Map shows seawater threat to California Central Coast aquifers

Sentinel wells are also expensive: Four of the wells used to monitor saltwater intrusion along Monterey Bay cost nearly $1 million to drill. In contrast, the Stanford team’s survey cost approximately $200,000.

ERT surveys could also be repeated at the same spot to observe changes over time, said study co-author Adam Pidlisecky, who is the chief research officer at Aranz Geo. “Looking at differences through time can often be more helpful than trying to understand a single image,” said Pidlisecky, who conducted the research while at the University of Calgary in Canada.

Phone tip
For Knight, the study, which is the longest and deepest of its kind, represents the culmination of an ambitious experiment that began with a phone tip six years ago. “A hydrologist working in the Monterey area told me that there was a lot of interest in what’s happening with saltwater intrusion along the coast and suggested it could be an interesting project for my group,” said Knight, who is also affiliated with the Stanford Woods Institute for the Environment.

When Knight contacted local city officials for permission to start her survey, some of them couldn’t quite believe their ears. “One’s response to me was, ‘I just spent hundreds of thousands of dollars drilling a well, and you’re telling me you can do this by walking along the beach?”’ she said.

Securing the required permits from cities and private landowners took close to a year, but in the fall of 2014, the team was ready to begin their survey. Over the course of about two weeks, researchers from Stanford University and the University of Calgary worked with engineers from the engineering company WorleyParsons to lay out a long string of electrical cables along the beach. The cables were punctuated by 4-foot steel electrodes sunk into the sand.

After a series of electrodes were in place, the team pumped tiny pulses of electricity through the connecting wire. The current spread below the ground, and by measuring the voltage between pairs of electrodes the team was able to create a resistivity map of the subsurface. Less resistance meant more saltwater lay below. Once the team surveyed a section of coastline they pulled out the stakes and repeated the process elsewhere.

The survey involved spending long hours in the sun hauling heavy equipment and painstakingly covering nearly the entire Monterey Bay coastline one mile at a time, but the effort paid off. “This is the first regional-scale view of the subsurface that clearly shows the impact of geology on fluid distribution as well as the influence of near-coast pumping,” Pidlisecky said. “Throughout the process we have actively connected with groundwater managers and technical professionals in the region, as well as the public at large. We are excited to take this engagement further with the publication and sharing of these results through an interactive 3-D web portal.”

Stanford says that the data the team collected yielded a colorful, two-dimensional resistivity map of the coastline that reveals complex patterns of saltwater intrusion in the region down to a depth of 1,000 feet and highlights the interplay between the geology and human activities controlling the region’s distribution of fresh- and saltwater. For example, the team discovered a dynamic body, or “lens,” of freshwater near Marina formed through infiltration from overlying storm water runoff ponds in an area that was otherwise undergoing saltwater intrusion. The map also showed that efforts by the Pajaro Water Management District to reduce groundwater pumping by providing recycled water to irrigators was working as planned.

Keith Van Der Maaten, general manager of the Marina Coast Water District, said the new findings could help fill in numerous gaps in their data of groundwater resources. “The current mapping of the freshwater-saltwater interface in our region has many issues and is misleading,” Van Der Maaten said. “The ERT data will help give us a more complete picture so we can move forward with our water supply planning and groundwater sustainability efforts.”

Follow-up airborne study
Since electrical resistivity is a property present in all materials, ERT could have applications beyond water management. “Any time what you are trying to image in the subsurface has a different resistivity than the material surrounding it, you should be able to use this method,” Goebel said.

Knight is not finished with the Monterey Bay region just yet, however. Her team has partnered with the Marine Coast Water District to conduct a follow-up airborne helicopter survey that employs a different geophysical technique to map subsurface electrical resistivity in the region’s inland areas.

“Instead of just one slice, the airborne survey is going to give us hundreds of slices and allow us to put together a three-dimensional picture of the underground freshwater distribution,” Goebel said.

 Read more in Meredith Goebel et al., “Resistivity Imaging Reveals Complex Pattern of Saltwater Intrusion Along Monterey Coast,”Journal of Hydrology Energy (22 February 2017) (in press) (doi: org/10.1016/j.jhydrol.2017.02.037)