What policies should the City set regarding the discharge and loss of water (as well land settlement problems in neighboring properties) when basements are being built?
Dewatering is an issue of an individual property owner consuming a disproportionate amount of a community resource to the detriment of the larger community. For example, the dewatering for a basement at 736 Garland pumped out 38.8 million gallons (Staff report). That is as much as the average annual usage of 400 residences. Or about the average annual irrigation usage of 1300 residences. If you put that 119 acre-feet of water in a column over that 0.24 acre property, it would be almost 500 feet high. Or almost 400 years of rainfall on that property at our long-term average of roughly 15-inch annually (for visualization: not all rainfall goes into the aquifer).
Development has severely curtailed natural processes for recharging the groundwater: There is far less land where rain can soak in, and percolation from creeks has been greatly decreased (except for San Francisquito Creek, the creeks are in concrete channels from El Camino to the Bay). The Pulgas fault that runs roughly under Foothill Expressway and Junipero Sera Blvd has folds that divert groundwater from higher up into the deep aquifers. The shallow aquifer under the developed portion of Palo Alto is dependent on the rain that falls on it.
The oak trees that make up so much of our canopy have a dual root system: shallow roots to harvest rainfall and a long taproot that reaches into the shallow aquifer. Excessive pumping will lower the water table to the extent that it is out of reach of those trees and thus making them increasingly vulnerable to drought.
Local experience is that subsidence (settlement) from excessive pumping of water is often permanent—Alviso is infamous for having sunk 13 feet in the early 1900s.
How much pumping is too much? I don't know. I am not a geologist (but some of my friends are). The geology under Palo Alto is complex – there two (inactive) earthquake faults in the block I live on, and two more faults further along the street (Matadero). Water moves at different rates in the aquifer—in some places it may be only a few feet per year, in others much faster. Land settlement near pumping sites is not just a problem for those property owners, but a leading indicator of problems for the larger community.
What policies would I support? It is the moral obligation of the person wishing to have a basement to not adversely impact the immediate neighbors or the larger area. Since this hasn't been enough, the City needs to require and enforce this principle. This includes accounting for the risk of subsidence in the immediate vicinity and effect of lowering the water table on vegetation and the cumulative effect of excessive removal of water from the aquifer.
I would support impact/development fees for basement building and at high water table locations that the contractor is required to pump water into a cistern or container to then replenish the aquifers or to be used by the city to water city trees.
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