Disinfection By-Product Concentrations in Finished Drinking Water


DBP concentrations in finished water from the CRW and LO DWTPs were similar, about 0.024 ± 0.006 mg/L for THM4 and 0.022 ± 0.008 mg/L for HAA5 (fig. 11). Maximum DBP concentrations in finished water also were similar for the two classes of DBPs, about 0.04 mg/L for both THM4 and HAA5. Concentrations of THM4 and HAA5 for the two samples collected from within the distribution systems were, however, higher (fig. 11).


Following the winter high-flow period, streamflow in the Clackamas River declined through summer as the snow packs diminished and groundwater made up a progressively greater contribution to the flow in the river. This seasonal transition for the Clackamas River and other rivers draining the Cascade Range was first described by Piper (1942). The lower turbidity and carbon levels that resulted (fig. 7) produced lower concentrations of DBPs in finished water (fig. 11). Lower concentrations of DBPs during a time when (regional) groundwater dominates flow is consistent with findings from a USEPA study (U.S. Environmental Protection Agency, 2005) that reported generally lower concentrations of THMs in finished water when source water was from groundwater rather than from surface-water sources.


The seasonal patterns in concentrations of THM4 and HAA5 and overall composition of DBPs in finished water also were similar at the two DWTPs. Chloroform was the dominant THM in finished water, making up 86–97 percent of the THM4 for both DWTPs; bromodichloromethane made up the remaining 3–14 percent. For HAAs, DCAA and TCAA were the primary DBPs, making up 40–60 percent each. MBAA was detected in finished water from both DWTPs at concentrations equal to the detection limit of 0.001 mg/L; no other DBPs were detected.


An examination of individual concentrations and Benchmark Quotient (BQ) values can reveal times when concentrations are most elevated, and how close they may be to existing standards. BQ values indicated no drinking-water standards were exceeded (fig. 12). The maximum BQ values were somewhat higher for HAA5, which has a lower MCL (0.060 mg/L) than THM4 (0.080 mg/L). Drinking-water standards for DBPs are based on the annual running average of the maximum concentrations from within the distribution system, not the “time zero” finished-water samples collected from within the treatment plant. The quarterly monitoring data for the CRW and LO DWTPs showed similar maximum BQ values—about 0.35 for THM4 and 0.46 for HAA5 (Oregon Health Authority, 2012) as those reported here. The two samples collected from within the distribution system (one from each DWTP) had the highest BQs of 0.76 and 0.93, which indicate that HAA5 concentrations approached but did not exceed USEPA standards.


Storm Effects on Organic Carbon Concentrations and Disinfection By-Products


Several large storm events during the study caused surface runoff (see photographs 5a-b), including one high-flow event in June 2010 that resulted in a peak DOC concentration of about 2.5 mg/L (fig. 7). In October, with the onset of autumn precipitation, DOC concentration increased about 50 percent in the lower Clackamas River during the initial storm (fig. 13). This storm, the first significant precipitation event in months, caused moderate runoff that increased turbidity in the mainstem from 0.5 to 4.5 FNUs, more than doubling the TPC and TPN at the CRW DWTP intake (table 8). Although a shift in DOM composition was observed at the CRW intake during this storm, it did not produce any notable change in finished-water DBPs (fig. 11). 


The second, larger storm at the end of October 2010 (sampled on November 1st) was more characteristic of a true soil “flushing” that mobilized carbon from the watershed into the hydrologic system. Although turbidity was not as high during this second storm, the DOC increased to 2 mg/L at the DWTPs, or about 2.5 times higher than during the summer base-flow period (table 8 and fig. 7). More importantly, the DOC was reactive, producing the highest concentrations of DBPs in finished water during this study: 0.04–0.05 mg/L THM4 and 0.03–0.04 mg/L HAA5. The two samples collected from within the distribution system had higher concentrations (fig. 11). In addition to increased DBPs, moderate taste and odor issues began at this time, along with increased demands for chlorine and coagulant doses (Kari Duncan, City of Lake Oswego, written commun., 2011). Even though the first storm mobilized quantities of carbon, concentrations of DBPs were higher in finished water later in the autumn after the ground was saturated.


First posted February 11, 2013