency, 1976) The soluble salts content of overburden matermls m western strip manes such as the Edna Mine m Colorado (McWhorter et al, 1975) may have the most stgraficant effect on water quahty Although reformation on the quahty of coal and overburden and the quantRy of prectpitatlon has permatted speoalauon on the nature of envtronmental perturbatmns from coal mmmg m the West, detailed studies are needed to assess their magmtude Speafically, a paucRy of mformatmn exmts on the quahty of waters originating from mined areas (Gdley et ai, 1976) and tht~r potent|al impact on the hmated surface water resources In November 1975, mvesngauons were maUated on water quahty alteratmns assoctated wRh the actlvmes of a s~zable open pR mme located in the Fort Umon FormaOon of the Northern Great Plains Coal Province The role of a coal mine sedlmentaOon pond m influencing the chemical composmon of mme dtscharge waters has been deterwaned to improve pond design and apply the Improvements to the operatmn of future mines

INTRODUCTION

Approxtmately one-half of the Umted States' known coal reserves are located m a s~x-state sector of the West whmh includes Colorado, Montana, North Dakota, South Dakota, Utah and Wyomang This geographical area is des|gnated by geologmts as the Northern Great Plains and Rocky Mountmn Coal Provinces and contains coal deposRs ranking from hgnlte to bitummous (U S Environmental Protection Agency, 1976) Major scale efforts to remove these resources by surface extracUon methods are ongoing and proposed (Atwood, 1975, Gnflith & Clarke, 1979), Both the quahty of the coal and the effects of land disturbance caused by surface coal mamng differ m the western reserves compared to those m the East and Midwest Coal from the Northern Great Plains has a lower sulfur content (by weight) than the eastern and wadwestern coals, averaging 0 6% and ransmg from 0 1 to 4 0% m those resources currently mined In these low sulfur coals, the relanve percentage of organm sulfur tends to be higher, thus, there is a comparaUvely lower amount of morgamc sulfur such as that found m the manerals pyrRe and marcame The potent|al for acid productaon from ox~chzed pyrite assocaated with subsurface waters exmts m coalbeanng strata throughout the Umted States Because of the lower pynUc content of overburden materials and coal and the high buffering capac|ty of surface waters m coal-rich regmns of the West, acid mane drainage is not w|despread In addition, rubbhzed overburden Is not subjected to high annual preclpltanon m the sema-and western states Lower potenuals for water-mmated erosion may also extst, however, the sparseness of vegetatmn decreases the stabd-

SITE D E S C R I P T I O N

Szgmficant quantmes of subbRummous coal are extracted from the West Decker Mine located m southeastern Montana lmmexhately adjacent to the Tongue River Reservoir (Fig. 1)~ As of July 1977, the area of the actively mined pit was 193 hectares. Pre. wous to mamng actmty at West Decker, groundwater flowed along an hydranhc gradient dlschargqng toward

1023

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the Tongue Rwer (VanVoast, 1974). The first mmmg cut directly intercepted groundwater flow m two aquifers, the Dletz-1 (D-l) overburden and the D-I coal, resulting m lateral movement of water into the mine pit from all chrecUons In adchtlon, groundwater entered the pit vertically from below as leakage from another aquifer, the D-2 coal. Thin groundwater was pumped out of the mane pit into a sump wbach also recaved groundwater from the ailuwum along the Tongue River and from small seepage channels emanaUng from rubbhzed overburden and an exposed coal wall. Sump water was pumped into the sedimentation pond, the level of which was controlled by a standpqm at the southern end. Overflow water, referred to as the mane effluent, was discharged onto the Tongue Paver flood plato at the upper end of the reser¢oir (Fig 2) At the tune of this study, the sedunentatton pond was 1 m deep wtth an area of 1 2 hectares RetenUon tune was approx|mately 6 2 days. From July 1975 to July 1976, the outflow averaged 0017m 3 s -I (Decker Coal Company, personal commumeation). During the course of thin mvesUgntton, the sedimentatmn pond system did not operate continually. The pond itself underwent slight changes m morphometry as well These deviations from mJUal condmom mciuded pond stagnaUon during the autumn 1976 through the ranter 1977 and pond dredging m December 1976

