*3.1. SWP THM Study*

In anticipation of this new federal THM regulation, the California Department of Health Services (now CSWRCB's Division of Drinking Water), Contra Costa Water District, and the Metropolitan Water District of Southern California conducted independent studies

of Delta source waters [14,33]. These studies found that Delta source waters had significant potential to produce THMs when chlorinated as part of conventional water treatment processes. Furthermore, these studies found that treated Delta waters, especially during drought conditions when bromine-containing THMs sharply increased, could exceed the MCL established under the 1979 regulation. These findings led to questions concerning the sources and magnitudes of THM precursors in the SWP and the possible benefits of a peripheral canal or other through-Delta conveyance facility in reducing precursor concentrations in the SWP. To answer these and other questions, CDWR conducted a study from September 1981 to January 1982 to determine the sources of THM precursors in the Sacramento River, Delta, and the SWP [33]. This study, which confirmed high concentrations of THM precursors in Delta source waters, concluded that waters exported from the southern Delta are higher in THM-producing substances than are waters tributary to the northern Delta. Specifically, the study identified Delta island agricultural drainage and seawater intrusion as key contributors of NOM and bromide, respectively, in southern Delta channels and drinking water intakes. Because Delta island peat soils are rich in NOM, they are high in THM precursors [13,21]. The drainage from these islands is higher in THM precursors than the channel waters. Because seawater is very high in bromide, seawater intrusion significantly raises the level of bromide in Delta water [15]. Chlorination of bromide forms bromine, which is more reactive in forming THMs than chlorine [34]. Moreover, bromine-containing THMs weigh more than chlorine-containing THMs, a significant fact given that the MCL is regulated on a weight-based total. Confirming earlier work, the study concluded that a peripheral canal would provide a source water to the SWP with reduced concentrations of THM precursors relative to baseline conditions. The report recommended that a monitoring program be initiated for the measurement of THM formation potential in the Delta and the SWP [33]. Formation potential, an indicator of precursor levels under laboratory conditions, does not measure formation under full-scale water treatment operations.
