Interaction between different water bodies in a small catchment in the Paris basin (Brévilles, France): Tracing of multiple Sr sources through Sr isotopes coupled with Mg/Sr and Ca/Sr ratios

Intensive monitoring of the Cuise Sands Aquifer in a small agricultural catchment (3 km2, where agricultural practices are well documented) over nearly 5 a showed significant disparities for major element concentrations, Sr concentrations, isotopic compositions and 3H values, depending on the groundwater sampling locations. Lower 87Sr/86Sr values (0.7079-0.7085) associated with low NO3- concentrations, low Mg/Sr molar ratios (<100) and low 3H values are attributed to natural end-members. Groundwater with higher 87Sr/86Sr values (>0.7085) and higher Mg/Sr molar ratios (>250) suggest a contribution of Sr from anthropogenic sources. Strontium isotopes coupled with Mg/Sr and Ca/Sr ratios of groundwater in this catchment successfully demonstrated the capacity to trace fertilizer contributions, which significantly modify the natural Sr isotopic signature of groundwater. The specific geochemical signature of groundwater revealed by Sr isotopes, in addition to major and trace element concentrations (in particular Mg/Sr and Ca/Sr molar ratios), demonstrated the partial isolation of groundwater bodies in this catchment. Compartmentalisation of the aquifer is compatible with fault distribution as defined by geological and geophysical investigations. Thus, the Sr isotopic tool coupled with Mg/Sr and Ca/Sr ratios has been applied with success to trace water pathways and recharge and to provide scenarios for the hydrological functioning of the system.