Title
Review of the 2024 Seawater Intrusion Contour Maps
Report
SUMMARY/DISCUSSION:
The Monterey County Water Resources Agency (Agency) is responsible for data collection and analysis of groundwater data throughout the Salinas Valley to support the ongoing groundwater level contouring, seawater intrusion mapping, and other programs related to current groundwater conditions. Conditions are assessed throughout the year to better understand how aquifers are responding during different hydrologic conditions as well as the relative groundwater storage fluctuations that occur on an annual basis. These activities align with Strategic Plan Goals B7, Use of data and analysis to make informed decisions based on science and E1, improve public outreach to increase transparency, communication, education and information about Agency projects and programs. Activities associated with this program are included in Funds 111 & 116 of the FY 24-25 budget.
Program Background
Seawater intrusion was first detected in the Salinas Valley Groundwater Basin in 1933. As seawater intrudes into an aquifer there is a transition zone where seawater and freshwater mix. The chloride concentrations in this transition zone are between 50 milligrams per liter (mg/L), which is the native water quality of the 180-Foot and 400-Foot Aquifers, and 19,000 mg/L, which is the concentration of chloride in Pacific Ocean water.
The Agency uses the 500 mg/L chloride iso-contour as the threshold for determining seawater intrusion into the coastal aquifers. This definition of “seawater intruded” is described in Agency Ordinance No. 03790, Section 1.01.13. A chloride concentration of 500 mg/L is roughly ten times the native concentration of chloride in these aquifers, two times the regulatory limit set by the E.P.A Drinking Water Secondary Maximum Contaminant Level (250 mg/L), and exceeds the USDA concentration (350 mg/L) for water considered to be of “Class III - injurious or unsatisfactory” quality fo...
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