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.

HighlightsAnimal effluents can be detected in soils after minimal applications using non-intrusive characterization techniques.Swine effluent in soil produces measurable electrical signatures in the absence of chemical signatures.Electrical resistivity imaging can determine soil thickness and identify rapid transport pathways in the subsurface.Dietary additives may be good proxies to detect swine effluent in soils.Abstract. Land application of animal manures is useful for increasing many beneficial properties of soil that include fertility, productivity, and overall soil health, but can pose risks to water quality if not properly applied, especially in karstic formations. A study was carried out to assess the use of electrical resistivity and soil sampling to better characterize the transport of applied swine effluent within a mantled karst system and to assess the potential for rapid transport of swine effluent represented by Zn and other metals commonly found in effluent. This hydrogeophysical investigation was conducted at three sites, all permanent pastures, along Big Creek near Mount Judea, AR, USA, that are underlain by the Boone Limestone formation that is home to the Buffalo National River. To determine if surface electrical geophysics could be utilized to monitor the applied swine effluent within a mantled karst riparian system, this study measured the bulk electrical properties of the subsurface, characterized soil properties at the surface, and used reactive transport modeling. Electrical resistivity imaging (ERI) showed soils at sites with applied effluent were more electrically conductive than a background site. A comparison of ERI and soil sampling bulk datasets demonstrated strong trends between bulk resistivity and fluid conductance. These results enabled a more accurate definition of soil characteristics, the epikarst zone, and bedrock, along with anticipated subsurface chemical transport. Keywords: Electrical Resistivity Imaging, Epikarst detection, Soil characterization, Swine Effluent.