GSP002_GSP_PlanArea.jpg

Map of the boundaries of Tracy Subbasin, a groundwater basin located in the western part of the San Joaquin Valley.

The status of the groundwater aquifer around Tracy will be up for review in the coming months, with an online forum next week the public’s chance to talk about how use of that aquifer could affect local groundwater wells in and around Tracy.

The state of California’s Department of Water Resources requires local jurisdictions to draft groundwater sustainability plans (GSPs) as a way of predicting if aquifers are in danger of becoming overdrafted, which could lead to wells going dry or the ground on top of these aquifers sinking. Water management agencies in parts of the state where aquifers were considered to be “critically overdrafted” had to have their plans to the state by the end of January 2020.

The Tracy Subbasin – a 345,000-acre area with the San Joaquin River on the eastern boundary and Old River to the west, extending north to Mandeville Island – includes the City of Tracy, is considered a medium priority “non-critically overdrafted” basin, meaning that the groundwater sustainability plan for the subbasin must be completed by the end of January 2022.

On July 20 the Tracy City Council approved its notice of intent to adopt a groundwater sustainability plan. It effectively serves as notice that the city will adopt the plan within 90 days.

Tracy is one of seven agencies that manage water use within the Tracy Subbasin. The Draft Tracy Subbasin Groundwater Sustainability Plan provides details on how groundwater fits into the water use plans for all seven agencies within the basin.

The city of Tracy serves water users over about 15,000 acres. Of the more than 20,000 acre-feet of water the city was estimated to have used in 2020, most comes from surface water supplies, such as water from the Stanislaus River through the South San Joaquin Irrigation District and the Sacramento-San Joaquin Delta through the U.S. Bureau of Reclamation.

Though the city is expected to nearly double its water usage beyond 2040, it expects to continue filling its need for water through surface water supplies, as well as increased use of recycled water.

About 767 acre-feet used in Tracy was estimated to come from groundwater in 2020, including wells that go anywhere from 155 feet to more than 1,000 feet deep. That amount is also expected to nearly double to more than 1,400 acre-feet at the city’s buildout beyond 2040.

The city of Tracy and most of the rest of the lower one-third of the 345,000-acre subbasin south of Old River is where most of the subbasin’s domestic wells are located. Of the 2,405 production wells documented in the plan, 1,958 are domestic wells, 373 are for agriculture and 74 are municipal wells.

Semi-rural areas next to Tracy show the highest concentration of domestic wells with 10 or more wells per-square-acre in many areas surrounding Tracy. A couple areas, including a section southeast of MacArthur Drive and Valpico Road, and another section between Interstate 205 and Von Sosten Road northwest of town, show 141 wells per-square mile in developed areas just outside of city limits, while a section of the Larch-Clover neighborhood shows 44 wells in a one-square-mile section bisected by Interstate 205.

The other six jurisdictions included in the subbasin are:

• County of San Joaquin. The county’s groundwater sustainability agency (GSA) cover all of the areas not covered by any of the other agencies. It is about two-thirds of the 345,000-acre subbasin made up of delta islands and waterway, with few groundwater wells.

• City of Lathrop, with about 25,000 residents, accounts for about 14,000 acres and uses and estimated 9,000 acre-feet per-year. Lathrop’s wells could produce more than 7,000 acre-feet of groundwater, though the city is expected to reduce that demand by using surface water provided by the South San Joaquin Irrigation District water.

• Stewart Tract is 14,000 acres bordered by Old River and Paradise Cut. It includes the River Islands Development, which will get its potable water from the city of Lathrop.

• Byron-Bethany Irrigation District covers about 29,000 acres at the junction of the San Joaquin, Alameda and Contra Costa county lines, and includes the community of Mountain House. The district gets its water from Old River through pre-1914 water rights, and also contracts for surface water through the state’s Department of Water Resources.

• Banta-Carbona Irrigation District serves about 18,000 acres of farmland between the city of Tracy’s southeastern border and the Stanislaus County Line. The district has pre-1914 rights to draw water from the San Joaquin River, and also contracts for surface water with the U.S. Bureau of Reclamation. It also includes agriculture production wells, which are put into service during drought years.

• West Side Irrigation District covers about 6,800 acres within the city of Tracy sphere of influence, and gets water from 1916 rights to pull water from Old River, with some groundwater wells in the agricultural area.

• Contact Bob Brownne at brownne@tracypress.com, or call 209-830-4227.

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