Technical Article

Introduction

The study area comprises the Davis Mine site,described below, and the watershed of Effluent Creek,which was named for the discharge of acidic minewater originating from the abandoned mine and itsassociated spoil heaps (Figure 1). Effluent Creek(watershed area 0.16 km²) drains into Davis MineBrook, a much larger second-order watershed, totalling4.37 km² above the confluence. At the confluence, themine water typically has a pH of 2.9, and contains:9.8 mg/L of total iron and 293 mg/L of sulphate. Othermetals are present in much smaller concentrations:principally copper (0.5 mg/L), zinc (5 mg/L), andaluminium (4.3 mg/L).

Effluent Creek appears to originate from seeps in thevicinity of Shaft 1 and is severely degraded along itslength of 250 m, with no evidence of invertebrates orfish and an ochre-stained reddish-yellow substratematerial (attributed to goethite precipitation, Bloom(2005)). Downstream of the confluence, theMassachusetts Dept of Environmental Protection(MADEP) reports that the Davis Mine Brook containsno fish and most of the invertebrate species normallyfound in local streams. Its substrate comprises reddishstained boulders and cobbles for up to 2.6 km until itflows into the larger Mill Brook (MADEP 2004).MADEP concluded that the “impaired” status of the

stream can be attributed solely to the Effluent CreekAMD discharge. The 2.5 km inspected length of DavisMine Brook above the confluence was reported to beunaffected by AMD (MADEP 2004), although recent

Investigating the Potential for Ongoing Pollution from an Abandoned Pyrite Mine

Russell Adams, David Ahlfeld, and Ashmita Sengupta

Dept of Civil and Environmental Eng, Univ of Massachusetts, 18 Marston Hall, Amherst, MA 01003, USA; corresponding

author’s e-mail: russelladams68@gmail.com

Submitted October 19, 2006; accepted December 26, 2006

Abstract. Davis Mine was the largest working pyrite mine in the state of Massachusetts during its lifetime between1882 and 1911. Since abandonment, a highly-polluting mine water discharge has emerged from the site of an oldmine shaft and a waste rock pile and is contaminating the nearby Davis Mine Brook. During the past 90 years, noattempt has been made to implement any pollution abatement measures. This paper assesses the likely currentvolume of mine waste on the site and compares this figure with the amount of mine waste produced during thelifetime of the mine based on old mine plans and production figures. A simple mass balance model allowed us tocompute the loadings of contaminants into Davis Mine Brook and to calculate the ratio of loadings from differentsources of pollution, namely the mine shaft discharge and ground water discharge from the waste pile. Results for2004 indicated that the proportion of mine water flowing from the shaft varies seasonally, with the greatestdischarge in spring and lowest in summer. These results allow us to assess the potential lifetime of the dischargeif left untreated and determine what flow pathways are important if a treatment scheme were to be implementedat the site.

Key words: Abandoned mine; acid mine drainage; geochemical tracers; iron; Massachusetts; pyrite mine;sulphate; waste rock

Figure 1. Map of Davis Mine site showing sampling points

and monitoring wells

Mine Water and the Environment (2007) 26: 2–13 © IMWA Springer-Verlag 2007

field investigations discovered that a large spoil heap20 m upstream of the confluence was contributing asmall quantity of ferruginous surface water to DavisMine Brook.

Current research at the site has focussed on the role ofmicrobial processes on the natural attenuation of theAMD discharge (Barreto 2005; Bloom 2005; Ergas etal. 2005). This paper aims to support this research bycalculating the potential lifespan of the AMDdischarge (based on the amount of spoil currently inthe valley) and the current pollutant loadings generatedfrom different sources. Certain species (sulphate,manganese, and chloride) have been used as naturaltracers, as suggested by Bencala et al. (1987), toestimate the proportion of the mine water attributableto ground water seepage through the waste rock pileand the proportion flowing from the discharge nearShaft 1. Measurement of these flows directly isdifficult to determine accurately but the total runoff inEffluent Creek and the concentration of mine waterdischarging from the Creek into Davis Mine Brook hasbeen successfully measured. The work described herewill also support subsequent design of systems forremediation of this AMD site.

Description of Davis Mine

Davis Mine was the largest working mine in the stateof Massachusetts during its 29 year lifetime between1882 and 1911. Ore deposits were discovered in themid-1800s in the Ordovician Hawley Schist formation.These deposits were thought to be of hydrothermalorigin and associated with quartzite replacement(Farquhar 1967). In the north of the mineral belt, theore contained large amounts of pyrite, makingcommercial mining viable at a time when industrialmanufacture of sulphuric acid was switching fromusing elemental sulphur to pyrite as the source material(Rowe Historical Society (RHS) 1977). Smalleramounts of other sulphide minerals, principallychalcopyrite, were found as well, and the Davis Mineproduced about 25,000 pounds (11.4 tonnes) of copperduring the first decade of the 20th Century (Farquhar1967). The operators of the mine recorded that duringmost of the year, the mine was fairly dry, except duringthe spring thaw when up to 0.1 MGal/day (4.4 L/s) ofmine water was pumped from the mine. Inflows offresh water were only recorded at the upper levels ofthe mine, indicating that ground water seepage fromthe host rock was negligible (Rutledge 1906).

