Groundwater Quality: Remediation and Protection (Proceedings of the Prague Conference, May 1995). IAHSPubl. no. 225, 1995. 97

Biogeochemical processes in the Torgau aquifer adjacent to the River

THOMAS GRISCHEK & WOLFGANG NESTLER Institute for Groundwater Management, Dresden University of Technology, Mommsenstrasse 13, D-01062 Dresden, ANDREW APLIN, GORDON MACLEOD & RICHARD CLAYTON Newcastle Research Group in Fossil Fuels and Environmental Geochemistry, Drummond Building, The University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK

Abstract Infiltration of water from the River Elbe into the unconfined Torgau aquifer has resulted in the following sequence of biogeochemical reactions, driven by the presence of both dissolved and particulate organic carbon: aerobic respiration, denitrification, reduction of manganese and iron oxide, and sulphate reduction. Computer-based geochemical modelling suggests that the waters are out of redox equilibrium, namely that the waters are supersaturated with respect to Fe(III) minerals and undersaturated with respect to Fe(II) minerals. Levels of Fe(II) are controlled by the rate at which Fe is produced from Fe oxide rather than by mineral precipitation reactions.

INTRODUCTION

The area of study is an unconfined aquifer system adjacent to the River Elbe, close to Torgau, Germany (Fig. 1). The Elbe basin in this area has a width of up to 4 km, a length of about 9 km and is filled with medium and coarse, unconsolidated glacial sands. Four water works are situated along the left side of the River Elbe and abstract groundwater for the domestic water supply of the Halle- and Torgau region. Extensive abstraction of groundwater has induced artificial recharge from the River Elbe into the aquifer via bank infiltration. In order to monitor the effects of this process, several observation wells were installed along assumed groundwater flow paths between the river and wells. Comprehensive hydrochemical analyses of groundwaters and river water were carried out between 1992 and 1994, in addition to field experiments. The purpose was to determine the flow velocities at different depths and to support groundwater flow modelling studies. The aims of the study were to (a) characterize the main physical and chemical processes occurring in the aquifer (e.g. adsorption, biodégradation, mixing), (b) use geochemical measurements to determine flow patterns in the aquifer, (c) give a prognosis of future raw water quality, and (d) design monitoring networks and water protection zones/strategies. The preferential flowpaths and retention time of the infiltrate were determined by calculating flow velocities from groundwater head data and hydraulic conductivity 98 Thomas Grischek et al.

Fig. 1 Location of the Torgau aquifer and monitoring profile.

(result of 120 sieving curve analyses according to the method of Beyer (1964)), pumping tests, measuring EDTA concentrations and stable isotope ratios (Grischek et al, 1993). The water quality problem in the River Elbe was discussed by Socher et al. (1991). As a result of the termination of industrial activities in the Elbe's headwaters (e.g. paper mills), plus the construction of waste water treatment plants within the last 3 years, there has been a significant improvement of river water quality. Of particular importance here is the decline in the level of dissolved organics in the river water and also the high levels of dissolved iron in the central parts of the aquifer. An accurate prognosis of future raw water quality requires consideration of both changes in river water quality and the high concentrations of dissolved iron, since both can result in treatment problems and increased treatment costs. This paper focuses on the redox geochemistry of the aquifer because redox reactions involving oxidized inorganic species such as oxygen, Fe(III) and nitrate and reduced organic carbon can strongly influence the levels of both organics and of Fe(II) in groundwater. Biogeochemical processes in the Torgau aquifer 99

MATERIALS AND METHODS

Sampling

The monitoring profile includes nine observation wells with sampling points at up to five different depths (Fig. 2). While the River Elbe and observation wells 2 and 3 were sampled every two weeks, the other wells were sampled monthly. During floods, which have an influence on groundwater flow, sampling was intensified. In the observation wells, packers are installed above the well screen and a membrane pump is fixed in the screen to prevent contact between water and air. The sampling and conservation of samples have been done according to German guidelines. Strict precautions were taken to prevent oxidation of samples.

