Biogeochemical Processes in the Torgau Aquifer Adjacent to the River Elbe
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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 Elbe THOMAS GRISCHEK & WOLFGANG NESTLER Institute for Groundwater Management, Dresden University of Technology, Mommsenstrasse 13, D-01062 Dresden, Germany 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-Leipzig 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).