MATERIAI.S AND METHODS

Sample collection

Water samples from three to mx mt~ m the sedtmentafiou pond system at the West Dee~er Mine were collected apprommately once a mouth from November 1975 to April 1977 Sttes were chosen to

represent sources of mflovang, intermediate and outflowing water Measurement of chemical constltuents

Cakaum, magnesium, hardness, sodmm, potassium, alkahmty, bicarbonate, carbonate, sulfate, chloride, &ssolved oxygen, total ~ron, total non-filterable reindue, pH value and mercury were determined by the methods described m Standard Methods (1976) Nttrate-mtrogen was measured by the procedure m Barnes (1959). The method of Stnckland & Parsons (1972) was used for sulfide Methods for the measurement of other nttrogen species, phosphorus species, fluoride, rcacttve silica, organm carbon, specific conductanc¢, turbidity, lead, cadmmm, arsenic and selemum are listed in Turbak et al (1979) RESULTS AND DISCUSSION

Extensive chermcal data were generated from this study (Turbak et al, 1979), however only summarized resuRs are discussed herein General

The water quahty of the sedunentat~on pond reflected changes m rmnmg actiwty and in pond operation and morphometry ConcentraUons of dmsolved oxygen, bicarbonate, sodium, sulfate and mtrogen species (prunarlly nltrate-mtrogen) differed m the November 1976-February 1977 samples compared to prcvtous data The most marked dLfferences are Illustrated in Fig. 3 With the ex~ptton of dissolved oxygen, these constgtu~nts may have showed increased levels m response to pond dr~gmg whw,h probably caused solutes m the sediments to be released into the water column Reduced chemical spe~es m these sediments appeared to exert a high

Impact of western coal minlng--I

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Fig, 3 Graph of the concentrations of selected chemical constituents m water samples collected from the sedimentation pond at the West Decker Mine

1026

S C TURBAK,G J OLSONand G A MCFFTERS

chemical oxygen demand, thus decreasing the &ssolved oxygen to an average concentration much lower than that reported for the previous year Pond stagnatmn and ~ce cover may have reduced atmospheric oxygen exchange and allowed processes which consume oxygen to exceed those which release ~t The dramatic increase m levels of mtrate-mtrogen was not solely the result of that released by dredging VanVoast & Hedges (1975) observed sporadically high concentrations of nitrate m the pond effluent and attributed them to an input of e~ther ammonium mtrate residues from explosives used for overburden blasting or leaching of mtrate available from other disturbed materials Because ammonium nitrate explosives were stored m vats approximately 50m away from the pond, runoff containing these materials may have enriched the pond water in mtrate-nltrogen Higher concentraUons of nltrate-mtrogen m the pond compared to prewous data were most likely the result of increased mining activity nearby Despite the limited opportanmes to collect both pond mfluent and effluent samples, the mfluent was chemacally &fferent from the effluent &scharged onto the Tongue River flood plain Effluent samples and those collected from the pond were slmalar Average analyses of four mfluent samples collected from July to October 1976 revealed that mflowmg water was characterized by relatively low &ssolved oxygen (51mg 1-'), lower pH value (75 standard units), higher concentration of total iron (021 mg 1-1) and the presence of sulfide (021mg 1-1), compared to average values of &ssolved oxygen (105 mg 1-1), pH (84 standard umts), total iron (0 11 mg i -1) and undetectable level of sulfide in effluent samples collected on the same occasions Water entenng the pond underwent aerataon w~th the concomitant loss of sulfide The &fferences in pH values observed between the mfluent and pond or effluent may have been due partrolly to a moderate level of photosynthetic acUwty within the pond during those months (Turbak et al, 1979) The West Decker pond functioned primarily as a se&mentaUon basin, however, greater quantmes of suspended parUculate matter were detected m the pond (17 mg 1-~ average total non-filterable residue) than in the mfluent (3 mg I-1) This observatmn suggested that fugmve dust and se&ment in runoff may have contnbuted to the levels of particulates m the pond Heavy truck traffic was observed continually on the roads adjacent to the pond In ad&tton, nearby blasting produced clouds of dust which usually drifted over the pond The pond was shallow and was subjected to considerable wind-driven rmxmg wluch may account for relatavely h~gh concentraUons of paruculates Suspended materials were largely abmuc m nature Samphng may have been too infrequent to collect data on mtermattent, turbid mflowmg water (referred to as secondary mfluents) which contamed sagmficantly l'agher concentrauons of suspended materials

