A unique approach to estimating lateral anisotropy in complex geohydrologic environments

• Published in: Journal of Hydraulic Research 2004-01, Vol.42 (sup1), p.77-81
• Main Authors: Halford, Keith J., Campbell, Bruce
• Format: Article
• Language: English
• Subjects: anisotropy; Aquifer test; ASR; Charleston; Earth sciences; Earth, ocean, space; Exact sciences and technology; Hydrology. Hydrogeology; numerical model; parameter estimation
• ISSN: 0022-1686; 1814-2079
• DOI: 10.1080/00221680409500050

Aquifers in fractured rock or karstic settings are likely to have anisotropic transmissivity distributions. Aquifer tests that are performed in these settings also are frequently affected by leakage from adjacent confining units. Finite-difference models such as MODFLOW are convenient tools for estimating the hydraulic characteristics of the stressed aquifer and adjacent confining units but are poor tools for the estimation of lateral anisotropy. This limitation of finite-difference methods can be overcome by application of the spin method, a technique whereby the positions of the observation wells are rotated about the production well to estimate anisotropy and orientation. Formal parameter estimation is necessary to analyze aquifer tests because of the number of parameters that are estimated. As a test, transmissivity, anisotropy, and orientation were successfully estimated for a simple hypothetical problem with known properties. The technique also was applied to estimate hydraulic properties of the Santee Limestone/Black Mingo (SL/BM) aquifer and a leaky confining unit beneath Charleston, South Carolina. A 9-day aquifer test with an average discharge of 6441/min was analyzed numerically. Drawdowns in the SL/BM aquifer and confining unit were simulated with a 12-layer MODFLOW model that was discretized into 81 rows of 81 columns. Simulated drawdowns at seven observation wells that ranged from 23 to 2700 m from the production well were matched to measured drawdowns. Transmissivity estimated along the minor axis ranged from 10 to 15 m 2 /day and along the major axis ranged from 80 to 10Om 2 /day. The major axis of transmissivity was oriented along compass heading 116° (degrees clockwise from north), which agrees with geologic interpretations. Vertical hydraulic conductivity and specific storage estimates for the overlying confining unit were 4 x 10 -5 m/day and 2 x 10 -4 1/m, respectively.