The mine site has been abandoned for nearly 100 yearsand no attempt at remediation has been made over thatperiod of time. The legacy of the former mineworkings are most strikingly the large spoil heaps,which are still largely bare of vegetation and contain

scattered lumps of pyrite ore, host rock, and remnantsof timber structures used in the mining operations.Also left behind are the shells of former mine buildingsand the cellar-holes and foundations of houses lived inby the mine owners and workers. Old mine plansindicate that four shafts between 366 and 549 m deepwere sunk, two for the transport of ore, workers, andmaterials, and two for ventilation and prospecting(RHS 1977; Rutledge 1906). The shafts were unusualin that they were sub-vertical, sunk into the ore body atan angle of 80º. The ore body was mostly worked usingthe “Cornish” method of underhand stoping (Rutledge1906), where tunnels are dug horizontally through thevein from the shafts and then “winzes” (vertical shafts)are dug down into the vein to extract the ore. Theremnants of at least three mine shafts can be seen anda series of mine collapses during and after theoperation of the mine have resulted in dramaticsinkholes that mark their location.

Mine and Spoil Volume Calculations

The volume of the mine workings and waste rock(spoil) were calculated using three different methods.These calculations are presented as being “order ofmagnitude” in accuracy:

(i) The area, volume, and mass of the waste rock pilebased on current (1994 onwards) survey data,photographs, and field observations.

(ii) The mined-out volume (and hence the volume ofrock removed) from the mine based on theproduction figures from various sources.

(iii) The mined-out volume, based on historical mineplans and data showing the former extent of themine workings.

We used figures from calculations (ii) and (iii) toestimate how much spoil was deposited on the groundduring the 29 years of operation of the mine. Theestimate of the current extent of the spoil fromcalculation (i) was used alongside the results ofcalculations (ii) and (iii) to estimate the rate at whichmine spoil is removed from the site by naturalprocesses. Table 1 summarizes all the data fromhistorical sources and recent surveys used in thecalculations.

Calculation of Area and Volume of Spoil Based onSurvey Data

These calculations provide an estimate of the volumeand mass of spoil currently deposited in the EffluentCreek valley. Some large spoil heaps to the NE and Nof the current field site have been identified from field

3

observations and aerial photos, though they fall outsidethe scope of the current project. The calculations werebased on surveyed and estimated areas of waste rockand thicknesses recorded by drilling logs (Bloom2005; Gál 2000; unpublished project data 2005), aseismic refraction survey with three transects acrossthe site (Gál 2000), and results of an unpublishedsurvey carried out more recently by a graduate student.Survey data consisting of a series of coordinates andspoil thickness were imported into the Surfer™program (Golden Software Corp.), and the thickness ofmine spoil calculated across the site by interpolatingbetween the points using triangulation. Figure 2 showsthe resulting contours of spoil thickness overlain on aplan of the mine area.

The total area of the mine site including the shells ofabandoned buildings approximately 5 times greaterthan the area of spoil calculated in Effluent Creekvalley (0.65 ha; Bloom 2005). The total areaincludes:

(i) a large spoil heap to the NE of the confluencebetween Effluent Creek (EC) and Davis MineBrook

(ii) two smaller heaps near the remnants of Shafts 2and 3 (visible on Google Earth™ and discoveredduring a field survey in Sept. 2006).

The thicknesses of these additional piles are notknown; however, they all possess steep (greater than45º and up to 10 m high) slopes. Some of the spoil wasdeposited on steep natural slopes, other piles weredeposited on flatter terrain and could comprise up to 10

m of spoil. After nearly 100 years since mining ceased,there is still a significant amount of waste rock in pilesdeposited on the site.

Calculation of the mass and volume of the spoil heapfrom the current survey data requires estimates of boththe porosity of the spoil and the density of the solidrock. The porosity of the spoil is an unknown quantity;a range of values from 0.35 to 0.55 have been used,representing the wide range of materials found in theDavis Mine spoil, which vary from clay and silt tolarge fragments of intact rock and ore. An unknownvolume of mine spoil was removed for pyriteextraction after the mine was closed.

Calculation of Spoil Volume Based on TotalProduction Figures

Table 1 summarizes known production figures fromthe mine during its working lifetime. Several sourceswere available. Total production figures are assumed tobe the total tonnage shipped to the nearest railhead atCharlemont, not the total extracted tonnage from themine. Critical to these calculations is an assessment ofthe ratio between how much ore was shipped and howmuch became waste rock.

The amount of waste rock deposited as spoil woulddepend on the ratio of ore to total rock extracted fromthe mine. Field (1985) stated that the ore veincontained more than 60% pyrite. When waste rock andlow grade material are factored in, it appears thatapproximately 40% or more of the mined rock was leftas spoil. Past studies of the mine (Bloom 2005; Gál2000) documented the presence of pyrite and

Property Value Source

Area of mine site 3 ha Bloom (2005); Gál (2000)

Depth of spoil 0.5-4 mBloom (2005); Gál (2000); survey data from

site (unpubl.)

