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Will Walt Ranch hurt groundwater supply?

Will Walt Ranch hurt groundwater supply?

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Is it possible the proposed vineyard at the Walt Ranch could negatively impact the groundwater in the severely depleted Coombsville/MST basin aquifer?

The United States Geographical Survey discovered in 2001 the Combsville/MST aquifer is recharged by rainfall runoff from the eastern hills that border it. John Muir said “when we try to pick out anything by itself we find it hitched to everything else in the universe.”

Information concerning the status of groundwater was presented to the residents of Coombsville in a meeting at Silverado Middle School in early 2000. USGS studied the MST groundwater basin running from Napa State Hospital to Silverado Country Club to determine why the aquifer is not recharging adequately.

USGS determined the MST basin includes three aquifers fed by runoff from the eastern hills, starting at Howell Mountain and running to Skyline Park. The water percolates into the aquifer by underground fissures, strata and springs fed by aboveground streams.

The USGS reported that, due to the draw of thousands of wells from residents, vineyards, wineries and two golf courses, the MST basin aquifer is reaching critical overdraft.

The valley floor has lost much of its ability to absorb water. In order for water to percolate into the aquifers, there needs to be large areas of undeveloped land. Due to urban development and agriculture, which shunts water into holding ponds, drains and sewers, this ability has been compromised.

Consistent with the findings of USGS, aboveground streams that recharge valley floor aquifiers with runoff from the hills will probably be severely impacted by the proposed vineyard development on the Walt Ranch.

As with other hillside vineyards, in the Walt development, rainwater will be collected in reservoirs by rainfall and underground drains, for the purpose of watering vines. Underground drains don’t allow water to percolate.

In moderate to severe rainfall, full reservoirs overflow into streams, creeks and rivulets, where the runoff overwhelms existing waterways, cascading down the hills, scouring streams, causing erosion, and sedimentation buildup in dams and rivers.

To make matters worse, on the Walt development, there will be huge agricultural wells further burdening the aquifer. Hillside vineyard development impacts groundwater under the valley floor in two ways: by not allowing rainwater to percolate and by drawing water out before it can reach the valley aquifers.

Intact watersheds absorb water, then release it relatively slowly. Water is held back by the vegetation canopy and root system. The burrows of underground rodent populations add to the sponge effect of undeveloped land. To protect the existing aquifers there needs to be a moratorium on vineyards and related development

As there is little undeveloped land on the valley floor, the burden on the valley’s aquifers will increase as more hillside vineyards are established.

Susan Todd


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