Although the federal government (US Environmental Protection Agency, 1977a, Heenan. 1977) has advanced gmdehnes for the water qualtty of effluents from surface coal mines, m&vldual standards for each mine may be modified at the reglon or state level based on local condmons (rate of effluent &s~.harge soil condmons of the receiving area, etc) As of June 1977, the effluent from the West Decker sedimentation pond had to meet the fotlowmg criteria (Montana Department of Health and Enxaronmental Soences, 1977) (1) total suspended sohds (total non-filterable residue) shall not exceed 30 mg l- 1 for a dally maximum, (2) the pH value shall not be less than 6 0 standard units nor greater than 9 0 standard units, (3) total iron is hmated to 3 5 mg1-1 for a daily average and 7 0 mg 1-1 for a daily maximum, and (4) the concentration of oll and grease shall not exceed I0 mg 1The first three cnterm are those suggested by the U S Envaronmental Protection Agency (1977a) although the level of total non-filterable residue for the West Decker, Mine has been mo&fied by U S Environmental Protection Agency Region VIII reqmrements The fourth cntenon has been set by the State of Montana Analyses for oil and grease were not performed on samples of sedimentation pond water Although the pond operation was erratic and thus hmlted opportunities to sample the effluent, the other criteria were met Acl&c condmons were not observed at West Decker This is &scussed m the succeeding section The chemical characteristics of the effluent from West Decker's se&mentatlon pond have also been reported in a hmnolog~cal study of the Tongue River Reservoir Whalen & Leathe (1976) found that the concentrations of s o & m , sulfate, ammoma-mtrogen, mtrate-mtrogen and bicarbonate were elevated m the &scharge of the pond m comparison to those in samples of Tongue R~ver water These solutes were the same as those which were higher in the November 1976-February 1977 se&mentataon pond samples most likely as a result of increased mmmg acnvaty and changes m pond morphometry and operation The Big Horn Mine, located approximately 50 nver-km upstream from West Decker, is shown m Fig 1 of Olson et al (1979) Discharges from this ,mane contained higher levels of so&urn, potassaum, sulfate, ammonia and nitrate (Dettman & Olsen, 1977) relative to those m Tongue River water Based on theoretical loading calculations, the water quahty of the Tongue River was not measurably impacted by mine effluents (Whalen & l.e.athe, 1976, Dettman & Olsen, 1977) In facq, VanVoast & Hedges (1975) have ,clewed the discharge of the West Decker sedimentation pond into the Tongue River flood plato as merely an acceleratmn of naturally occurring groundwater flow which does not slgmficantly affect the quahty of the river water The mvestlganons cated

Impact of western coal mmmg--I above have reported that the pH values for wane discharges were around or exceeding 8 standard units whtch were s~mtlar to the finclmgs of this study Cations and anions

Cations and antons are expressed as meqmv 1-1 m Table 1 to facd~tate comparison The tonic compos~tton of roland surface waters consists of four major cartons, Ca 2+, Mg 2+, Na + and K + and amons, HCO~, CO~-, SO~- and C1- (Wetzel, 1975) The relative abundance of these ~ons ts generally d~strlbuted as follows Ca > Mg >_ Na > K and HCO3 > SO4 > CI (all carbonate considered as b~carbonate) Analyses of Tongue R~ver water samples collected m close proximity to the West Decker Mme (Table 1) indicated that eqmvalent portions of calcram and magnesmm were greater than that of so&urn (Whalen & Leathe, 1976) Water from the sedtmentat~on pond, however, showed a d~fferent order of cat~omc species w~th sodmm exeeechng other cations by an order of magmtude (Table 1) In add~Uon, magnesmm was greater than calcmm and both these anions were s~gmficantly h~gher than potassmm The an~omc compos~tton of pond water followed the general tendency for proportions of'btcarbonate to dormnate, w~th the sulfate eqmvalent exceethng that of chloride W~thm the pH ranges measured, b~carbonate accounted for almost all of the total alkahmty In pond samples collected from November 1976 to February 1977, the equtvalent portions of mtrate were greater than those of chloride and fluoride combined (Turbak et al, 1979) Th~s was unusual because, although mtrate ts of immense btolog~cal ~mportance, it does not contribute s~gmficantly to the amomc components'of natural waters unless they are htghly enriched (Wetzel, 1975) In typical roland waters of temperate regions, total hardness ts approximately equal to total alkahmty when both are expressed as mg l-~ of calcmm carbonate It ts evident from the sedimentation pond data presented m Table 1 that stgmficant amounts of another cauon, obviously sodmm, were present The sedimentation pond water can be characterized as btcarbonate vath sochum as the predominant cauon and, m th~s respect, was hke the groundwaters of the same region A smulanty between the two waters was not unexpected constdenng the source of the pond water Tlus s~rmlanty, however, was hmltecL D~fferences between groundwater and surface water are best dlustrated by data from VanVoast & Hedges (1975) and VanVoast et ai (1978) included m Table 1 These groundwater data were derived from observations on twelve test wells drdled into the undisturbed D-1 and D-2 coal and on ten wells m the West Decker spods Although groundwater was sampled at several locaUons w~tlun and near the West Decker Mine, there are hnuted observaUons at any one site so that the comparative data presented m Table 1 can only show general trends The data m Table 1 demonstrate that sodmm was