Analytical methods

The main parameters measured included pH, Eh, alkalinity, dissolved oxygen, conductivity, dissolved organic carbon (DOC), major ions, iron (total and Fe(II)), manganese, ammonium, nitrate, nitrite and chemical oxygen demand (COD). In general, analyses were carried out according to German guidelines and DIN methods. Sediment samples from various depths of well 5 (Fig. 2) were chosen to determine total iron, easily reducible iron and extractable sulphate. The 6-hour leaching of the sediments at 96°C in 0.04M hydroxylamine chloride (in 25% acetic acid, volumetric) removes exchangeable iron, iron bound to carbonates and iron (and manganese) oxides. This iron is considered to be the iron in the aquifer matrix which can be utilized by iron reducing bacteria. The total iron concentrations were determined using a hydrofluoric/ perchloric acid digestion (Jackson, 1958). The leachates were analysed using Flame Atomic Absorption Spectroscopy (Varian SPECTRAA 3400).

650 550 450 350 250 150 50 O

Fig. 2 Monitoring profile Torgau-Ost I. 100 Thomas Grischek et al.

Microscopy and mineralogy

All sediment samples were examined under a binocular microscope and selected samples were impregnated with resin and examined under transmitted light. Representative samples from each part of the aquifer were analysed by X-ray diffraction between 2-60° 20 using Cu Ka radiation.

ANALYTICAL RESULTS

Water chemistry

Aquifer cross-sections showing concentrations of key analytes are shown in Figs 3 and 4. The processes occurring in the aquifer cause a decrease of pH, Eh, dissolved oxygen, nitrate and DOC concentrations and an increase in sulphate and iron concentrations of the infiltrate. The groundwater in the uppermost zone of the aquifer, mostly at wells 1, 6, 7 and 8, shows an influence of agriculture (calcium, magnesium, sulphate). As a result of a corn field, high nitrate concentration were observed at sampling point 8/1. Sampling point 7/5 is affected by saline water flow. The groundwater at sampling points 1/5, 11/5, 3/5, 4/5, 5/5 and 6/5 has significant lower concentrations in major ions, compared to the sampling points located above and to 7/5 and 8/5. The general trends in the distribution of redox sensitive species conform to those predictable from the well established sequence of microbially mediated redox reactions in which organic matter is sequentially oxidized by oxygen, nitrate, manganese oxide, iron oxide and sulphate (e.g. Champ et al, 1979). As measured Eh decreases with increasing depth, oxygen and then nitrate are initially removed from the waters. Deeper within the aquifer, and away from the main inferred flowpaths, concentrations of dissolved Fe(II) increase to a maximum of 30 mg l"1 in the central part of the aquifer (as do ammonium and manganese) and then decrease towards its base. Except at sampling point 7/5 sulphate concentrations decrease slightly in the basal section of the aquifer. Throughout the aquifer, the distribution of dissolved organic carbon (DOC) is variable. The River Elbe contains the highest amount of DOC (6.8 mg l"1), although this is much lower than levels in 1989 when the DOC content was as high as 15 mg l"1 (Socher et al., 1991). Alkalinity is highest in the uppermost and lowermost aquifer sections.

Sediment analyses

Iron concentrations decrease from 3.5 mg g"1 in surface sediments to a relatively constant level of 1.5 mg g"1 below 69 m NN (NN is German reference level). This is consistent with the observation of extensive coatings of red iron oxides in the surface part of the aquifer. Concentrations are slightly higher in the basal aquifer section. Easily extractable iron comprises 41 + 11.5% (la) of total iron. XRD revealed no crystalline iron minerals. Biogeochemical processes in the Torgau aquifer 101 350 250 150 50 0 m 450 350 250 150 50 0 100 450 (a) (b) 90

80 7 77 +253 217 554 +529 -52f 2 513 -i 630 + +527 + 682 225 70 581 524 / 1 9 ? +527 +202" + ° + +531 V E-60 +148 150 188 u + 189 +179 | 50 Silt Layer t 16! +176 ^9+187Iff 40 174J6J +167 ^+171 -*

