In recent years, cracks have continually appeared in the walls and foundations of at least two dozen homes in the same Pleasanton neighborhood.
Neighbors who live just southeast of Sutter Gate Park, frustrated that their homes values are declining due to this damage, claim pumping from two nearby Zone 7 Water Agency wells caused the ground near their homes to sink.
The water agency, however, said a recent study proved the sinking is caused by natural soil water loss due to the intense years-long drought, not from use of the wells.
The issue will be researched and discussed further by city staff, said Vice Mayor Karla Brown.
Brown said it will take some time to collect the existing research and to determine what, if any, additional data needs to be collected, so there is no timeline for when this issue may appear before the Pleasanton City Council.
"We don't know if Zone 7 is even part of the problem, but they (residents) obviously have a problem, so the source still needs to be determined," she said. "We understand their concerns and the ball is in our court now."
About 20 residents, who live just southwest of Santa Rita Road and Stoneridge Drive, filled a Zone 7 conference room Wednesday evening for an informational presentation to learn about the study and to express their frustrations. Two City Councilwomen, Brown and Kathy Narum, attended the presentation.
Residents said the soil under and around their homes has been sinking by inches in different areas over the years. Some blame Zone 7's Mocho Wells 3 and 4, which are used to pump potable water and are located within 100 feet of some neighbors' homes.
Uneven sinking is causing stress to the foundation of their homes and their roadways, leading to cracks and potholes.
"It's affecting our properties. It's affecting our homes," said homeowner Kathy Gunn. "What about the crack in my banister? The crack in my wall?"
The area is spotted with homes with cracks, but not every home in that neighborhood has damage, residents said.
A contracted study, which cost $45,000 and was funded by Zone 7 through water rate funds, was released in June. It looked at all the ways the ground could sink in Pleasanton and surrounding areas, and the cause of the uneven sinking these residents are experiencing seems to be caused by natural drying out of topsoil, Zone 7 assistant general manager Kurt Arends said.
"I don't believe our pumping is what's causing your problems," Arends told residents.
The homes in question, including houses along Sutter Gate Avenue, Lin Gate Street and Laramie Gate Circle, appear to have been built on silty, clay-like soil -- a common culprit of foundation damage to homes across the East Bay during droughts, Arends said. When that type of soil dries out, it shrinks and cracks.
Homes built on this type of soil often experience trouble with their foundation in droughts because the soil on the edges of the home dries out and sinks faster than the soil in the center of the home, causing the foundation to warp, said Phillip Gregory, principal engineer for Cal Engineering & Geology, a Walnut Creek firm that conducted the ground movement study.
The study determined three factors can cause the ground to move: First, the topsoil dries out and shrinks, the soil hundreds of feet underground can shift when water is withdrawn and movement of tectonic plates can make the earth move. In this case, tectonic activity isn't causing any noticeable changes, said G. Reid Fisher, geologist for Cal Engineering & Geology.
Pumping out groundwater at the aquifer level can cause ground in Pleasanton to sink if water is taken out past a certain unsafe level, but it would probably happen evenly and across the entire city, Gregory said.
He also said water agencies are tasked with making sure water never falls below that level of dangerous sinkage. Zone 7 General Manager Jill Duerig said the district's water levels are within a safe range.
Uneven sinking in different parts of a house is more indicative of topsoil drying out during the extreme drought, Gregory said.
He noted the study didn't survey these residents' specific residences but said the reasons soil could sink are the same across the valley, and the valley as a whole didn't appear to have evidence of sinking. If groundwater pumping was causing the ground to shrink, there would be evidence like wells collapsing, and wellheads would be sticking out of the now-sunken ground, which researchers didn't observe.
Resident Michael Grossman said at the meeting he's not convinced topsoil drying is causing the damage to his home. Residents started noticing damage in 2004, about two years after Mocho Well 3 and Mocho Well 4 were installed and started pumping water.
Mocho Well 3 pumps water from 300-400 feet underground, and Mocho Well 4 pumps from 500-700 feet underground, Duerig said.
The homes experiencing damage are near the wells, Grossman said, and he believes it's unlikely the timing is a coincidence.
He said afterward that he was disappointed because the contracted study looked at soil and ground movement in the entire valley, and he and his neighbors were interested in a study that looked at their neighborhood and whether the nearby wells were causing an adverse effect.
However, he said the best course of action now is to wait and see what happens after the city evaluates the situation.