Hydrogeochemical changes of seepage water during artificial recharge of groundwater in Berlin, Germany
The spatial and temporal evolution of the seepage water chemistry below an artificial recharge pond was investigated to identify the impact of dynamic changes in water saturation and seasonal temperature variations. Geochemical analysis of the pond water, suction cup water and groundwater showed that during summer, nitrate and manganese reducing conditions dominate as long as saturated conditions prevail. Iron and sulphate reduction occur only locally. When the sediment below the pond becomes unsaturated, atmospheric oxygen penetrates from the pond margins leading to re-oxidation of previously formed sulphide minerals and enhanced mineralisation of sedimentary particulate organic carbon. The latter promotes the dissolution of calcite. During winter, both the saturated and the unsaturated stage were characterised by aerobic conditions. Thereby, nitrification of sedimentary bound nitrogen could now be observed because nitrate is not immediately consumed, as is the case during summer. This suggests that nitrification below the pond might be less affected by seasonal temperature changes than nitrate reduction.
Increasing water demands and pollution of water resources are some of the most serious challenges of the modern world. Facing these problems, techniques of artificial groundwater recharge such as river bank filtration (e.g., Ray et al., 2004), aquifer storage and recovery (ASR, e.g., Pyne, 1995), deep well injection (e.g., Stuyfzand et al., 2002) and infiltration ponds (e.g., Bouwer, 2002) are becoming increasingly popular. Like other recharging techniques, infiltration ponds are commonly used either to enhance the quality of surface water or to purify partially treated sewage effluent (Bouwer 1991; Asano, 1992). While the fate of specific organic substances during surface water infiltration into unsaturated porous media has been investigated intensively (e.g., Fujita et al., 1998; Lindroos et al., 2002; Långmark et al., 2004), only a few studies exist which simultaneously give attention to the prevailing inorganic chemistry, including redox conditions (e.g., Brun and Broholm, 2001). However, as the local redox state is known to affect the fate of various organic pollutants such as pharmaceutically active compounds (PhAC’s, e.g., Holm et al., 1995, Massmann et al., 2005), pesticides (e.g., Tuxen et al., 2000), or halogenated organic compounds (e.g., Bouwer and McCarty, 1983), it is important to understand the development of redox zones and their constraints in artificial recharge systems.
The aim of this study was to characterise and understand the dynamics of the hydrogeochemical evolution of the seepage water below an artificial recharge pond, which result from transient hydraulic conditions and from seasonal temperature variations of the surface water.
The study site is one of three recharge ponds surrounded by 44 production wells located near Lake Tegel, Berlin, Germany. Its infiltration surface extends over an area of approximately 8700 m2 and has an elevation of 3 m below the adjacent ground surface. Surface water of Lake Tegel is discharged into the pond after it has passed a microstrainer. With time, clogging processes at the pond’s floor lead to a continuous decrease of infiltration rates i.e. from 3 m/d to as low as 0.3 m/d. As soon as the infiltration rate decreases to below 0.3 m/d the site operator Berliner Wasserbetriebe (BWB) abrades the clogging layer to restore the original hydraulic conductivity of the bottom sediments. This operational cycle is repeated every 3-4 months.
The sediments in the adjacent area of the pond are of Quaternary age and consist of fluvial and glacio-fluvial, medium sized sand deposits. Fragments of an up to 5m thick till layer are locally found in depths of approximately 15 m below the ground surface (Pekdeger et al., 2002). The hydraulic conductivities of the aquifer sediment are about 10 -100 m/d. A more detailed description of the sediment hydraulic properties is given in Pekdeger et al. (2002).
The organic carbon content of the sediment below the pond is highly variable and ranges from 0.2 g/kg to 20 g/kg. Reducible forms of iron and manganese minerals are found in concentrations of 0.2 - 1.2 g/kg and 0.01 - 0.1 g/kg, respectively. Total sulphur concentrations are in the range of 0.1 - 2.1 g/kg and are highest in organic-rich layers. A more detailed description of the geochemical composition of the sediment below the pond can be found in Greskowiak et al. (2005, Chapter 3).
A detailed monitoring program was carried out over a period of more than one year. As part of this program (i) the pond water, (ii) groundwater at a depth of 7 m below the pond and (iii) water extracted from four ceramic suction cups located at depths of 50 cm, 100 cm, 150 cm and 200 cm below the pond were analysed for major ions and dissolved organic carbon (DOC) every week. Anions and cations were measured by ion chromatography (IC, DX 100) and atomic adsorption spectrometry (AAS, Perkin Elmer 5000), respectively. DOC was measured photometrically with a Technicon autoanalyser. Dissolved oxygen (DO) was measured by optical oxygen sensors (Hecht and Kölling, 2001) placed next to the suction cups. Water contents and pressure heads at 50 cm and 150 cm depths below the pond were recorded continuously by TDR (Time Domain Reflectometry) probes and pressure transducers respectively. The temperature of the pond water and the groundwater as well as the piezometric head at 8 m below the pond were continuously measured daily with data loggers. The location of the measurement devices are schematically shown in Figure B.1.
