1. Is groundwater pumped for residential basement construction? Yes. Very large amounts of groundwater from the shallow surface aquifer are pumped to build basements when below ground soils are saturated to provide dry soils using a commercial-scale construction process termed “dewatering.” This technique is now being permitted for constructing residential basements in Palo Alto at a rapidly increasing rate, from an average of five (5) per year (2006 – 2008) to at least 14 this year. Dewatering is used only at those sites with water saturated soils; it is not used at drier sites.
2. Why should I care about groundwater pumping for basement construction? Aquifers and groundwater are a community and public trust resource that, although unseen, play an important role literally supporting structures and infrastructure, draining storm water, and storing and providing moisture for our canopy and plants.
3. What are the effects of removing groundwater? Removing groundwater has a variety of impacts. The forces exerted by groundwater literally support the ground, structures and infrastructure and through capillary action, provide water to our trees. The shallow surface aquifer pressure increases the recharge of the deeper aquifer which is used for irrigation and on which Palo Alto relies for emergency water.
Lowering the water table locally causes ground settling. This settling may not be uniform across structures, which may then develop either tight doors or windows, or permanent cracks in foundations, walls or masonry. Settling of even less than an inch is adequate to cause permanent structural damage. Lowering the water table below the seasonal normal fluctuation can cause irreversible compression of the soil (hysteretic soil compaction).
4. What are the effects of lowering the water table on vegetation? Water available for trees and plants is reduced. Soils wick water up, much like sponges, resulting in increased soil moisture several feet above the water table, well into the root zones of trees in much of the area in which dewatering is occurring.
5. What are the impacts of these basements after construction? Both the City of Palo Alto and the Santa Clara Valley Water District provide incentives to install permeable pavement to reduce the amount of storm water entering storm drains and instead soak into the ground, thereby reducing flood risks and recharging aquifers. Basements displace soils that would otherwise be available to absorb rain water, increasing the probability that rain water will flow into the storm drains.
Much of Palo Alto is known to have covered gravel beds from former creek beds. Basements are dams in the unseen rivers that flows through the soils, gravel beds and aquifer beneath Palo Alto. Water needs to flow around these basements. If water cannot flow through the soil fast enough, it will flow above the soil, into the storm drain system, and if the storm drain capacity is exceeded, will flood our streets and properties. The water table/water pressure surrounding a basement is locally higher, in the same manner as water in a flowing river is higher as it flows around an obstacle. The locally higher water table increases the risk that basements in neighboring properties will flood.
6. What can I do if my property is damaged by ground settling caused by groundwater pumping? You’re on your own. You must resolve any damage claims directly with the party that caused the damage. The City will neither order the dewatering to stop nor help you with any damage claims. You may sue. In that case it will be necessary for you to prove that the specific dewatering operation was the cause of the damages, and most likely pay attorney’s fees, which might be reimbursed if you obtain a judgement in your favor.
7. How much water is pumped? In total, it is estimated that 126 million gallons (16,000,000 ft3) of groundwater has or will be pumped out for the construction of 14 basements in Palo Alto in 2015 alone. This is enough to cover a football field 275 feet deep, or fill 50,400 water tank (2,500 gallon) trucks, or provide enough water for 18,000 average Palo Alto residences for the entire month of July, 2015 (equivalent to 40-50% of the state-mandated water conservation goal for all single family residences in Palo Alto for a year) or lower the aquifer by more than 1 foot over an area of 1 square mile. This estimate is based upon the midpoint of City’s estimate of 8 – 10 million gallons (1.2 million cubic feet) per basement. For some basements, more than 20 million gallons is pumped. The amount of water being pumped out is not metered.
8. Where is groundwater pumping occurring? Most of the residential dewatering projects are concentrated in an area of approximately 1 square mile bounded by Webster Street, Louis Road, Colorado Avenue and Channing Avenue, although two are near Middlefield Road further south.
9. From where is the water pumped? Groundwater is typically pumped from 15 to 25 feet below grade, and the groundwater table locally lowered about 2 feet below the bottom of the basement in the area to be excavated. The “bottom” of the basement is generally 10 – 20 feet below grade; some are below sea level. Groundwater is typically pumped at a rate of 50 – 100 gallons per minute continuously for 3 – 6 months.
