HR: 1340h
AN: H43D-0400    [Abstracts]
TI: Monitoring and Modeling the Fluctuations in Apparent Groundwater Age During a 30-Day Pumping Test.
AU: * Perle, M E
EM: meperle@ucdavis.edu
AF: Hydrologic Sciences, University of California Davis, One Shields Avenue, Davis, CA 95616 United States
AU: Zhang, Y
EM: yong.zhang@dri.edu
AF: Hydrologic Sciences, University of California Davis, One Shields Avenue, Davis, CA 95616 United States
AU: Fogg, G E
EM: gefogg@ucdavis.edu
AF: Hydrologic Sciences, University of California Davis, One Shields Avenue, Davis, CA 95616 United States
AB: Recent research shows that dispersion due to geologic heterogeneity can cause large (10's to 100's of yrs) variations in actual groundwater age within individual samples drawn from a well, even if well bore mixing is not significant. We hypothesize that the presence of such large ranges in groundwater age may cause the mean apparent age as estimated from environmental tracers such as CFC's, SF&_{6}$, and $^{3}$H-$^{3}$He to drift measurably during long-term, continuous pumping. This hypothesis was confirmed by 3-D numerical experiments wherein variation in groundwater ages under high-rate ($\sim$0.06 m$^{3}$/s; 1,000 gpm) long-term pumping was modeled using backward-time random walk particle tracking techniques combined with geostatistical simulations of hydrofacies heterogeneity. Results indicate that the age distribution within a water sample and the mean apparent age implied by environmental tracers is strongly influenced by historical atmospheric concentrations of environmental tracers and subsurface heterogeneity. As a partial implementation of this same experiment in the field, an abandoned well was pumped at a low rate ($\sim$0.005 m$^{3}$/s; 75 gpm) during 53 days. Water samples were collected from the top and bottom of a 25 foot well screen at 12 hour intervals for the first 30 days and were analyzed for CFC's, SF&_{6}$, and $^{3}$H-$^{3}$He. The measured tracer ages indicate that 1) CFC-11 apparent ages increased with time within the first five days of pumping and then remained constant for the remainder of the pump test; 2) trends in CFC-12, CFC-113 and SF&_{6}$ indicate a discrepancy in apparent ages with CFC-11, 3) water reaching the top interval is younger than water reaching the bottom interval. Gas samples were collected from the unsaturated zone to investigate possible CFC contamination and tracer concentration spatial variations in the pumping well recharge zone. Potential effects of heterogeneity and local CFC contamination on the monitoring results will be discussed.
DE: 1803 Anthropogenic effects
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting