Arsenic mobility in groundwater surface water systems in carbonate rich pleistocene glacial drift aquife

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Arsenic mobility in groundwater/surface water systems in carbonate-rich Pleistocene glacial drift aquifers (Michigan)

Kathryn Szrameka,* ,Lynn M Waltera,Patti McCallb

a Department of Geological Sciences, 2534 C.C Little Bldg., University of Michigan, Ann Arbor, MI 48109, USA

b Insight Environmental Services, Inc., 5892 Sterling Drive, Howell, MI 48843, USA

Abstract

Within the Lower Peninsula of Michigan,groundwaters from the Marshall Formation (Mississippian) contain As derived from As-rich pyrites,often exceeding the World Heath Organization drinking water limit of 10 mg/L Many Michigan watersheds,established on top of Pleistocene glacial drift derived from erosion of the underlying Marshall Formation,also have waters with elevated As The Huron River watershed in southeastern Lower Michigan is a well characterized hydrogeochemical system of glacial drift deposits,proximate to the Marshall Fm subcrop,which hosts carbonate-rich groundwaters,streams,and wetlands (fens),and well-developed soil proﬁles Aqueous and solid phase geochemistry was determined for soils,soil waters,surface waters (streams and fens) and groundwaters from glacial drift aquifers to better understand the hydrogeologic and chemical controls on As mobility Soil proﬁles established on the glacial drift exhibit enrichment in both Fe and As in the oxyhydroxide-rich zone of accumulation The amounts of

Fe and As present as oxyhydroxides are comparable to those reported from bulk Marshall Fm core samples by pre-vious workers However,the As host in core samples is largely unaltered pyrite and arsenopyrite This suggests that the transformation of Fe sulﬁdes to Fe oxyhydroxides largely retains As and Fe at the oxidative weathering site Groundwaters have the highest As values of all the waters sampled,and many were at or above the World Health limit Most groundwaters are anaerobic,within the zones of Fe3+and As(V) reduction Although reduction of Fe(III) oxy-hydroxides is the probable source of As,there is no correlation between As and Fe concentrations The As/Fe mole ratios in drift groundwaters are about an order of magnitude greater than those in soil proﬁles,suggesting that As is more mobile than Fe This is consistent with the dominance of As(III) in these groundwaters and with the partitioning

of Fe2+into carbonate cements Soil waters have very low As and Fe contents,consistent with the stability of oxy-hydroxides under oxidizing vadose conditions When CO2charged groundwaters discharge in streams and fens,dis-solved As is eﬀectively removed by adsorption onto Fe-oxides or carbonate marls Although Fe does not display conservative behavior with As in groundwaters,a strong positive correlation exists between As and Sr concentrations

As water–rock interactions proceed,the As/Fe and Sr/Ca ratios would be expected to increase because both As and

Sr behave as incompatible elements Comparisons with groundwater chemistries from other drift-hosted aquifers proximate to the Marshall sandstone are consistent with these relations Thus,the Sr content of carbonate-rich groundwaters may provide useful constraints on the occurrence,origin and evolution of dissolved As in such systems

doi:10.1016/j.apgeochem.2004.01.012

Applied Geochemistry 19 (2004) 1137–1155

www.elsevier.com/locate/apgeochem

* Corresponding author.

E-mail addresses: kszramek@umich.edu (K Szramek),lmwalter@umich.edu (L.M Walter).

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1 Introduction

Elevated As levels in surface and groundwater systems

can be derived from both anthropogenic and natural

sources Although anthropogenic As contamination

from mining operations,fossil fuel processing,and

pes-ticides/herbicides applications is typically local in extent,

contamination can reach levels thousands of times of

that from natural sources (e.g.Smedley and Kinniburgh,

2002) Natural As sources have recently received

increas-ing attention due to the discovery of regional-scale As

contamination of groundwaters,with As enrichment far

above the World Health Organization (WHO)

max-imum contamination limit (MCL) of 10 mg/L (0.13 mM)

in drinking water (Welch et al.,1999,2000; Nordstrom,

2002; Smedley and Kinniburgh,2002) Perhaps the most

widely known problem of naturally occurring As

enrichment of groundwater occurs in unconsolidated

deltaic sediments in Bangladesh Here,potentially 30

106people have been exposed to levels of As up to 2500

mg/L in the groundwater (Nordstrom,2002)

