A study of seawater intrusion using direct-current soundings in the southeastern part of the Oxnard Plain, California

Abstract

On the basis of groundwater quality data, previous investigations have estimated that more than 60 square kilometers of the Oxnard aquifer have been intruded by seawater to distances reaching several kilometers inland from the shoreline of the Pacific Ocean. A direct current resistivity survey was made to investigate whether the increased salinity in the inland wells is caused by lateral invasion of seawater or by downward leakage of saline water from near surface formations through some wells. The resistivity survey, which consisted of 94 Schlumberger soundings, helped solve the problem by mapping the three dimensional geometry of subsurface materials having different resistivities down to depths of about 500 m. The results of the resistivity survey showed that over most of the study area, near-surface layers (down to a depth of about 10 m) often have very-low to low resistivities (<1 to 4.5 ohm-m); but at depths greater than 30 meters, where the Oxnard aquifer is found, the very-low resistivity layers (seawater-saturated layers) do not extend inland as far as previously suspected. The resistivity survey showed that it is possible to detect freshwater aquifers beneath saline-water aquifers, and that the freshwater aquifers in the study area mostly are characterized by resistivities in the 30 to 70 ohm-m range.

Integrated approach for demarcating subsurface pollution and saline water intrusion zones in SIPCOT area: a case study from Cuddalore in Southern India

“…6 Showing multi-electrode resistivity images for a MERI 1, b MERI 2, and c MERI 3 in the northern part of the study area Na + , 946.2 mg/l; and Cl − , 1,290 mg/l, whereas the residual level is 1.500 m (amsl), but the Uppanar River located at 750 m distance. Zohdy et al 1993 found the resistivity ranges for various sediments with the help of TDS concentration of groundwater. The resistivity of sandy saturated or sandy clay with brackish water (TDS, 1,500-10,000 mg/l) ranges from 4.5 to 10.0 Ω m. The data base obtained at Semmankuppam village has given a good agreement with the resistivity values of sediment type (sandy saturated) and salinity of water (salty brackish water).…”

Advancing process‐based watershed hydrological research using near‐surface geophysics: a vision for, and review of, electrical and magnetic geophysical methods

“…Many surface studies image resistivities from a metre below ground surface down a few tens of metres, and cross-well studies commonly have boreholes spaced on a similar scale. ERI has been used to determine the extent of conductive contaminant plumes or saltwater intrusion (Zohdy et al, 1993;Frohlich et al, 1994), and for locating voids, such as fractures, mine shafts, and karst terrain (Smith, 1986). Because ERI is sensitive to changes in fluid electrical conductivity and water content if the water is conductive, it has been used for monitoring time-varying processes, such as changes in moisture in the vadose zone (e.g.…”

The use of vertical electrical sounding resistivity method for the location of low salinity groundwater for irrigation in Chaj and Rachna Doabs

“…Resistivity of water may vary from 0.2 to over 100 X m depending on its ionic concentration and the amount of dissolved solids (Palacky 1987). Resistivity of natural water and sediments without clay vary from 1 to 120 X m (Zohdy and Martin 1993). Because of these confusing effects, the lithology and groundwater quality effects cannot be differentiated by the geoelectric resistivity survey alone (Choudhury and Saha 2004).…”