Santa Rosa may be zeroing in on the cause of the contamination in the water supply of a Fountaingrove neighborhood devastated by the October wildfires, but there are troubling signs the problem may extend into the surrounding area.
New tests have recently detected benzene just outside an area north and south of Fountain Grove Parkway around Fir Ridge Drive, where residents were instructed in November to stop drinking and boiling water while the city investigated the source of the contamination.
In response, the city is launching a more aggressive regimen of water tests covering every burned neighborhood in Santa Rosa, including Coffey Park, in its effort to make sure other burn zones aren’t experiencing similar problems.
“We just don’t want to leave any stone unturned going forward,” Ben Horenstein, the director of Santa Rosa Water, said Thursday.
Since Jan. 24, when the city last released detailed test results, the city has found 58 additional instances of benzene in the drinking water in Fountaingrove. The vast majority were found within the boundaries of a 184-acre advisory zone where the city first discovered elevated levels of benzene, a chemical commonly found in plastics and gasoline that can cause cancer. Only 13 of the 350 homes in the area survived the Oct. 8 wildfires.
Six of the new contaminated samples were found just outside the advisory zone. The samples were drawn from water service lines to burned homes on Chateau Court, Llyn Glasen Place, Royal Manor, Horizon View Way, Sedgemore Drive and Millbrook Drive, city officials said.
The city has now detected elevated levels of benzene in 145 samples taken from the water system in Fountaingrove. The newest results are similar in range to those that have had been discovered before Jan. 24, with some exceptions.
The first batch contained four test results showing benzene levels over 500 parts per billion, one of which was as high as 918 parts per billion. The maximum contaminant level (or MCL) for benzene in drinking water in the state is 1 part per billion. This time, however, there were no results over 500 parts per billion.
There were three between 10 and 500 ppb; 10 between 25 and 100 ppb; 16 between 5 and 25 ppb; and 29 between 1 and 5 ppb.
In what was a surprise to city officials who for months have stressed that the contamination appeared isolated to the advisory area, six of those results were from outside the existing advisory area.
All were under 10 ppb except one, something of an outlier at 255 ppb, said Jennifer Burke, the city’s deputy director of water and engineering resources.
Before last week, the city had only received one positive test for benzene outside the advisory area. However, it disappeared and didn’t return after some equipment was replaced.
The city is now replacing the high-density polyethylene service lines that lead to the six locations outside the existing advisory zone and plans to test them to see if it fixes the problem, Burke said.
A forensic chemist analyzing the results has helped the city rule out one possible source. None of the samples have shown any isooctane, suggesting the contamination source is not from a petroleum product like gasoline, Horenstein said. As a result, the city believes the contamination was not caused by a leaky underground storage tank, he said.