Looking Deep into Never Explored Granite for Groundwater

A GEOPHYSICS PROFESSOR AND STUDENT RESEARCHERS USE SPECIALIZED RESISTIVITY EQUIPMENT TO DETECT DEEP GROUNDWATER ON THE CENTRAL COAST<

APRIL 2025
BY NICK WILSON

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On a rural ranch in Paso Robles, California, one might not wonder about what lies deep down under the soil. Cal Poly researchers captivated by the question are going to great lengths to answer it.  

The ranch owner wants to drill a water well, but the property sits atop a site not usually associated with water above a subsurface of granite weathered and fractured by millions of years of geological movement from seismic fault activity.  

Physics Professor John Jasbinsek and students went to work searching for the best place to drill for water and explore a bigger question concerning the fascinating subsurface hydrogeology associated with fragmented Salinian Block granite.  

Jasbinsek, who teaches courses in geology and geophysics, said the rock is special for how it has come to be exposed on California’s Central Coast. 

“This granite has been transported, over the course of millions of years, from the western Mojave Desert along faults related to the formation of the San Andrea Fault plate boundary,” Jasbinsek said. “This Paso Robles ranch is an interesting site, because it’s not a typical aquifer site, which tends to be sediment. Weathered and fractured granitic rock, where groundwater might be found, underlies a lot of this property.”

Alluvial aquifers, from which groundwater is typically drawn, come from formations of sand and gravel. But hard rock — granite that is an intrusive igneous rock formed from magma that cooled slowly deep within the Earth’s crust — hasn’t frequently been explored for water. Jasbinsek said Salinian Block granite in California has never been searched with geophysical methods for groundwater resources.

As governing agencies, farmers, vineyard operators and landowners grapple with managing water resources strained by overdrafting and the effects of climate change, identifying ways to diversify water resources has become a growing concern, along with conservation and recycling efforts through multi-faceted groundwater management programs.

Many of Jasbinsek’s students have gone on to jobs using hydrogeology because of the demand in the field, including work involving geophysical surveys — non-invasive ways to gather information about subsurface land areas.

  From left, Lee Knudtson, Iason Pitsillides, Owen Cheng, Breanne
Evans, Leannah Hill and Addison Burke next to SuperStingR8
Multi-Electrode Resistivity System equipment as it collects
resistivity data from a 1.1 km long electrical resistivity
survey in a field site in Paso Robles, California. Top photo is
from the same field site. Courtesy photos

The ongoing study in Paso Robles allows students to explore aquifers, develop skills in geologic mapping and acquisition, and analysis of near-surface geophysical data. They did so with the help of electrical resistivity tomography, which uses electrical currents to measure variation in subsurface resistivity in areas of groundwater interest. The team uses equipment such as the AGI SuperSting R8 Multi-Electrode Resistivity System purchased with Frost funding.

“Patterns and variations in subsurface resistivity values can indicate areas of groundwater,” the professor said. “Because resistivity values also depend on characteristics of the geologic medium the groundwater is hosted in, such as composition and permeability, it can be counterintuitive that zones of relatively higher resistivity are the water-bearing regions. Therefore, understanding the local geology as much as possible is fundamental to interpretation of the geophysical data.”

Using surveys to explore sites from 100 to 300 meters below ground, Cal Poly researchers identified areas in the Paso Robles field site with low resistance to electrical flow. They also used seismic refraction tomography, a prospecting technique using seismic waves, to study subsurface conditions.

“The research in Paso Robles was interesting in that it was a unique case study,” said former student Addison Burke (Environmental Earth and Soil Sciences, ’24), physics minor who received a Cal Poly Teacher-Scholar Mini Grant supporting experiential learning. “It was a complex system that we were working with. I was also able to see the project all the way through, which was very rewarding. The landowner recently drilled at our site recommendation and found water that is sufficient for a well.”

After the drilling results, the researchers plan to write a manuscript to submit for publication.

Burke said that geophysical exploration research can lend insights into other potential aquifers to help communities and property owners in need of a water supply.

“The main source of groundwater supply is through alluvial aquifers, but many are being overdrafted faster than they are being recharged,” Burke said. “Hard rock aquifers can provide an adequate resource for local water needs. I hope to continue work similar to this study throughout my career.”

In a previous study highlighted in a 2021 journal article, Jasbinsek and Cal Poly faculty and student researchers specializing in hydrology examined groundwater and stream conditions at Cal Poly’s Swanton Pacific Ranch. Located in Santa Cruz County, the 3,200-acre ranch site is used for educational and research in sustainable agriculture.  

The team examined whether agricultural pumping of the fields for the Jacobs Farm (a site at Swanton Ranch leased by Cal Poly to Jacobs Farm/Del Cabo Inc. for organic farming) and use of wells, had any impacts on stream flow of Scotts Creek adjacent to where the pumping occurs.  

Jasbinsek worked with professors Bwalya Malama and Christopher Surfleet, and students in the Natural Resources Management and Environmental Sciences Department, to conduct monitoring, testing and imaging. Malama’s work includes focus on groundwater hydrogeology and soil biophysics. Surfleet’s career as a hydrologist has spanned academic, research and private land and federal forest land management.  

They concluded that the overall connection between pumping and stream flow was relatively limited at the time, but prolonged dry periods could lead to significant depletions of streamwater sources and fish habitat. 

“The results of this work illustrate the importance of site-specific investigations to effectively assess stream-aquifer interactions and stream depletion due to groundwater pumping for crop irrigation,” the co-authors wrote in their journal article.

Research supported by: Cal Poly Teacher-Scholar Mini Grant Program, USDA McIntire-Stennis Program, the California State University Agricultural Research Institute (ARI).