Estimation of groundwater recharge is a critical component of the hydrogeological characterization of contaminated sites, because recharge governs the rate of groundwater available to transport contaminants and its spatial variability influences flow path trajectories. To properly constrain recharge rates and to understand its spatially and temporal variability, application of multiple, complementary data sets using different techniques is needed. This is particularly true in semi-arid areas where fluxes are small and highly variable in space and time.
In this study, we present the results of a multidisciplinary approach that led to the development of a conceptual model of groundwater recharge in a sedimentary bedrock aquifer, located on an upland ridge in southern California. The area is semi-arid with potential evapotranspiration (1400 mm y-1) exceeding the average annual precipitation (450 mm). Information about the spatial and temporal variability of recharge are required to inform the upper boundary of a 3-D groundwater flow model and to simulate plume fluxes.
Initially, a long-term site-wide (11.5 km2) recharge value was obtained by the application of the chloride mass balance based on measurement of bulk atmospheric chloride deposition, groundwater samples from 206 wells, and measurements of chloride in surface runoff during rain events. Secondly, the analysis of high-resolution porewater chloride concentration profiles from the vadose and groundwater zones at 11 locations, provided borehole-scale, long-term recharge values and indications about the portion of recharge through the matrix versus fractures in the vadose zone and the effect of land use changes on recharge. Finally, a high-resolution, spatially-distributed hydrologic numerical model (MIKE SHE) representing surface water and groundwater was used to simulate responses to precipitation from 1995 to 2014.
We found that the average annual site-wide recharge is 19 mm. However, simulated recharge values across the study area span over three orders of magnitude, from 0 to >1000 mm y-1, as a function of topography, surface geology, and land use. This recharge occurs mainly at the end of the wet season and, occasionally, after exceptionally high-intensity precipitation events. Upon reaching the deeper vadose zone, on average, 80% of the flow towards the water table occurs as slow (decadal to century) intergranular flow in the rock matrix and 20% as faster fracture flow.
The use of different types of techniques (geochemical, modeling and physical methods), at the proper resolution, was fundamental to uncover the hydrologic processes in the unsaturated zone, to investigate the influence of the surface and subsurface factors on recharge and, thus, to assign rules for recharge across the entire site in the flow model. This is particularly important because this quantitative conceptual model helps to understand recharge conditions and contaminant fluxes from source areas that affect the rate of contaminant migration and attenuation in groundwater.