The Aftermath of California Wildfires: Unexpected Effects on Water Quality Hinders Compliance with Stage 2 D/DBP Rule

摘要

Wildfires have become typical events during summer in Southern California. Therndevastation produced by these events is evident, especially when they affect areas nearrnurban centers, as demonstrated by the recent Station Fire in Los Angeles. However,rnwildfires that affect areas distant from urban centers are less known but not lessrndevastating. The Zaca fire in 2007, affected thousands of acres of lands located north ofrnLake Cachuma, in the Los Padres National Forest, northwest from the City of SantarnBarbara. Lake Cachuma is a large reservoir operated by the U.S. Bureau of Reclamation,rnwhich receives State Project Water from the Polonio Pass WTP in the Central Coast uprnnorth, as well as runoff collected from nearby areas. The California Aqueduct runs inrneast-west direction on the south side of the Santa Ynez Mountains and feeds two majorrnWTPs, namely the Corona del Mar WTP in the City of Goleta and the Cater WTP in thernCity of Santa Barbara. Lake Cachuma is used as a reservoir to provide additional storagernto feed these two WTPs.rnThe Corona del Mar WTP was especially affected after the Zaca fire, when the waterrnquality analysts observed that the plant was having serious issues meeting the 80 μg/Lrntotal trihalomethanes limits determined by the Stage 2 D/DBP Rule. The cause wasrnrelated to significant changes in the influent TOC of the water, which had increased fromrnnormal levels of 2.0-4.0 mg/L to approximately 5.0-6.0 mg/L. This increase in TOC wasrnassociated with the washout of ash dispersed throughout the watershed by the winter rain,rnwhich ended up accumulating in Lake Cachuma.rnUnder this unexpected scenario, the City implemented a series of remedial actions tornadjust their operation and meet the DBP permit limits, while keeping chlorine as thernprimary disinfectant. Some of these actions include utilizing enhanced coagulation byrnraising the alum dose, as well as using significant doses of powdered activated carbon forrnTOC removal. The City also implemented a monitoring program to track the TOCrnchanges in the Lake. Although the Zaca fire occurred nearly two years ago, the TOCrnlevels in the Cachuma Lake are still as high as 4.5 mg/L. This suggests that the washoutrnof the ashes from the lake will have a very slow rate, given the inherent limnologicalrncycles. The question then becomes: although the WTP currently meets the Stage 1 DBPrnregulation, will it be able to meet the Stage 2 D/DBP Rule when it comes into effect inrn2012? This paper will present a summary description of the proposed plan of action tornmeet this goal, as well as design considerations that should be part of the analysis in areasrnprone to wildfires.