Manteca’s ability to support existing development by resorting to just using groundwater is likely to end by the time 2017 rolls around.
When that happens, the city can no longer assume they could maintain current drought-level uses without using treated surface water delivered by the South San Joaquin Irrigation District. It means Manteca’s economic fate and its ability to keep water flowing through faucets will rely more and more on the SSJID’s ability to protect its senior adjudicated water rights from efforts by the state to commander it for unimpaired water flows in a bid to boost fish counts.
The 2014 Manteca Urban Water Master Plan recently adopted by the City Council underscores that reality.
With the Eastern San Joaquin Ground Water Basin (ESJGWB) that municipal wells tap into dropping 1.7 feet a year, hydrology experts hired by the city have set the safe yield of water that Manteca can pump in any given year at 13,570 acre feet. An acre foot is the amount of water that would cover a standard football field 12 inches deep.
Manteca actually exceeded the safe yield threshold of 13,570 acre feet when they pumped 14,900 acre feet in 2004. That was a year before Manteca started taking water from the Nick DeGroot South County Surface Water Treatment Plant. Since then Manteca groundwater pumping dropped down to 8,463 acre feet in 2010 and 7,429 acre feet in 2015 as the city now gets 52 percent of its water from SSJID in any given year.
The surface water is one reason why the portion of the ESJGWB that is directly under Manteca is currently not dropping. In Ripon, where the city relies 100 percent on groundwater, the aquifer has been dropping a foot on an annual basis for the past three years hence the higher urgency to conserve water in the community of 15,000. Ripon next month goes to once a week irrigation of landscaping while Manteca plans to remain at three times a week through the winter. Ultimately Ripon also wants to tap into SSJID surface water.
Based on water use projections against the safe annual groundwater pumping yield of 13,570 acre feet and the maximum 11,500 acre feet from the first phase of the water treatment, the city would need to have the second phase of the treatment plant located on Dodds Road 16 miles northwest of Manteca near Woodward Reservoir in place by 2031. The second phase would give Manteca 7,000 acre feet of additional water for a combined capacity of 32,250 acre feet of water in a normal year.
The city anticipates using the full current phase one allotment of 11,500 acre feet by 2020 and then working up toward the groundwater safe yield rate until the second phase is in place. The safe yield is likely to be cast in stone by the state-mandated groundwater sustainability law that restricts pumping to the amount that is replenished each year in an aquifer. That means additional groundwater could not be taken out to support growth unless it is replaced on an annual basis.
by the year 2040
Meanwhile projections to ramp up the use of recycled water to the point in 2040 when 2,240 acre feet of such water is excepted to be used for public landscaping and the golf course will mean Manteca will need 27,530 acre feet of water from the surface treatment plant and groundwater to serve anticipated growth.
That is enough water to meet the needs of 127,740 residents in 2040 that Manteca is expected to have based on historic annual growth rates of 2.3 percent. That is 52,740 more residents than the current population.
When the second phase is finished at the treatment plant and coupled with groundwater Manteca would have a 4,720 acre foot cushion. Although not stated in the report given it looks out only 25 years, that cushion should it all be used based on per capita annual water consumption numbers in the report could support up to another 20,000 residents for a population of just under 150,000.
Given the state mandate on groundwater pumping to essentially take no more from a basin than is replenished in a given year, Manteca would only have three other ways to further increase water availability for growth: Harnessing stormwater in some manner, using treated wastewater to recharge aquifers or reduce portable water for irrigation, or more effective water conservation.
If Manteca maintains per capita usage that they had in 2015 of 139 gallons a day, the city’s water use forecast will be fairly conservative given the plan us built on a state target of 179 gallons of water a day being used by every Manteca residents on a per capita basis. Last year’s use was a 28 percent reduction on per capita consumption in 2013. By comparison Manteca’s average daily use between 2003 and 2007 was 221 gallons per capita.
Manteca has 15 potable water wells and 31 irrigation wells that draw from aquifers that extend down to 600 feet. At 600 feet and lower there are more water tables but they have salt water.