Area (Effluent Creek spoil) 0.65 ha Gál (2000); survey data from site (unpubl.)

Life of mine 29 years Rowe Historical Society (1977)

Pyrite production over 10 years

ending 1892 334,552 tons (Farquhar 1967)

Daily pyrite ore production

100 tons (average), 20 tons

(Jan 1883), 500 tons/week

(Sept 1883)

RHS (1977); Rutledge (1906)

Annual ore production 37,084 tonnes Calculated from daily totals

Monthly ore production 3,000 tons Rutledge (1906)

Lifetime ore production (shipped) 1,075,436 tonnes Based on production figures above

Estimate of production 1 million t (1,016,000 tonnes) Rowe Historical Society (1977)

Density of ore 2747 kg/m3 Rutledge (1906)

Table 1. Summary of historical data

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formation of secondary minerals by pyrite oxidation inthe spoil heap leading to the generation of AMD, andpyrite crystals are still visible in the spoil today.

Calculation of Mined-out Volume of UndergroundWorkings from Documents and Mine Plans

The second set of calculations used actual dimensionsof the mine from Rutledge (1906) and RHS (1977) to

estimate the mined-out volume underground and themass of rock and ore removed. Plans showing thesurface workings of the mine from Rowe HistoricalSociety (Rowe, MA) and the sources described abovewere also used to estimate the volume of theunderground workings and the volume of rock and oreremoved during operation (Table 5 below tabulatesthese dimensions). Here we assumed that the workingswere prismatic since underground mine plans wereunavailable. Therefore it was assumed that the cross-section of the workings (across the strike) did not varywith length along the mine axis, i.e. viewed in plan themine “footprint” was rectangular and orientated alongthe strike at an approximate angle of 18º to the NE ofgrid north. Based on the available plans, mine volumewas calculated by multiplying the depth of workingsby the width of workings in plan by the length ofworkings (along the strike of vein). We assumed theextraction ratio (percentage of mine volume removed)equalled 65%, for workings using the underhandstoping method of extraction (Younger and Adams1999).

Calculation of AMD Loadings into Davis MineBrook

Calculation of AMD loadings requires measurementsof the concentration of metals and sulphate associatedwith AMD and also an estimate or measured values ofthe mine water discharge rate from Effluent Creek intoDavis Mine Brook. In addition to identifying the AMDloadings, the calculations below will attempt to splitthe Effluent Creek AMD discharge into componentsoriginating from Shaft 1 and ground water dischargefrom the exposed spoil.

Hydrology

Long term averages of precipitation and temperature,for the time period 1971-2000 were obtained forMassachusetts from the NOAANational Climatic DataCenter (NCDC), http://www.ncdc.noaa.gov. Thenearest observations to the mine site were from Heathrain gauge about 5 km from the site, mean annualprecipitation for the period was 1312 mm, distributedfairly evenly over the year with a maximum of122 mm in May and a minimum of 84 mm in February.Hourly precipitation data are also available from NorthAdams Airport meteorological station, which isslightly further away from the mine. Real-time datafrom this rain gauge are currently available and thetotals have been compared with those measured at arain gauge in the Effluent Creek spoil site. Over asix-month period from June to Dec. 2004, totals fromboth sources matched within 3%. Rainfallmeasurements at the mine site were discontinued afterthis time.

Figure 2. Map of the Effluent Creek valley showing extent

of spoil heap and monitoring points (contours indicate

thickness of spoil in meters)

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Effluent Creek was gauged continuously for a sixmonth period in 2004 using a v-notch weir installed atsite C2 close to the confluence with Davis Mine Brook(Figure 1). After Dec. 2004, flow measurements wereno longer continuously recorded and the area behindthe weir plate filled with sediment from the spoil pilemaking flow measurement difficult. Measurements ofrunoff were also made sporadically in both Davis MineBrook and Effluent Creek using a PygmyTM currentmeter or dye tracing tests to calculate velocity andestimates of the cross-sectional area of the flow. Thesemeasurements, summarized in Table 2, indicate thatgenerally, flow in Davis Mine Brook is about 30 timesgreater than flow in Effluent Creek. Recent fieldsurveys during dry weather have indicated thatEffluent Creek emerges from two seeps located nearShaft 1 (Figure 1), the seep nearer (approximately 5maway) the abandoned shaft being noticeably moreferruginous than the seep further (approximately 10m)away. Above this location there is evidence of anintermittent stream, which may flow during wetweather periods and during the spring thaw. BetweenShaft 1 and Davis Mine Brook, Effluent Creek flowincreases considerably; the source during dry weatherof this runoff appears to be drainage from a shallowground water system in the spoil and possiblysuperficial deposits (glacial drift) along the valleyfloor, discussed below. The groundwater level must bebelow the top of Shaft 1 under these conditions.

Because of the sporadic nature of runoff measurementsin both watersheds, especially Davis Mine Brook, itwas necessary to develop a simple model of monthlyand seasonal runoff in both watersheds using readilyavailable hydrological and meteorological data.

