HR: 13:40h
AN: H13L-01    [Abstracts]
TI: The Significance of Wellbore Flow in the Santa Clara Valley, California
AU: * Hanson, R T
EM: rthanson@usgs.gov
AF: U.S. Geological Survey, 5735 Kearny Villa Rd, Suite O, San Diego, CA 92123 United States
AB: Interaquifer flow through water-supply wells screened across multiple aquifers is an important component to the flow of ground water in many developed aquifer systems. Wellbore flow in the Santa Clara Valley, California, was examined using flow measurements, hydrochemistry, and numerical simulations. Interaquifer flow was assessed locally using a combination of wellbore flow measurements and depth-dependent water-chemistry sampling at three water-supply wells and depth-specific samples from multiple-well monitoring sites. Interaquifer flow owing to wellbore flow in hundreds of multi-aquifer supply wells was assessed regionally using a simulation model of regional ground-water flow. Wellbore flow and depth-dependent chemical and isotopic data indicate that flow into the well from multiple aquifers, as well as capture of artificial recharge by pumping of water-supply wells, predominantly is occurring in the upper 500 feet of the aquifer system. Uncorrected carbon-14 data indicates that the ground water in the upper 500 feet generally is less than 2,000 years old, and ground water in the deeper aquifer layers generally ranges from 16,700 to 39,900 years old. Results of depth-dependent and depth-specific sampling collectively suggest that wellbores may be the main path for vertical flow between aquifer layers. In addition, on the west side of the valley, the ground water in the shallower aquifers (< 500 ft depth) contains a substantial amount of water from artificial recharge. Deuterium and oxygen isotopic data indicate as much as 60 percent of water pumped from production wells in this area originated as artificial recharge. Depth-dependent samples from water-supply wells also helped to better delineate trace amounts of anthropogenic contaminants coming from shallower aquifers . In addition to wellbore measurements, the regional effects of wellbore flow were assessed with a regional ground-water flow model that included the simulation of wellbore flow from wells screened across multiple aquifers, using the multi-node well package in MODFLOW-2000. The simulated intra-wellbore flow accounts for about 19 percent of the average net simulated regional ground-water flow, accounts for almost all interaquifer vertical flow, and occurs in both pumped and unpumped multi-aquifer wells. Because wellbore flow in wells screened over multiple aquifers is the main component of vertical flow between aquifers, this also becomes the main conduit for recharge to the lower aquifers. In addition, the incorporation of wellbore flow in the simulation significantly affects the simulation of stream-aquifer interaction, subsidence, and other model components because wellbore flow supplies some of the water that otherwise would have been simulated as water from aquifer and aquitard storage.
DE: 1803 Anthropogenic effects (4802, 4902)
DE: 1828 Groundwater hydraulics
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1876 Water budgets
SC: Hydrology [H]
MN: Fall Meeting 2005