Agency: August 2024 survey shows modest groundwater rises; seawater intrusion largely stable with small advances near Salinas

Monterey County Water Resources Agency hydrologists presented the agency ugust 2024 groundwater elevation contours and seawater intrusion maps at the board meeting, reporting modest gains in groundwater elevations in many areas and limited expansion of chloride indicators near the city of Salinas.

The presentation described the August survey as a single‑day snapshot of the basin when aquifers are typically most stressed. "The August survey is done to create the groundwater elevation contours and it's done in a single day in August," said Guillermo Diaz, hydrologist with the agency. He said the effort samples roughly 135 wells across the coast to Chualar to map groundwater surfaces.

The board was told that for the 180‑foot and east‑side shallow aquifers the red 10‑foot increment contours show near‑coast increases of about 1 to 3 feet compared with 2023, and that a 0‑foot contour now runs along parts of the coast, indicating groundwater elevations just above sea level there. For the 400‑foot aquifer staff reported smaller increases near the coast (about 1–2 feet) and noted that groundwater elevations at the coast remain below sea level in the deeper zone.

Ricardo Carmona, hydrologist, explained the seawater intrusion monitoring program and the agency dopted threshold. "As defined in ordinance number 3790, the agency uses the 500 milligram of chloride per liter as the threshold for establishing seawater intrusion in the area," Carmona said. The agency also maps a 250 mg/L contour as an early indicator of increasing chloride concentrations.

Carmona reported that most wells in the 180‑foot aquifer showed no change in intrusion phase compared with 2023; the 500 mg/L contour did not change substantially in 2024. He said the 250 mg/L leading edge shows a slight expansion toward the western city limit of Salinas. For the 400‑foot aquifer he reported a small southeastern expansion of the 500 mg/L contour identified in 2023 and that seven wells ahead of the eastern 250 mg/L contour progressed from not intruded to Phase 1 in 2024.

Staff described sampling totals for the transition zone: "During the summer season we collected groundwater samples from 36 wells in the 180‑foot aquifer and 74 wells in the 400‑foot aquifer," Carmona said. All samples were analyzed by Monterey County's consolidated chemistry laboratory.

Directors and members of the public pressed staff for additional data layers. Several directors asked that future presentations include whole‑valley contours and river flow data to better link reservoir releases and river flows to groundwater gradients. Guillermo Diaz said the August contours were produced for the same geographic area used in the intrusion analysis but acknowledged a correlation with conditions in the upper valley and explained that groundwater movement acts on slower time scales.

Staff and a public commenter referenced the agency nnual/quarterly conditions reporting. "The quarterly conditions report incorporates groundwater levels as well as flow conditions for the bigger picture," said Amy Woodrow, a staff member who raised her hand during the public comment period.

Why it matters: seawater intrusion can permanently reduce freshwater storage and raise treatment or supply costs for coastal wells. The agency—ontinues year‑to‑year mapping and geochemical phase analysis to detect trends and to inform management actions.

Provenance: Topic introduced 01:17:58 (presentation start with Guillermo Diaz) and discussed through 01:49:56 (closing of item).