Arsenic is not an abundant element in the earth’s

continental crust (Wedepohl,1995) It can,however,be

concentrated in sulﬁde-bearing minerals such as pyrite

(Savage et al.,2000) The most common sources of As in

the natural environment include volcanic

rocks,speciﬁ-cally their weathering products and ash (Nicolli et al.,

1989; Smedley et al.,2002); marine sedimentary rocks

(Smedley and Kinniburgh,2002); hydrothermal ore

deposits and associated geothermal waters (Korte and

Fernando,1991); and fossil fuels,including coals and

petroleum (Korte and Fernando,1991; Smedley and

Kinniburgh,2002) Although igneous and metamorphic

rocks contain As,the average concentrations (1.5 and 5

mg kg1,respectively) (Ure and Berrow,1982; Smedley

and Kinniburgh,2002) are lower than the average range

from sedimentary deposits (5–10 mg kg1) (Webster,

1999) Typically the As concentrations in sedimentary

rocks increase with increasing amounts of sulﬁde

minerals,oxides,organic matter and clays (Smedley and

Kinniburgh,2002)

Arsenic has 4 oxidation states in aquatic systems,

III,0,+III and +V with the two main inorganic

species found in water being arsenite (III) and arsenate

(V) (e.g.Cullen and Reimer,1989; Drever,1997; Kim,

1999; Stollenwerk,2003) Thermodynamics predicts that

arsenite is stable under reduced conditions and arsenate

is stable under oxidized conditions However,both

spe-cies can be found regardless of the redox conditions,

suggesting that kinetic or microbial processes are

impor-tant controls on speciation (Smedley and Kinniburgh,

2002; Stollenwerk,2003) The geochemical behavior of

arsenate is often compared to that of phosphate,while As

acid is comparable with boric acid (e.g.Drever,1997)

Thus,arsenate is much less mobile under intermediate

pH conditions

Arsenic contents in groundwaters depend on the source of As,the geochemical evolution along the ﬂow path,and the redox state of the system Many diﬀerent mechanisms of As release have been observed in natural systems Focusing on sedimentary occurrences,the two main pathways are the reductive dissolution of

Fe oxyhydroxides (FeOOH) that releases adsorbed As (Nickson et al.,1998,2000; McArthur et al.,2001; Dowling et al 2002; Kolker et al.,2003) and the oxida-tive dissolution of As-rich pyrite (Mallick and Rajagopal, 1996; Mandal et al.,1996; Chowdhury et al.,1999) In anaerobic laboratory experiments,high HCO3 con-centrations promote release of As from sulﬁde minerals (Kim,1999; Kim et al.,2000) BothMcArthur et al (2001)

andHarvey et al (2002)report that the reducing con-ditions associated with organic matter decomposition may increase As mobility Similarly, Dowling et al (2002)

observe that high levels of dissolved As and Fe are positively correlated with NH3and CH4,suggesting that microbial breakdown of FeOOH releases As Taken together,any or all of these processes could reasonably inﬂuence the mobilization and transport of As in groundwater systems

The formation of the source FeOOH material under-going oxidative dissolution in the subsurface commonly occurs in oxidizing soil proﬁles Here,As released by oxidative weathering can be adsorbed to the product Fe oxyhdroxides in the zone of accumulation Some researchers have examined soils developing on parent materials rich in As and Fe from both anthropogenic and natural As sources (Strawn et al 2002; Courtin-Nomade et al.,2003; Ne´el et al.,2003) These researchers indicate that successive oxidation and re-precipitation processes can also occur,with progressive loss of As in the solid phase,i.e.,Fe-oxides This loss of As is a result