30 Production Well Production Well Torgau Aquifer Torgau Aquifer Confining Bed Detail of Cross-Section Confining Bed Detail of Cross-Section ! 20

150 50 0 n- 450 350 100r 450 350 250 250 150 50 0 m (0 (d) 90- Elbe River 9.6 ^ 80: V. 2.3 +5.4 3.1 5 S*BX~^+ 0.2 °+ oTi- -1- 0 +0 .70 +0.1 0 + + + +0 -60 0 +0 0 + + .. ». °. 50 Silt Layer 0 +0 + +0 2 * 40 0 + 0 0 + 0_+_ 0 +0 + + +0 _J^~ r 30 Production Well Production Well Torgau Aquifer Torgau Aquifer Confining Detail of Cross-Section Confining Bed Detail of Cross-Section 20- Bed

450 350 250 150 30 0 450 350 250 150 50 0

(f)

0.09 +0.06 0.1. 0 ^3--0.05°+1C + 0 06 n in^2J°——+0.1C +a09^1°+^lf +^

0.10 0.1f +0.29 0.12 0.11 +0.14 Silt Layer 0.40 0.33 *°25 0+26*O.2i ^^

+0.24 „+21+0.22 ' """

Production Well Torgau Aquifer Confining Bed Detail of Cross-Section I

Fig. 3 (a) Mean conductivity (uS cm"1); (b) mean redox (mV)' (c) mean oxygen concentrations (mg l"1); (d) mean nitrate concentrations (mg 1" ); (e) mean Fe(II) concentrations (mg l"1); (f) mean ammonium concentrations (mg l"1). 102 Thomas Grischek et al.

450 350 250 150 50 0 450 350 250 150 50 0 (a) (b)

106 +109 ipo-'gy / 16g< 2.7 + » v ^ïr-y 2.7v» 1^5 96 +/225 2> +112 IP1* ... / + + 104

113 +120 117

117 183 15! +142 ini(+229_ -^"._ ^ 112

Production Well Production Wet Torgau Aquifer Torgau Aquifer Confining Bed Detail of Cross-Section Confining Bed Detail of Cross-Section Fig. 4 (a) Mean sulphate concentrations (mg l"'); (b) mean DOC concentrations (mg I"1).

The amount of 6.4 mg g"1 iron were extracted from the zone where lignite particles were found at 48.5 m NN, at least twice that in the overlying sediments. Microscopic examination showed that the material dissolved was a crumbly black material, released about 4.7 mg g"1 sulphate, compared to 0.25 mg g"1 in other parts of the aquifer. Anhydride (CaS04) was also identified in this zone. We infer from these results that pyrite was originally present in the lignite zone but was oxidized to Fe oxide and anhydride during the extensive post-sampling storage period.

Biogeochemical redox processes

In this section, we consider the biogeochemical reactions within the Torgau aquifer and use suggested stoichiometrics of the microbial redox reactions in an attempt to achieve a mass balance of reductants and oxidants. This is not a straightforward exercise since mixing along the infiltration flowpath, plus the natural variability of the infiltrating water are complicating factors. For the purposes of discussion, however, the aquifer is divided into three parts based on the locally dominant reactions: an upper section (surface to 65 m NN), where aerobic respiration and denitrification dominate, a central section rich in dissolved iron and a lower section below 35 m NN where there is some evidence for sulphate reduction. For the interpretation of water quality data, it is important to note that there is an old groundwater flow below the silt layer from the east side of the River Elbe beneath the river to the production well. At sampling point 3/3, there seems to be mixing of infiltrating water with groundwater coming from sampling point 11/3. The sampling points 11/3, 11/5, 1/3 and 1/5 are not affected by infiltrating water. Most of the infiltrating river water flows to the well through the upper part of the aquifer (gravels). Higher nitrate concentrations at sampling point 6/2 compared with 5/2 indicate groundwater flow from sampling point 5/1. The main redox reactions in the upper part of the aquifer involve the reduction of dissolved oxygen and nitrate by organic matter, described here as CH20: Biogeochemical processes in the Torgau aquifer 103

