Regional Water Quality Patterns in an Alluvial Aquifer: Direct and Indirect Influences of Rivers
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Published in Journal of Contaminant Hydrology 169, 123-131, 2014 which should be used for any reference to this work 1 Regional water quality patterns in an alluvial aquifer: Direct and indirect influences of rivers ⁎ A. Baillieux, D. Campisi, N. Jammet, S. Bucher, D. Hunkeler Centre for Hydrogeology & Geothermics (CHYN), University of Neuchatel, Rue Emile Argand 11, CH-2000 Neuchatel, Switzerland abstract The influence of rivers on the groundwater quality in alluvial aquifers can be twofold: direct and indirect. Rivers can have a direct influence via recharge and an indirect one by controlling the distribution of fine-grained, organic-carbon rich flood deposits that induce reducing conditions. These direct and indirect influences were quantified for a large alluvial aquifer on 2 Keywords: the Swiss Plateau (50 km ) in interaction with an Alpine river using nitrate as an example. The hydrochemistry and stable isotope composition of water were characterized using a network of 115 piezometers and pumping stations covering the entire aquifer. Aquifer properties, land use and recharge zones were evaluated as well. This information provided detailed insight into the factors that control the spatial variability of groundwater quality. Three main factors were identified: (1) diffuse agricultural pollution sources; (2) dilution δ18 δ2 processes resulting from river water infiltrations, revealed by the OH2O and HH2O Dilution contents of groundwater; and (3) denitrification processes, controlled by the spatial Denitrification variability of flood deposits governed by fluvial depositional processes. It was possible to Alluvial aquifer quantify the dependence of the nitrate concentration on these three factors at any sampling Flood plain Stable isotope point of the aquifer using an end-member mixing model, where the average nitrate concentration Diffuse pollution in recharge from the agricultural area was evaluated at 52 mg/L, and the nitrate concentration of infiltrating river at approximately 6 mg/L. The study shows the importance of considering the indirect and direct impacts of rivers on alluvial aquifers and provides a methodological framework to evaluate aquifer scale water quality patterns. 1. Introduction and water is abundant for irrigation. Hence, a large fraction of the recharge area might be used for growing crops. As water Alluvial plains frequently harbor productive aquifers tables in alluvial aquifers are often shallow and deposits that play an important role for drinking water supply and permeable, alluvial aquifers are vulnerable to contamination. irrigation. River sediments often form shallow aquifers Elevated nitrate concentrations can be observed in many composed of gravel, sand, silt or clay deposited in river aquifers, impacting the quality of drinking water supply wells. channels or on floodplains (He and Walling, 1997). Alluvial It is important to understand the factors that control nitrate aquifers close to mountain ranges are often particularly levels and groundwater quality patterns in general in view of permeable and produc-tive due to the presence of coarse the long-term exploitation of alluvial aquifers. material. Alluvial plains are often also hotspots of human Different factors can influence groundwater quality pat- activity with traffic routes, settlements and intense terns in alluvial aquifers. Here nitrate patterns are discussed agriculture. Soils formed in organic-rich river deposits are although spatial patterns of other substances (e.g. pesticides) fertile (Schaetzl and Anderson, 2005) are influenced by similar factors. Alluvial aquifers often interact ⁎ Corresponding author. with rivers. In humid areas, nitrate levels in larger rivers are E-mail address: [email protected] (D. Hunkeler). frequently fairly low especially in headwater areas since 2 agriculture is usually less intense compared to downstream and water inputs. Furthermore, a dense network of monitoring plains. Hence, groundwater–surface water interactions can wells is available throughout the aquifer. The direct influence of lead to a zone of lower nitrate concentration along rivers, in river water was mapped based on the stable isotope composi- contrast with nitrate-rich groundwater recharged through tion of water as direct recharge by precipitation and river water farmlands (Debernardi et al., 2008; Hosono et al., 2013; Pinay are expected to have a distinctly different isotope signature due et al., 1998). At locations dominated by direct recharge, to the origin of the river from a high altitude. Based on the evapotranspiration and water input including irrigation are mixing ratios, expected nitrate concentrations due to dilution the main physical