Dissolved and Particulate Carbon Concentrations


DOC concentrations were generally low in the Clackamas River, typically about 1.0–1.5 mg/L; during storms, concentrations occasionally increased up to about 2.5 mg/L (fig. 9). DOC concentrations were much higher in the tributaries during the one storm sampling in October 2010; Rock Creek had the highest concentration (7.8 mg/L). Within the mainstem, carbon concentrations (DOC and TOC) increased downstream to the drinking-water intakes (fig. 10). Longitudinal increases between the Carter Bridge and Estacada sites, although not large, may be attributed to an effect of the hydroelectric project reservoirs, including North Fork Reservoir, but also could be caused by input from Wade Creek (fig. 1), which drains the city of Estacada and receives treated effluent from the wastewater treatment facility that discharges to the Clackamas River upstream from River Mill dam (fig. 3).


There was a strong correlation between concentrations of DOC and TOC (r = 0.98, p <0.001). In general, the TOC pool was dominated by the dissolved fraction (greater than 70 percent); there were, however, times when the particulate fraction (TPC) was significant, making up 30 to 40 percent of the TOC (fig. 10). In the tributaries, the highest percentage of carbon as TPC was during the October 10, 2010, initial storm event when turbidity also was relatively high. The TPC fraction of the TOC pool was also high (approximately 40 percent) in several of the samples from North Fork Reservoir in 2010. In mid-March 2011, an unusually high TPC value in the source-water sample from the CRW DWTP intake (2.0 mg/L) resulted in a relatively high TOC value (3.6 mg/L) that was largely made up of TPC (55 percent). Given that just 2.2 mi downstream at the LO DWTP, TPC was 0.5 mg/L and TOC was 2.0 mg/L that day, this relatively high TPC value at CRW DWTP was likely the result of resuspension of particles accumulated within the intake vault rather than being reflective of river conditions.


Seasonal variations in streamflow had a pronounced effect on turbidity and carbon concentrations in the mainstem and tributaries and on DBPs in finished water. The DOC concentrations in source water were highest (2.0 to 2.6 mg/L) in June and November 2010 and January 2011—all samples affected by storm runoff (fig. 10). Carbon concentrations were commonly closely tied to streamflow (fig. 7) and resulted in a significant positive correlation between streamflow and DOC (r = 0.82, p <0.001). The effect was, however, complex because higher streamflow also occasionally diluted carbon concentrations. The net effect of this situation is that over the course of a storm, or longer periods of time with successive storms, there is a “hysteresis effect.” Relative to streamflow, carbon concentrations are initially high as the storms mobilize organic matter from the watershed, but as the storm continues, concentrations are lower, relative to streamflow, because less carbon is flushed later, leading to lower concentrations from the effect of greater dilution (Pellerin and others, 2012).


First posted February 11, 2013