Time Domain Electromagnetic Soundings for Mapping Sea‐Water Intrusion in Monterey County, California

Abstract

ABSTRACTIn November 1986, a surface geophysical survey was performed in Monterey County, California using Time Domain Electromagnetic (TDEM) soundings. Prior to this time, the Monterey County Flood Control and Water Conservation District had relied largely on water quality samples from monitoring wells to determine the extent of sea‐water intrusion in the Salinas Valley. This monitoring program had found that of the three aquifers in the study area, two aquifers, the “180‐ft” and the “400‐ft,” had been intruded. The third aquifer, the “900‐ft” aquifer, had not shown signs of sea‐water intrusion. The District decided to use surface geophysics to obtain information about water quality between monitoring wells and to contour the position of the 500‐ppm isochlor in both of the upper aquifers.Because the conductivity of water increases with chloride concentration, electrical methods are well‐suited for mapping chloride concentration in ground water. The TDEM sounding method was selected as the surface electrical measurement technique. It had proven successful for mapping interfaces between fresh water and salt water on surveys in Cape Cod, Massachusetts, in several areas in Florida, and in southern California. A total of 100 soundings were measured in the project area, requiring three weeks of field work. The soundings were located by using the information available from monitoring wells. Lines of soundings were run approximately perpendicular to the anticipated position of the 500‐ppm isochlor to better define the location of the isochlor.Several soundings were measured adjacent to wells to aid in correlating the geophysical interpretations to known water quality data. By correlating the surface geophysics to water quality data from monitoring wells, the position of the 500‐ppm isochlor could be mapped in both the “180‐ft” and “400‐ft” aquifer.The positions of the contours of the 500‐ppm isochlor derived from monitoring wells and from 100 TDEM soundings show excellent agreement. There is more local detail in the isochlor contours derived from geophysical interpretations than in the contours derived from monitoring wells, mainly because of the higher TDEM stations density.