of the incomplete sorption of As back onto the FeOOH

as it is re-precipitated as a solid phase

There has been increasing concern about elevated

As levels in the groundwaters of the glaciated mid-continent region (Fig 1 A) Arsenic levels exceeding the WHO MCL have been observed in groundwaters from bedrock and glacial aquifers in the southeastern Lower Peninsula of the state of Michigan (Fig 1B,C) The source of As in the region is thought to be oxic weath-ering of As-rich pyrite (as high as 8.5 wt.% As) from the Marshall Fm and Coldwater Shale,both of Mis-sissippian age (Kolker et al.,2003) Iron oxyhydroxides found in glacial deposits that contain rock fragments of the Marshall Fm and the Coldwater Shale have As concentrations up to 0.7 wt.% (Kolker et al.,2003) The highest value reported for Michigan groundwater comes from the Marshall Fm and is 220 mg/L (2.94 mM) (Kim, 1999,2002; Kolker et al.,2003) Most prior studies of

As in Michigan groundwaters (Kim,1999; Kim et al., 2000,2002; Welch et al.,2000; Kolker et al.,2003) have focused on watersheds in the eastern-most part of the

1138 K Szramek et al / Applied Geochemistry 19 (2004) 1137–1155

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state,locally known as the ‘‘Thumb’’ region (Fig 1B).

Heterogeneity in As levels is to be expected,given the

complex interplay between Pleistocene glacial history,

erosion,deposition,and ﬂuctuations in recharge rates to

drift and bedrock aquifers

In this contribution,the authors explore the patterns and causes of elevated As concentrations in ground-waters from unconﬁned glacial drift aquifers in the Lower Peninsula of Michigan (seeFig 1) in proximity

to the Marshall Fm To gain a fuller understanding of

Fig 1 Hydrogeological framework of the upper Midwest (US) and Lower Peninsula of Michigan (A) Average Pleistocene drift thickness in the Midwest Region The Lower Peninsula of the state of Michigan is marked with a box (B) Middle to Upper Paleozoic bedrock geology of the Lower Peninsula of Michigan The Marshall Formation sub-crop forms a circle around the state The two main bedrock aquifers are in Devonian carbonates and sands of the Marshall Fm (C) Shaded relief map of Michigan marked with outlines of the 4 USGS reference watersheds identiﬁed by name Each watershed is marked with a line of section referred to in Fig 2

K Szramek et al / Applied Geochemistry 19 (2004) 1137–1155 1139

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the processes regulating the As contents of glacial drift

groundwater systems in Michigan,the authors

investi-gated the geochemical relations between As and other

geochemical variables in groundwater,surface water,

and soils in a well constrained portion of the Huron

River watershed in southeastern Michigan This is part

of a larger study of C cycling and transformations in the

Huron watershed (Szramek,2002) As shown in the

following section,the Huron River watershed is

estab-lished on top of heterogeneous glacial deposits and has

groundwaters which exhibit a large range of As

con-centrations,many above the WHO MCL of 0.13 mM

2 Hydrogeologic framework of arsenic occurrence in

lower Michigan groundwaters

The Michigan Basin is a cratonic depression ﬁlled

with mainly Paleozoic era sedimentary bedrock and

mantled by Pleistocene glacial deposits (Dorr and

Eschman,1970) As shown in Fig 1B,the principal

bedrock aquifers in the basin are Devonian carbonates,

Mississippian and Pennsylvanian units,including the

Marshall Fm.,and the glacial deposits (Rheaume,

1991) A major aquitard,the Coldwater Shale,underlies

the Marshall Fm sandstones Due to the bowl-like

shape of the basin the sub-crops of the Marshall Fm

and the Coldwater Shale form a nearly concentric ring

within the Michigan Basin Bedrock is mantled by a

sequence of Pleistocene glacial deposits up to 300 m

thick which show the record of 2 Ma of Pleistocene ice

sheet advances and retreats (e.g Dorr and Eschman,

1970) These glacial sequences exhibit a range of

hydrologic properties and include permeable sands and

gravels in outwash deposits,less permeable tills,and

highly impermeable lakebed clays The glacial deposits

are also the primary control on the topography of the

state,and glacial depositional features commonly deﬁne

watersheds (Fig 1C)

