Electrical resistivity imaging of an enhanced aquifer recharge site

Abstract

Enhanced aquifer recharge (EAR) is defined as any engineered structure or enhanced natural feature designed to convey stormwater, surface water or wastewater directly into an aquifer (e.g. aquifer storage and recovery (ASR) wells) or into the vadose zone eventually percolating to an aquifer (e.g. spreading basins, dry well, etc.; USEPA 2021). Identifying the storage and flow capabilities of complex aquifers can improve the efficacy of many conceptual site models (CSM) for sites considered for ASR projects. In a karst setting, the EAR process may be able to take advantage of natural surficial features and the increased storage capacity of karst aquifers to improve recharge to groundwater. However, the suitability for an EAR project in a karst setting depends on the maturity of the karst and its preceding epikarst. The focus of flow within the epikarst causes enlargement of fractures and karst conduits. Thus, the storage and transmissivity within the karst vary greatly. Electrical resistivity imaging (ERI) is a well-known geophysical tool for mapping fractures and sinkholes, typical in karst settings. Locating enhanced water conveyance structures of a karst aquifer can improve the design and operation of an EAR site. This study investigated the hydraulic connection between shallow and deep groundwater using ERI to identify potential flow pathways and to improve our understanding of the storage mechanisms of the epikarst. The results presented in this paper validate the effectiveness of ERI in characterizing karst/epikarst and delineating soil, bedrock and local faults and fractures in the subsurface.

Keywords: aquifer recharge; climate change; electrical resistivity imaging; karst; vadose zone.

Figure 1.

Idealized cross-section depicting karst conduits, karst and epikarst sinkholes, water table and the contacts between soil and weathered bedrock (S-WB contact), and between weathered bedrock and competent bedrock (WB-CB contact).

Figure 2.

Aerial map of the EAR sinkhole relative to BMS. The mapped fault (yellow, solid line) trending northeast to southwest that ends between the EAR sinkhole and BMS is the Ham Fault. Other mapped faults (yellow lines) trend southeast-northwest and abut the Ham Fault. BMS spring and intermittent drainage features mapped in teal (short dashed lines). GoogleEarth image: imagery date: 3/11/2018; 34°35’27.10’ N, 96°40’19.89’ W; elevation 0 m; eye altitude 2.27 km.

Figure 3.

EAR field site near Fittstown, OK. (a) Aerial photo indicates electrode positions of 4-m spaced ERI surveys (red cross), sinkholes (yellow diamonds), sinkhole intended for use for EAR (pink triangle) and groundwater wells (black circles with crosses). (b) Topographic map demonstrates the low-lying nature of the site, suitable for runoff (dashed lines) collection during precipitation events.

Figure 4.

(a) Resistivity values on logarithmic scale for ERI datasets. Green and purple represent more electrically conductive locations and blues and grays represent more electrically resistive locations. (b) Electrical gradient values for ERI data (log(Ω m⁻¹)/m). Darker pink represents increasing electrical gradient strength and while represents a weak electrical gradient.

Figure 5.

Depiction comparing two points along ERI survey EAR0408 of a strong electrical gradient (yellow cross) and a weak electrical gradient (yellow diamond). (a) ER data. (b) Laplacian zero-crosses (black line) over Gradient (pink; dark with higher gradient).

Figure 6.

Direct-Push Electrical Conductivity Array tool readings (in resistivity, Ω m⁻¹) and ER values (Ω m⁻¹) from corresponding locations along ERI surveys at eight different boring locations at the EAR site.

Figure 7.

ERI Survey EAR0408 with epikarst interpretations for (a) soil boring EC log and corresponding ER, (b) inset of soil boring over ERI, (c) inset of soil boring over zero-crosses and electrical gradient, (d) 2-m survey, (e) 4-m survey and (f) aerial map with ERI surveys (red crosses), EAR0408 (orange solid line) and inferred fractures (yellow dashed lines) (Ham Fault extension is bolded).

Figure 8.

ERI Survey EAR0404 collected across the EAR sinkhole with (a) inset of ERI directly below the EAR sinkhole, (b) inset of Gradient and Laplacian grids directly below EAR sinkhole, (c) 2-m survey, (d) 4-m survey and (e) aerial map with ERI surveys (red crosses), EAR0404 (orange solid line) and inferred fractures (yellow dashed lines) (Ham Fault extension is bolded).

Figure 9.

EAR site with ERI surveys (red crosses), monitoring wells (black circles), sinkholes (yellow diamonds), EAR sinkhole (pink triangle) and localized fractures picked from ERI results (yellow dashed lines; Ham Fault Extension in larger, bolded dashed line).

Figure 10.

ERI survey EAR0414 collected 20 m south of the EAR sinkhole: (a) image from downhole video at 13.2 m, (b) image from downhole video at 20.8 m, (c) 2-m survey, (d) 4-m survey and (e) aerial map with ERI surveys (red crosses), EAR0408 (orange solid line) and inferred fractures (yellow dashed lines).