Several rivers are gauged by the USGS in the vicinityof the mine site. The nearest gauging station witharchived data from an unregulated river is located onthe North River at Shattuckville, less than 10 km fromthe mine. The North River is a tributary of the largerDeerfield River, which flows east through Charlemont(6 km south of the Davis Mine). Over the period 1940-2005, mean annual runoff was 731 mm (source USGS

Surface-Water Annual statistics http://waterdata.usgs.gov/ma/nwis/annual), with a maximum in April of180 mm, and a minimum in August of 17 mm. Thisseasonal variation is typical of the region: the highApril total is usually attributed to snowmelt and theAugust low to high evapotranspiration. Potentialevapotranspiration data for the region was obtained forthe locality of the mine from a regional model based onthe Penman-Monteith equation (Fennessey and Vogel1996) and averaged 785 mm/year. Monthly runoff (Q)and precipitation (P) data allowed us to calculatemonthly runoff coefficients (i.e. Qi/Pi for each month,i), which were used to estimate monthly runoff in bothDavis Mine Brook and Effluent Creek watersheds. Itwas assumed that runoff coefficients in thesewatersheds were identical to those calculated for theNorth River watershed. Monthly measured totals fromEffluent Creek compared well with estimated monthlyrunoff obtained using the runoff coefficients. From themodel, mean annual runoff in Effluent Creek waspredicted to be 3.2 L/s and 88 L/s in Davis Mine BrookMinimum runoff was predicted to be 0.7 L/s and19 L/s, maximum runoff 11 L/s and 310 L/s in EffluentCreek and Davis Mine Brook respectively, (see Table 2for measured runoff in these streams). Mean modelledannual runoff was greater than the measured runoff inEffluent Creek, mainly because the period ofmeasurement missed much of the spring snowmeltrunoff. Monthly runoff was then aggregated toseasonal runoff for the loading calculations. For thepurposes of this study, the New England seasons weredefined as: Winter: Dec. to March; Spring: April andMay; Summer: June to Aug.; and Autumn (Fall): Sept.to Nov.

Hydrogeology

Ground water levels in three aquifers were monitoredfrom 2003 to 2005 using up to 15 wells; nine of thesewells were equipped with multi-ports so that samplingand head measurement was possible from up to 3different depths. The aquifers monitored were: (i) thespoil along Effluent Creek valley (ii) superficialdeposits (glacial drift) either below the spoil or

Stream Period of Measurements Interval or Number of

Measurements

Min/mean/max

flow (L s-1)

Effluent Creek

(continuous)

27th June 2004-26th, Dec. 2004 10 minutes 0.5 1.66 51.1

Effluent Creek

(spot gauging)

16th June 2004-25th, Sept. 2006 12 0.6 2.0 4.9

Davis Mine Brook

(spot gauging)

16th June 2003-25th, Sept. 2006 15 (more than one cross section

measured on some days)

24.4 83.0 285

Table 2. Summary of measured runoff data

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exposed along the Effluent Creek valley sides wherespoil are absent (iii) the bedrock below the glacial drift(Bloom, 2005). The multi-port wells also allowed thevertical movement of ground water in the vicinity ofeach well to be estimated. The ground water flowpatterns are complex but generally indicate drainagethrough the spoil (which is the most pervious of thethree formations) towards the Effluent Creek stream.

Field Sampling and Laboratory Analysis

Surface water and groundwater samples for anion andcation analysis were filtered in-line in the field using0.45µm filters (Millipore, Billericay, MA, USA).Samples for cation analysis were acidified with 1 dropof 6 N nitric acid in the field to prevent metalprecipitation. Field samples were returned to thelaboratory for cation and anion analysis, as discussedbelow. Samples were also taken from selected wells formicrobial community analysis (Ergas et al. 2005).

Principal cations (Na, K, Ca, Mg), transition elements(Fe, Cu, Mn, Zn), silicon (Si) and other AMD indicatorspecies (Al, Pb) were measured using a Spectro M120inductively coupled plasma optical emissionsspectrophotometer (Spectro Analytical Instruments,Kleve, Germany). Anions (Cl, SO4, NO2, NO3, F, Br,PO4) were determined using a Lachat 5000 IonChromatograph (Hach Company, Loveland CO, USA)(Ergas et al. 2005).

Geochemical Data

This study began collecting samples for water qualityfrom surface water (several sites on Effluent Creek andDavis Mine Brook) in November 2002. Samples from9 multi-port wells were collected from early 2003 untilspring 2005. Three of these wells: #3, #8, and #5, weredrilled directly through the spoil pile into the bedrockbelow (Figure 2). Figures 3a and 3b show seasonalmean concentrations of total iron and sulphate ingroundwater and surface water in Effluent Creek andDavis Mine Brook. Surface water quality wasmeasured by an earlier study in the mid 1990s (Gál2000) and also from a MADEP investigation on thewater quality of Davis Mine Brook in summer 1996(MADEP 2004). Together, these data indicate that overthe period 1994-2006, levels of pH, iron, and sulphatein both Davis Mine Brook and Effluent Creek arereasonably constant. Samples from the wells indicatethat water quality in the spoil ground water issignificantly worse than the water quality in both theglacial drift and weathered bedrock. The presence of alarge quantity of secondary minerals (mostly goethiteand jarosite) in the upper layers of the mine spoilwhere pyrite oxidation has taken place (Gál 2000) isthe cause of this poor quality ground water. Where the

slope of the spoil heap is less than 1%, a 5 cm thickhardpan layer of Fe-oxides has formed at the surface ofthe waste rock; this reduces the infiltration rate of oxicsurface water (Gál 2000).

