Présentations Colloques

In this study, multiple environmental-tracers were used to estimate the amount of recharge and evaluate its spatial and temporal variability, to investigate the different mechanisms influencing recharge and to quantify groundwater travel times in a fractured sandstone aquifer located in southern California. The study area is 11.5 square km and located on an upland ridge in the Simi Hills about 400 m above the surrounding valley. Because of its morphological and hydrological features, it provides ideal conditions for recharge studies and comparison between different methods. Moreover, this site and its data set is uniquely given the abundance of multi-temporal chemical analysis of groundwater from an incredibly dense network of monitoring wells, multilevel systems, seep piezometers and core samples. The motivation for this study was prompted by the need to understand the rate of groundwater flow and mechanisms affecting transport of contaminants resulting from site operations between 1949 and 2006. Moreover, considering the severe drought that is currently affecting California in the last four years, the estimation of groundwater recharge has increased scientific relevance in this area. Through the application of the Chloride Mass Balance Method for the saturated zone, a site-wide annual recharge of 19 mm (4.2% of the average annual precipitation) was estimated. Given that surface runoff is continuously monitored and represents only 6.1% of the mean precipitation, the resultant average annual evapotranspiration is 408 mm (89.6% of precipitation). These values are corroborated with other site data and consistent with other study sites in semi-arid regions reported in the scientific literature, showing a robust technique for improved water balances and groundwater flow system inputs, especially in regions where water is scarce. This recharge estimate, when used in a 3-D groundwater flow model, imposes limits of the total volumetric groundwater flow rate available to transport contaminants in the plumes in the study area.< p>