Groundwater Recharge Assessment in a Fractured Sandstone Aquifer in Southern California

Abstract:

This abstract summarizes the preliminary results from groundwater recharge estimates at an inactive industrial research facility located in southern California: the Santa Susana Field Laboratory. The research activities, carried out at this site from 1949 to 2006, caused chemical contamination of the aquifer. For this reason, the correct assessment of groundwater recharge and fluxes is a key topic in order to better understand the mechanism controlling the rate of contaminant transport and fate toward offsite receptors. The study area is about 11.5 km² and is formed by fractured sandstone with interbedded shale and siltstone on a topographic ridge approximately 300 m above adjacent valleys. The bedrock is mostly exposed at surface with irregularly covered by thin alluvial deposits and vegetation, mostly chaparral. The Chloride Mass Balance method was used to estimate the long term average annual site-wide recharge considering the hydrogeological features of the area, the availability of chloride concentration data and the inherent assumptions of the method. Daily data of precipitation and runoff from 2008 to 2011, chloride concentration data in precipitation, dry deposition, surface water and in hundreds of monitoring wells across the site were available. The average spatial and temporal recharge estimated for the site is 4.7% of the average annual precipitation (452 mm) with a range of variation between 2% and 7%. This result matches other values of recharge at the site and reported in the literature for arid and semi-arid environments in different areas of the world (Scanlon et alii, 2006; Sharda et alii, 2006). Furthermore, the annual average runoff constitutes a small percentage (8%) of total inflow volume and therefore, the main “loss” of water is represented by the evapotranspiration. This result is consistent with the climatic characteristics of California where annual evapotranspiration is reported to be greater than annual precipitation volume (Hidalgo et alii, 2005). These results represent a first step in the study of groundwater flow and contaminant fluxes to potential receptors from the site. Further quantitation of recharge and its spatial variability across the site will be assessed in the next several months by different independent approaches, such as physical and isotopic methods.