Delineating Recharge to a River Valley Aquifer by Riverine Seismic and EM Methods

Abstract

The City of Fredericton, New Brunswick, Canada operates a water well field adjacent to the Saint John River. Water is extracted from a semi-confined aquifer that consists of a buried sand and gravel esker ridge discontinuously covered by a clay∕silt aquitard. Geological, hydrogeochemical, and hydraulic data have long pointed to infiltration of river water as the main source of aquifer recharge but the shape and size of the recharge zone has, until recently, remained speculative. A combination of three geophysical surveys, including resistivity imaging along the shoreline, and seismic and electromagnetic profiling on the river itself, have now revealed the complex outline of the area beneath the river where the infiltration likely occurs. Broadband seismic reflections, with dominant frequencies of [Formula: see text] were recorded from depths as great as [Formula: see text] using a boomer source and 1KB Seistec line-in-cone receiver well suited to operation in the shallow water environment. EM31 and EM34 terrain conductivity meters, deployed in an outboard-motor-powered canoe and raft, revealed the subsurface conductivity structure in the same depth range. While the seismic method imaged the clay aquitard stratigraphy including its termination against the esker ridge, the electromagnetic method provided a more complete plan view of the extent of the clay window recharge zone. The two riverine data sets are highly complementary; as a start towards joint interpretation, we introduce the concept of bathymetry-corrected apparent conductivity and show how it helps to sharpen the boundary of the inferred recharge area. Confirmation of the geophysical interpretation is provided by the presence or absence of aquitard material in boreholes that have recently been drilled from a barge on the river.

River sediment salt-load detection using a water-borne transient electromagnetic system

“…Hurwitz et al (1999) have described results obtained from a floating TEM system first described by Goldman et al (1996). Their system investigated between depths of 10 and 100 m. In a study of aquifer recharge Butler et al (2004) employed the Geonics EM31 and EM34 (Frequency domain EM systems) which allowed efficient mapping of horizontal variations in apparent conductivity, but this allowed no vertical resolution of conductivity. DC Resistivity arrays have previously been employed in floating configurations (Allen and Merrick, 2003;Dyck et al, 1983;Amimoto and Nelson, 1970).…”

River sediment and flow characteristics near a bank filtration water supply: Implications for riverbed clogging

“…Using resistivity imaging along the shoreline, and seismic and electromagnetic profiling on the river, Butler et al (2004) have confirmed the presence of several windows in the riverbed in the vicinity of the municipal well field.…”

“…No municipal production wells appear on this cross section. (source: Butler et al, 2004). during the data collection period shown in Fig.…”

“…The locations of the eight production wells (denoted by the prefix PW) and selected monitoring wells are represented as black dots, while the window areas, as delineated by Butler et al (2004), are represented by the shaded regions. Fig.…”

Technical Evaluation of the Interaction of Groundwater With the Columbia River at the Department of Energy Hanford Site 100-D Area

“…Use geophysical surveys to identify geological variations. Use boat-towed continuous seismic reflection, electromagnetic (EM) methods, and ground penetrating radar (GPR) to determine geological contacts and clay versus sand contents, "Delineating Recharge to a River Valley Aquifer by Riverine Seismic and EM Methods," (Butler, et al 2004), "Use of Ground-Penetrating Radar and Continuous Seismic-Reflection Profiling on Surface-Water Bodies in Environmental and Engineering Studies," (Haeni, 1999). Although attempts to use GPR along the shoreline were deemed unsuccessful, because of lack of penetration (perhaps due to attenuation of the signal by clay), the method should be reevaluated for use, because in places where GPR does penetrate, it may indicate either loose fluvial sandy deposits or perhaps preferential discharge locations, (Conant 2004) where more permeable deposits are present and the armored layer is absent.…”