Here’s an interesting article about the aquifer that underlies Golden Gate Park:

“The ongoing California drought and the constant threat of earthquake have driven home the need for The City, which draws 85 percent of its water from the Tuolumne River in Yosemite National Park, to have a local supply on standby. In 2016, the SFPUC expects to begin delivering a mixture of Hetch Hetchy water and groundwater pumped from the underground aquifer that stretches from Golden Gate Park south past Lake Merced and beyond the San Mateo County line.

About two-thirds of The City will receive the new blend of water. Eventually, groundwater pumped from six wells could provide as much as 5 percent of The City’s daily water needs and also provide a 30-day emergency backup supply in case the Hetch Hetchy flow is cut off.

But the local groundwater is not as clean as the legendary Sierra Nevada snowmelt that is touted as some of the best drinking water in the U.S. Thanks to leaks from sewer mains and runoff from fertilized parks, local groundwater contains levels of nitrate and coliform bacteria “that exceed U.S. Environmental Protection Agency’s drinking-water standards,” according to a review conducted by the U.S. Geological Survey in 1990.

Untreated and unblended, the local groundwater is “suitable for irrigation and other nonpotable uses,” but not for drinking, the USGS wrote. However, when blended with Hetch Hetchy water — at about a 10 percent groundwater to 90 percent Hetch Hetchy ratio — the water meets national safety standards, according to Steven Ritchie, the SFPUC’s assistant general manager for water.

“This is perfectly safe water,” he recently told a Board of Supervisors committee, adding that in preliminary taste tests, most residents could not tell the difference. Most of the western side of The City would receive the blended water, according to the SFPUC.

Water is considered unsafe to drink at nitrate concentrations of more than 10 milligrams per liter, and harmful to infants at over 20 milligrams per liter. During tests during the past decade, nitrate levels of up to 55 milligrams per liter were detected from test wells near Golden Gate Park and near Lake Merced, according to the Groundwater Supply Project’s environmental impact review.

Four wells — one in Golden Gate Park, one near Lake Merced and two in the Sunset — are slated to be dug beginning in August. Later stages call for two wells in Golden Gate Park that pump water for irrigation to be converted to pump drinking water. San Bruno, Daly City and the California Water Service Co. — a privately held utility based in San Jose — all draw drinking water from the same groundwater basin.

Cal Water uses a 60-40 blend — 60 percent Hetch Hetchy purchased from the SFPUC and 40 percent groundwater — that’s also filtered onsite to meet water quality standards, said Tony Carrasco, a district manager for the agency.

The SFPUC’s blend planned for future use isn’t going to go through a filtration process before it hits taps and toilets. However, according to SFPUC spokesman Tyrone Jue, it will be chlorinated slightly to kill coliform bacteria. But even without the chlorine step, the mixture is perfectly acceptable, according to professor Jason Gurdak, a hydrogeologist at San Francisco State University.

Most California cities draw from local groundwater tables into which sewage and agricultural runoff seep, he said. Mixing the groundwater with higher-quality water for a final product that meets quality standards is a routine practice. A much bigger threat to local water quality is seawater, Gurdak noted. Seawater, which has leeched into over-pumped groundwater tables in Santa Cruz and Monterey counties, would “defeat the purpose of the project,” Ritchie said.”

via Blending contaminated SF groundwater with Hetch Hetchy supply makes it safe to drink, experts say | Other News | San Francisco | San Francisco Examiner.