On a single block of Webster Street in the Old Palo Alto neighborhood, a tale of two basements is unfolding — one that illustrates the city's evolving debate over groundwater.

At 2189 Webster St., near North California Avenue, contractors building a basement began pumping water out of the ground on July 18, according to data obtained by the Weekly. By the time the pumping concluded on Oct. 7, they had extracted 22.3 million gallons of groundwater, nearly all of which was discharged into the city's storm drains.

At a basement project on the north end of the block, near Santa Rita Avenue, the groundwater pumping began on June 29 and concluded on Sept. 15. Here, only 133,000 gallons were pumped out. The number of gallons discharged into the storm drains? Zero.

Palo Alto generally doesn't celebrate the construction of residential basements — private amenities that have become both more commonplace and more scrutinized over the past several years. Yet for city officials, the basement at 2121 Webster St. is kind of a big deal: the first residential basement project in Palo Alto to use a cutoff wall.

A December report notes that all of the extracted water had simply soaked into the backyard.

"A typical large groundwater pumping site produces about 100 times that amount, 20 million gallons in total," the report states. "As a result, staff will convey to other applicants that this cutoff wall achieved very good results."

The 2121 Webster project may have been the city's first residential cutoff wall, but it hasn't been the only one. Two other basements have since been completed using their own cutoff walls — one at 3845 Carlson Court and another one at 1462 Edgewood Drive, a property owned by Facebook CEO Mark Zuckerberg. In each case, not a drop of groundwater went into the gutter, according to Public Works data. (By contrast, the six projects last year that used traditional basement-construction techniques discharged 115 million gallons of water into the storm drains.)

To construct a cutoff wall, builders drill a sequence of holes in the ground (in some cases, as deep as 30 feet), install I-Beams into these holes, and then fill up the space between the beams with grout, which solidifies to create a rocky curtain composed of cement and water. A basement then gets built inside the confines of this wall, obviating the need to pump water from a much broader area.

The cutoff wall technique isn't exactly new, Assistant Public Works Director Phil Bobel said. They long have been used at contaminated Superfund sites and in other areas where contractors want to restrict groundwater pumping. The 2121 Webster project was, however, the first time that such a technique was used for a residential basement in Palo Alto, Bobel said.

The use of cutoff walls in Palo Alto can be attributed in large part to a citizens group called Save Palo Alto's Groundwater, formed in 2015 to protest the growing number of dewatering projects and to promote new restrictions. Their advocacy prompted the city to gradually add requirements for projects that require groundwater pumping. Now, before dewatering occurs, applicants must complete a hydrological study that evaluates the impact of groundwater pumping on trees and neighboring properties and predicts the volume of the water drawdown. They are also required to create fill-up stations at the construction site (with strict water-pressure guidelines) so the pumped-out groundwater can be used for irrigation or construction clean-up; and to limit pumping to 10 weeks (after a two-week "start-up" period) between April 1 and Oct. 31.

Unless, that is, a builder uses a cutoff wall. As of last year, anyone who chooses this technique doesn't need to do the hydrology study, pay water-discharge fees or be subject to the strict time restrictions.

Building a cutoff wall is generally more expensive than the using the broad-based water-pumping method, but the city's requirements, such as the hydrology study, have dramatically lessened the cost differential, said Gus Carroll, whose company, Carroll Custom Homes, handled the basement project at 2121 Webster.

"If it was three years ago, it would've been much more expensive to do the cutoff walls," Carroll said. "Because we didn't have to pay any of the additional fees, the delta wasn't that dramatic."

Bobel said the goal of the city's required studies was to obtain better data about water drawdown. But the requirement also appears to have incentivized builders to pursue a wall option.

Even so, Bobel noted that cutoff walls may not always be the best approach to basement construction. Things could get problematic if the builder encounters a fluid layer, like a stream bed or gravel.

"If you were to go down and hit a layer like that, groundwater would flow under the wall very easily and it wouldn't be a good place to do a cutoff wall," Bobel said.

For that reason, Bobel said, staff has opted not to require cutoff walls — or any other specific technique — for basement construction. Rather, the city decided last month to continue to rely on its system of incentives and required studies.

That position has earned the support from Save Palo Alto Ground Water. Keith Bennett, the group's founder, called the new cutoff walls a "step in the right direction." He warned, however, that they come with their own challenges. For instance, several cutoff walls constructed in close proximity to each other could function like bricks in a bathtub.

"The disadvantage of cutoff walls is that underground construction displaces and blocks groundwater flow," Bennett said. "We see them as a good tool, but not as a one-size-fits-all solution."

It's a tool that could become even more common next year, though. Dan Garber, a local architect and former chair of the Planning and Transportation Commission, joined Bennett two years ago in exploring the feasibility of cutoff walls. Since then, Garber has partnered with Pete Moffat Construction and America Drilling, a shoring subcontractor, to create a cost model and trial methodology for using cutoff walls in residential projects. Carroll, the builder at 2121 Webster, said Garber's spreadsheet helped him arrive at the decision to use a cutoff wall.

Last month, Garber stated in a letter to the council that given the burdensome city requirements of traditional dewatering, "I do not know why any homeowner looking to build in an area of high groundwater would not pursue a cutoff wall." He told the Weekly that his group had calculated that in high-aquifer areas, the economics of doing a cutoff wall compare well with the traditional techniques particularly because the walls could cut the work time by as much as half.

Like Bennett, Garber sees the new trend as extremely promising, but both warn there is still much work to be done when it comes to limiting groundwater discharge — particularly in commercial projects.

"A single commercial project doing traditional broad-area dewatering can easily eclipse all the single-family housing projects using traditional broad-area dewatering methods," Garber wrote in an email.