Disinfection By-Product Formation Potentials


During each basin-wide survey (table 4), laboratory measurements of DBPFP were made to compare the propensity for waters from a variety of locations within the watershed to form DBPs upon chlorination. These instantaneous concentrations, and the load and yield calculations based on these concentrations, reflect stream conditions at the time of sampling; because the discrete sampling only captured one point in time, there likely are considerable variations in these estimates. These variations were probably limited to periods of dynamic river conditions, especially during the October 2010 storm; data from the three other basin-wide surveys, conducted during stable streamflow, were less affected by such temporal variability. Additionally, the DBPFP values represent DBPs formed from non-coagulated water under controlled laboratory conditions and do not necessarily equate with DBPs formed during actual water treatment.


By far, the highest DBPFP values were from the tributaries (fig. 14), particularly Deep, Rock, and Sieben Creeks, where DOC concentrations were also highest. Concentrations were particularly high—up to almost 1.0 mg/L in Deep Creek—during the October 10, 2010, storm event, when longitudinal increases in main-stem DBPFP values were observed. These high concentrations are likely the result of these tributary inputs.


DBPFPs were measured on filtered and unfiltered water, allowing for an evaluation of the relative importance of particles (greater than 0.7 µm) to total DBP formation. Particulate carbon occurs in the form of sediment, detritus, and plant cells including floating and detached benthic algae. Although the majority of the DBPs that formed (60–100 percent of the THMFP and 40–100 percent of the HAAFP) were attributed to the dissolved fraction, particulate carbon also contributed DBPs. Considering just main‑stem sites, on average, 10 percent of the unfiltered THMFP and 32 percent of the unfiltered HAAFP were attributed to particles. A comparison of the formation potentials for filtered and unfiltered samples also generally showed higher values in unfiltered samples for the three primary DBPs in finished water—chloroform, DCAA, and TCAA (fig. 15). This finding suggests there would be some reduction in DBPs by introducing a coagulation step prior to chlorination during water treatment.


It should be noted that although they contribute a relatively small percentage to the total DBPs in treated Clackamas River water, brominated DBPs are of potential concern because of their high molecular weights and potentially more problematic human-health effects compared with chlorinated DBPs. Brominated DBPs form if bromide is present in the source water because it reacts with chlorine and organic matter to form DBPs more quickly than chlorine alone. Although bromide in the Clackamas Basin could, potentially, originate from marine influences (air currents that deliver salts in sea spray), the major source is probably Austin Hot Springs, a hydrothermal suite of springs located along the upper Clackamas River about 3 mi upstream from the Two Rivers Campground sampling site (fig. 1). Chemical analyses conducted 40 years ago showed a bromide concentration of 1.2 mg/L in the spring, which discharged 275 gal/min (U.S. Geological Survey, 2006). The upper Clackamas River at Two Rivers campground had the highest proportion of brominated DBPs during this study, likely reflecting the higher bromide levels at this site compared with downstream locations.


Specific Disinfection By-Product Formation Potentials


STHMFP and SHAAFP values provide information about the degree to which carbon from various locations in the basin reacts with chlorine to form DBPs. Shifts in these values reflect changes in DOM composition and, thus, its source and processing. Surprisingly, there was a significant negative correlation between STHMFP and SHAAFP for both filtered and unfiltered samples (appendixes F5 and F6). This suggests precursor sources for these two classes of DBPs can differ from each other in the Clackamas Basin, as has been reported in other studies (Krasner and others, 2006; Kraus and others, 2008, 2010).


The STHMFP values for filtered samples were highest in September 2011 (fig. 16 and table 9). The highest STHMFP value was associated with the 80-ft depth release point within North Fork Reservoir during a seemingly small blue-green algae bloom. STHMFP values at downstream main-stem sites were also elevated at this time, including source water for the CRW and LO DWTPs. The two basin-wide sampling events at this time represented the summer low-flow period and the seasonal drawdown of Timothy Lake, which contributed about 20 percent of the flow in the lower mainstem. The two STHMFP measurements at Carter Bridge and Estacada in September 2011 (table 9) were higher than the storm-water samples collected during the October 10, 2010, storm. These measurements suggest DOM with a high content of THM-forming carbon entered the river at that time, although water‑column chlorophyll-a levels were not that high: 1 µg/L or less in the mainstem and less than or equal to 2.5 µg/L in North Fork Reservoir. While these specific DBP values were elevated, the overall carbon concentrations were low at this time, which resulted in relatively low absolute DBPFP values (fig. 14). 