1027

pr0Portlonately lugher m groundwater originating m ~ae~Co~l-bearmg aquifers than m surface waters and spoils groundwater samples collected adjacent to or within the West Decker Mine VanVoast & Hedges (1975) have reported that the D-I and D-2 coal aqmfers contain htgh concentrattons of sodmm and bicarbonate The elevated levels of sochum are probably the result of chssolutlon of sodmm-nch feldspars and other sodmm rmnerals and, to some extent, base exchange reacttons (Roger Lee, U S Geological Survey, personal commumcatton) Water from the sedtmentatton pond, hke groundwater from rome spods, contmned higher proporUons of magnesmm and calcram m compartson to those m groundwater from the coal-bearing aqmfers This observation agrees wtth the contenUon of VanVoast et al (1975) that rmnerals (such as those containing calcmm and magnesmm) are more easaly dissolved in rubbhzed overburden than they are in the undisturbed materials McWhorter et al (1975) suggested that the calcmm and magnesmm tn runof from spods sampled at the Edna Mine m Colorado resulted from the exposure of the rmnerals gypsum, epsomtte, dolomite and calcite to leaching and weathering action This phenomenon may be operable at West Decker although the U S Geological Survey has reported no epsomtte m the spods or se&ments of this area (Roger Lee, personal commumcatlon) The relative proportions of the major cattons (sodmm, calcmm and magnesmm) are reflected m the calculation of the sodmm absorptton ratio SAR This ratio was derived to predict the magmtude to which ~rr~gat~on water undergoes cation exchange reacttons m the sod (Hem, 1971) SAR values greater than eight mdtcate that the water may adversely affect the permeabthty of sods contmnmg slgmficant quantlttes of clay (U S Enwronmental Protection Agency, 1972) Although West Decker's pond functioned prlmardy as a sedunentat~on basra and as a source of water for the nune's dust abatement program, ~t was also intended for ~mgat~on of reclamatton areas ~f needed m tames of drought Smce the average SAR vahle for the pond water was relatively high, this water was not ideal for irrigation Gflley et al (1976) found that the SAR values of water collected from Impoundments at the In&an Head Mine in North Dakota were generally between threshold and hmltmg values for trrlgatlon water standards and suggested cauuous use of the water for this purpose The relative proporttons of sulfate were varmble m the chfferent waters considered m Table 1 The concentrat~ons of sulfate m the sed~mentatton pond water were not explainable solely by the rmxmg of d~fferent groundwaters collected m the sump (VanVoast & Hedges, 1975) Sulfate, hke calcmm, may be leached from gypsum as observed by McWborter et al (1975) The mlcrobtally-mediated oxidation of pyrite occurring m exposed overburden and coal ts another explanatmn for the htgher levels of sulfate m the sedtmentaUon pond water and the spods groundwater

1 32

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Total alkahmty and hardness expressed as mg 1- ~ as CaCO3

SAR=

* All catmns and anmns expressed as m ¢qmv i t Computed by the follow/ng formula (Hem, 1971)

