The May 6 diesel spill that contaminated Matadero Creek in Palo Alto continues to show signs that it is abating, according to a mid-July joint update published Monday by the city and VA.
The spill occurred after a malfunction in a generator structure at the Veterans Administration Palo Alto Health Care System campus on Miranda Avenue. An unknown quantity of diesel flooded outside of the building and contaminated the creek, raising concerns for the welfare of wildlife.
A boom system across multiple areas of the creek is staying blue rather than turning pink, an indication that the structures are no longer picking up the diesel fuel or any of its chemical constituents. There is no longer any diesel fuel odor anywhere along the creek, according to the update.
Monitoring by a consultant hired by the VA primarily found total petroleum hydrocarbon as diesel (TPH-diesel) in the creek, at one point thousands of times higher than San Francisco Regional Water Quality Control Board screening level standards of 640 for fresh water. One location in May showed a screening level 296 times the standard level. TPH-diesel is a measurement of multiple hydrocarbon compounds that typically make up diesel fuel, according to the consultant's reports.
The testing has not found benzene, toluene, ethylbenzene and xylene compounds except for in one spot, when xylene was found on May 7 near the original source of the spill. It has not been detected in samples since, according to the consultant's reports. Those individual compounds would be expected to impact wildlife, based on discussions with the state Department of Fish and Wildlife, the city and VA's update said.
Creek water samples are continuing to show that diesel fuel is above "normal" limits in certain sections of the creek about halfway between the VA generator building and the Bol Park Path footbridge. A fourth area near the Bol Park Path footbridge also previously showed high levels of diesel.
The diesel was thought to be trapped in creek bank soil and vegetation. It didn't break down because the areas weren't exposed to adequate sunlight, which would help dissipate the volatile fuel, investigators said, according to the update. To expose the fuel and help it to dissipate, crews used hand tools to remove vegetation, lightly tilled the top 2 inches of soil and flushed the vegetation with water to help move any contaminant into areas where it would be trapped by the booms.
The results of those measures are still pending, but the levels of diesel appear to be going down. The areas between the three booms tested at 4,000, 6,600 and 74 micrograms of total petroleum hydrocarbons-diesel per liter on June 28 and 30. Samples taken on July 7 showed the levels at two of these areas had dropped to 2,600 and 2,800 micrograms per liter, respectively. In the area that tested 74 micrograms per liter in June, the July 7 sample did not detect any diesel. The other area near the footbridge also dropped from 2,000 micrograms per liter on June 25 to 200 micrograms per liter on July 7. A few tests are still pending, including for benzene, toluene, ethylbenzene and xylene, according to the report.
Testing and future reports will continue regularly, the update noted.