Description
Local surface water and stormflow runoff were infiltrated intermittently from a 40 hectare basin between September 2003 and September 2007 to determine the feasibility of recharging alluvial aquifers pumped for public supply, near Stockton, California. Infiltration of water from the basin produced a pressure response that propagated through unconsolidated alluvial-fan deposits to 125 meters (m) below land surface in 5 days and through deeper, more consolidated alluvial deposits to 194 m below land surface in 25 days, resulting in increased water levels in nearby monitoring wells. The depth to the top of the saturated zone at the basin fluctuates seasonally from depths of about 15 to 20 m. Since the start of recharge, water infiltrated from the basin has reached depths as great as 165 m below land surface. Based on sulfur hexafluoride tracer-test data, water from the basin moved downward through the saturated alluvial deposits until reaching more permeable zones about 110 m below land surface. Once reaching these permeable zones, water moved rapidly to nearby pumping wells at rates as high as 13.2 meters per day (m/d). Flow to production wells through highly permeable material was confirmed on the basis of flowmeter logging, and simulated numerically using a two-dimensional radial ground-water flow model with particle-tracking. Arsenic concentrations increased slightly as a result of recharge from 2 to 6 micrograms per liter immediately below the basin. Although few water-quality issues were identified during sample collection, high groundwater velocities and short travel times to nearby wells may have implications for groundwater management at this site and at other sites in heterogeneous alluvial aquifers.
