title: Regional water quality patterns in an alluvial aquifer: direct and indirect influences of rivers.
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 the Swiss Plateau (50km(2)) 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 processes resulting from river water infiltrations, revealed by the δ(18)OH2O and δ(2)HH2O contents of groundwater; and (3) denitrification processes, controlled by the spatial variability of flood deposits governed by fluvial depositional processes. It was possible to quantify the dependence of the nitrate concentration on these three factors at any sampling point of the aquifer using an end-member mixing model, where the average nitrate concentration in recharge from the agricultural area was evaluated at 52mg/L, and the nitrate concentration of infiltrating river at approximately 6mg/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.
authors.affiliation: Centre for Hydrogeology & Geothermics (CHYN), University of Neuchatel, Rue Emile Argand 11, CH-2000 Neuchatel, Switzerland.
authors.forename: A
authors.initials: A
authors.lastname: Baillieux
authors.affiliation: Centre for Hydrogeology & Geothermics (CHYN), University of Neuchatel, Rue Emile Argand 11, CH-2000 Neuchatel, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Campisi
authors.affiliation: Centre for Hydrogeology & Geothermics (CHYN), University of Neuchatel, Rue Emile Argand 11, CH-2000 Neuchatel, Switzerland.
authors.forename: N
authors.initials: N
authors.lastname: Jammet
authors.affiliation: Centre for Hydrogeology & Geothermics (CHYN), University of Neuchatel, Rue Emile Argand 11, CH-2000 Neuchatel, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Bucher
authors.affiliation: Centre for Hydrogeology & Geothermics (CHYN), University of Neuchatel, Rue Emile Argand 11, CH-2000 Neuchatel, Switzerland. Electronic address: Daniel.hunkeler@unine.ch.
authors.forename: D
authors.initials: D
authors.lastname: Hunkeler
mesh_terms: D000383:Agriculture; D058440:Denitrification; D004784:Environmental Monitoring; D060587:Groundwater / Q000737:chemistry*; D007554:Isotopes / Q000032:analysis; D009566:Nitrates / Q000032:analysis; D045483:Rivers / Q000737:chemistry*; D013557:Switzerland; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000032:analysis*; D060753:Water Quality
chemical_list: D007554:Isotopes; D009566:Nitrates; D014874:Water Pollutants, Chemical
keywords: Alluvial aquifer; Denitrification; Diffuse pollution; Dilution; Flood plain; Stable isotope