Of special interest in framing this study were areas

where glacial drift aquifers overlie the Marshall Fm In

Fig 1C,the locations of 4 watersheds (Thumb region,

Huron River,Kalamazoo River,and Manistee River)

established on top of the Marshall Fm subcrop are

displayed Groundwater chemical data including As

concentrations are available for each of these 4

water-sheds from the United States Geological Survey NWIS

Web database (2001) and Kim (1999) Schematic

hydrogeologic cross-sections (Fig 2 A–D) show that the

watersheds fall along a continuum between highly

permeable open systems to those with signiﬁcant

per-meability contrasts within the drift aquifer materials to

those with virtually no permeability The Thumb area is

primarily covered with lakebed clay deposits,reducing

contact between surface ﬂow systems and the underlying

bedrock aquifer The Thumb area is part of a regional

groundwater discharge system and saline water is commonly encountered within 60 m of the surface (Rheaume,1991) This situation is unusual for Michigan because most other areas have significant communi-cation between surface waters and groundwater flow systems as exemplified by the Huron,Manistee and Kalamazoo watersheds

The groundwater As concentrations in each of the

4 watersheds are displayed in Fig 3 Although the Marshall Fm is located within each of these reference areas,As concentrations vary widely Diﬀerences in the permeability and transmissivity of the glacial drift deposits would be expected to play a large role in the variability of As levels These factors encourage or inhibit the oxidization and reduction processes known

to mobilize As from sulﬁde and oxide minerals Groundwater from the Thumb region has the max-imum As value reported in the Lower Michigan area (2.94 mM As) and 70% of the 100 wells sampled have As concentrations in excess of the WHO MCL of 0.13 mM (Kolker et al.,2003) The median As concentrations in groundwaters from the 4 watersheds are,from lowest to highest: 0.010 mM in the Manistee watershed,0.0134

mM in the Kalamazoo,0.029 mM in the Huron,and 0.121 mM in the Thumb region The hydrogeology of the Huron watershed study site is similar in many ways to the Manistee and Kalamazoo watersheds and oﬀers an interesting counterpoint to prior geochemical investigations of the Thumb region

3 Materials and methods 3.1 Study location The Portage Creek catchment is located in the wes-tern portion of the Huron watershed (Fig 4A),where the Marshall Formation sub-crops beneath glacial drift (Fig 1B) The groundwater in the Huron watershed is mainly hosted in glacial drift aquifers (Twenter et al.,

1976) The area has high topographic gradients as it is one of the headwater catchments of the Huron River The mean annual temperature for the region is 10 C and the average annual precipitation is 80 cm The drainage area for Portage Creek is approximately 205

km2and is mainly comprised of hardwood forests,with limited urban development Portage Creek ﬂows through lakes and wetlands on its course toward the main stem of the Huron River

The work focused on the Hell Fen area (Fig 4B) The fen is located along Tiplady Road near Hell,Michigan (W83 590 0800 and N42 2600 36000) Fens are ground-water-fed wetlands that have high concentrations of

Ca2+and HCO3and circum-neutral pH (Glaser et al., 1990,Komor,1994; Almendinger and Leete,1998)

Fig 4Ashows the surface drainage in the fen with small

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creeks ﬂowing along the surface The fen is surrounded

by topographic highs allowing for the discharge of

shallow groundwaters The groundwater discharges at

this point because of the drift impermeability and

heterogeneity in the location of the fen (Fig 5)

3.2 Sample sites

Groundwater wells are all producing from glacial

drift Well sites were chosen based on their proximity to

Hell Fen (shown in Fig 4B) and on the availability of

well driller’s information As seen inFig 5,the hetero-geneity of the drift allowed for waters being drawn from diﬀerent drift types Sampled wells were mainly unconﬁned and varied from 16 to 60 m deep

