Understanding mechanisms of recharge through fractured sandstone using high-frequency water-level-response data

High-frequency time series analysis and cross-correlation identified the relationship between precipitation and water-level responses at 16 sandstone wells in southern California, USA. The time series analysis suggested that the water table rises only when a threshold value of precipitation is reached during the rainy season that likely represents the water content deficit from the previous 7-month dry season being replenished before generating a water-table response. The cross-correlation indicates two statistically significant lag-times: 0-3 and 20-50 days. Confidence in these results was augmented by unprecedented and exceptionally high-resolution sampling frequency. Water pressure readings were collected every second and then analyzed to identify and remove the effects of barometric pressure changes, Earth tides and earthquakes on water levels. These effects are usually considered noise in recharge studies, but their accurate quantification helped assess the unconfined nature of the wells, minimize uncertainties of the results, and isolate the groundwater responses to precipitation. Diffusivity values for the thick unsaturated zone, based on the time lags, suggest quick responses are related to flow through fractures and longer time lags are associated with piston-type movement in the matrix. Fast responses were more likely for shallow water tables in response to high-intensity precipitation events and vice versa. These findings are consistent with those found, using lower resolution data, for the Chalk aquifer in England (UK), despite the contrasting fracture and matrix properties, hydrogeological setting and climatic conditions. Thus, the same style of response to precipitation is expected globally where similar fractured porous media are present.

Automated summary

This study uses high-frequency time series analysis and cross-correlation to investigate the relationship between precipitation and water-level responses in sandstone wells in southern California. The results suggest that the water table only rises when a certain precipitation threshold is reached, indicating replenishment of water content from a previous dry season. The study also identifies different flow mechanisms based on lag times. These findings are consistent with previous research on Chalk aquifer in England, indicating similar precipitation responses in fractured porous media.