The Science of Sustainability

What Is That Unusual Smell In Walnut Creek?

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Walnut Creek is a nice, clean suburban city today, but people who live there long enough eventually ask why it smells like rotten eggs around John Muir Medical Center. That smell has been there longer than Walnut Creek.

To get to the hospital, you take Ygnacio Valley Road east from downtown and over Shell Ridge. On the other side of Shell Ridge is the hospital, overlooking the wide, flat Ygnacio Valley. Here's the valley today as it appears on Google Maps.

John Muir Medical Center sits in the neighborhood labeled "San Miguel." The valley made up the heart of the Rancho San Miguel, given its name by Doña Juana Sanchez de Pacheco for her late husband Miguel Antonia Pacheco, but originally it was the Rancho Arroyo de Las Nueces y Bolbones (Walnut Creek and Bolbones [tribe]). Here's the Rancho San Miguel as it was mapped in 1850, turned to place north up.

We see the creek (Arroyo de las Nueces) on the west, Shell Ridge on the south, Ygnacio Valley in the middle, and Lime Ridge (mapped as Sierra de los Golgones) on the east. What's that notation at the end of Shell Ridge? Let's zoom in.

The "warm sulphur spring" was where Ygnacio Sibrian, Doña Juana's son, established Sulpher [sic] Springs Ranch on his share of the Rancho San Miguel. The official records put "Sulpher Springs" a bit north of Heather Farms, but U.S. Geological Survey Water-Supply Paper 338, Springs of California, offers the following description of the springs as of 1915:

"A group of sulphur springs lies near the northeastern base of a low ridge about 2 miles northeast of the town of Walnut Creek. The largest spring is on the ridge about 100 yards from its eastern base and 25 yards north of the county road [Ygnacio Valley Road]. When the place was visited, the water rose in a board-curbed pool protected by a latticed house and was piped to a cattle trough a few yards away. It yielded about 3 gallons a minute of mildly sulphureted water, 81° in temperature. . . . Five other smaller springs issue in a belt extending 350 yards along the base of the ridge, in and near the barnyard of Sulphur Springs farm. Two of them have been piped to watering troughs near by. The other three are of seeping flow and form only small marshy places."

From that description, the spring was closer to where Clarke Swim Center sits today in Heather Farm Park, another health-centered water attraction.

Rotten-egg smell is hydrogen sulfide gas (H2S), produced by certain bacteria from sulfur minerals in the absence of oxygen. Normally H2S is quickly oxidized by another set of bacteria into H2SO4, sulfuric acid, which proceeds to eat away at limestone and other rocks. But if the gas can move quickly to the surface it can reach your nose. Such is the case at Shell Ridge, as the Contra Costa County geologic map shows.

Shell Ridge consists of the following rock units, in order of age: upper Domengine Formation (Tdu), Cierbo Formation (Tc, with sandstone/conglomerate (Tcsc) at the base), Neroly Formation (Tn), and Green Valley/Tassajara group (Tgvt). The strike-and-dip symbols show that the rocks are actually overturned—tilted beyond vertical.

What is sulfur doing in Walnut Creek's rocks? Sulfur minerals, mostly gypsum (CaSO4·2H2O) or pyrite (FeS2), are widespread in sedimentary rocks. Around here, the sulfur is pyrite associated with organic matter, and the Domengine Formation in particular is a well-known petroleum reservoir rock. It stands to reason that a suitable set of cracks would support a sulfur spring here. The smell of rotten eggs used to be considered part of the healthful power of a warm spring. Savor that as you visit the hospital today.

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Category: Blog, Environment, Geology

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Andrew Alden

About the Author ()

Andrew Alden earned his geology degree at the University of New Hampshire and moved back to the Bay Area to work at the U.S. Geological Survey for six years. He has written on geology for About.com since its founding in 1997. In 2007, he started the Oakland Geology blog, which won recognition as "Best of the East Bay" from the East Bay Express in 2010. In writing about geology in the Bay Area and surroundings, he hopes to share some of the useful and pleasurable insights that geologists give us—not just facts about the deep past, but an attitude that might be called the deep present. Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.
  • Joe Queirolo

    Interesting! What happened to the springs? Are they still flowing?

  • Beth

    I'm thinking of buying in the area and I have an infant. Would it be dangerous for my family to live near these fumes? I can smell it from our potential home depending on the wind.

    • Andrew Alden

      Beth, I can't give medical or environmental advice. Perhaps the city's health department has advice for residents. Personally, unless my home were permeated with the smell, I would consider it annoying rather than "dangerous," but that may just be ignorance speaking.

  • Bob

    Thanks for the write up! We just moved into the neighborhood and have been wondering where the smell was coming from. Good to know its a natural spring.