84

No of talcs samples No of dctcrmmah'ons per site

202 308

pH

Total aLkalimty aS CaCOs~ Tota/hardness as CaCOa

of EaonE s p e ~

Order of prcdommanoe

SIt~¢

1

1141 149 84 12

222 064 573 O.53 H C O ~ > SO~ > C O 3 > CI

390 013 347 0O8 H C O ~ > SO4 > C O 3 > CI

08

SARt

Ca ~ Mg > Na > K

Calcmm* Magncmum So~mn Potmmum

O t a ~ of prodormnano~ of oatmmc specu~

305 3 11 133 0 10

Parameter

Groundwater from D-1 and D-2 coal-bearing aqmfers (1974-1975) (VanVoast & Hedges, 1975) 1 17 1 80 261 0 19 403 Na > Mg > Ca > K

reservotr [1975-19"/6) (Whalen & Leathe, 1976)

Tongue River-inflow to

11 7 026 6 79 0 15 HCOz > S O , , > CO~ > CI 600 285 84 1-5 19

995 601 70 10 1-6

234 3 73 13 7 021 79 Na ~ - M g > Ca > K

369 4 23 367 039 266 Na>Ca>Mg>K 23 6 037 203 072 HCO3 > S O , , > C I > CO3

Sedimentation Pond (1975-1977)

Groundwater from spoils (1975-1977) (VanVoast et al, 1978)

Table 1 Selected consUtuents of sedlmentaUon pond water and those of surface and groundwatees coBect~d wnhm the vicinity of the West Decker Mine

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Impact of ~estern coal rmnmg--I

In the second paper of this series, Olson et al (1979) discussed the detection of high numbers of the bacterrain Thzobacdlus ferrooxutans m water and sediment samples collected at the West Decker Mine This orgamsrn, largely responsible for the production of acid mine drainage problems m coal mmmg operataons m the eastern and mldwestern Umted States, ~s beheved to be active m wacrozones w~thm the coalbearing strata at West Decker Klmble (1978) ~solated slmdar orgamsms m overburden samples from the West Moorhead deposit of the Fort Umon FormaUon m eastern Montana Sulfuric acid produced from pyrite oxldaUon would enhance the leaching of calcram, magnesmm and other elements, however, extreme acid concht~ons have not been observed m the Fort Umon FormaUon because of rap~d neutrahzat~on by relatively high concentratzons of rmneral carbonate present m the overburden materials The importance of carbonates m influencing the quahty of wane drmnage waters has been well recognized (Carucc~o, 1968, Geldel & Caruccao, 1977, Rogowskl et al, 1977) There was a great deal of vanab~hty m the groundwaters which supply the sedimentation pond water creating chfflcultaes in the comparison of ground and surface waters Average values of selected consUtuents m groundwater samples collected from the D-I and D-2 coal aquifers, however, were different from the pond water (Table 1) Although d~fferences may be explained m part by the contributions from additional groundwater sources such as from the spoils, recharge areas or the D-1 overburden, other processes hke leaching and wacrob~al acuwty appear to be ~mportant m deterwanmg the quahty of water m the sedimentaUon pond

1029

gram of coal) Discharge from the wane did not present a source of mercury polluUon to the aquaUc environment outside of the West Decker Mine because of slgmficant dduuon of the effluent m the Tongue River flood plato A U S Fish and Wddhfe Serwce study (Gregory, 1977) proposed that the sedimentation pond be used as a source of water to fill and maintain a northern pike spawmng marsh and waterfowl nestmg site The effect of mercury m the pond water on northern pike has been investigated Phdhps (1978) found that fingerlings reared m the sedimentation pond attained relauvely low body burdens of mercury (013/tg of Hg per g of muscle Ussue) after four months exposure The fate of metals m the pond water may be m part deterwaned by the acUvlUes of wacro-organlsms The finding that relaUvely htgh numbers and acUvltles of sulfate reducing bacteria are detected m the pond sediments Is reported m the second paper m this series (Olson et al, 1979) The sulfide produced by these orgamsms may have precipitated metals as highly insoluble metallic sulfides. This reactton may have effectively removed metals from the water column, thus, the actlwty of sulfate reducing bacteria possibly served as a detoxlficatlon mechamsm for pond water This posslbihty Is discussed further m Olson et ai (1979)

