Groundwater Management

In September 2014, Governor Jerry Brown signed The Sustainable Groundwater Management Act of 2014 (SGMA). SGMA sets the framework for statewide sustainable groundwater management by local agencies. SGMA requires, among other things, the formation of Groundwater Sustainability Agencies (GSA) and the preparation of Groundwater Sustainability Plans (GSP).

In 2017, member agencies of the Stanislaus and Tuolumne Rivers Groundwater Basin Association (STRGBA) - MID, City of Modesto, City of Oakdale, Oakdale Irrigation District, City of Riverbank, City of Waterford and Stanislaus County – formed as a GSA. STRGBA GSA has the authority and responsibility to manage the groundwater basin. Many groundwater basins in the San Joaquin Valley have experienced heavy groundwater pumping – especially during droughts. Several are now in a condition of critical overdraft. The Modesto Subbasin isn't considered to be critically overdrafted, but since most of the cities within our basin rely solely on groundwater, we're considered a high-priority basin. Due to that designation, SGMA requires that we adopt and begin implementation of a GSP by January 31, 2022.

STRGBA GSA obtained a $1 million grant from the Department of Water Resources (DWR) and hired Todd Groundwater, Inc. to prepare a GSP for the basin. The GSP will help to identify areas within the basin that need better groundwater monitoring and provide a path to preventing undesirable results over the next 50 years.

For updates regarding groundwater activities and the GSP, please visit the STRGBA website.

The loss of recharge that has occurred due to the conversion from agricultural to urban land use (almost 28,000 acres over the last 60 years) and the ensuing increase in urban demand (met by groundwater) resulted in a continually expanding and deepening groundwater “cone of depression” in the Modesto urban area.

That adverse effect, in part, led to the planning and ultimate construction of the Modesto Regional Water Treatment Plant (MRWTP), owned and operated by MID. Phase 1 of the MRWTP, completed in 1994, supplies more than 30 million gallons per day (mgd) to the City of Modesto (roughly 50% of the annual average demand). The resulting relaxation of groundwater withdrawals by the City allowed groundwater levels to recover by more than 40 feet in the local urban area.

Phase 2 of the MRWTP, completed in 2016, expanded the capacity of the plant to 60 mgd.

MID owns and operates more than 100 water wells. About half of these are used for water table control on the west side of the MID irrigation service area where there is a high water table. Without being able to control the elevation of the water table via pumping from shallow wells the soil conditions would be waterlogged and crops would not be able to be grown.

The remaining 50 or so wells are deeper production wells that are used to supplement the surface water supply in the canal distribution system. Most of these deeper wells are located immediately adjacent to the canal system in MID-owned rights-of-way.

MID pumps on an average annual basis about 38,000 acre-feet.

These facilities are replaced and/or upgraded on an ongoing basis at the rate of three replacements/upgrades per year.

MID as a part of STRGBA participated in several programs and projects over the years to improve knowledge and operation of the subbasin’s groundwater. These include but are not limited to:

• Integrated Regional Groundwater Management Plan
  • Completed in 2005, this plan encouraged all local agencies to work together to protect the groundwater quantity and quality by developing and implementing Basin Management Objectives. This plan was expanded and further developed as part of the Groundwater Sustainability Plan (GSP) process completed by STRGBA GSA.
• Hydrogeologic Characterization of the Modesto Area
  • The U.S. Geological Survey (USGS) conducted a thorough, multi-phased study of the Modesto Groundwater Sub-Basin in 2004 titled “Hydrogeologic Characterization of the Modesto Area, San Joaquin Valley, California”. One of the key findings of this report is that the primary means of recharge in the Modesto Subbasin is via the application of surface water for agricultural irrigation purposes.
• Wellfield Optimization Project
  • A separate study took a look at how MID can get the greatest benefit out of its own water wells in concert with surface water, under both dry and wet conditions. This resulted in the Wellfield Optimization Decision Support System and was paid for under a grant that MID received under the umbrella of the STRGBA.
• Recharge Mapping
  • Another study that was performed for STRGBA was the mapping of the zones of recharge within the Modesto Subbasin. The resulting maps show that the areas with the greatest recharge occur primarily to the north and east of the City of Modesto, and in the area surrounding Modesto Reservoir. This study was performed under the guidance of DWR via a local assistance program and at no cost to any of the agency members of STRGBA.

These documents and additional groundwater management information can be found on the STRGBA website.

Water that is found beneath the surface of the Earth in saturated rock formations is called groundwater. It is a very important and valuable water supply for much of the world, and particularly in California's Central Valley.

Groundwater is a significant component of our overall water supply. In fact, many of the cities in the Central Valley rely 100% on groundwater for their drinking water supply, as did the City of Modesto for most of its history.

According to information contained in California Department of Water Resources Bulletin 118, the estimated amount of groundwater stored within the Modesto Subbasin, in just 100 feet of saturated thickness, is 5 million acre-feet or 2.5 times greater than the maximum storage capacity of Don Pedro Reservoir. The Modesto Subbasin is defined as that area of land lying between the Stanislaus River on the north, the Tuolumne River on the south, the Sierra Nevada Mountain foothills on the east, and the San Joaquin River on the west.

MID water service boundary is entirely located within the Modesto Subbasin. The primary source of recharge (60%) in the basin occurs through agricultural irrigation using surface water supplied by the MID. Other sources of recharge in the basin occur via rainfall and seepage from surface water bodies such as Modesto Reservoir. Some reaches of the Tuolumne River are supplemented with groundwater inflows.