Trends in pollutant concentrations in relation to time of recharge and reactive transport at the groundwater body scale

SummaryTo quantify the threat of pollutants to present and future groundwater quality requires detecting trends in groundwater quality. We related the measured concentrations of pollutants from selected monitoring locations in the Sand Meuse groundwater body to the time of recharge determined by 3H/3He as a means to aggregate the measurements and detect trends that are significant for the entire groundwater body. The aim of this study was to (1) define under what conditions the concentrations of reactive solutes may be related to travel times; (2) identify solute specific trends; (3) assess which geochemical processes are responsible for these trends using a geochemical model (PHREEQC); and (4) distinguish between different types of trends.The observed trends in the concentrations of Cl are solely the result of historical changes in chloride concentrations in recharging groundwater. Cation exchange influences the trends in the concentrations of K and Ca, and heavy metals, and to a lesser extent also Na and Mg. Trends in the concentrations of NO3 and SO4 are the combined result of changing recharge concentrations and the process of pyrite oxidation. Fe, As, Ni and Zn are released during pyrite oxidation and these trends are indirectly the result of changing land use practices.We distinguish three types of trends in the concentrations of solutes related to time of recharge. These are the result of: (1) the anthropogenic trends in historical concentrations in recharging groundwater unaltered during conservative transport through the groundwater body, (2) anthropogenic trends in recharge concentrations altered by reactions during transport and (3) geochemical reactions in the subsurface that are triggered by anthropogenic changes in the recharge concentrations of other solutes.