City staff presented the Groundwater Basin Storage and Sustainable Yield Study update to the Water Commission Oct. 26, reporting modeled sustainable pumping rates and an estimate of maximum basin storage capacity and discussing next steps for operational guidance.

Jasmine Showers, a water resources analyst, explained that USGS updated the 2018 groundwater model to produce Excel-based dashboards allowing staff to explore two objectives: sustainable pumping (minimizing drawdown in the Foothill Basin and minimizing seawater intrusion in Storage Unit 1) and maximum storage capacity (modeled without pumping). The basins were modeled under three initial conditions — “empty” (1990), “recovered” (1998) and “recent” (2013) — and runs used precipitation sequences from 2004–2013.

Showers reported that the Foothill Basin sustainable pumping average over 10 years is about 5,270 acre-feet (all pumpers); the city’s share of the Foothill sustainable pumping estimate is about 280 acre-feet per year. For Storage Unit 1, staff said the sustainable pumping figure tied to chloride constraints at a sentinel monitoring well (23F4) is about 700 acre-feet per year under recent conditions. Combined, staff summarized sustainable pumping at roughly 980 acre-feet per year, or about 9% of current city potable annual demand.

To estimate storage capacity, USGS simulated filling the basins without pumping, linking multiple 10-year model runs until the incremental change in stored volume flattened after approximately 40 years. That approach produced a modeled maximum storage capacity of about 16,000 acre-feet across both basins (approximately 9,500 acre-feet in Storage Unit 1 and 6,400 acre-feet in Foothill) with the largest storage increase occurring in the first 10 modeled years.

Staff emphasized policy and operational choices remain. Showers and other staff said the 2018 “maximum drought pumping” approach does not align with city goals — which favor avoiding deep drawdown and seawater intrusion — and the 2024 dashboard allows staff to evaluate sustainable pumping consistent with those goals. Staff noted the updated dashboard and modeling outputs will be made publicly available when the USGS furlough-related access issue is resolved.

Commission discussion covered when a basin might be considered “recovered” and how to correlate monitoring-well elevations with modeled basin volume; staff said additional work is needed to translate modeled storage into operational triggers and advised that turning on well pumps for sustained extraction requires staffing and procedural readiness. Staff also discussed a completed pilot of injecting treated drinking water, saying the pilot demonstrated no adverse groundwater impacts to regulators; staff said wider injection or managed recharge can help recovery but that natural recharge often provides more benefit than small-rate injections.

Commissioners asked whether a “GSP light” (a limited groundwater sustainability plan) would be useful; staff recommended studying whether to establish guidelines and to consult other pumpers in the basins. Staff said they will return during spring water-supply planning with proposals on timing and operational thresholds and that any plan should weigh administrative costs, regulatory obligations and potential funding for shared basin management.