A sequence of shallow groundwaters that discharge into the fen were sampled using 5 PVC piezometers transecting the fen to cover aerial variability (Fig 4C) Care was taken to prevent surface water contamination

by packing swelling clay around the outside of the upper half of the set pipe The piezometers sampled water at a depth of approximately 90 cm

Fig 2 Schematic bedrock geologic and glacial drift cross-sections for the four reference watersheds in the Lower Peninsula of Michigan In each case,the Marshall Formation aquifer is conﬁned on either side by shale aquitards However,glacial drift thickness, lithology and permeability diﬀer markedly among the 4 watersheds (A) Manistee River watershed has the thickest and most permeable drift section comprised of outwash sands and gravels,with till in moraine deposits (B) ‘‘Thumb’’ area has the lowest drift permeability and the thinnest cover over bedrock,as lakebed clays comprise most of the Pleistocene section (C) Kalamazoo River watershed has drift comprised of permeable sands and gravels but is a relatively thin cover such that the eastern side of the area has Paleozoic bedrock very near the surface (D) Huron River watershed has a relatively thick drift section characterized by impermeable lenses of till spread throughout sandy outwash and moraine deposits.

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Surface water samples were collected in conjunction

with a larger study on the C systematics of the Huron

River watershed The surface sampling locations

(Fig 4A) were selected based on their relationship to

conﬂuences with the main stream,at points before and

after the stream passed through a lake system Care was

taken to collect upstream of large roads and

develop-ments to limit potential contamination from local runoﬀ

The soil water sample sites are all located in upland

areas that surround the fen The sites H-1,H-2,and H-3

are shown in Fig 4B Ceramic-cup tension lysimeters

(Soil Moisture Corp.) were installed in these sites for the

collection of waters at depths ranging from 23 to 100 cm

Soil samples were collected from two locations,H-1

and H-2 The soils were sampled every 10 cm to a depth

of 1.5 m using a large-diameter auger Samples were

taken from the center of the augured material to limit

contamination from adjacent soil layers All samples

were bagged in air tight Bitran bags and then frozen A

representative subsample of this material was ground to

pass a 63 mm mesh prior to geochemical analyses

3.3 Water collection

Well water was generally collected from the well

owner’s outdoor tap A laminar ﬂow of water was

allowed to run into the collecting vessel until

temper-ature and dissolved O2were stabilized,typically taking

around 20 min,depending on distance to the well-head

and presence and size of the holding tank for the household Samples were only taken after temperature and dissolved O2levels stabililized,indicating that the water sample was representative of in situ conditions Aliquots were immediately taken and transferred into crimp sealed glass bottles ﬁlled with no headspace to limit O2contamination

Shallow groundwater was collected from the piezo-meters using a peristaltic pump Pumping was main-tained on the wells for approximately 15 min to allow for the removal of stagnant water

Stream and fen water samples were collected over 3 seasons (10/00,5/01,and 6/01) to capture variability in the system Base ﬂow of the streams in the Huron watershed is during the summer months,however,fre-quent thunderstorms can interfere with capturing the stream at base ﬂow

Soil waters were collected from 3 nests of ceramic-cup tension lysimeters (Soil Moisture Corp.) Approximately

48 h before sample collection,tension was pulled on the lysimeters to 30 cbars to draw water into the ceramic-cup If soil water was present,it was extracted using acid washed syringes

3.4 Field measurements and sample preservation Temperature,conductivity,dissolved O2 (DO),and

pH were determined at the ﬁeld location Temperature and DO was measured using a YSI model 58 meter and

Fig 3 Range of groundwater As concentrations in the 4 reference watersheds ( Kim,1999; USGS,2001 ) Number of samples and median As concentrations are as follows: n=13; 0.01 mM (Manistee); n=24,0.0134 (Kalamazoo); n=18,0.029 (Huron); and n=25, 0.121 (‘‘Thumb’’) The minimum As concentration is constrained at 0.01 mM,the As detection limit for these data sets The World Health Organization MCL of 0.13 mM is indicated.

