Using geophysics to map saltwater intrusion along a coastal river

In 1991 a small water district wanted to drill a new water supply well in the bank of a coastal river in northern California. A drilling site was picked a few miles from the river's mouth at the Pacific Ocean. The site was a wide bank in the river close to existing infrastructure. Saltwater intrusion was a possible problem. Prior to drilling, the district needed to determine if brackish water had infiltrated the river bank. We were engaged to perform a geophysical investigation to map saltwater infiltration along the river bank.

We preformed a geophysical investigation using a combination of electrical resistivity and seismic refraction. These techniques were chosen for their low cost and usefulness in defining the expected targets. Saltwater is very electrically conductive and is easily seen with electrical techniques. Electrical resistivity was used to determine if the groundwater within the river bank was brackish or fresh. Seismic refraction was used to determine the depth to the water table and the depth to bedrock below the river bank. Combining the results of the two techniques allowed us to determine the top of the water table, the top of the saltwater intrusion and the thickness of the river deposits.

Resistivity Methods

Resistivity Method

Seismic Refraction Method

Refraction Method

We collected data at eleven spots along the river. We chose spots where the bank was wide enough to accommodate the instrument arrays used to collect the data. The first location was near the mouth of the river where saltwater infiltration was expected. The last location was approximately six miles up stream where fresh water was expected. We used the first and last locations to calibrate the data collected in between. Electrical data were collected using Schlumberger arrays with outer electrode spacings to 1800 feet. Seismic refraction data were collected using spreads 300 to 600 feet in length.

Sounding LocationsSounding Locations

The results of the data collected at one location are shown below. This location was slightly over a mile from the river's mouth. The seismic data indicated three layers. The top layer was from 8 to 20 feet thick and had a compressional (P) wave velocity of 950 feet per second (fps). The slow P-wave velocity suggested the top layer consisted of partially saturated sediments. The middle seismic layer was from 35 to 110 feet thick and had a P-wave velocity of 5600 fps. The P-wave velocity, 5600 fps, is typical of unconsolidated saturated sediments. The top of the middle seismic layer corresponded to the top of the water table. The lower layer 's P-wave velocity was 8800 fps. The P-wave velocity suggested the lower layer was partially weathered bedrock.

Example ProfileSeimic and Resistivity Profile

The resistivity data also showed three layers with resistivities of 110 Ohm-feet, 11 Ohm-feet and 700 Ohm-feet, respectively. The upper layer resistivity, 110 Ohm-feet, indicated sediments partially to fully saturated with fresh water. At a depth of about 40 feet the resistivity dropped sharply to 11 Ohm-feet. This low value indicated a layer of sediments saturated with brackish water. The third resistivity layer corresponded to the bedrock beneath the river deposits.

From the information provided by the seismic and resistivity data shown above, we were able to determine the river sediments were 40 to 130 feet thick and the thickest section was approximately 200 feet from the edge of the river. The water table was 13 feet below grade and there was a 60-foot layer of brackish water underlying a thin layer of fresh water. The data indicated that saltwater had intruded the river bank and that this location would not produce fresh water.

Below are the results of the first nine soundings. The data showed the first five locations were underlain with brackish water. The brackish water was again found at the seventh location. Only fresh water was found after the eighth location.

Investigation ResultsResults from 9 Locations

The extent of the salt water intrusion beneath the river bank is shown below. Wells drilled within the green zone would encounter brackish water and would not be suitable water supplies. Wells drilled upstream from the intrusion zone would encounter fresh water. The initial location for the water well was scrapped because it fell within the intruded zone. A new location away from the intrusion zone was found. Our geophysical investigation saved the district tens of thousands of dollars in exploratory drilling costs.

Intrusion MapZone of Saltwater Intrusion

Seismics and resistivity are just two of many geophysical techniques offered by J R Associates. Please contact us to discuss how our services could benefit you.

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