Internal and external sources of THM precursors in a midwestern reservoir

This research investigated the source of natural organic compounds that serve as precursors to disinfection by-product formation. Samples taken from a mid-western reservoir showed that inputs from tributary streams accounted for 80% of precursors leading to the formation of trihalomethanes (THMs) and that internal processes generated the remaining 20%. The highest THM formation potential (THMFP) was found in samples collected below the thermocline, suggesting that decomposition of sedimenting organic matter enhanced precursor production. Study findings have implications for water providers managing thermally stratified reservoirs in agricultural landscapes. Because THMFP was largely found to be associated with the dissolved fraction, filtration to remove particulates would have little influence on precursor concentrations. Hypolimnetic withdrawals as a means of mitigating THMFP are not recommended because of deepwater THM concentrations arising from the production of precursors through microbial decomposition of natural organic matter. Implementation of best management practices in order to mitigate nutrient loading likely would diminish the formation potential of THMs by reducing algal abundance in tributaries and other source waters.