A hidden reservoir helps keep dishes clean, yards green, and flush toilets in Manteca, Ripon, and Lathrop.

The reservoir — a giant underground aquifer fed primarily by seepage from along the San Joaquin and Stanislaus rivers — is poised to become the source of last resort for farmers and even cities facing curtailment of surface water deliveries.

That is why water experts anticipate a minimum 20-foot drop in the water table as 2014 unfolds. That is how far the water table dropped during the 1976-77 drought when there were far less crops and less people in the South County.

As the river flow drops significantly due to the Sierra snowpack being at a historic low dating back to  the 1850s, three things are expected to happen.

• Water levels will drop.

• Underground aquifer salinity will increase since there will be less water to keep the much saltier water in aquifers beneath the Delta from pushing eastward. Salt has already been detected in wells as far east as Ripon. Salinity has also jumped in Lathrop’s city wells.

• New wells will have to go even deeper adding significantly to power bills required to run pumps to extract water. Current PG&E costs to bring an acre foot of water to the surface runs between $100 and $200.

The underground water level is 10 feet deep nearest the river. It drops to 20 feet beneath Manteca, 80 feet in Escalon and 200 feet in Lockeford.

And while the dropping water levels in South San Joaquin County are alarming they are nothing compared to the rest of the San Joaquin Valley. A report by the U.S. Geological Survey indicated that the valley lost 60 million acre-feet of groundwater since 1961. That is the equivalent of filling New Melones Reservoir 25 times.

That study also noted that not only is the Central Valley the nation’s largest farming region but it is also the largest single area of ground water pumping. The study indicated about 20 percent of all ground water pumped in the United States is taken from beneath the Central Valley.

That has led to land subsidence issues throughout the valley. The federal report noted land has dropped 29 feet in areas near Mendota. Typically, land does not bounce back from subsidence. At the same time, the compacted water table often isn’t able to effectively store much water.

The drought, now in its third year, is expected to see higher concentration of salt in wells throughout the South County. For urban domestic uses, that isn’t expected to become a major issue at least on the consumption end. But it can pose serious problems with the treatment of wastewater as it would increase the salinity level in water that goes through wastewater treatment plants. The state has salt standards that must be met before treated water can be returned to the river.

While higher salt levels in domestic water aren’t classified by the Environmental Protection Agency as a contaminant that must be kept below certain levels to protect human health, the agency recommends that sodium in domestic water sources shouldn’t exceed 20 milligrams per liter. The biggest problem is that high levels of salt in irrigation water can render soil poisonous for agricultural crops and other vegetation such as ornament trees, shrubs and grass.

Rising salt levels in the Division 9 service area south of Manteca and west of Ripon prompted the South San Joaquin Irrigation District to invest $14 million in a state-of-the-art closed delivery system. It reduced surface irrigation water use substantially to allow farmers who were on wells to connect to the SSJID system. The reason they needed to switch is the well water they were putting on orchards was slowly starting to kill the trees.

There is a silver lining of sorts to the dropping water table. The SSJID will not have to run the roughly three dozen wells they operate as much. Most of those wells are located south of Manteca and west of Ripon to address high water table issues.

The high water table is a potential threat to everything from swimming pools and water,  sewer and storm system pipes to building foundations and even streets.

Without the district’s dewatering efforts, SSJID General Manager Jeff Shields noted “swimming pools and pipes would be popping up.”

And while Friday’s big storm isn’t expected to put a major dent in the drought, it did accomplish one thing. The National Weather Service indicated the current weather year that started Oct. 1 is no longer the driest on record. It’s now the second driest.