Other studies of temporal changes in mine waterdischarges (Younger 1997; Younger et al. 2002) haveindicated that pH rises and concentrations of sulphate,iron and other cations fall exponentially over severaldecades following the first emergence of the dischargeafter closure. At Davis Mine, older chemistry datafrom the decades immediately following closure orfrom mine water pumped during its operation are notavailable to support this hypothesis, more recent dataindicate that the water quality of the Effluent CreekAMD varies somewhat over a year, but over a decadesince a MADEP survey in 1996 (MADEP 2004) hasremained fairly constant.

Geochemical data from nearby rivers unaffected bymine water have been collected sporadically by theUSGS and MADEP as part of their ongoingmonitoring obligations and are available on-line( h t t p : / / w a t e r d a t a . u s g s . g o v /m a / nw i s / qw ;www.mass.gov/dep). Concentrations of iron andsulphate in these rivers were generally 2-3 orders ofmagnitude lower than those measured in EffluentCreek and 1-2 orders of magnitude lower than thosemeasured in Davis Mine Brook downstream of theinflow of Effluent Creek. The pH generally rangedfrom 6 to 8.

Conceptual Model of Runoff and Pollutant Generationin Effluent Creek

Based on observations made at the mine site andmeasurements of discharge and ground water levels inthe Effluent Creek watershed, a conceptual model offlow in Effluent Creek watershed was developed. Thismodel partitions the total runoff entering Davis MineBrook into components, which have differentpathways through the Effluent Creek watershed. Theflow pathways are important for calculating theloadings into the Brook since the geochemical datadiscussed above indicates some variations in pollutantconcentration between surface and ground watersamples in the watershed. Moreover, a potentialtreatment scheme for the Effluent Creek mine watercould be developed considering the most severe sourceof AMD pollution and designed accordingly, e.g. usinga permeable reactive barrier (PRB) (Amos et al. 2003)to treat contaminated ground water, or using a passivetreatment wetland (Hedin et al. 1994) to treatcontaminated surface runoff.

In general, a pollutant loading (L) can be calculated bythe simple relationship (see Eq. 1) if a corresponding

7

flow value (Q) and the pollutant concentrations (C) areavailable. This equation can be applied to any site onDavis Mine Brook or Effluent Creek where these termsare measured or estimated using the monthly runoffmodel.

L = CQ (1)

An equation (2) was derived for the differentcomponents of runoff in Effluent Creek watershed inthe conceptual model:

Qe = Qs + Qgw (2)

Qe = total runoff from Effluent Creek (gauged atlocation C2 and/or estimated from monthly runoffcoefficients)

Qs = runoff emerging at the site of Shaft 1 andsampling location C3 (comprising surface and groundwater and not measured).

Qgw = ground water discharge through the spoil pile,emerging in Effluent Creek.

The corresponding equation for total loading Lt (of apollutant) from Effluent Creek watershed is given by(3), based on (1) above.

1000

10000

100000

1000000

Spring,

2003

Sum

mer,

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2003

Win

ter,

2004

Spring,

2004

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2004S

ulp

hate

Concentr

ation

(µg/L

)A4 C1 C2

C3 G/W

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2003

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ter,

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Iron

Concentr

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(µg/L

)

0

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250

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2003

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Autu

mn,

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ter,

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hate

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(Kg/d

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Iron

Load

(Kg/d

)

Figure 3. (a) Sulphate and (b) iron concentration, in Effluent Creek runoff (C1 and C2), Davis Mine Brook runoff (A4) and

spoil groundwater (G/W), see Fig. 1 for locations ; (c) sulphate and (d) iron load, in Effluent Creek runoff (C1 and C2) and

Davis Mine Brook runoff (A4)

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Lt = Cc3Qs + CgwQgw = CeQe (3)

where Ce are concentration data measured at theEffluent Creek weir, Cc3 and Cgw are mean sulphateconcentrations measured in surface water at site C3and in spoil groundwater respectively.

In equations (1) and (2) all the quantities are eithermeasured or estimated from the conceptual model ofrunoff described above apart from Qs and Qgw. Theratio, R=Qs/Qgw can be calculated using theconcentration data by rearranging (3), assuming thatthere are no transformation processes taking place inthe Effluent Creek watershed (e.g. streambeddeposition of precipitates). The use of (3) is valid whenthe species is conservative. We considered which of themajor constituents at the site could be treated with thisassumption. Chloride is a clear candidate. Othersconsidered, iron, sulphate and manganese, are subjectto precipitation so are not strictly conservative(Bencala et al. 1987).