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Fig 4 Location of the Huron Watershed field study sites showing elevation and physiography (A) Portage Creek catchment at the northeastern edge of the Huron Watershed (shown as small inset map) Surface water sampling locations are shown in the gray circles The indicated sample numbers correspond to those in Table 1 The Huron River was sampled after the Portage Creek confluence at the USGS gage station 4173000 (Huron R near Dexter,MI) (B) Topography of the study area at the town of Hell,MI An extensive wetland area studied is indicated as ‘‘Hell Fen’’ on the map Groundwater and surface water locations are indicated by black and white circles,respectively Soil water and soil profile sampling sites are located at H-1 and H-2 The line shown from H-1 to H-2 sites indicates the location of the driller log lithologic sections shown in Fig 5 (C) Expanded scale view of Hell Fen with fen surface drainage sample locations indicated by the open circles The dashed line across the fen is the location of the piezometers,locations are numbered from 1 through 5,spaced roughly equally,going from west to east.

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a YSI 5239 DO probe with high sensitivity membrane,

directly at the source,either in the stream or at the

groundwater well Conductivity was observed in the

ﬁeld using a Corning 316 meter with a two point

cali-bration 0 and 1413 mS,mostly to provide a rapid

geo-chemical reference point Dissolved O2 measurements

were precise to 5% saturation and conductivity

measurements to within 5%

A Corning 315 high sensitivity pH meter with an

Orion Ross combination pH electrode calibrated with

low ionic strength buﬀers of 4.1 and 6.97 were used to

measure pH in the ﬁeld as close to the water

temper-ature as possible The pH of a sample can change due to

degassing and warming; therefore,the samples were

placed in a large volume airtight container and

mea-sured at least twice to ascertain electrode stability The

precision of pH determinations is 0.01 pH units

Samples for later chemical analysis in the lab were

collected in HDPE bottles The bottles and ﬁlters used

for the As samples underwent a 3-step acid wash

proce-dure and were dried in a laminar ﬂow hood Aliquots

for analyses were ﬁltered through a 0.45-mm nylon ﬁlter

into their respective bottles while still in the ﬁeld and

refrigerated until analyzed Samples collected for total

dissolved As analyses were acidiﬁed down to

approxi-mately a pH of 2 with optima grade HNO3 (Fisher

Scientiﬁc) Samples intended for As speciation (As

III/As V) determinations were ﬁltered into dark glass

bottles,ﬁlled with no headspace,and were not acidiﬁed

Dissolved inorganic C (DIC) and ICP–AES aliquots

were preserved in the ﬁeld with CuCl2,and HNO3,

respectively The DIC aliquots were placed in serum

vials,ﬁlled with no headspace,and then crimp-capped

using Teﬂon-lined septa Aliquots for titration alkalinity and ion chromatographic analyses were placed in HDPE bottles ﬁlled with no headspace without any acid treatment Refrigeration on site and rapid analysis back at the University laboratory was essential for

As speciation and for alkalinity titrations to prevent oxidation and carbonate/hydroxide precipitation 3.5 Geochemical analyses

Arsenic was measured on a Thermo-Finnigan Ele-ment 2 mass spectrometer using a modiﬁed method of hydride generation (Klaue and Blum,1999) Most ana-lyses were for total As concentrations Here,all species

of As in aqueous solutions are oxidized to As(V) using 10% (v/v) HNO3 and ultraviolet oxidation in a con-tinuous-ﬂow reaction vessel A small suite of samples were collected for determination of As speciation The same method of hydride generation was used (e.g.Klaue and Blum,1999),but the aqueous sample is passed through column pretreatment to separate the As (III) from As(V) prior to the oxidation step As(V) is then reduced with 1% (w/v) NaBH4in 0.1 M NaOH to form AsH3gas The AsH3gas is then swept with Ar into the mass spectrometer after passing through a liquid/vapor separator (Klaue and Blum,1999) A few of the samples were run without hydride generation on a Finnigan Element 1 ICP–MS The detection limit for As run on high resolution is about 0.004 mM

