County consultants propose new drought‑focused proof‑of‑water testing after coastal groundwater update

Consultants for Mendocino County presented a comprehensive update to coastal groundwater conditions and proposed a revised proof‑of‑water testing procedure intended to better predict whether private wells will perform during droughts.

The presentation, led by Audra Bardsley of Larry Walker Associates and hydrology lead Gus Yates of Todd Groundwater, described 21 new resource maps, a public data‑management system (DMS) that aggregates historical well records, and a parcel development‑likelihood analysis to inform the county’s Local Coastal Program update. "This is the first comprehensive reevaluation of coastal groundwater conditions since that 1982 study," Bardsley said during the presentation.

County and consultant officials said the study was prompted by recent drought impacts and grant funding from the Coastal Commission to update technical data used in coastal planning. The DMS is public and searchable, the team said, and will accept future proof‑of‑water tests to improve countywide records.

The consultants proposed a new proof‑of‑water workbook and test format designed to assess how a well would perform in conditions similar to the 2020–2021 drought. The workbook standardizes data entry and uses statistical relations derived from local water‑level records to estimate a well’s likely fall water level and yield under drought conditions. Gus Yates described the approach as a way to "peek at how well this well would perform in a drought," and said the method combines measured pump‑test results with calculations that estimate drought‑period water levels.

A significant technical finding presented to the commission was that on‑site septic return flows can reduce summer water‑level declines in some populated coastal areas: in densely developed clusters, indoor water use returned via septic can maintain shallow aquifers more than previously expected. The consultants also reported counterintuitive correlations: higher building density did not always mean greater dry‑season declines, in part because irrigation — not household indoor use — is the most consumptive summer demand.

The recommended proof‑of‑water changes include testing for all parcels that will rely on wells (not only certain lot sizes), a 16‑hour pumping test tied to calculated maximum‑day demand, seasonal pumping‑rate adjustments (higher pumping rates if tests are conducted in wetter months), specific recovery criteria (95% recovery within 24 hours), and a requirement that test results be uploaded into the DMS. The workbook estimates the well’s performance in a two‑year drought scenario to help planners and applicants assess risk.

Commissioners and coastal planners pushed back on data‑presentation issues. Fort Bragg and Point Arena staff asked the consultants to add map disclaimers and overlay park and forest lands because dynamic aggregation in the DMS can visually cluster many wells at a single dot. Consultants acknowledged known location‑accuracy limits in statewide well records and agreed to clarify the public interface.

Marina Douly, Mendocino County Environmental Health director, said the department supports the new procedures and the DMS. "Right now we get these in paper. We're scanning them and we're reading them, but it's not being loaded or saved in a way that can help us learn more," she said, noting that digital proof‑of‑water records could expand the county’s capacity for statistical analysis and long‑term monitoring.

Public commenters and local planners raised policy questions about whether winter tests in unusually wet years should be restricted or adjusted, and whether the workbook should incorporate an explicit rainfall‑to‑date variable to avoid passing tests based on unrepresentative wet conditions. Consultants said they would consider how to address high‑rainfall years in the workbook and that stricter pumping requirements in winter are already part of the method.

The county will accept public comments on the final reports and the proof‑of‑water workbook as staff integrates the technical products into the Local Coastal Program update and any future policy changes. The consultants emphasized that any procedural changes would require board approval before becoming mandatory.