10. How much do government agencies collect in fees and permits for construction dewatering? The City of Palo charges approximately $710 for a dewatering permit for 6 months. There is no usage-based fee or assessment for discharging the groundwater pumped out for construction into the storm drain. The total cost to the developer for removing this resource from our aquifer is about $710.
11. How much do residents pay for equivalent water disposal in the storm drain? The Storm Drain Fee for 1 equivalent residential unit (ERU) is $12.63 / month ($151.56 / year). A single dewatering site will dump as much water down the storm drains as the city estimates would go into the storm drains from 480 residences (1 ERU / residence) in a year. Developers are not currently required to pay any additional fees to compensate for the heavy use of the city’s storm drains, even though a “fair share” payment would be $72,748 for a typical basement.
12. How much would Santa Clara Valley Water District charge for a resident to pump non-potable groundwater for irrigation? Santa Clara Valley Water District charges about $600 / acre-foot (43,560 ft3) for a permit to pump groundwater. For the amount of water pumped for a typical basement, the cost would be approximately $16,500. However, a specific exemption from fees is provided for construction dewatering in the shallow aquifer. The fee to builders is zero.
13. Is this groundwater pumping sustainable? The amount of water removed from the aquifer in 2015 is roughly the same as would be available to recharge the aquifer from average (not drought) rainfall for one year, after allowing for runoff and evaporation over an area of 1 square mile.
14. What happens to the pumped groundwater? Approximately 99% is dumped into the storm drains, which then flows to the Bay.
15. Isn’t this pumped water available for irrigation for free? The City requires faucets with hose connections and fill stations for water tank trunks at each dewatering site. There are no requirements for the actual use of the water or the pressure supplied to hose connections for neighborhood use; City policy effectively condones wasting water. In practice, the water is not substantially used. Although the water is of high quality and usable, it is wasted.
16. How and when is the shallow surface groundwater replenished? Primarily from rain and landscape irrigation. Precise recharge rates are not known, but it is believed to be in the range of months to years.
17. Doesn’t the water flow to the Bay anyway, and therefore doesn’t pumping the groundwater improve the environment of the Bay? The aquifer and soils have an important role in transporting storm water to the Bay; more water flows in the unseen river beneath our homes to the Bay over the course of a year than down the creeks. However, during the summer, there is little flow in the aquifer (there almost no flow in creeks either). Dewatering locally lowers the water table below its normal historical low level, and in some cases below sea level, much as pumping water from a lake could lower the lake level below the outlet level.
18. Hasn’t the City already carefully studied dewatering? The City commissioned a study in 2004, and City staff reviewed the study in 2008 after receiving citizen complaints. Not only are several important issues especially on localized effects not addressed, there are important differences between the current situation and the time of the original study. Existing City dewatering policy does not anticipate the current number or water volume of dewatering activities within the City. Despite acknowledgment by the study that there will be “temporary and local effects,” the study does not meaningfully address localized impacts, including ground settling, reduced soil moisture for trees, flood risks and storm water management, public compensation for the use of the water, or public policy in an era of climate change. Furthermore, it is incorrectly assumed that short-term effects will not cause permanent damage.
19. From where did this information come? All information in this document is either provided by or derived from the City of Palo Alto, the Santa Clara Valley Water District, USGS topographical maps, the US National Oceanographic and Atmospheric Administration, and materials provided by degreed professionals in soil sciences or hydrology, including documents in the Public Record for the City of Palo Alto.
20. What is the objective of Save Palo Alto’s Groundwater? Save Palo Alto’s Groundwater is requesting that the City of Palo Alto require that developers/owners building basements in areas with high water tables use best construction practices such as limiting the duration and depth of dewatering and/or using “silent piling” and other techniques to minimize the amount of water extracted during this construction. Furthermore, we request that all water extracted be used, for example, for irrigation, for fire hydrants and other non-potable uses and/or recharged back to the aquifer.
21. Is a more detailed document available? Yes, please check out our White Paper.
22. How do I obtain further information or help with this effort? Send an e-mail with your name and contact information to [email protected]