Major element chemistry on waters was measured by ICP-AES for cations and ion chromatography for anions A Leeman Labs,Inc.,Plasma-Spec ICP-AES 2.5 was used to analyze for Ca,Mg,Na,Sr,and Fe with

a precision of 2% for major and 5% for minor ele-ments Anions (Cl and SO42) were analyzed on a Dionex 4000I series ion chromatograph (IC) with an AS14 column to a precision of 2% Aliqouts of soil leaches were analyzed using a Finnigan Element 1 ICP–

MS at a precision of 1.5 to 2%

Total alkalinity was measured within 24 h of sample collection by electrometric endpoint titration using a Radiometer TitraLab automated titration system with a TIM900 titration manager and ABU91 or ABU93 autoburette Due to the given measurement precision ( 0.01 meq/kg),the pH range of the samples,and the ionic composition of the solutions,HCO3 was calcu-lated as equivalent to total alkalinity Charge balance calculations performed on water chemistry data to check for internal analytical consistency were within 5%

3.6 Solid soil collection and analysis Soils were extracted for hydroxide and carbonate bound metals using a modiﬁed strong acid leach descri-bed by Hossner (1996) In a study by Chen and Ma

Fig 5 Lithologic heterogeneity of the drift is shown in this

schematic cross-section of the Hell Fen area Driller well log

records for private wells were used to construct the

cross-section The locations of the two soil proﬁles and lysimeter

sites are shown as H-1 and H-2.

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(2001),a similar method was tested on 20 diﬀerent soils

and shown to be an eﬀective way to determine total As

The extraction method uses approximately 0.5–0.8 g of

soil ground to ﬁner than 63 mm which is treated with 5 ml

of ‘‘aqua regia’’ (3 HCl:1 HNO3) in an acid-cleaned

125-ml polypropylene bottle The soils were reacted for 3 h

on a shaker table at room temperature After that time,

the reaction was stopped by the addition 95 ml of H2O

to form a 5% acid solution to prevent cation

precipita-tion The solutions were then ﬁltered through a 0.45-mm

polypropylene ﬁlter into acid cleaned vials Blanks

(same procedure without soil) were carried out in the

same manner and subtracted from the ﬁnal calculations

This cold acid extraction technique primarily

dis-solves the most reactive fractions in the soil (hydroxides

and carbonates) and does not signiﬁcantly attack silicate

or sulfide minerals The effectiveness of the modified

technique was conﬁrmed via repeat extractions on the

solid residue No additional As was recovered in repeat

digests Additionally,several samples from the base of

the soil column were analyzed by S-coulometry to

determine if sulﬁdes were present in the bulk parent

material Results of S analyses were below detection,

consistent with the maturity of the weathering zones in

these well developed soils

4 Results and discussion

4.1 General water chemistry

Major element chemistry of waters from the Portage

catchment [soil water (lysimeter),groundwater (well and

piezometer),and surface water (Portage Creek and Hell

Fen)] is dominated by Ca2+,Mg2+ and HCO3

(Fig 6A) The stoichiometry of the dissolution reaction

for carbonate minerals with CO2yields 2 mol HCO3for

each mole of divalent cations (Ca2++Mg2+) and most

waters are close to this ideal value The Mg2+/Ca2+

ratio of the waters (see Table 1) falls very close to 0.5,

suggesting that 1 mol of dolomite dissolves per 1 mol of

calcite The glacial drift contains fragments of Paleozoic

carbonates (calcite and dolomite),and this is evident

from the soil extract data presented later in this section

Aqueous speciation and carbonate mineral saturation

state calculations indicate that groundwaters are all near

equilibrium with respect to dolomite and approximately

twice saturated with respect to calcite (Szramek,2002)

Given the average groundwater temperature around

10C,dolomite is more soluble than calcite,permitting

dolomite dissolution concurrently with calcite

precipita-tion As will be discussed later in this section,carbonate

mineral recrystallization would be expected to occur

along the groundwater ﬂow path,and is evidenced by

the signiﬁcantly elevated Sr2+/Ca2+ratios in many of

the groundwaters (seeTable 1)

