Consultants propose geophysics, test holes to locate backup wells for Rancho Murieta

Consultants Pat Dunn (certified hydrogeologist, NV5) and Sage (licensed geophysicist, Terracon) told the Rancho Murieta Improvements Committee that a targeted program of ground-based electrical resistivity and seismic refraction, combined with existing aerial EM data, would let the district prioritize locations for test holes and, if warranted, production wells.

Dunn said earlier work from 2012–2014 had identified several favorable areas and two proposed boreholes (PWA and PWB) that, had they been completed, were estimated to deliver the district’s previous drought‑supply target. "We were asked back then to look for 600 acre feet of water," Dunn said. "With those 2 boreholes ... we felt we could accomplish 200 to 300 GPM per well, which would give you that 600 acre feet." He and Sage emphasized that geophysics can narrow where to drill but cannot confirm yield or water quality until test holes are drilled, cased and pumped.

Sage outlined the two field methods proposed: electrical resistivity surveys using surface electrodes to map resistivity contrasts that often correspond to sands and gravels, and seismic refraction to better resolve denser, fractured bedrock. "The warmer colors are gonna be your higher resistivity zones ... more likely going to be your gravels, your sands," Sage said, describing cross sections the team would produce. He recommended avoiding utility easements and keeping lines as straight as possible to preserve data quality.

Cost and scope: Dunn said Terracon’s base geophysics proposal was roughly $74,000 and that he had added nearly $18,000–$20,000 for site-access, project management and work‑plan revisions. He recommended first doing ground geophysics on proposed "yellow‑line" areas and using previously collected red‑line data where available; the team proposed up to six geophysical lines and two test borings (pilot holes) for ground‑truthing before committing to steel casing and full well construction.

Water quality and treatment: Committee members pressed on arsenic, iron and manganese reported in earlier test holes; Dunn said water‑quality isolation zone sampling during test-hole drilling would tell the district whether blending or pretreatment would be necessary. "When we do the test hole, we tell you what the water quality will be and whether treatment may be necessary or not," Dunn said.

Next steps: Directors supported sending the proposal to the full board for discussion and formal approval of funding (anticipated to come from the water‑augmentation account). Multiple members urged staff to coordinate access agreements and to seek grant funding before committing to full well construction, while recognizing the high capital cost of drilling and treatment if wells prove productive.