HR: 0800h
AN: H41E-0461    [Abstracts]
TI: Exploring Aquifer Heterogeneity and Anisotropy Using Sequential Pumping Tests
AU: * McLin, S G
EM: sgm@lanl.gov
AF: Los Alamos National Laboratory Water Quality and Hydrology Group, P.O. Box 1663 MS-K497, Los Alamos, NM 87545 United States
AU: Keating, E H
EM: ekeating@lanl.gov
AF: Los Alamos National Laboratory Hydrology, Geochemistry, and Geology Group, P.O. Box 1663 MS-T003, Los Alamos, NM 87545 United States
AB: A sequential pair of traditional aquifer tests was performed using both municipal water supply and numerous observation wells. Initially, a 25-day aquifer test was conducted at well PM-2 at a constant discharge rate of 79 lps (1,249 gpm), while supply wells PM-4 and PM-5 were used as observation wells. Then a 21-day aquifer test was conducted at well PM-4 at a constant discharge rate of 94 lps (1,494 gpm) while supply wells PM-2 and PM-5 were used as observation wells. These data reveal horizontal propagation of drawdown in the regional aquifer beyond 2,650 m (8,700 ft) from each production well, and a pronounced resistance to vertical drawdown propagation at shallower depths. Hydraulically, the regional aquifer seems to behave like a semi-confined aquifer with leaky units located above a highly conductive layer that averages about 260 m (850 ft) in thickness. Analyses of drawdown and recovery data from individual observation wells in the first test suggest that the highly conductive layer between wells PM-2 and PM-4 has a transmissivity of about 400 m2/day (4,250 ft2/day) and a storage coefficient of about 0.00035. Analyses of data from the second test suggest a transmissivity of 600 m2/day (6,450 ft2/day) and a storage coefficient of about 0.00039. The aquifer thins between PM-4 and PM-5 to an effective thickness of about 150 m (490 ft), while the aquifer transmissivity and storage coefficient increase only slightly. It is unclear if these differences are indeed significant, or if they simply reflect combined parameter uncertainty arising from natural variability and the type-curve solution method. While these analyses yield excellent type-curve matches, they fail to explain differences in measured vertical pressure responses from multi-screened observation wells. In addition, multi-dimensional numerical flow models mimic both horizontal and vertical hydraulic responses very well, and suggest that the aquifer is actually phreatic. These tests demonstrate that the regional aquifer below Los Alamos is strongly heterogeneous, and exhibit pronounced horizontal and vertical anisotropy in hydraulic transmitting properties. These results also illustrate that model dimensionality can influence inferred aquifer behavior, and reinforce theoretical predictions (Neuman, 1975) that say unconfined aquifers may exhibit confined or leaky-aquifer behavior at early times. Finally, the transition from leaky-confined to phreatic conditions is estimated to occur after about 300 days of continuous pumping.
DE: 1829 Groundwater hydrology
DE: 3260 Inverse theory
DE: 3360 Remote sensing
SC: Hydrology [H]
MN: Fall Meeting 2005