Stream Bottom Resistivity Tomography to Map Ground Water Discharge

Abstract

This study investigates the effectiveness of direct current electrical resistivity as a tool for assessing ground water/surface water interactions within streams. This research has shown that patterns of ground water discharge can be mapped at the meter scale, which is important for understanding stream water quality and ecosystem function. Underwater electrical resistivity surveys along a 107-m stream section within the Burd Run Watershed in South Central Pennsylvania identified three resistivity layers: a resistive (100 to 400 Omega m) surface layer corresponding to the streambed sediments, a conductive (20 to 100 Omega m) middle layer corresponding to residual clay sediments, and a resistive (100 to 450 Omega m) bottom layer corresponding to the carbonate bedrock. Tile probing to determine the depth to the bedrock and resistivity test box analysis of augered sediment samples confirmed these interpretations of the resistivity data. Ground water seeps occurred where the resistivity data showed that the residual clays were thinnest and bedrock was closest to the streambed. Plotting the difference in resistivity between two surveys, one conducted during low-stage and the other during high-stage stream conditions, showed changes in the conductivity of the pore fluids saturating the sediments. Under high-stream stage conditions, the top layer showed increased resistivity values for sections with surface water infiltration but showed nearly constant resistivity in sections with ground water seeps. This was expressed as difference values less than 50 Omega m in the area of the seeps and greater than 50 Omega m change for the streambed sediments saturated by surface water. Thus, electrical resistivity aided in characterizing ground water discharge zones by detecting variations in subsurface resistivity under high- and low-stream stage conditions as well as mapping subsurface heterogeneities that promote these exchanges.

Investigating sediments and rock structures beneath a river using underwater ERT

“…Nyquist et al [38] identified three resistivity layers in similar settings: a resistive (100 to 400 Ωm) surface layer corresponding to the streambed sediments, a conductive (20 to 100 Ωm) middle layer corresponding to residual clay sediments, and (unrelated to the current case study) a resistive (100 to 450 Ωm) bottom layer corresponding to the carbonate bedrock. Crook et al [39] could show that, for similar settings, the boundary between the lower two resistivity regions correlates well with the interface previously logged between the alluvial gravels (95 to 1500 Ωm) and underlying weathered limestone (10 to 75 Ωm).…”

“…In addition to their field case study, Kwon et al [38] presented a numerical study evaluating the potential of underwater ERT surveys when using floating electrodes or electrodes placed at the bottom of the river bed with regards to electrode spacing, sensitivity and electrode spreads. Nyquist et al [39] used stream bottom ERT to map groundwater discharge and assess groundwater-surface water interaction within streams. They show that patterns of groundwater discharge can be mapped at the meter scale.…”

“…Therefore, electrical resistivity and geometry of the water column must be known for an accurate robust inversion [12,39,44,45]. The electrical resistivity and geometry of the water column is fixed in the earth model and the inversion program attempts to determine the electrical resistivity of the cells that would most accurately reproduce the observed electrical resistivity measurements [46].…”

Boat-towed radio-magnetotellurics — A new technique and case study from the city of Stockholm

“…They develop an automated algorithm to extract the depth to bedrock using borehole and AEM data. There are a few engineering examples that demonstrate the capability of geophysical measurements, especially electric and electromagnetic (EM) methods, on lakes, rivers, or streams (e.g., Nyquist et al, 2008;Toran et al, 2010). Day-Lewis et al (2006) present results of two case studies that demonstrate the usage of continuous resistivity profiling for identifying the spatial distribution of submarine groundwater discharge and subbottom freshwater.…”

Technical Note: Deciphering the Hydrologic Response of Riverbeds across Changes in Recharge with Electrical Resistivity Imaging

“…Hydrogeologic and environmental factors such as the water content, porosity, salinity, clay content, pore geometry, and pore-fluid temperature control the electrical resistivity of a medium (Everett, 2013). ERI surveys can be used to delineate 15 groundwater discharge areas (Nyquist et al, 2008), recharge pathways through mantled sinkholes (Schwartz and Schreiber, 2009), and riverbed sediment architecture (Crook et al, 2008). Daily et al (1992) showed that ERI can effectively map changes in water content within the vadose zone by analyzing the temporal changes in electrical resistivity as a result of an infiltration event.…”