On a late June evening, when Rachel Bratt first smelled the odor, she was inside her Mountain View home. A sniff outdoors revealed notes of sulfur and rotten eggs.

She called a plumber to investigate – twice – and her neighbors near the El Monte Avenue-El Camino Real intersection did as well, because they believed their homeowners association’s sewage lines might be the cause. They weren’t. Weeks later, the stench persists at night in parts of south Mountain View and central Los Altos.

“When the smell is bad, it has severe health detriments,” Bratt said last week. “Me and a couple of people, we got migraines and physically sick, and there’s no acknowledgment from the city that there’s even a problem.”

The official source of the smell remains a mystery. Residents of Mountain View and Los Altos have not lodged any formal complaints with those cities, and city representatives could offer no information regarding the cause when contacted by the Town Crier. Neither could a spokesman from Valley Water, the county’s wholesale water and groundwater management agency.

Cal Water recently warned Los Altos District customers about a temporary “different” taste or odor to their tap water caused by seasonal changes from Valley Water, its wholesale supplier.

“While unpleasant, any taste and odor related to this are aesthetic issues only; we rigorously monitor water quality, and your water continues to meet all federal and state standards set to protect public health,” company officials wrote in an Aug. 6 email to customers. “On our end, we are increasing monitoring in our system and will notify you of any expected changes.”

Los Altos resident Jennifer Kavanagh is among those who received the Cal Water notice. She said her tap water has seemed a little odd lately, but its smell hasn’t risen to the level of the septic, sewer-type funk she’s detected at night for several months on Parma Way. Like Bratt, she initially thought she had a plumbing problem.

“I’m so surprised that somebody else noticed it because I thought, is it just me?” Kavanagh said.

Following the funk

Yvonne Kingman, Cal Water director of corporate communications, confirmed the mystery smell and tap water changes are likely unrelated.

“If residents had a taste or odor connected with the seasonal changes in surface water, it would be limited to their drinking/tap water but not limited to a certain time of day (it would be all day),” Kingman wrote in an email to the Town Crier sent Friday. “Our local office also has not been receiving any more taste or odor complaints recently.”

Another possible culprit is the natural biological processes that occur in the San Francisco Bay. While representatives from the Save the Bay organization, the Don Edwards San Francisco Bay National Wildlife Refuge and the South Bay Salt Pond Restoration Project indicated they are unaware of any known issues related to run-off or water quality, the restoration project’s executive project manager was willing to hypothetically entertain the suggestion of an algae overgrowth.

“If you have a really big algal bloom followed by a die-off, it can happen that it becomes sort of the dominant smell of the bayshore for a couple days,” Dave Halsing said. “That does happen from time to time.”

Hot conditions with stagnant air flow can be conducive to lingering scents, especially during the late summer or early fall, but the mystery smell’s predominance at night is a bit puzzling, Halsing said.

Bratt, for one, is content with the potential bay connection because it seems to jive with what she uncovered through her own sleuthing.

“A few nights ago, when it was really bad, I drove around to see where the source was,” she said. “I went down Miramonte, the opposite side of Shoreline, and it just got significantly better the farther you got from the bay.”