SEASONAL AND SPATIAL TRENDS IN MANGANESE SOLUBILITY IN AN ALLUVIAL AQUIFER

The hydrogeochemistry of river water and groundwater in a hydraulic co nfiguration where river water infiltrates into an alluvial aquifer was monitored monthly over a 14-month period. Trends could be seen for te mperature, pH, and dissolved O2 and Mn. Wells located within 15 m of t he river show a significant seasonal variation in dissolved Mn. A thre shold temperature of 10-degrees-C seems to be necessary in order to tr igger and maintain Mn solubility (microbiologically mediated reactions ). Farther from the river, where the temperature is relatively constan t, there is little seasonal variation in dissolved Mn. Some of the bor eholes contain little Mn (0.5 mumol L-1) while in others the Mn concen tration is very high (up to 25 mumol L-1). Hypotheses proposed for the high dissolved Mn content in these wells are (a) a local control of t he redox conditions by organic matter in aquifer sediments and (b) a d ifferent mineralogical source of Mn.