Using noble gases to understand recharge mechanisms in a fractured sandstone aquifer

Noble gas recharge temperature (NGRT) and excess air (EA) values, derived from the analysis of noble gases in groundwater, were used to improve the understanding of groundwater recharge mechanisms through the vadose zone of a sandstone aquifer in southern California (USA). The wide range of NGRT and EA values suggests that complex recharge mechanisms exist between two end members: matrix and fracture flow. In particular, combining NGRT and EA values, four groups of wells with different recharge mechanisms were identified: high EA combined with low NGRT suggests recharge is dominated by fast flow through an interconnected fracture network (group A), low EA with high NGRT suggests recharge is controlled by slow flow in the rock matrix (group B), low EA and intermediate NGRT suggests percolation through fractures followed by imbibition into the unsaturated matrix and subsequent matrix flow (group C), and high EA combined with high NGRT in group D suggests flow of water that originally resided in the matrix being pushed into the fracture network by a strong infiltration event (group D). These interpretations were corroborated with tritium groundwater dating and consideration of the potential influence of well completion characteristics on flow processes and measured noble gas concentrations due to mixing. This study demonstrates the contribution of noble gas analysis to identify recharge mechanisms in complex aquifers. This is crucial to inform three-dimensional numerical flow and transport models and to predict future hydrological scenarios in response to climate change.