Comparison of solutes, nutrients, and bacteria inputs from two types of groundwater to the Rhone river during an artificial drought

Solute, nutrient and bacterial inputs to the River Rhone from the interstitial habitat of a gravel bar and the floodplain aquifer were investigated during an artificial drought. Eight springs were investigated: four groundwater-fed springs in the floodplain, located at the bottom of the bank; and four interstitial-fed springs located at the downstream end of a gravel bar. During this period, the inflows of groundwater to the river represented an average input of 0.77 mg l(-1) of nitrogen (of which 93.3% were nitrates), 0.0187 mg l(-1) of total phosphorus (of which 42.2% was orthophosphate), 3.56 mg l(-1) of silica, 2.315 +/- 0.703 mg l(-1) of dissolved organic carbon (DOC, of which 47% was biodegradable) and 7.3 x 10(4) +/- 3.7 x 10(4) bacteria per ml (of which 8.8% were active). Silica, DOC, biodegradable DOG, and bacteria concentrations displayed temporal variations during the study, which seem to be linked to the biological activity of the groundwater biofilm. There was a strong heterogeneity between the two types of groundwater that flow to the river: concentrations of calcium and alkalinity were higher in bank springs than in gravel bars springs. In these latters, sulfate, sodium, nitrogen, phosphorus were significantly higher.