California’s drought has led some Palo Alto residents to press for a ban on the dumping of millions of gallons of groundwater into storm drains, a result of some basement construction.
Dewatering — a standard construction practice in projects where basements are built in areas with high water tables — is under fire for the second time in six years.
In 2008, neighbors concerned that dewatering might kill their trees, crack their foundations and wastewater spoke up. This year, residents are pressing the city again.
“I’ve always been concerned about [dewatering], but now I’m really concerned about it because of the drought,” said Jeffrey Koseff, a Stanford professor of civil and environmental engineering who wrote to the mayor in August questioning the practice. “Groundwater is really our savings account: it’s the last resort, it’s what we have to use when there’s no precipitation.”
Dewatering involves continuous pumping of thousands of gallons a day to keep basement construction sites that sit below the top of the water table dry. Palo Alto has two water tables: a deep water table, which contains potable water and sits 200 feet underground, and a shallow water table, the top of which can be as little as five feet below ground and which contains non-potable water.
City officials have allowed the Public Works department and the local environmental nonprofit Acterra to recycle the water to irrigate trees and sweep streets. Officials described the amount used for these purposes as minimal.
The city’s Utility Advisory Commission — a body made up primarily of residents that advises the city council — has suggested boosting the amount of water from dewatering projects used to water parks and to supplement the recycled wastewater the city uses for irrigation.
But some city officials are pushing back on building a broader infrastructure to achieve such goals, saying the basement dewatering projects are too limited in scope to justify the investment.
Building one basement in an area where the shallow water table is close to the surface can require the pumping of millions of gallons. Thirty to fifty gallons per minute are pumped from basements around the clock during dewatering projects, which can last months.
The city approved nine dewatering projects last year and four in 2014, according to a review of public documents by Peninsula Press.
“We’ve had numerous calls from people…who were certainly aware that it doesn’t look good to be throwing that much water away,” said Ken Torke, Palo Alto’s former watershed protection manager. “[The irrigation plan] was more of a gesture than a real change in how much water was going down the storm drain.”
Torke said installing the necessary pipes to move water from a dewatering project to a place where irrigation is needed is impractical, because dewatering projects are meant to last just months and it is hard to predict beforehand what areas will need dewatering if residents build basements. It would also be a bureaucratic headache, he said: “Most of our regulations generally aren’t that nimble.”
Still, Steve Eglash, a member of the Utilities Advisory Committee said the city could do more. Officials “are choosing not to explore that benefit because we never have and because we think it’s expensive and that it’s not worth doing,” Eglash said.
Groundwater hydrologist Jim Ulrick explained that the shallow groundwater being pumped out wouldn’t be potable.
A layer of clay separates Palo Alto’s shallow water table from its deep groundwater in most parts of the city, and it’s the deep groundwater that is considered drinkable. Shallow groundwater contains more minerals and is more likely to be contaminated by pesticides and fertilizers that penetrated the ground.
Using the water for onsite irrigation is impractical, too, Ulrick said, because the thousands of gallons dewatered from a site each day are far more than landscaped plants can absorb.
Palo Alto senior engineer Mike Nafziger said he didn’t expect the city to change its approach to dewatering. Nafziger noted that the city has already taken steps to limit the extent of dewatering by banning pumping after construction is finished.
Koseff wants officials at least to study the issue more. “If at the end they conclude that it’s OK, it’s fine, at least we would have done it out of the knowledge that we looked at it in a more rigorous and complete way,” Koseff said. “[We need to consider] what are the potential uses of this water? Do we use it to irrigate trees, or do we leave it there as a resource that could potentially be tapped into at some future time?”
(Homepage photo courtesy of Kaushik Narasimhan on Flickr via Creative Commons.)