NS23C-03 Geophysical Imaging for Assessing Restored Wetlands' Soil Properties, Infiltration Rates, and Potential Surface Groundwater Interaction

Abstract

Efforts to reduce nutrient loading into Lake Erie include restoring old farm fields within Northwestern Ohio to wetlands. With a history of intensive farming, these fields are characterized by legacy nutrients, altered soil structure and drainage tiles. Inadequate knowledge of soil properties distribution, drainage tile locations and potential surface groundwater interaction at the sites limits restoration projects' effectiveness. This study combined a rapid geophysical site characterization framework with in-situ soil cores and hydraulic tests to assess variations in soil properties and hydraulic processes. We used a 250MHz PulseEkko ground penetrating radar (GPR) system and an EM38-MK2 conductivity meter to acquire GPR and electromagnetic imaging (EMI) measurements at selected wetlands with the equipment towed behind a utility terrain vehicle to allow extensive spatial coverage. Electrical resistivity (ER) data were acquired along transects using a Supersting R8 resistivity meter with an 84-electrode switch box. The ER measurement was done using a dipole-dipole array with 1m electrode spacing. Co-located soil cores were collected along the transects for laboratory soil moisture content, organic matter and texture measurements. In contrast, in-situ measurement of soil-saturated hydraulic conductivity was done using a SATURO infiltrometer. The apparent electrical conductivity (ECa) and ER distribution show comparable subsurface structures and parameter zones, and ECa correlates strongly with soil moisture, organic matter and silt contents (R2 >0.7; p-value <0.002). The ECa also notably correlates with the soil saturated hydraulic conductivity (R2 = 0.85), indicating the possibility of using EMI to rapidly characterize potential water retention zones (low Ksat) in restored wetlands. The spatial distribution of geophysical parameters depends linearly on soil properties distribution. This geophysical-soil-property relation, developed through a rapid site characterization framework, allows for improved soil sampling and monitoring plans. This study shows the effective application of EMI, GPR, and ER for pre- and post restoration characterization of old farm fields with legacy nutrients and drainage tiles that contribute to nutrient loading into Lake Erie.