Imaging Saltwater Intrusion Using Electrical Resistivity Tomography
The Place
Along the coast of the Monterey Bay in California, groundwater is the primary source of freshwater (up to 99% in Monterey County), and it supports a multi-billion dollar agricultural industry. Groundwater extraction over the past century has resulted in significant saltwater intrusion along the coast.
The Problem
Saltwater intrusion is the process by which saltwater migrates into a portion of a coastal aquifer that previously contained freshwater, often as a result of anthropogenic changes such as increased groundwater pumping or diversion of recharge sources. Saltwater intrusion can have significant economical and ecological impacts, and therefore it is a critical issue to investigate and manage. Traditionally intrusion is mapped and monitored using wells, which provide point information and can be spatially disparate, meaning they may fail to capture the extent of complexity of intrusion. Because of this there is a need for additional cost effective methods of locating regions of intrusion.
Our Approach
We are using the geophysical method of Electrical Resistivity Tomography (ERT) to accomplish this. This method results in continuous profiles of electrical resistivity with depth, and because of the large contrast in the resistivity of freshwater saturated sediments and saltwater saturated sediments, is ideally suited to mapping out intrusion in coastal aquifers.
Results
2012 Pilot Study:
In 2012, ERT data was acquired along a 7 km segment of beach between Seaside, CA and Marina, CA. The results from this acquisition, shown in the figure below, both demonstrate the viability of this geophysical technology for this application, and clearly show the value of geophysical imaging for understanding the geologic controls on the distribution of saltwater and freshwater from the ground surface to 150 m. The success of the 2012 pilot study motivated collection of ERT data along the whole length of the bay. For more information see Pidlesecky et al., 2015
2014 Full Scale Study:
In 2014, 40 km of ERT data were collected along the Monterey Bay coast. Inversion of these data resulted in resistivity sections with reliable results to a depth of approximately 300 m.b.s.l. These data show a complex distribution of fresh and salt water, influenced by geology, groundwater pumping, recharge, and land-use. Interpretation of the resistivity sections in conjunction with existing data sets and understanding of the region, allows us to identify the factors controlling the observed resistivity along the coast. For more information see Goebel et al., 2017.
