Using hydraulic head, chloride and electrical conductivity data to distinguish between mountain-front and mountain-block recharge to basin aquifers

Numerous basin aquifers in arid and semi-arid regions of the world derive a significant portion of their recharge from adjacent mountains. Recharge can effectively occur through either stream infiltration in the mountain front zone (mountain-front recharge, MFR) or subsurface flow from the mountain (mountain-block recharge, MBR). While a thorough understanding of the recharge mechanisms is critical for water resource management, distinguishing between MFR and MBR is typically difficult. Here we present a relatively simple approach that uses hydraulic head, chloride and electrical conductivity data to distinguish between MFR and MBR. These types of data are inexpensive to measure, and in many cases are readily available from hydrogeological databases. In principle, hydraulic head can inform on groundwater flow directions and stream-aquifer interactions, while chloride can help to distinguish between different groundwater pathways if the sources have distinct concentrations. Electrical conductivity values can be converted to chloride concentrations using an empirical relationship, and hence can be used in a similar manner to chloride, thereby significantly increasing the data set. The practical feasibility and effectiveness of this approach are tested through the case study of the Adelaide Plains basin, South Australia, for which a wealth of historical groundwater level, chloride and electrical conductivity data is available. Hydraulic head data suggest that streams are gaining in the adjacent Mount Lofty Ranges and losing when entering the basin. They also indicate that not only the Quaternary sediments but also the underlying Tertiary sediments receive significant recharge from stream leakage in the mountain front zone. Chloride data also reveal clear spatial patterns suggesting that MFR dominates recharge of the low salinity groundwater found in the basin. This interpretation is further supported by stream water chloride analysis. This study demonstrates that both hydraulic head and chloride data can be effectively used to distinguish between MFR and MBR.