CH20+ 02 = = > HCOJ+ H+ (1)

+ 5CH20 + 4NO3 = = > 2N2 + 5HCOJ + 2H20 + H (2) We consider the flowpath between the river and sampling point 2/3, which is situated 5.3 m below the river bed. The observed losses of oxygen (0.302 mmol l"1) and nitrate (0.037 mmol l"1) require the consumption of 0.348 mmol l"1 of carbon, whereas the observed decrease in dissolved organic carbon (DOC) is only 0.199 mmol l"1. This implies that other reductants are present within the aquifer, most probably particulate organic carbon which forms an organic rich layer in the upper centimetres of the river bed proved by analyses of TOC in sediment samples at different depths (Grischek et al., 1993). The amount of water infiltrating in the central part of the aquifer is much lower, the retention time of the infiltrate is more than 100 days. Next we consider the flowpath between sampling points 2/3 and 4/3. The most obvious and likely source of iron in the groundwater is by the reductive dissolution of amorphous iron oxides, of which there are significant amounts. Simple calculations show that each cubic centimetre of aquifer contains about 950 /xg of solid phase extractable iron, compared to only 4.5 /xg of iron dissolved in the groundwater (mean concentration of 15 mg l"1 Fe(II) in the central part of the aquifer). Of particular relevance to the water treatment industry, the future potential for the presence of substantial amounts of dissolved iron is clear. However, the high iron concentrations occur in what are probably relatively stagnant parts of the aquifer, suggesting that the rate of reduction influences iron concentrations. Consideration of the stoichiometry of the microbial reduction of iron by DOC:

2+ CH20 + 7H+ + Fe(OH)3 = = > 4Fe + HCOJ + 10H2O (3)

suggests that the observed decrease in DOC of 0.023 mmol l"1 would release 0.092 mmol l"1 of dissolved iron, compared to the observed increase of approximately 0.261 mmol l"1. Since there is very little organic matter in the sediments within this part of the aquifer (0.014% org. carbon estimated for fraction <2 mm), additional iron could be supplied from the oxidation of pyrite by nitrate. This is supported by a decrease in nitrate concentration (0.319 mmol l"1), accompanied by an increase in sulphate (0.436 mmol T1). Taking a reaction stoichiometry for the oxidation of pyrite by nitrate:

2+ 5FeS2 + I4NO3 + 4H+ = = > 7N2 + 5Fe + 10SO|" (4)

and combining this with reaction (3) predicts the release of 0.228 mmol l"1 sulphate and 0.206 mmol l"1 iron. The observed increase in sulphate is significantly higher, which could be a result of mixing with a portion of water flowing from direction of sampling point 3/3. However, carbonate alkalinity is also released by reaction (3) and yet alkalinity is lower in the iron-rich zone than in other parts of the aquifer. This is not easily explained unless carbonate is lost by mineral precipitation. The large amounts of readily extractable iron and sulphate in the lignite rich layer (35-40 m NN) suggests the presence of pyrite which has been oxidized subsequent to sampling. Sulphate reduction by organic matter can be written as:

2CH20 + SO;;- = = > 2HCO3 + H2S (5) 104 Thomas Grischek et al. and pyrite formation can be written in many ways of which one possibility is:

+ 8SO^~ + 4FeOOH + 15CH20 + H ==> 4FeS2 + 15 HCO7 + 10H2O (6)

Pyrite formation thus results in increased alkalinity and lower contents of dissolved sulphate. No pore water samples were taken within the lignite layer but waters below the lignite rich layer are depleted in sulphate (by 2.703 mmol l"1) and iron (by 0.359 mmol l"1) and have higher alkalinity (0.42 mmol l"1) comparing sampling points 4/4 and 4/5. These results are qualitatively consistent with the reaction 6. However, since the increased alkalinity is much less than the predicted from the apparent decline in sulphate, we must either infer that alkalinity has been lost by carbonate precipitation or that the water composition in the aquifer is insufficiently homogeneous to allow the type of electron balance that we are trying to demonstrate. The heterogeneity of the aquifer causes concentration variations even within the investigated monitoring profile. Chapelle (1993) noted that, as the oxidation of organic matter drives the simultaneous dissolution and precipitation of mineral phases, there is no net change in water composition. A full interpretation of all observed changes within the monitoring profile would require a more detailed mineral analysis.

