Application and evaluation of electromagnetic methods for imaging saltwater intrusion in coastal aquifers: Seaside Groundwater Basin, California

Abstract

Developing effective resource management strategies to limit or prevent saltwater intrusion as a result of increasing demands on coastal groundwater resources requires reliable information about the geologic structure and hydrologic state of an aquifer system. A common strategy for acquiring such information is to drill sentinel wells near the coast to monitor changes in water salinity with time. However, installation and operation of sentinel wells is costly and provides limited spatial coverage. We studied the use of noninvasive electromagnetic (EM) geophysical methods as an alternative to installation of monitoring wells for characterizing coastal aquifers. We tested the feasibility of using EM methods at a field site in northern California to identify the potential for and/or presence of hydraulic communication between an unconfined saline aquifer and a confined freshwater aquifer. One-dimensional soundings were acquired using the time-domain electromagnetic (TDEM) and audiomagnetotelluric (AMT) methods. We compared inverted resistivity models of TDEM and AMT data obtained from several inversion algorithms. We found that multiple interpretations of inverted models can be supported by the same data set, but that there were consistencies between all data sets and inversion algorithms. Results from all collected data sets suggested that EM methods are capable of reliably identifying a saltwater-saturated zone in the unconfined aquifer. Geophysical data indicated that the impermeable clay between aquifers may be more continuous than is supported by current models.

Coupling distributed stormwater collection and managed aquifer recharge: Field application and implications

“…Groundwater overdraft, which occurs when aquifer outputs (including groundwater extraction) persistently exceed inputs, has many negative consequences (Konikow and Kendy, 2005;Zektser et al, 2005;Werner et al, 2013;D€ oll et al, 2014). Overdraft has caused saltwater intrusion, subsidence, and permanent storage loss in California's aquifers (Galloway et al, 1998;Nenna et al, 2013;Faunt et al, 2016). California enacted its first statewide groundwater management legislation in 2014, the Sustainable Groundwater Management Act, which creates a framework for local agencies to bring groundwater basins into balance.…”

Differentiating tidal and groundwater dynamics from barrier island framework geology: Testing the utility of portable multifrequency electromagnetic induction profilers

“…Comparatively few studies have used EMI methods in coastal environments (Paine et al, 2004;Seijmonsbergen et al, 2004;Vrbancich, 2009;Christensen and Halkjaer, 2010;Nenna et al, 2013;Delefortrie et al, 2014b), with most of these focusing on mapping saltwater intrusion. Most of these studies use Geonics EM31, 34, 38, and similar frequencydomain sensors; Geonics EM47, 63, and similar time-domain electromagnetic (TDEM) sensors in addition to various airborne electromagnetic (AEM) systems.…”

“…AEM surveys are important for coastal studies but are beyond the scope of this paper. Previous coastal EMI studies have explored subsurface σ because it is related to framework geology (Seijmonsbergen et al, 2004;Vrbancich, 2009), classification of coastal wetlands (Paine et al, 2004), and investigation of coastal groundwater dynamics and pollution (Goldman et al, 1991;Fitterman and Deszcz-Pan, 1998;Christensen and Halkjaer, 2010;Nenna et al, 2013). Seijmonsbergen et al (2004) use the EM34 (albeit not a portable multifrequency EMI profiler) at 20 m station spacing and 20 m coil separation to acquire a 14.5 km transect along a segment of the Dutch coast, Netherlands.…”

“…For example, Nenna et al (2013) test the feasibility of TDEM methods to identify hydraulic communication between a confined freshwater aquifer and an unconfined saline aquifer using a Geonics PROTEM 47. Data were acquired using center loop and offset receiver geometries.…”

“…EMI profiling for other uses is rapidly increasing for applications such as groundwater, environmental, and engineering studies (see Huang and Won, 2000;Everett, 2013). In the coastal environment, the use of EMI has focused on imaging saltwater intrusion for groundwater resource management (Nenna et al, 2013). Coastal EMI surveys for other purposes have been underused for several reasons including lack of awareness of the method by nongeophysicists and data reliability (see George and Woodgate, 2002).…”