HR: 0800h
AN: H51A-0336    [Abstracts]
TI: Accounting for Aquifer Heterogeneity in Analysis of the Skin Effect
AU: * Barrash, W
EM: wbarrash@cgiss.boisestate.edu
AF: Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725 United States
AU: Clemo, T
EM: tomc@cgiss.boisestate.edu
AF: Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725 United States
AU: Fox, J J
EM: jessicafox@velocitus.net
AF: Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725 United States
AU: Johnson, T C
EM: tcj@cgiss.boisestate.edu
AF: Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725 United States
AB: Because results of hydraulic tests in boreholes can be influenced by wellbore skin, understanding and quantification of wellbore skin improves our ability to estimate hydraulic conductivity (K) with such tests. We have investigated positive skin at 10-cm ID PVC wells with slotted casing that were emplaced in a 15-m thick coarse fluvial aquifer at the Boise Hydrogeophysical Research Site (BHRS), and we have concluded that the skin effect derives from sand grains partially clogging the slotted casing. We use the WTAQ code (Moench, 1997; Barlow and Moench, 1999) with a redefinition of the term for delayed observation well response to include skin effects at observation wells (in addition to pumping wells) in order to analyze aquifer tests for average skin conductivity (K_s) values at individual wells at the BHRS. Systematic differences in K_s values are recognized in results at pumping (K_{s Q}) and observation (K_{s obs}) wells: larger values are seen at observation wells (average K_{s obs} = 0.0023 cm/s) than pumping wells. Two possible causes are recognized for the occurrence of higher K_s values at observation wells than pumping wells: (1) flow diversion between aquifer layers on approach to a pumping well with a positive skin; and (2) larger portion of flow passing through lower-K zones in the heterogeneous aquifer near the pumping well than the observation wells due to strongly radially convergent flow near the pumping well. Because of the increased drawdown at a pumping well compared with an observation well due to heterogeneity in the aquifer, observation wells provide better K_s estimates than those from pumping wells for the well-aquifer system at the BHRS.
DE: 1828 Groundwater hydraulics
DE: 1847 Modeling
DE: 1849 Numerical approximations and analysis
SC: Hydrology [H]
MN: Fall Meeting 2005