Given the high topographic gradients in the Hell fen area,it is common for groundwaters to discharge into surface ﬂow systems with attendant degassing of dis-solved CO2,especially in the summer when there are large temperature increases during discharge Under these conditions,calcite supersaturation (IAP/K) can increase

to values as great as 16,which produces the CaCO3marl

of the Hell fen surface sediments (Szramek,2002) Car-bonate precipitation has an important regulating eﬀect

on the nutrient cycling in fens (e.g.Boyer and Wheeler,

1989) because phosphate has a very strong aﬃnity for adsorption on carbonate mineral surfaces (e.g.DeKanel and Morse,1978,Walter and Burton,1988)

Fig 6A shows that surface waters commonly have

Ca2++Mg2+ concentrations greater than those in the groundwaters A plot of Na+vs Cl(Fig 6B) shows that groundwaters and soil waters tend to have very low

Clconcentrations,but the surface waters can be extre-mely enriched in Cl Approximately 20,000 t of salt are added each year by the Washtenaw County transporta-tion department (Mulcahy,2003) and CaCl2 is also commonly used to deice walks and driveways Two fen water samples shown inFig 6B have Clin excess of all other samples These two samples are taken from

a location close to the road and experienced larger input of salt as a result All the surface waters have

Cl in excess of Na+ suggesting both salts contribute

to the overall solute load of the surface waters Thus, water chemistry in the Portage catchment is dominated

by inputs from carbonate mineral dissolution and anthropogenic salt sources

4.2 Arsenic in soil profiles The geochemistry of soil extracts (Al,Fe,As,Ca,Mg) for the two profiles H-1 and H-2 is reported inTable 2 The trace metal,As and Al relations vs depth for the two soil profiles are shownFig 7 A–D The zone of accumu-lation (B horizon) for the soil is evident by the increased concentrations of Al,Fe and As between 50 cm and 125

cm (Fig 7A–C) In this zone the Ca and Mg concentra-tions indicate that the carbonates have been selectively weathered out of the soil column until at least 130 cm in H-1 and 160 cm in H-2 Below the zone of accumula-tion,the Ca and Mg contents rapidly increase towards relatively unaltered parent glacial drift values (Table 2) The ultimate source of the As in the soil is from As-rich pyrite from the Marshall Fm that was incorporated into the drift and then oxidized and re-precipitated as FeOOH within the drift (Kolker et al.,2003) Soil pro-ﬁles H-1 and H-2 have an average As/Fe ratio within the zone of accumulation of 0.5 (mol 103) This value is similar to values reported byKolker et al (2003)in the Thumb area for the bulk Marshall Fm.,taken from core cuttings that range from 0.9 to 1.8 (mol 103) and till derived from the Marshall Fm approximately 1.5–2.7

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(mol 103) The bulk Marshall Fm hosts As in unaltered

As-rich pyrite and arsenopyrite,whereas the till and soil

proﬁles host As in Fe oxyhydroxides The similar values

indicate that the oxidation of Fe sulﬁdes to Fe

oxy-hydroxides appears to closely follow the value of the As

and Fe of the precursor phase

4.3 Relations between arsenic and iron in waters Arsenic and Fe concentrations in the water samples from the Portage catchment are reported inTable 1 In

a plot of As vs Fe (Fig 8A),each water type appears clustered in composition space with respect to Fe and

Fig 6 General major element geochemistry of the water samples (A) Carbonate geochemistry: All waters fall near the 1:2 mol ratio

of Ca 2+ +Mg 2+ :HCO 3indicating that dissolution of carbonate minerals is the major process controlling the water chemistry This pattern is typical of surface waters and shallow groundwaters in the carbonate-rich drift deposits of the upper Midwest (e.g Rheaume, 1991; Szramek,2002 ) (B) Anthropogenic salt inputs into the Portage Creek catchment include NaCl and CaCl 2 Surface waters are most inﬂuenced by additions of these two salts,explaining why many plot above the 1:2 stoichiometric line in Fig 6A Most groundwaters and soil waters have low Cl contents,with most Na + derived from plagioclase feldspar dissolution.

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