GROUNDWATER RECHARGE POTENTIAL USING SECONDARY TREATED WASTEWATER: METHODS AND CASE STUDY IN THE SOUTHERN SAN JOAQUIN VALLEY, CALIFORNIA

Water scarcity in a period of climate uncertainty necessitates exploring new avenues for recharging depleted groundwater. The Western United States, including the agriculturally rich San Joaquin Valley (SJV), is highly dependent on winter precipitation and accumulated snow pack to refill reservoirs for use during peak summer agricultural operations. However, severe weather patterns (such as the current drought) have drastically reduced both the amount and longevity of the snow pack resulting in the over-drafting of groundwater. This study investigated the potential for managed aquifer recharge (MAR) using secondary treated wastewater. The approach employs an infiltration basin water balance (WB), soil columns (SC) infiltration tests, subsurface simulations (Hydrus modeling), and geographic soil survey data (NRCS SSURGO) for regional upscaling. We address the following key questions: i) Is there a correlation between a field-scale water balance and a soil column study? ii) What portion of applied wastewater effluent leads to recharge?, and iii) What is the overall potential of MAR to supplement regional recharge, using the southern SJV as a case study? For the soils of the southern SJV, there were strong correlations between soil column study and water balance approach (Slope=0.75 and Intercept=22.35 with r2=0.99). A WB method showed that basin infiltration rates dropped from 125.2 mm/day on day 7 to 40.6 mm/day on day 330. Given these infiltration rates, we estimated that 225-1220 hectares of land will be required to successfully dispose of 0.5-0.6 million m3/day of wastewater (generated from the four regional waste water treatment facility in southern SJV). Hydrus 2D simulation package also successfully modeled field measurements of water content and the observed water balance percentage with recharge rates were 9.7-68.9 mm/day depending on area, soil type, and duration of use of the reclamation basin. Utilizing NRCS SSURGO data, recharge rate, and volume of wastewater, it is estimated that 5.8-42.2 % of the incoming wastewater could contribute to recharge if directed at all the candidate infiltration zones. California overdrafts approximately 2 million acre feet of groundwater annually, this associated regional MAR in SJV would reduce that volume by approximately 0.8-2.5%. It is important to add, however, that health concerns associated with reclaimed wastewater remain and need to be addressed before this approach can be fully embraced