However, we considered if the changes inconcentration observed between C3 and C2 could bedominated by mixing with groundwater so that anyprecipitation that does occur can be assumednegligible. This notion can be tested by examining thechange in concentration between C1 and C2. Over thisreach of Effluent Creek the iron concentrationschanged significantly (as can be seen in Figure 3b)suggesting that significant precipitation is occurring. Incontrast, the sulphate concentration over the samereach changed little suggesting that the overall changesin sulphate concentration are dominated bygroundwater mixing. Furthermore, sequentialextraction analysis of streambed sediments showedthat very little manganese precipitated in the low pHwaters of Effluent Creek.

Sulphate and specific conductivity have traditionallybeen used as indicators of mine water contamination insurface waters and ground water (Gray 1996). It ispossible for negative values of R to be calculated undercertain conditions, i.e. if the concentration recorded atC2 is greater than the concentration recorded at bothC3 and in the groundwater. Negative values wereremoved from the results since these would indicateflow reversing in direction, and values of R were notcalculated when the difference in concentrationbetween sites C2 and C3 was less than the 10%measurement error inherent in the data.

In the absence of flow measurements along EffluentCreek, it can also be assumed in the most simple casethat R is equal to As/Agw, where As and Agw are thewatershed areas above Shaft 1 and the remainder of thewatershed (i.e. A – As , where A is the total area of

Effluent Creek) respectively (R = 3.3). If we assumethat the ratio of runoff from the area above Shaft 1 tothe runoff from the spoil equals R, then we assume thatall runoff generated below Shaft 1 flows through thespoil to reach the stream and that this runoff is aroundone quarter to one third of the total discharge inEffluent Creek.

Recent field visits have confirmed that this is probablythe case during dry weather. However this relationshipmay not always hold true; calculating R usingconcentration data provides an alternative figure.Another source of error in calculating R simply basedon watershed areas is that Qgw, that is baseflowreaching Effluent Creek from the spoil, may beoverestimated due to ground water flow bypassing theEffluent Creek weir (C2) and flowing into the westernside of Davis Mine Brook. There also exist at least twomore extreme and several intermediate cases when theassumption that R = As/Agw may not be true. Underhigh runoff or flood conditions, the majority of theAMD loading could originate from surface runoff, sothat the ground water component becomes negligible.Under low flow conditions, Qs may become negligibleso that the loading term representing ground waterseepage through the spoil pile dominates.

Results

Spoil and Mine Volumes

The current volume of mine spoil deposited in theEffluent Creek valley was calculated using the survey

Dimension and source Value

Porosity of spoil 0.35-0.55

Density (of rock) 2.65 t/m3

Total volume in Effluent Creek 8322 m3

Mass of rock (depending on porosity)

9924-14335 t

Additional area of spoil piles (Shafts 2 and 3) (source: field survey/Google Earth™)1

256 m2/ 416 m2

Additional area of spoil pile 40 m to E of Shaft 3 (source: Google Earth™)

139 m2

Additional area of spoil pilenear Davis Mine Brook (source: Google Earth™)

475 m2

1Aerial maps on Google Earth™ identified an additionalspoil pile not located by the survey so the total area calculated from this source is greater.

Table 3. Calculation of current volume and mass of spoil.

9

data described above. The results are presented inTable 3. Results from the calculations of spoil volumefrom the production figures, and the volume of themine workings, from the historical data on the minesize, are presented below in Tables 4 and 5.

AMD Loadings and Runoff Ratios

Runoff values and contaminant loadings werecalculated using two different methods, firstly usingrunoff estimated from the monthly model andmeasured concentrations from samples taken fromEffluent Creek and Davis Mine Brook. Figures 3cand 3d shows mean seasonal loadings of iron andsulphate (respectively) at sites A4, C1, and C2 (i.e. Ltalready defined above and computed using (1)).Loadings at monitoring site C1 (Lc1) which isapproximately 100 m upstream of C2 were calculatedby first calculating Qc1 by applying a factor of 15/16

to Qe to account for the smaller watershed area to C1(calculated to be 15 ha). Seasonal runoff at EffluentCreek weir (i.e. Qe) and from Davis Mine Brook (Qd)was estimated using the monthly runoff coefficientsderived above.

The second method computed runoff and contaminantloadings in Effluent Creek watershed using Equation(3) above. Table 6 shows estimates of R using differentchemical species, first for the three seasons in 2004averaged and then on a seasonal basis. We did notcalculate values for winter 2004-5 since there was onlyone sample taken that season (Jan. 2005) at C3 andconcentrations of contaminants measured in thelaboratory from this sample were questionable.

Blank values in the table indicate seasons where theconcentrations of a particular species at C2 and C3were within 10% of each other, indicating that thedifference between the concentrations from the twosites was less than the error margin; therefore, theseresults were discarded since they gave spuriouslyhigh or low values of R. The computed ratios can becompared to the value of R (3.3) that we calculatedassuming that R is equal to As/Agw discussed above.

Figure 4 shows loadings of (a) sulphate and (b)manganese for the three seasons in Table 6. Flows (Qsand Qgw) from the shaft and ground water andcorresponding loadings were calculated usingEquations (2) and (3). Loadings were calculated usingseasonal mean concentration data and Eq. (1). Meanvalues of R (averaged from the values computed usingthe three different species) were used to calculate Qsand Qgw.