Patterns in the STHMFP measurements for unfiltered samples were similar to filtered samples (fig. 16). The values were highest in samples collected September 2011 from main‑stem and source-water sites (table 9), in samples from lower-basin tributaries (Eagle, Clear, and Rock Creeks), and from the 80-ft release depth within North Fork Reservoir.


The highest SHAAFP measurements for filtered samples were from Deep, Eagle, and Clear Creeks during the initial October 10, 2010 storm. Three samples collected from North Fork Reservoir (mid-depth, release point, and near the bottom) also had relatively high SHAAFP values during an algae bloom in August–September 2010. As with STHMFP, patterns in SHAAFP were similar for unfiltered and filtered samples; the values were highest in lower-basin tributaries during the October 2010 storm. At this time, SHAAFP values were also high in the mainstem from Carter Bridge downstream to the CRW and LO DWTP intakes. Prior studies found that HAA precursors can be linked to soil-derived, degraded DOM as well as to DOM more recently added by algal production (Kraus and others, 2008, 2011).


The correlations between STHMFP and SHAAFP were weak, which again emphasize that sources of these two classes of DBPs may differ substantially in the Clackamas Basin. Prior studies have suggested that aliphatic structures play a more important role in THM formation and aromatic structures play a more important role in HAA formation (Croué and others, 2000; Liang and Singer, 2003). A strong positive correlation was found between SHAAFP and SUVA, which suggests HAA precursors are associated with ultraviolet absorbing, aromatic compounds. This type of DOM is strongly associated with soil-derived, humified organic matter.


Higher HAA5 formation per unit carbon seen at downstream tributaries during the initial October 2010 storm, when DOC concentrations and SUVA values were elevated, agree with prior studies in this region linking HAA precursors to the flushing of organic material from soils (Kraus and others, 2010). Higher THM4 formation per unit carbon seen in the main-stem samples during September and October when SUVA values were low suggest a potential link between THM precursors and contributions of DOM from algae.


Loads and Yields of Organic Carbon and Disinfection By-Product Precursors


The highest measured concentrations of DOC and TOC were from the tributaries (fig. 9), especially Deep, Rock, and Sieben Creeks, but the limited amount of streamflow from all these tributaries resulted in relatively low DBP loads to the main‑stem Clackamas River during three of four samplings (fig. 17). During storm periods, however, loads from the tributaries can become important. During the October 2010 storm, for example, the loads of DOC and TPC from Deep Creek alone accounted for about 50 and 70 percent, respectively, of that measured at the CRW DWTP intake (table 10). This carbon reacted to produce DBPs, accounting for 56 and 65 percent of the unfiltered THMFP and HAAFP loads, respectively, at the CRW DWTP intake. At this time, about half (47 percent) the total carbon was in particulate form, including duckweed fragments (see photograph 6) possibly associated with over-spillage from ponds within the Deep Creek Basin. Other streams including Eagle, Clear, and Rock Creeks contributed 28, 9, and 9 percent of the DOC load at the CRW DWTP intake, respectively, during the initial autumn storm. Eagle Creek contributed 28 and 37 percent of the unfiltered THMFP and HAAFP load, respectively, at the CRW DWTP intake (fig. 17).


THMFP in the lower mainstem for unfiltered samples increased 44 percent between Estacada and Barton on all four dates, including times when the tributaries were seemingly unimportant. This suggests a particulate source—such as sloughed benthic algae, for example—might be contributing to higher downstream THMFP. Although Eagle Creek did produce higher THMFP values compared with the mainstem at Estacada, concentrations were even higher downstream at Barton (fig. 14). 


The carbon and DBP precursor yields also varied according to hydrologic condition. Forested areas with relatively higher amount of flow (upper basin, Eagle Creek, and main-stem Clackamas River downstream to Barton) had the highest DOC yields during the May 2010 spring high-flow period; Sieben, Rock, and Deep Creeks had the highest yields during the October 2010 storm (fig. 18). During the summer base-flow period, however, the lowest DBP yields were from the tributaries, and the highest yields were from the upper and middle basin. The THMFP for unfiltered samples increased from Carter Bridge downstream to Estacada and to Barton at this time (fig. 14), which could be from both phytoplankton within North Fork Reservoir and sloughed benthic algae in the reach upstream from Barton.


First posted February 11, 2013