RIVER-GROUNDWATER EXCHANGES, BANK FILTRATION, AND GROUNDWATER QUALITY- AMMONIUM BEHAVIOR

In many countries, bank-filtrated water is an important component of t he drinking water production, In this case, most of the water pumped f rom the alluvial aquifers originates from the adjacent river. Bank fil tration is generally considered beneficial both quantitatively and qua litatively. However, in some cases bank filtration ran cause deleterio us effects to groundwater quality. This paper describes such a case, f ocusing 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 how line path and pore water of river bed sediments was colle cted 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 (less than or equal to 20 mg NH4 L-1), with higher concentratio ns near the bank. The high NH4 concentration in the sediment is relate d to the heavy organic load in the river and the mineralization of thi s organic matter by benthic microflora. Among the different mechanisms that influence NH4 transport and retention in the porous medium, it e merges that sorption by the alluvial sediments (K-d approximate to 1 - 10 x 10(-3) m(3) kg(-1)), or even the chalk (K-d approximate to 48 x 10(-3) m(3) kg(-1)) seems to be effective in retaining NH4, This is il lustrated by a model of NH4 transfer with retardation adapted to the c onditions of the site, precipitation of NH4 salts is probably not invo lved in regulating NH4 concentration at this site.