Modelling redox conditions

The general reactions described in the previous section are useful as a means of identifying the main biogeochemical processes operating within aquifers. They are of less value for quantifying the controls on the concentrations of individual solutes and thus predicting their abundance elsewhere within the aquifer and in the future. This can be done with computer-based geochemical models which allow one to calculate equilibrium concentrations of solutes in waters in contact with a given suite of minerals, plus the saturation states of minerals. Here we attempt to understand the controls on the concentration of iron in the Torgau aquifer using the solubility-speciation modelling computer model, Geochemist's Workbench (Bethke, 1994). Input data to the code are elemental analysis, pH, Eh and the temperature of water being modelled. The code will then calculate the speciation of the solution and the saturation state of minerals. Prediction of supersaturation suggests that mineral precipitation should occur unless this is hindered for kinetic reasons. Undersaturation suggests that mineral dissolution should occur until saturation (equilibrium) is achieved. Understanding the controls on the abundance of iron in natural waters is complicated by its redox geochemistry. For a redox sensitive element such as iron, Eh is a critical but notoriously difficult input parameter to geochemical models. In addition to the measured Eh, which may not accurately represent the true system Eh of the water (Wolery, 1992; Appelo & Postma, 1993), it is possible to calculate Eh from measured concentrations of redox-sensitive species. If the system is at redox equilibrium, all redox couples should be, but very rarely do give the same Eh. For sample 2/3 (Fig. 2), the measured Eh (platinum electrode) is 248 mV. For the Fe3+/Fe2+,NO J /NO J ,NH4/NO J redox couples the calculated values are 338, - 540 and 95 mV, respectively. For samples 3/3, 4/3, 3/5 and 4/5 (Fig. 2), the measured Eh values and Fe3+/Fe2+ calculated redox values are 179, 258, 150 and 280 mV, 171 and 292 mV, Biogeochemical processes in the Torgau aquifer 105

157 and 300 mV, respectively. Values calculated from the NO 3 /NO4 redox couple are likely to be unreliable because NO J/NO4 ratios are controlled primarily by microbial and adsorption processes. Eh values calculated from the Fe3+/Fe2+ redox couple are likely to give the most realistic estimate of the redox state for the water samples, along with field measured Eh. Iron-rich water samples from sample points 3/3, 4/3, 3/5, 4/5 (Fig. 2) have been modelled using measured Eh values and Eh values calculated using the Fe3+/Fe2+ couple. The results are similar in each case. The model predicts that the waters are supersaturated with respect to haematite (Fe203) and goethite (FeOOH) and that to reach equilibrium, essentially all the Fe3+ should precipitate as one of these minerals. In contrast, no Fe(II) mineral would precipitate (in this case siderite, FeC03). In the sulphate-rich portions of the aquifer, e.g. sample point 3/4, the models suggest that the water is slightly undersaturated with respect to gypsum. It is impossible to state definitively why the water is supersaturated with respect to haematite. Either the measured abundance of Fe(III) is too high due to post-sampling oxidation or haematite precipitation has been inhibited. It appears that the concentration of Fe(II) is controlled by production rather than precipitation reactions. These are most probably the amount of microbially available organic carbon and the rate of reaction between oxidized iron minerals and organic carbon. The relatively high iron concentrations can be maintained for two reasons. Firstly, the waters have low alkalinities, reducing the potential for siderite precipitation. Secondly, there is sufficient ferric iron in the aquifer to preclude the onset of energetically less efficient sulphate reduction and thus the production of sulphide, which would cause the precipitation of insoluble iron sulphides.

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