Hydrogeochemistry And Arsenic Mobilization Of Shallow Groundwater System Of The North Yinchuan Basin, P.R. China

Although high As groundwater has been found in the Yinchuan Basin for around20years, little is known about hydrogeochemical contrasts between low and high Asgroundwaters and their connection to As mobilization.Distribution of groundwater Asand As mobilization would not only provide scientific support for ensuring drinkingwater safey, but also help in better understanding As transport in aquifer systems.Investigations were carried out at three regions (piedmont proluvial fans, dryfarmland and paddy farmland along the Yellow River) in the northern Yinchuan Basinfor evaluating As distribution and geochemical processes for As mobilization.Ninety-two groundwater samples,4surface water samples, and66sediments sampleswere collected and analyzed for chemical and/or isotopic components. Results showthat low As groundwater is generally found in the piedmont proluvial fans. However,high As concentrations (up to105μg·L-1) are mainly observed in groundwater fromlow-lying parts of the basin along the Yellow River with alluvial sand aquifers andlow groundwater flow rate, which are associated with reducing conditions. High Asgroundwater is characterized by high concentrations of NH4+, dissolved Mn, dissolvedFe and Fe(II), but relatively low concentrations of NO3-and SO42-. The intensiveirrigation in the paddy recharges the aquifers by vertical infiltration of the divertedYellow River water, and leads to the higher redox potentials and the lower dissolvedAs in comparison with those in the dry farmland. Environmental isotopes (δ18O andδD) have been used to trace groundwater recharge and hydrogeochemical processesfor As enrichment. Results show that evaporation due to the intensive irrigation playsa minor role in As enrichment. The positive correlation between As and dissolved Fesuggests that groundwater As would result from the reductive dissolution of Fe oxides.Besides, HCO3-and dissolved P may be involved in competing with As for bindingsites on Fe oxide minerals.Sediment As ranges between3.94and75.2mg·kg-1(average of11.5mg·kg-1),which are not unusually high compared to average values elsewhere. HCl-leached Asaccounts for68%of total As in the sediments, while H3PO4-leached As accounts for 5%. Depth-matched samples show a good correlation between dissolved As ingroundwater and H3PO4-leached As in sediments, demonstrating that groundwater Aswould mainly come from P-mobilizable As in aquifer sediments. Arsenic distributioncoefficient (Kd), calculated from H3PO4-leached As and dissolved As, ranges between5.08and17.3cm3·g-1, which generally depends on groundwater redox potentials. Inreducing conditions with low redox potential, low values are found with As beingpreferentially partitioned into groundwater.