Recently we heard, “Could massive dewatering in San Francisco be causing the Millennium Tower to sink and tilt? If that’s part of the problem why isn’t it a problem here?” After all, Palo Alto has quite a bit of dewatering going on.
It has happened to us: When Palo Alto was founded in 1894, 100% of its water supply was groundwater. Because of groundwater overdraft, Palo Alto subsided 2 – 4 feet in the 1960s.
But, there are two reasons why subsidence stopped: Palo Alto started getting 100% of its potable water from the Hetch Hetchy and the Santa Clara Valley Water District (the groundwater under our feet is part of the Santa Clara Water Basin) imports water to recharge the Basin. In 2015 it imported 54,000 acre feet for groundwater recharge of the Basin (not only Palo Alto). Paid for, of course, with taxpayers’ money.
We are throwing away that water and then buying water to recharge the aquifer. We don’t have area wide subsidence as in the 1960s because of water recharge but there’s a financial and environmental cost. Environmental? Among other impacts, we have a net transfer of water from the earth to the sea. That groundwater that ends up in the Bay contributes to sea level rise.
So why is dewatering permitted? We have heard several arguments for dewatering – our responses are in bold:
- People have a right to do with their property as they wish. Yes, people have a right to do with their property as they wish as long as it’s their property and doesn’t adversely impact others. Groundwater, however, is a community resource. On average, less than 0.2% of the groundwater that is pumped for basement construction comes from the site being dewatered, the rest comes from neighboring properties. People next to dewatering sites have reported impacts such as uneven floors and cracked foundations and windows.
- This water would flow into the Bay anyway. Not necessarily. The shallow groundwater pumped out during dewatering, normally recharges and is recharged by our creeks, streams, wetlands and the deeper aquifer. Removing groundwater from the ground disturbs this balance and can cause among other problems, local settling, subsidence and sinkholes. Reducing the freshwater flow through the soils increases salt water intrusion.
- Shallow groundwater, where dewatering occurs, is “nuisance” water and has no connection with the deeper aquifer which is the designated source of our emergency groundwater. For many years conventional wisdom has been that in Palo Alto the shallow and deeper aquifer levels are separated by a confining layer from around El Camino Real to the Bay. A recent study of the data from hundreds of wells by the consulting firm, Todd Groundwater, found that such confining layer does not exist as a solid unbroken “floor”. Instead, there are many smaller aquitards throughout the area. The bottom line is that the shallow and the deeper aquifers levels are connected – indeed is it water from the shallow aquifer that recharges the deeper aquifer levels. Thus there is no valid reason not to value ALL our groundwater.
Fortunately, the City seems to be moving in the right direction and we expect City Staff will propose better rules for construction in areas with high water tables this December (or maybe late November) to City Council. We must remain vigilant that the proposals are effective in reducing water waste and are enforceable. We request that you keep writing to City Council with your concerns so that they are aware that this issue is of importance to residents. Thank you for your support and advocacy!