TitleNS22A-07 - Application of Electrical Resistivity Tomography for Aquifer Characterization at Shellmound, MS
Session Start2022-12-13 09:00:00 CST
Session End2022-12-13 10:30:00 CST
LocationMcCormick Place, S105bc
AuthorMd Lal
Presentation TypeOral
AbstractGroundwater is being extracted for a managed aquifer recharge (MAR) pilot project utilizing riverbank filtration and groundwater transfer and injection located near the Tallahatchie River at Shellmound, MS. The extraction well for the project is located about 40 m away from the river and screened into the Mississippi River Valley Alluvial (MRVA) aquifer. The MAR project aims is to pump groundwater and induce infiltration from the Tallahatchie River by means of bank filtration. The MRVA aquifer is highly heterogeneous and typically varies from confined to unconfined within short distances. It is important to characterize the MRVA aquifer, but existing geophysical information is incomplete. Previously collected Airborne Electromagnetic (AEM) data did not provide continuous high-resolution data of the subsurface near the extraction well and were muted close to the river due to power lines. Bore logs provide discrete data and cannot be used for continuous spatial characterization of the aquifer. Electrical Resistivity Tomography (ERT) provides high-resolution subsurface images that can be used for aquifer characterization, such as determining aquifer extent, identifying preferential flow paths, and detecting subsurface voids or variations in lithology. ERT fills the gaps in bore logs and AEM data at the study site. In this study, data for the three ERT profiles were collected with 4 m electrode spacing. Data for the 444 m long ERT profile-1 were collected across and on the west side of the river, starting from the riverbank with the Wenner array types. For the 668 m long ERT profile-2 and profile-3, data were collected with a roll along of 50% overlapping along the river using the Dipole-Dipole array types. ERT data were inverted with the finite element method and Cholesky Decomposition solver with Dirichlet boundary conditions. The inverted ERT profile-1 and profile-2 were calibrated with the extraction well, and nearby monitoring well bore log to delineate the aquifer. Available AEM data for the extraction site was also used to calibrate the inverted ERT profiles for aquifer delineation. Profile-1 shows the MRVA aquifer transitioned from confined to unconfined in the agricultural field about 380 m west of the river. Profile-2 shows that the aquifer is confined along the profile and thicker on the northern side compared to the south side of the extraction well, and this profile also shows four higher resistivity anomaly zones whose depths range from 20 to 32 m. These higher resistivity zones might be attributed to preferential groundwater flow paths or loosely packed sand and gravel zones. Profile-3 shows the aquifer varies from confined to unconfined in multiple places along this north-south profile west of the river. This study characterizes and delineates the vertical extent of the MRVA aquifer near the extraction well and will provide an improved understanding of groundwater flow to support the assessment of the MAR project.