HR: 10:40h
AN: H32A-02    [Abstracts]
TI: Effect of Transient Leakage From Confining Units on Apparent Groundwater Age
AU: * Zinn, B A
EM: bzinn@usgs.gov
AF: United States Geological Survey, 432 National Center 12201 Sunrise Valley Drive, Reston, VA 20192 United States
AU: Konikow, L F
EM: lkonikow@usgs.gov
AF: United States Geological Survey, 431 National Center 12201 Sunrise Valley Drive, Reston, VA 20192 United States
AB: The use of environmental tracers to estimate the age of water has become a popular method to help calibrate groundwater flow models. Because of heterogeneity and dispersion, however, fluid age at any particular point in an aquifer will represent a mixture of travel times, with the distribution of ages determined by the detailed structure of the flow field. The flow field is prone to transient changes, particularly in locations of significant human influence. The effects of pumping on fluid age distribution in a hypothetical regional groundwater system are assessed numerically using direct age simulation with a solute-transport model. A hypothetical system with layers of aquifers and confining units possessing different hydraulic conductivity and porosity values created a heterogeneous distribution of fluid ages and a steady-state predevelopment age distribution under constant regional flow conditions. Pumping was introduced by means of a well field that extracted water from a confined aquifer. The pumping caused leakage into the aquifer of older water from an overlying low-conductivity confining unit and created large changes in age concentrations in the aquifer. Age concentrations changed by more than an order of magnitude in a large fraction of the confined aquifer, and the effects propagated a substantial distance downgradient from the well field location. These ages are much larger than the regional travel time through the confined aquifer. Both the pumping rate and the age ratio between the old (confining unit) and young (aquifer) regions of the system were found to be key factors influencing overall age change in the aquifer, whereas the distribution of pumping locations had a smaller effect. The results suggest that environmental age tracers may give inaccurate estimates of regional flow behavior in groundwater systems with substantial well development - ages in developed aquifers may be dominated by transient effects of leakage from confining units because the leaking older water mixes with or displaces younger formation water as it flows downgradient.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1873 Uncertainty assessment (3275)
SC: Hydrology [H]
MN: Fall Meeting 2005