GEOPHYSICAL DATA FOR CHARACTERIZING INFILTRATION PROCESSES BENEATH A MANAGED AQUIFER RECHARGE POND

With increasing use of managed aquifer recharge as a means of augmenting groundwater supplies, there arise questions about the operational efficiency of these systems. In particular, we are focused on improved understanding of the factors that govern the efficiency of artificial recharge ponds. A common challenge with the operation of artificial recharge ponds is that infiltration rates decrease over time due to clogging at the base of the pond. This clogging is typically due to either infiltration of fine grain material or biological growth at the base of the pond (or a combination of both). In order to better understand these clogging mechanisms, we have explored the use of geophysical methods to monitor the spatial distribution and temporal evolution of clogging in an artificial recharge pond.

The Harkins Slough Recharge Pond, near Watsonville California, presented a unique opportunity to develop and test geophysical methods, specifically for improved understanding of clogging processes. At this site we deployed a series of geophysical tools aimed at addressing the two challenges at the site. We first addressed the question: What is controlling the decrease in filtration rate? The development and deployment of electrical conductivity probes beneath the pond allowed us to monitor changes in the top ~1 m over a 4-month period. This dataset revealed that clogging in the top ~10 cm was responsible for the decrease in infiltration rate, most of which occurred in the first 30 days of operation. The second question: What is the spatial variability of infiltration at the site was addressed using 2D time-lapse ERT, acquired using an ERT cable buried beneath the pond. The results of the ERT imaging suggest that there is significant variability in infiltration properties across the site. The findings at the site demonstrate the value of geophysics in obtaining information regarding clogging during the operation of artificial recharge ponds.