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Aquifer mapping and drilling depth estimation for monitoring wells using electrical resistivity tomography
Proceedings of the 2023 Mississippi Water Resources Conference
Year: 2023 Authors: Alim M.S., Mamud M.L., Holt R.M., Wodajo L.T., Hickey C.J., O'Reilly A.M., Samad M.A.
Goodwin Creek is located in the northwestern part of the Yocona Watershed (Panola County, Mississippi) where the Sparta Sand aquifer outcrops. The creek is a tributary of the Yocona River, which flows into the Yazoo River which is a tributary of the Mississippi River. Surface and internal soil erosion are very common problems near the creek due to the surface runoff and groundwater flow. This soil erosion generates soil pipes that collapse and gullies that directly affect the agricultural fields and the pasturelands. To monitor groundwater flow in the unsaturated and saturated zones and to understand surface-groundwater interaction, multidisciplinary geophysical and hydrogeological research is being conducted at Goodwin Creek. An electrical resistivity tomography (ERT) survey was carried out at the study site to map the underlying aquifer, characterize its heterogeneity, estimate drilling depths for proposed monitoring wells, and propose an optimal location for a groundwater extraction well. A 501-m long ERT profile with 3 m electrode spacing was acquired using a mixed dipole-dipole and strong gradient array, starting at the northwestern bank of Goodwin Creek and towards the pastureland. ERT results demonstrate that the aquifer is relatively thin (<10 m) near the creek and thicker (>30 m) under the pastureland. The aquifer outcrops over a short distance (<10 m) in the pastureland but is confined on both sides of the outcrop. Disturbed and undisturbed soil samples were collected down to 1.5 m at multiple locations along the ERT profile. Soil samples collected at the outcrop location are composed of sand and gravel which is consistent with the high resistivity values on the ERT tomograms. Soil samples collected away from the outcrop consisted of clayey soil consistent with low electrical resistivity values from the ERT survey. The underlying aquifer is delineated assuming a threshold value for the resistivity of non-aquifer materials. The aquifer map is used to determine the drilling depths for monitoring wells and provide a recommendation of an optimal location for installing an extraction well.