Speiran, G.K., 2000, Dissolved organic carbon and disinfection by-product precursors in waters of the Chickahominy River Basin, Virginia, and implications for public supply: USGS Water-Resources Investigations Report 00-4175, 60 p.
Surface water that is treated for use in public supplies typically is disinfected with chlorine to help ensure potability. When water containing dissolved organic carbon (DOC) is chlorinated, chlorinated disinfection by-products (DBPs), such as trihalomethanes, are formed. Many of these DBPs are carcinogenic. Only certain types of dissolved organic compounds, however, produce DBPs; these compounds are commonly called DBP precursors. Water treatment removes a large part of the DOC from the water but cannot selectively remove DBP precursors. Although treating water before chlorination reduces concentrations of DBPs in finished water, concentrations can still exceed drinking-water standards. Knowledge of factors affecting the spatial and temporal variability in concentrations of DBP precursors in source waters will allow utilities to withdraw water when concentrations are low. This strategy will facilitate water treatment, reduce water-treatment costs, and provide finished water having lower concentrations of DBPs.
Results of field and laboratory studies conducted from April 1995 through September 1998 were used to evaluate spatial and temporal variability in concentrations of DOC and DBP precursors in the Chickahominy River Basin, as an example watershed having extensive wetlands in the Coastal Plain of Virginia. During base-flow periods, concentrations of DOC and DBP precursors decreased as much as 50 percent where the river flowed through extensive wetlands. During stormflow periods, concentrations of DOC and DBP precursors increased to peak values at, or following, peak streamflow.
Variability in concentrations of DOC and DBP precursors in stream water depended on hydrologic conditions. During base-flow periods, ground-water discharge and stream-bed detritus were the principal sources of DOC and DBP precursors to the streams, while organic litter distributed across the wetlands was only an indirect source because it had little contact with stream water. Concentrations of DOC and DBP precursors decreased downstream because of dilution from tributary inflow having a low organic content and because of decomposition of DOC as water flowed through the broad and shallow channels of the Coastal Plain reaches of the Chickahominy River.
During stormflow periods, infiltrating precipitation leached DOC and DBP precursors from the organic litter into the soil and shallow ground water. Discharge of DOC and DBP precursors with interflow and shallow ground water likely accounts for the peak in concentrations of DOC and DBP precursor at, or following, the peak in streamflow.
Study results have important implications for the use of water for public supply. Water treatment costs would be reduced and concentrations of DOC and DBPs in finished water would be lower if (1) withdrawal points are placed as far downstream as possible and (2) water is withdrawn and stored during the early parts of stormflow periods and withdrawal is discontinued when concentrations of DOC and DBP precursors increase.
U.S. Department of Interior, U.S. Geological Survey
Last modified: 10/11/01 10:16:50 AM