All over Santa Clara County, driving along the freeway or the expressway, you see ponds.

When I was very young, I wondered why lakes and streams didn’t just drain away into the ground, since they weren’t lined with concrete, like swimming pools. (I grew up in arid Southern California, where lakes and streams were somewhat of an exotic rarity.) Well, these ponds are designed to do exactly that.

They’re percolation ponds. Sited on gravelly soils, they allow water to seep down to recharge our groundwater sub-basins.

While the Water District has 10 surface reservoirs in the county, their storage capacity is dwarfed by that of the three groundwater sub-basins. Roughly half of the water that comes out when I turn on the faucet is rainwater that fell in a local watershed and made its way underground, from where it was pumped into the water system.

Groundwater is a multi-faceted issue. Our vast groundwater storage capacities give us an extra cushion even when the surface reservoirs look depleted. Percolation is a natural water purifier. But excessive pumping of groundwater causes the actual ground to sink, or subside. And our water supply is exquisitely sensitive to contamination, both from leaking underground storage tanks, and as well as from surface contamination. The aquifer is Coyote Valley, a major recharge zone, is very close to the surface.

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Working to build a local, sustainable food system in San José