Dimension and source Value

Volume of pyrite shipped(based on annual productionof 37.1 Kt/yr and oredensity of 2.75 t/m3)

391,559 m3

% pyrite in ore (Field 1985) 60Total volume of rockremoved

652,612 m3

Total volume of spoil 261,053 m3

Total mass of spoil 311,305- 449,664 t

Table 4. Calculation of volume of spoil generated from

production figures

Source Dimension

Rutledge (1906) Massachusetts Economic Geology (Farquhar 1967)

RHS (1977)

Depth of Shaft 1 1200 ft (366 m) 1800 ft (549 m) Depth of Shaft 2 Assumes the same as #1

Ore body width 30- 40 ft (at #1 shaft) (9.1-12.2 m)

24 ft “common” (7.3 m) 61 ft “maximum”(18.6 m)

20-40 ft (6.1 – 12.2 m)

Depth of mine (maximum) 1267 fta (386 m) 1400 ft (427 m) 19 levels

Length of mine 450 ft (137.1 m) 900 ft (274.2 m)

Cross-sectional area 4055 m2 3073 m2

Volume of mine 556,271 m3 843,388 m3

Volume of rock removedb 358,132 m3 544,578 m3

Total Volume of Spoil 143,252 m3 217,903 m3

Total Mass of Spoilc 170,829 – 246,753 t 259,849 – 375,338 t aRutledge (1906) was used to calculate the thickness of each level, based on information that the #18 level was at a depth of 1200 ft. A diagram in Rutledge (1906) shows a thickness of 62 ft; bExtraction ratio = 65% (Younger and Adams 1999). We also accounted for backfilled shaft #2, documented by Rutledge (1906); c depends on porosity of waste rock, range given as 0.35-0.55.

Table 5. Calculation of mine and spoil volume from plans and publications

10

Discussion

The results from the calculations of current and pastspoil volumes show that the current estimate of minespoil volume appears to be on the low side comparedto estimates of the volume of material extracted fromthe mine. There could be a significant amount of spoilunaccounted for in the NE and N spoil heaps, as thesepiles have not been surveyed in any detail.Alternatively, estimates of the amount of spoil left onthe ground during mining (40% of the total volume ofrock and ore removed) may be on the high side of thetrue figure.

The mine volume calculated from the mine plans(around half a million m³) agrees well with the figurecalculated from the records detailing amount of oreshipped from the mine (around 600,000 m³). Assumingthat 60% of the rock removed from the mine wasactually shipped off site results in an estimate of theamount of mine spoil left on the ground when the mineclosed to be approximately a quarter of a million m³.Over 90 years, this has decreased by an order of

magnitude, if the calculations of current and past spoilvolumes in Tables 3-5 are accurate.

It is possible that the units used for production figureswere “short tons”, i.e. 2000 lb rather than 2240 lb.Assuming this to be true would reduce the productionfigures by 12%. The current volume of the mine couldtherefore be slightly less than the maximum volumequoted in Tables 4 and 5.

Based on loadings of 94.8 kg/day of sulphate and3.37 kg/day of iron discharging into Davis Mine Brookfrom Effluent Creek and a total waste rock massbetween 9000 and 14000 tonnes, we can assume thatthe AMD discharge into Effluent Creek will continueto generate elevated acidity and sulphateconcentrations for many decades if no remediationscheme is implemented. At the confluence, loadings ofiron and sulphate in Davis Mine Brook (measured atsite A4) are of a similar order of magnitude to theloadings from Effluent Creek (Figures 3c and 3d)probably due to contamination from spoil heapsupstream of this location.

Ground water flow through spoil was estimated tocontribute between 41 and 64% of the total flow intoDavis Mine Brook based on the ratios (R) ofgroundwater flow in Effluent Creek watershed to flowfrom the Shaft 1 discharge, for an eight-month periodin 2004 (Table 6). Seasonally, flows from the Shaft 1discharge increased during spring so that this flowcomprised up to 80%, and decreased during summerdown to 26% of the total flow, as the water table in the

Species All Seasonsaveraged

Spring2004

Summer2004

Autumn2004

Sulphate 1.77 2.53 0.59Mn 1.26 3.9 0.69 0.58Cl 0.70 0.34 0.59

Table 6. Ratio R (shaft discharge to ground water discharge)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

Spring 04 Summer 04 Autumn 04

Manganese

Load

(Kg/d

)

Shaft (C3)

Groundwater

EC Weir (C2)

0

20

40

60

80

100

120

140

160

180

200

Spring 04 Summer 04 Autumn 04

Sulp

hate

Load

(Kg/d

)

Figure 4. Calculated manganese (a) and sulphate (b) loads in Effluent Creek stream from different sources in the watershed

11

watershed dropped. We postulate that mine wateremerging at Shaft 1 mostly originates from floodeddeep mine workings however during exceptionally wetspells it may include surface drainage from bothexposed waste rock piles to the N of the abandonedmineshafts and uncontaminated runoff from the mostlyforested upper Effluent Creek watershed. The besttracers for calculating R, based on consistent signals inthe three sets of samples, were manganese andsulphate. There was considerable variability, especiallyin sulphate concentrations, in groundwater samples soan average of all upper port (spoil) samples was taken.Standard deviations of sulphate measurements (around400 mg/L) and iron (around 80 mg/L) in groundwaterwere in fact greater than the average concentrations ofthese elements indicating the high variability insubsurface conditions. It was only possible to calculateR for two (using sulphate and chloride) or three (usingmanganese) seasons due to a limited number ofsamples from site C3.

