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River-Groundwater Exchanges, Bank Filtration, and Groundwater Quality: Ammonium Behavior

INRA, Science du Sol, Domaine Saint Paul, Site Agroparc, 84914 Avignon cedex 9, France;

E. Ledoux

Centre d'Informatique Geologique, Ecole des Mines, 77305 Fontainebleau, France;

M. Detay

Lyonnaise des Eaux, Cholet, France.

^* Corresponding author (doussan{at}avignon.inra.fr).

ABSTRACT

In many countries, bank-filtrated water is an important component of the drinking water production. In this case, most of the water pumped from the alluvial aquifers originates from the adjacent river. Bank-filtration is generally considered beneficial both quantitatively and qualitatively. However, in some cases bank filtration can cause deleterious effects to groundwater quality. This paper describes such a case, focusing on ammonium (NH₄) concentrations. The data were gathered at an experimental bank-filtration site which is part of a large well field along the Seine River (France). At this site, groundwater was sampled along a flow line path and pore water of river bed sediments was collected with peepers or by centrifuging core samples. The pore waters of the superficial river bed sediments have high ammonium concentrations (>30 mg NH₄ L^–1) whereas, in the groundwater, these concentrations are lower (20 mg NH₄ L^–1), with higher concentrations near the bank. The high NH₄ concentration in the sediment is related to the heavy organic load in the river and the mineralization of this organic matter by benthic microflora. Among the different mechanisms that influence NH₄ transport and retention in the porous medium, it emerges that sorption by the alluvial sediments (K_d = 1 – 10 x 10^–3 m³ kg^–1), or even the chalk (K_d = 48 x 10^–3 m³ kg^–1) seems to be effective in retaining NH₄. This is illustrated by a model of NH₄ transfer with retardation adapted to the conditions of the site. Precipitation of NH₄ salts is probably not involved in regulating NH₄ concentrafion at this site.