processes that influence nitrate concentra- only were calculated and compared to measured concentrations tion as shown in a study that compares five agriculture regions in order to locate denitrification zones. These zones were throughout the USA (Green et al., 2008). In addition, nitrate related to the alluvial soil quality and processes that control the levels might also be influenced by biogeochemical processes, architecture of the alluvial plain. This approach could be especially plant uptake and denitrification (Bohlke et al., 2002; applicable in future studies to better understand the sources Haycock and Burt, 1993; Kellogg et al., 2005; McMahon and and the attenuation mechanisms of groundwater nitrate pollu- Bohlke, 1996). Numerous studies have investigated denitrifi- tion in alluvial aquifers. cation in the riparian zone (Burt et al., 1999; Clement et al., 2003; Kellogg et al., 2005; Maitre et al., 2005) and in aquifers in 2. Study area general (Bohlke et al., 2002; Green et al., 2008; Tesoriero and Puckett, 2011). Denitrification in riparian zones has frequently The Seeland region is located in the northwestern part been attributed to shallow water table conditions and/or soil of Switzerland (Fig. 1). The unconfined North-Seeland quality. According to Pinay et al. (2000), soil texture in alluvial aquifer was formed by the deltaic depositional system of sediments, particularly the silt and clay fraction, is closely the Aare River, which took place after the last Pleistocene related to the denitrification potential. Similarly, Schilling and glaciations (Würm). Basement rocks underlying the aquifer Jacobson (2012) linked the lithology of alluvial sediments to are composed of the Tertiary Molasse of the Swiss Plateau, the potential of denitrification. They documented greater which constitutes most of the reliefs surrounding the organic content in fine-textured materials of swale sediments, aquifer. During the Riss glacial period, the Rhone Glacier known to promote denitrification. Hence, rivers might have a carved out this bedrock valley that is partly filled by ground two-fold influence on nitrate concentrations: a direct influence moraines. These deposits are covered with a series of via dilution and an indirect one by controlling on a long time- glacio-lacustrine sediments, repre-sented mostly by clay scale spatial patterns of fine-grain, organic-carbon rich sedi- with locally gravelly lenses, exceeding a thickness of 200 m. ments that promote denitrification. While the nitrate behavior During the last glaciations, large amounts of glaciofluvial in the vicinity of streams and at the scale of agricultural parcels gravel (‘Seeland gravel’ in Fig. 1) were deposited in front of the has attracted a great deal of attention so far, there is only Rhone Glacier. The successive avulsions of the meandering limited information on nitrate patterns at the scale of entire Aare River led to a variable lithology of the alluvial aquifers directly and indirectly influenced by rivers. unconsolidated Quaternary sediments (‘Aare deposits’ in Rationalizing the spatial variability of nitrate concentrations Fig. 1), which varies from well-sorted gravels to clay. The in the proximity of rivers can be challenging since both average thickness of these deposits in the alluvial plain is biogeochemical and dilution processes affect the concentration, approximately 25 m and can reach 50 m in the Aarberg– independently of the spatial variability of land use. Several Kappelen region. Up to 10 m of flood deposits (‘fine sediments’ studies have analyzed stable isotopes in nitrate to demonstrate in Fig. 1) are observed in the northeast border of the plain. The denitrification and to differentiate it from dilution (Hosono deltaic depositional system has led to a strong heterogeneity of et al., 2013; Mariotti et al., 1988). In another study, chemical the sediments, but on a larger scale, the aquifer is relatively parameters (e.g., chloride; Pinay et al., 1998)wereused to homogeneous (Kozel, 1992). Untilthe19thcentury and thefirst – quantify the mixing of surface water and groundwater. river engineering works (Jura water corrections, 1868 1891), The aim of this study is to identify and quantify the various this large area was a floodplain of the Aare River (Kozel, 1992), factors that influence the spatial distribution of nitrate limiting its exploitation for human activities. After the river concentrations at the scale of an alluvial aquifer, with a special engineering works, the swamps and marshes were drained and focus on the effect of a river crossing the aquifer. More turned into farmlands. specifically, the goal of the study is to evaluate the relative The Seeland aquifer is the largest aquifer in the Bern role of dilution vs. denitrification on spatial patterns of nitrate region. It is crossed by the Hagneck