Current Abstract

March 11th, 2025 Meeting Abstract

“Use of Geophysical Logs to Identify Base of Freshwater and Base of
Underground Source of Drinking Water in Central San Joaquin Valley, California“

Presented by:  Emily Imperato, Matthew Weingarten, Rafael Almeida

Abstract: 

Groundwater salinity trends within California’s Central Valley are not well constrained. This study
identifies the shallowest elevation of base freshwater (BFW), <3,000 parts per million (ppm) total
dissolved solids (TDS), and the shallowest elevation of base of underground source of drinking water
(BUSDW), <10,000 ppm TDS in Fresno, Madera, Kings and Tulare counties. Resistivity and lithologic logs
from over 600 oil and gas wells inside and outside of oil fields were analyzed. Resistivity values were
identified to represent these groundwater surfaces: 10 ohms for BFW and 3 ohms for BUSDW. These
representative resistivity values were derived using a temperature and porosity correction for thick (>10
ft) clean sands, specific to the study area. These chosen values are consistent with previous studies
conducted south of the study area in Kern County (Gillespie, 2017). The results reveal distinct salinity
trends north and south of the Kings River drainage divide. South of the divide, BFW and BUSDW are
relatively deep, reaching up to 6,000 feet (ft) below ground surface (bgs) on the eastern margin, rising
sharply to near surface elevation in the Tulare Lakebed basin. North of the divide, BFW and BUSDW are
shallower, particularly along a central ridge where these surfaces are about 1,500 ft higher than at the
margins. Along the western edge of the basin, BFW and BUSDW elevations vary greatly. In general, the
observed salinity patterns across the study area are interpreted to result from spatially variable
freshwater recharge from the Sierra Nevada and Coast Ranges. BFW and BUSDW depths follow the
basement along the eastern margin of the study area. We interpret the shallow depth of saline water in
the Tulare Lakebed basin to result from a combination of the low permeability Tulare Lake sediments
limiting freshwater recharge and the natural filling and evaporation cycle of the closed lake basin. In
general, BUSDW remains between 500–1,500 ft below BFW, except in the westside subbasin, where
historical (pre-1960s) groundwater pumping was substantial. We propose that historical groundwater
pumping of fresh groundwater in the westside subbasin has caused the BFW to become shallower, but
not BUSDW, as evidenced by an increased gap (approximately 2,500 ft) between the two surfaces.
Shallow groundwater pumping is more likely to result in upward movement of the BFW than BUSDW
because these groundwater wells target only shallower freshwater zones and low vertical permeability
inhibits upward flow of the deeper, more saline water. Study results suggest that overall, the date of
geophysical logs is not especially critical when mapping BUSDW, which may change little over time.

Biography:

I’m Emily Imperato and I hold a Bachelor of Science in Geology from
the University of California, Santa Barbara, and a Master’s in Geology
from San Diego State University, which I completed in December
2024. During my master’s program, I interned at the Electric Power
Research Institute (EPRI) on their carbon sequestration team and at
Chevron’s Bakersfield office, where I contributed to a carbon
sequestration prospect assessment.
For my master’s thesis, I focused on salinity mapping in California’s
San Joaquin Valley using geophysical logs. I continued this work at the
U.S. Geological Survey (USGS), extending salinity mapping efforts
across other regions of California.
I’m happy to be back in my summer home of Bakersfield and look
forward to presenting my master’s thesis work and the work I was
doing at USGS.