CONCLUSIONS

There are several considerattons winch make the study described hereto informative Decker Coal Company will be one of the world's largest producers of subbitummous coal when eastward and northward expansion of the present mine at West Decker is comPotentially toxic elements pleted. The two additional mines will also discharge Levels of cadwaum, lead, arsemc, selenmm and drainage waters directly or indirectly into the Tongue mercury were deterwaned m sedlmentat~on pond River Reservoir The Tongue River and the reservoir samples collected from July 1976 to April 1977 (Tur- are major resources m an area gath hwated surface baket al, 1979) Chetmcal data alone ts insufficient water supplies for deterwanmg if lead and selenmm exceeded the The reservoir was created primarily to store'irrigarecommendauons of the U S Environmental Protec- t~on water for downstream users. In this area, rural tton Agency (1977b) for the protecUon of aquattc hfe populatlons depend largely upon groundwater for and wfldhfe, bloassays mvolwng sens~tlve resident domesuc uses and hvestock (VanVoast & Hedges, species are suggested The concentrations of arsemc 1975). In addiUon to these needs, sufl~clent water supand selemum m sedimentation pond samples, how- phes are reqmred for the successful reclamation of ever, were below recommendations for domestic mined lands (Atwood, 1975; Jackson, 1977), Underwater supply This may have also been true for lead standing the impact that western coal mmmg has on although the recommended level was generally below the quahty of surface and subsurface waters Is of conthe detection hwats of the analys~s. Cadwaum levels m slderable slguiticance. the pond system were non-hazardous for aquatic life Thls study was designed to elucidate the function less sensmve than salmomd fishes and cladocerans of a surface coal mine sedimentation pond m altering The concentrations of mercury ranged from 0.09 the quahty of wane drainage waters. Clearly, mine to 0 81 ~g 1-1 and were consistently greater than U S. drainage waters underwent alteratmns prior to and Environmental Protection Agency (1977b) hm~t of during retentton m the sedimentation pond at West 0 05 ~g 1- ~ for the protection of freshwater aquatic Decker. It Is important that the function of the pond hfe and w~ldhfe The source of mercury m these be mamtalned and even improved so that the effluent waters may be the coal Joensuu (1971) reported rela- ~s non-hazardous and can serve a useful purpose A tively h~gh levels of mercury m coal (up to 33/tg per detailed consideration of specific design parameters

1030

S C TURBAK G J OLSON and G A MCFETERS

which would ~mprove the operatton of West Decker's sedimentation pond system was beyond the scope of th~s mvesttgatton This aspect of sed~mentaUon pond functaon ~s &scussed m Hdl (1976), Kathurm et al (1976) and Jamak (1977) Speofic recommendations are outhned by Turbak et al (1979), only the followmg ~s d~scussed m th~s pubhcatton The physical features of sedtmentat~on pond design should be strongly interfaced w~th a cons~derat~on of biological acttv~ty wtthm the pond system Although the following suggestion has not been evaluated on a pdot plant level, a two pond system may be more effioent for lmprowng water quahty A deeper prtmary pond could serve two purposes, one of whtch would be to facthtate sedimentation since ~t would be less subject to wind-driven mixing Zones of anaerob~os~s could develop m deeper waters and promote bacterial sulfate reduction Adequate contact between the sulfide produced and mflowmg waters would remove selected heavy metals and other elements from solutton A more shallow secondary pond would permtt the b~olog~cal uptake of numents such as those contributed from mtrogen explosives and carbonaceous materials m the coal This suggestton has recetved some consideration by the Bureau of Land Management (1978) m their environmental assessment of the North Decker extension Acknowledoements--Thls research was funded by the U S Enwronmental Protection Agency, Duluth, Minnesota, Research Grant No R803950 Gratitude is expressed to members of the CooperaUve Fishery Research Umt, F~shenes B~oassay Laboratory and Department of M~crob~ology at Montana State Umvermy and the Department of Chemistry at Colorado State Umvers~ty for their invaluable assistance Decker Coal Company of Sheridan, Wyoming permitted the undertaking of th~s study and personnel at the West Decker Mine were cooperative and mformauve REFERENCES

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1031

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