Retracing the development of raw water quality in water works applying reactive controlled material flux analyses

摘要

The Fuhrberger Feld aquifer in northern Germany provides the majority of water supply for the city of Hannover. Although parts of the recharge area have received a strong intake of nitrate from agricultural activities during the past 40 years, the extracted raw waters show little sign of nitrate contamination due to microbial nitrate reduction coupled with iron sulfide oxidation in the aquifer. Accordingly, iron and sulfate concentrations, in particular, are temporarily objectively high. To evaluate the contribution of quantitatively important processes to the development of raw water quality, all relevant natural and anthropogenic induced processes were identified, quantified and stepwise integrated in a reactive controlled material flux model using PhreeqC.Comparing measured and modeled data, long-term trends in raw water quality could be retraced. As a result, anthropogenic initiated processes such as varying nitrate influxes, conversion of wet grasslands into arable land coupled with geochemical processes (biomass degradation, oxidation of iron monosulfides in hydromorphic soils) and a partial nitrate breakthrough to the wells due to local complete consumption of iron disulfides were responsible for observed quality changes.