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

Abstract

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 × 104 ± 3.7 × 104 bacteria per ml (of which 8.8% were active). Silica, DOC, biodegradable DOC, 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.