Strategies for Managing and Reducing Disinfection By-Products


The strong terrestrial signature of the carbon in the Clackamas River suggests watershed-management strategies aimed at controlling soil erosion from forestland might reduce the input of DBP precursors to the river. Timber harvesting and associated road construction can cause erosion that commonly leads to sedimentation of streams and causes elevated turbidity. Soil erosion in the Clackamas River basin is widespread, and continuous monitoring since 2002 has shown high turbidity after storms every year (U.S. Geological Survey, 2012).


In many parts of the upper basin, unstable geology and steep slopes contribute to mass wasting and erosion, particularly in the Fish Creek and Collawash River Basins, and along portions of the upper Clackamas River where the potential for surface erosion is relatively high (Metro Regional Services, 1997). In the lower basin, tributaries become highly turbid after rain events, delivering sediment (and pesticides) to the main-stem Clackamas River (Carpenter and others, 2008); tributaries also contained high concentrations of DBP precursors. Clackamas County Water Environment Services and others are working to reduce storm-water discharge pollution, rates, and volumes in the Sieben and Rock Creek watersheds, which may reduce peak concentrations of DBP precursors in the Clackamas River (Andrew Swanson, Clackamas County Water Environment Services, written commun., 2012), although these controls were not evaluated for their efficacy in removing DBP precursors.


Certain forest-management activities such as fertilizing with urea-nitrogen, burning of post-harvest slash, and disturbing and exposing soils to direct precipitation could all contribute to leaching of DOM into the shallow groundwater system and eventual transport to the river. Although forest fertilization on Federal forestland largely ended with the Northwest Forest Plan, this practice was common until 1996 in the Clackamas River basin, where applications peaked at over 1 million pounds of nitrogen annually (Carpenter, 2003). Adams and others (2005) found that, although variable, fertilization with urea-nitrogen increased soil leaching of DOC in another Pacific Northwest Douglas-fir forest in Washington State, but it is unclear whether this has been important in the Clackamas River basin or not.


Suppression of wildfire also may increase abundance of microbes in forest soils as was found in another Douglas‑fir forest in Colorado (Switzer and others, 2012). Although there have been a few small fires in the Clackamas River basin over the past 20–30 years, there have been no large fires comparable to some of the larger burns in 1930 and 1940, for example (Taylor, 1999). From 1900 to about 1940, seven large fires burned 2,000 to over 11,000 acres in the Clackamas basin (Carpenter, 2003). It is not known whether the lack of large fires has increased microbial activity or the export of carbon and DBP precursors from forested areas, but this could be an area for future study.


It is important to understand the primary sources of DBP precursors and processes that control their transport to the river. It is equally important to determine the capacity of the forest to store rather than leach organic carbon, because this may ultimately control concentrations of DBP precursors in source water and DBPs in finished drinking water, in concert with precipitation, snowpack, and other factors that affect flow and dilution rates and other influences that affect the annual growth of benthic algae and phytoplankton in this system.


First posted February 11, 2013