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Overdependence and mining of groundwater has caused land to sink in parts of California
photo credit: (Credit: United States Geological Survey
There's likely a vast, unseen reservoir beneath your feet, built by centuries of rain percolating through the earth. Problem is, it's not limitless. In the Sonoma Valley, one of the county's three basins, that invisible supply has sunk 10 feet in two years.
In a webinar held Wednesday evening, officials with the Sonoma Valley Groundwater Sustainability Agency---an entity created under California's recent groundwater law said there are concerns beyond supply and demand. Arsenic, nitrate and boron have been detected., along with chloride, a marker for saltwater. Since 1970, officials estimate, the aquifer has lost 14,000 acre-feet, as wells pump faster than nature can replenish. That's equivalent to more than a tenth of Lake Mendocino's capacity. And also prompting salty, brackish water from San Francisco Bay, to seep in.
For now, officials say keeping close watch is sufficient, but mandatory water restrictions could come if the land above starts sinking--a process called subsidence that's rife in the Central Valley. There is a solution. Mimicking nature's process by redirecting storm run off and flood waters.
"In the winter, when grape vines are dormant, spreading that water out over agricultural fields and letting it infiltrate into the ground is one way of achieving recharge as well," said Marcus Trotta, a hydrologist with the agency.
But the area's complex geological formations make that less than straightforward.
"In our basins, some of the initial studies that we've done on stormwater recharge have proven, it is a bit challenging to find the best locations for recharging relatively large amounts of water, we have a lot of clay in our aquifers and in shallow soils that can inhibit and limit the amount that can be recharged," Trotta said. 
Aside from conservation, one of the few possible realistic solutions involves better storm forecasting, allowing water levels behind dams to rise higher. Currently, authorities release large volumes from reservoirs ahead of incoming storms---assuring capacity to absorb floodwaters. Sometimes though, those predicted storms fail to materialize.
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