|Figure B.1: Schematic cross-sectional view of study area and locations of measurement devices.|
Results and Diskussion
The hydraulic regime immediately below the pond was characterised by cyclic changes between saturated and unsaturated conditions, as indicated by (i) the varying piezometric head resembling the approximate groundwater table (Figure B.2) and (ii) the varying water contents and pressure heads below the pond (data not shown). These changes, which occurred during each operational cycle, resulted from the repeated formation of a clogging layer at the pond bottom (Greskowiak et al., 2005, Chapter 3). Each operational cycle was hydraulically classified into four different stages (Figure B.2). Stage 1 was characterised by a rising groundwater table resulting from the refilling of the recharge pond. During Stage 2 the groundwater table rose to the bottom of the pond and saturated conditions were established. As soon as the hydraulic resistance of the clogging layer became too high (around day 60 and day 200 after start of the monitoring program), air penetrated from the pond margins beneath the clogging layer, which resulted in a sudden drop of water contents (not shown) and piezometric head (Figure B.2). Thus, unsaturated conditions established below the pond (Stage 3). During Stage 3 the groundwater level continuously declined to approximately 4 - 5m below the pond (Figure B.2). Note that infiltration still took place during Stage 3, but continuously decreases to approximately 0.3 m/d (Figure B.2). During Stage 4 the pond was dry and the site operator abraded the clogging layer. During this Stage, the groundwater level was at its minimum.
|Figure B.2: a. Infiltration rate and piezometric head; b. temperature of the pond water; the numbers 1-4 refer to Stages 1-4.|
Surface water composition
Throughout the entire observation period, the pond water typically contained 7 - 16 mg/l of DO. Nitrate concentrations were highly variable and ranged from 0 - 12 mg/l. The lowest concentrations were attributed to nitrogen uptake by algae blooms during summer. With a few exceptions, dissolved iron (data not shown) and manganese were typically below detection limit. Sulphate concentrations were relatively high (~140 mg/l, data not shown). With calcium concentrations of 80 - 90 mg/l and total dissolved inorganic carbon (DIC) concentrations of 30 - 37 mg/l, the pond water was oversaturated with respect to calcite (SICalcite= 0.4 - 1.2). Dissolved organic carbon was rather low with concentrations of about 5 -11 mg/l. The pH was relatively stable and generally in the range of 8 - 8.5.
As soon as saturated conditions established below the pond in summer (stage 2), DO became entirely depleted at all observed depths below the pond (Figure B.3). Nitrate and manganese reducing conditions were dominant beneath the pond as long as saturated conditions prevailed. This was indicated by the total depletion of nitrate and the subsequent increase of dissolved manganese at several observation points (Figures B.3 and B.4). Iron and sulphate reduction and subsequent formation of iron sulphides occurred locally as a result of the sediment’s chemical heterogeneity and non-uniform flow (Greskowiak et al., 2005, Chapter 3). At the beginning of Stage 3, when the sediment below the pond became unsaturated, atmospheric oxygen entered the region leading to an increase of DO (Figure B.3) and subsequent cessation of nitrate and manganese reduction (Figure B.4). The iron sulphides that had formed during saturated conditions immediately became re-oxidised and led to a short peak of elevated sulphate concentrations at various depths below the pond. During Stage 3, nitrate concentrations at some observation points below the pond were considerably lower than the concentrations in the pond water and groundwater (Figure B.3). However, throughout the entire Stage 3, the presence of atmospheric oxygen greatly enhanced the mineralisation of labile particulate organic carbon (POC) below the pond originating from seasonal algae blooms, which promoted the dissolution of calcite (Greskowiak et al., 2005, Chapter 3). As a result, DIC concentrations of the groundwater steadily increased relative to the surface water concentrations until the end of Stage 3 (Figure B.4).
|Figure B.3: Concentrations of a. dissolved oxygen (DO), b. nitrate (NO3-) in the pond water, groundwater and at depths of 50 cm; the numbers 1-4 refer to Stages 1-4.|
During winter, DO was not entirely consumed in the presence of fully saturated conditions (Figure B.3), presumably because very low surface water temperatures (Figure B.2) caused a reduction in microbial activity. Therefore the consumption of electron acceptors such as oxygen and nitrate proceeded at much lower rates (see also, e.g., Prommer and Stuyfzand, 2005). Unlike in summer, aerobic conditions prevailed throughout the entire Stage 2 as a result of the lower water temperatures during winter (Figure B.2). Neither consumption of nitrate nor production of dissolved manganese occurred. Instead, during Stage 2 and Stage 3, nitrate concentration increased by 1 - 5 mg/l within the seepage water on its path from the pond bottom to the groundwater monitoring well (Figure B.3). Since ammonium concentrations within the pond water are generally small and range from 0 mg/l to 0.2 mg/l (unpublished data of BWB), additional sediment bound nitrogen might be oxidised under the prevailing aerobic conditions. The nitrogen source could either be exchangeable ammonium that was oxidised, as previously observed for other recharge basins (e.g., Bouwer et al., 1980), or it might also be organic nitrogen that was oxidised during breakdown of POC (e.g., von Gunten et al., 1991). The production of nitrate below the pond was only observable during winter when nitrate was not consumed simultaneously. During Stage 3, an enhanced production of inorganic carbon is observed, though not as intense as during the summer period (Figure B.4).
|Figure B.4: Concentrations of a. dissolved manganese (Mn2+), b. total dissolved inorganic carbon (DIC) in the pond water, groundwater and at depths of 50 cm; the numbers 1-4 refer to Stages 1-4.|
This study investigated the geochemical evolution below an artificial recharge pond with respect to its transient hydraulic behaviour and seasonal temperature changes. The results show that during the summer period the spatial and temporal development of different redox environments is impacted considerably by the hydraulic conditions prevailing below the pond. During the entire winter period, the redox environment below the pond remains aerobic despite variable hydraulic conditions. As a side effect of decreased biodegradation rates, nitrification of sedimentary bound nitrogen is only observed during winter conditions. This could indicate that nitrification below the pond is less affected by temperature changes than nitrate reduction. Since local redox conditions are the key control for the attenuation of some trace organic compounds, the study provided the base for further investigations on the fate of such compounds during artificial recharge, as demonstrated for the case of PhAC’s by the companion paper by Massmann et al. (2005).
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