There are adequate water supplies for Manteca to allow a 1,592-home development to proceed that would take city urbanization as far south as Sedan Avenue.
That’s the conclusion West Yost Associates Consulting Engineers  made based on a projected City of Manteca population of 127,700 in 2040. Manteca now has a population of 76,000.
The City Council when they met Tuesday at 7 p.m. at the Civic Center, 1001 W. Center St., is being asked to approve the water supply report for the project known as Griffin Park that also would include 65,340 square feet of commercial along what is now South Manteca Road less than a mile north of West Ripon Road.
Identifying and verifying a water source is a state requirement for all developments with 500 or more dwelling units. That is what the West Yost Associates study does.
It assumes, however that the city will be able to tap what water the 334 acres involved are either now using in the form of raw irrigation water from the South San Joaquin Irrigation District as well as the groundwater agricultural operations are now pumping to grow crops on the land that would be replaced by housing.
That may be a big “if”. Richland Communities thought it could automatically rely on what it incorrectly assumed were “water rights” based on the historic use of SSJID irrigation water for vineyards surround the Hat Mansion located at the end of Pillsbury Road when it wanted to annex that land to the city to build 800 homes. What stopped Richland were two things: The property in question was never included in the city’s water masterplan and buying water from SSJID doesn’t mean you are buying the historic water rights that the district secured more than 110 years ago.
There is also no current understanding between SSJID and the city that water used for irrigation would become available automatically for urban use when specific land is developed. Manteca as well as the cities of Tracy and Lathrop contract with SSJID for a set amount of treated water annually. There is additional water that Manteca could pay to secure in a planned second phase of the Nick DeGroot South San Joaquin County Water Treatment Plant operated by SSJID.
The other wildcard is the new state mandate for groundwater basin stability. While a general rule of thumb has an acre of urban uses consuming the same amount of water as an acre that is being farmed, work has yet to start on the actual specific overall water basin management plan that will essentially determine groundwater pumping caps for cities and farms that tap the basin. The study assumes that the safe yield for basin pumping within the city limits for all uses — Manteca Unified, agriculture, Eckert’s Cold Storage as well as uses by residents, businesses and the city — will remain at 13,570 acre feet of groundwater a year.
The danger in that assumption is no one has yet determined the base for groundwater pumping for the entire basin. There is also the question of existing over drafting and how cutbacks on historic pumping by all parties relying on the basin — cities and farmers — will be imposed to stop water levels from dropping further.
Based on current Manteca water supplies, there would be 21,894 acre feet available in 2020 with a surplus of 2,544 acre feet. That surplus would disappear sometime between 2025 and 2028. City studies conducted as part of the 2015 Urban Water Management Plan assumes 7,000 more acre feet, however, will be available by 2025 by expanding the surface water treatment plant or replacing current portable water for irrigation with recycled water from the wastewater treatment plant.
If a dry year occurred in 2020 based on current building trends and uses, Manteca would come up short of meeting its full water needs by 2 percent. The 7,000 additional acre feet would eliminate any single dry year deficits until sometime after 2030. A dry year in 2035 based on water demand and growth would result in a 7 percent shortfall. That water deficit would jump to 12 percent for a single dry year in 2040. With normal annual water supplies there would be a 2,035 acre surplus in 2035 that growth would whittle down to 1,364 in 2040.
Griffin Park would only take 334 acre feet of its annual water use estimated at 887 acre feet at build out from groundwater. The rest will be provided from the city’s allotment from treated surface water from the SSJID plant.
Griffin Park is bordered on the east by Manteca Road/South Main Street to Atherton Drive. It borders parts of Tinnin on the west and one point goes a quarter mile west of Tinnin bordering land Manteca Unified has secured for a possible high school.

To contact Dennis Wyatt, email dwyatt@mantecabulletin.com