Trihalomethane Compounds and Their Precursors in Salt Lake County: Evaluation of Trihalomethane Source and Production

Salt Lake County, Utah, watershed streams, drinking water treatment plants, and distribution systems were monitored for a period of one year or greater to determine the seasonal watershed precursor load, trihalomethane (THM) formation, and correlation between the two in the drinking water supplies. In addition, unit treatment processes were examined at Parleys, Big and Little Cottonwood water treatment plants, in the spring and late summer, to evaluate THM precursor reduction and THM formation. Additional studies were also conducted to appraise the potential contribution of natural sources to watershed THM precursor loading. Total organic carbon was used to estimate THM precursor concentrations. Instantaneous (time of sampling) and 7-day (incubated) forms of trihalomethanes were measured. Stream THM precursor production was greatest during spring runoff; the mean precursor concentration at this time was 3.4 mg/1. Stream discharge and precursor concentration frequently exhibited a linear correlation. high streamflow, as related to precipitation events, produced greater THM formation potential. Rainfall runoff from roads and other distrubed areas provided most of the instream precursor load. Leaf littler and surface soils near streams were shown to be potentially significant sources of THM precursors (leaf litter more than soils). THM formation at all water treatment plants peaked during March to June and September to October, corresponding to increated runoff in the spring, and precipitation, reservoir using reservoir water had greater (approximately 60 percent) mean annual effluent THM concentrations thatn those using solely stream water. THMs at all distribution stations averaged less than 40 ug/1 (greater than water treatment plants) during 1981. Groundwater had a comparatively small THM formation potential. The USEPA maximum contaminant level of 100 ug/1 was never exceeded at any utility or distribution station. The water treatment plants normally reduced the influent organic precursor concentration by about 30 percent, principally by sedimentation and filtration. Instandaneous THMs generally increased with process time. Precursor and chlorine concentrations for long term control of THM formation include sources control of surface water organics and nutrients, improving treatment plant efficiency, and minimizing chlorine dosages