Nitrogen-alkalinity interactions in the highly polluted Scheldt basin (Belgium)

We present results of one year observations in highly heterotrophic and oxy gen-depleted rivers of the polluted Scheldt basin. Monthly measurements rev ealed a high variability for dissolved inorganic carbon and nitrogen, with the following strong parallelism: highest alkalinity and NH4+ were associat ed with lowest NO3- and oxygen and vice-versa. In river water incubations, nitritication lowered the alkalinity whereas denitrification raised it; in an anoxic, NO3- -free incubation an increase of alkalinity was observed, pa rtially due to ammonification. A stoichiometric analysis, taking into accou nt the amount of protons produced or consumed by each process involving nit rogen, revealed that monthly variations of NO3- and NH4+ with ammonificatio n, nitrification and denitrification could explain the 28 and 62% alkalinit y variations at all stations, except one. The remaining part of the alkalin ity variations was attributed to other anaerobic processes (Mn-, Fe- and SO 4-reductions). This trend seems to be the result of the whole catchment met abolism (riverine waters and sediments, sewage networks and agricultural so ils). The observed HCO concentrations in the Scheldt basin were 2-10 times higher than the representative concentrations reported in pristine basins a nd used in chemical weathering models. This suggests the existence of an an thropogenic source, originating From organic matter decomposition. We concl ude that in highly polluted basins, nitrogen transformations strongly influ ence the acid-base properties of water. (C) 2001 Elsevier Science Ltd. All rights reserved.