Regional water quality patterns in an alluvial aquifer: Direct and indirect influences of rivers

• Published in: Journal of contaminant hydrology Vol. 169; pp. 123 - 131
• Main Authors: Baillieux, A., Campisi, D., Jammet, N., Bucher, S., Hunkeler, D.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier B.V 15.11.2014; Elsevier
• Subjects: Agriculture; Alluvial aquifer; Aquifers; Denitrification; Deposition; Diffuse pollution; Dilution; Earth sciences; Earth, ocean, space; Environmental Monitoring; Exact sciences and technology; Flood plain; Floods; Freshwater; Ground-water quality; Groundwater - chemistry; Hydrogeology; Hydrology. Hydrogeology; Isotopes - analysis; Mathematical models; Nitrates; Nitrates - analysis; Rivers; Rivers - chemistry; Stable isotope; Switzerland; Water Movements; Water Pollutants, Chemical - analysis; Water Quality; Switzerland; Switzerland, Swiss Plateau; Diffuse pollution; Denitrification; Dilution; Flood plain; Stable isotope; Alluvial aquifer; floods; mixing; fine-grained materials; ground water; nitrates; pollution; networks; variability; denitrification; land use; aquifers; hydrochemistry; plateaus; organic carbon; rivers; stable isotopes; models; sampling; concentration; infiltration; grains; pumping; dilution; water quality; frame structure
• ISSN: 0169-7722; 1873-6009
• DOI: 10.1016/j.jconhyd.2014.09.002

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 (50km2) 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 δ18OH2O and δ2HH2O 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. •Regional water quality trends were investigated in an alluvial aquifer.•Recharge sources and the alluvial plain architecture influenced water quality patterns.•Isotope analysis allowed us to separate the effect of dilution and reactive processes.