Napa County’s annual groundwater inspection finds monster storms in winter 2017 helped boost the supply in Napa Valley’s aquifer to the highest level in a decade.
Water from the big rains soaked into what amounts to a subterranean reservoir. A study by Luhdorff & Scalmanini calculated the Napa Valley subbasin alluvial aquifer in 2017 held 219,000 acre-feet of water, compared to a 20-year low of 191,000 acre-feet in 2014. One acre-foot is the equivalent of about 326,000 gallons.
That 219,000-acre feet is the equivalent of one-seventh of the water in a full Lake Berryessa, 71 billion gallons, enough to fill 107,000 Olympic-size swimming pools. Water depth ranged from a few feet to 15 feet in most places beneath the Napa Valley floor in the heart of wine country.
“It’s a quite full groundwater basin,” consultant Vicki Kretsinger Grabert told the Napa County Board of Supervisors on Tuesday.
That’s not due to the big 2017 storms alone. The county’s 2016 groundwater checkup also found the subbasin extending underneath the 45,928-acre valley from Calistoga to Napa to be just about full.
Rural wineries, vineyards, businesses and homes sink wells into the Napa Valley subbasin. One goal of the annual groundwater study done for Napa County is to keep these straws from sucking out more water from the subbasin than goes in.
Napa County can pump out only a fraction of that 219,000 acre-feet of groundwater in any one year without risking such consequences as soil subsidence. The 2017 groundwater study estimates the annual sustainable yield at 17,000 acre-feet to 20,000 acre-feet.
Groundwater use in 2017 came to 15,831 acre-feet, which is within the sustainable yield, the study said. During the 2012-2015 drought, groundwater use averaged 18,000 acre-feet annually.
Napa Valley farms, cities, rural homes and businesses used a total of 34,142 acre-feet of water from all sources in 2017, the study said. Of that, 46 percent came from groundwater, 37 percent came from reservoirs and waterways, 13 percent came from the State Water Project and 3 percent came from recycled water.
When all of the groundwater recharge and well pumping had been calculated for 2017, the ledger was 4,470 acre-feet on the plus side, the study said. In some years, people have used more groundwater than soaked into the sub-basin.
Napa County keeps track of groundwater levels in the Napa Valley sub-basin with 61 monitoring wells.
Still, not all parts of the Napa Valley sub-basin are equal. The 1,960-acre area at Petra Drive a few miles northeast of the city of Napa has seen groundwater declines over the years. Water levels in two wells studied dropped between 20 feet and 30 feet before stabilizing in 2009, the study said.
This area is different from other parts of the valley due to such factors as complex geology, Kretsinger Grabert said. The East Napa fault that parallels the Napa River and the Soda Creek fault affect groundwater levels. A significant amount of groundwater goes into Napa River and to San Pablo Bay.
The Board of Supervisors in 2017 declared the Petra Drive area a special groundwater management area that will be treated differently than the rest of the valley. Strategies could include increased conservation.
The Coombsville area – also called the Milliken-Sarco-Tulucay area – also faces groundwater problems, with groundwater levels in some areas having fallen up to 250 feet since the 1950s. The study said sinking groundwater levels have stabilized at wells since 2009.
Longtime environmentalist Chris Malan said the study failed to address such problems as salt water intrusion and water quality decline in the southeast county. She said people in the Milliken-Sarco-Tulucay area have had wells fail over the last few years.
Historically, the Napa Valley aquifer level came to the surface of the land, Malan said. The study doesn’t consider that water levels based on that historic baseline are down 15 feet.
“There are definitely problems in our aquifer,” Malan told the Board of Supervisors on Tuesday. “It’s too rosy of a picture. It’s not the full-true picture. We should have it.”
Jim Lincoln of the Napa County Farm Bureau also addressed the Board.
“We’re not afraid of good information,” he said. “If the groundwater levels are going down, we want to know that. It looks like they’re actually going up.”
Supervisor Alfredo Pedroza agreed that the news is good.
“It’s rosy, but it’s factual and it’s based on science,” Pedroza said.