Generally ground water in the upper ports of the wells(i.e. in the spoil) was of worse quality with higherconcentrations of metals and sulphate, than surfacewater in Effluent Creek (see Figures 3a and 3b for ironand sulphate concentrations from all sites). Therefore,the water quality in Effluent Creek between sites C3and C1 deteriorated due to addition of contaminatedbaseflow from the spoil. Downstream, between C1 andC2 there was a significant decrease in theconcentration of some species of contaminants,particularly total iron, the load of which were predictedto decrease by 35%, based on runoff estimates inEffluent Creek at C1 and C2 from the monthly runoffmodel (spring 2003-winter 2004-5). This decrease canbe attributed to the precipitation of goethite onto thestreambed. It is not possible to compute the loading ofiron from groundwater in Effluent Creek since Eq. (3)assumes that the species is conservative and thereforeany in-stream transformations are zero. Loads wereseasonally highest in spring and lowest in summer,mostly due to the seasonal variation in surface runoffnot in contaminant concentrations, which remainedreasonably constant throughout the year.

Corresponding loadings of sulphate and manganesefrom the shaft and groundwater were also calculatedusing the ratio R and Equation (1) (Figure 4). Loadingsfrom the shaft discharge were greatest in spring whenthe flow rate from the shaft was greatest, and loadingsfrom the groundwater were highest in summer andautumn 2004 when the groundwater flow rate from thewaste rock was greatest. Loadings of both manganeseand sulphate were predicted to increase between Shaft1 (C3) and Effluent Creek weir (C2) as the groundwater from the waste rock with high concentrations ofboth species flowed into the stream. Ongoing studies

are using tracer and geochemical equilibriummodelling studies, supported by field reconnaissanceof the stream bed, to determine the quantity of goethiteand other minerals deposited.

Conclusions

Amonthly model was developed to calculate runoff forthe ungauged Davis Mine Brook watershed and itsEffluent Creek tributary, where an uncontrolled minewater discharge has been identified. Readily availabledata sources were used to determine runoff coefficientsthat enabled estimation of runoff directly fromprecipitation data. A limited set of measured flowsfrom a weir in Effluent Creek were used to validate themonthly model. Using the monthly runoff data fromthis model and water samples collected during a recentmonitoring program, simple “order of magnitude”calculations for the abandoned Davis Mine site haveidentified that total contaminant loadings of iron fromEffluent Creek into Davis Mine Brook averaged 3.37kg/day and sulphate 94.8 kg/day between spring 2003and winter 2004-2005.

The mine water emerging from an additional dischargenear Shaft 1 was estimated over an eight month periodin 2004 to average between 36 and 59% of the totalEffluent Creek by mass loading, and to rangeseasonally from 26% to 80%. A simple mass balancemodel was developed to compute the ratio of shaftdischarge to ground water discharge. Three differenttracers were used to compute this ratio, and the threeresults were mostly consistent for three seasons in2004. Further down the valley, ground water in thespoil heap generally is of poorer quality than theEffluent Creek surface flow (originating from the Shaft1 discharge), based on samples collected from multi-port wells. The percentage of the sulphate loadoriginating from the shaft discharge was estimated tovary seasonally between 18 and 78%, using the modelresults for an eight month period in 2004.

The estimated mass of waste rock still present at thesite is between 9,000 and 14,000 t. There are stillfragments of pyrite ore in the waste piles and ongoingpyrite oxidation in the aerobic upper part of the piledue to microbial processes. Based on estimatedpollutant loadings and an estimate of the mass of thewaste material remaining on the site, the mine waterdischarge is forecast to continue to flow for manydecades unless abatement measures are implementedat the site.

By performing simple mass balance analysis on spoilvolumes, water flows, and pollutant loadings, theresults of this work have provided useful informationtoward the design of remediation strategies for this

12

site. It will not be sufficient to control leachate fromthe spoils in the immediate vicinity of Effluent Creekas originally thought. Instead, consideration must begiven to the substantial loadings that emanate directlyfrom Shaft #1. In addition, the fact that estimates of thevolume of spoil placed is substantially larger than thespoil identified in the Effluent Creek area, along withthe observed upstream loadings in Davis Mine Brook,suggest that there are other spoil piles, still unexplored,that must be considered in any remediation effort.

Acknowledgements

In this work, we have drawn from a body of datacollected by a number of students and other colleagues.In addition to those individuals whose work isreferenced, we note our appreciation to Kris Baker,Philip Dixon, and Lynne Stopen.

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