Granular activated carbon (GAC) pretreatment was evaluated for the removal of trihalomethane (THM) precursors from a surface water supply, the Occoquan Reservoir, in northern Virginia. The carbon contactors were operated in the upflow mode at flow rates of 2, 4, and 6 gpm which provided empty bed contact times (EBCT) of 26, 13, and 6.6 minutes, respectively. Reservoir raw water quality data was collected to determine what relationship existed between these measurements and the trihalomethane formation potential (THMFP) of the reservoir water. The results indicated that THM precursors, as measured by total organic carbon (TOC) and THMFP, could be removed from an untreated surface water supply by GAC contact. The degree to which THM precursors were removed was directly related to EBCT, the most effective being 26 minutes (2 gpm). GAC contact appeared to be selective for the removal of those precursors responsible for instantaneous THM concentrations i.e., those produced within a thirty minute chlorine-contact period. Those precursors responsible for THM concentrations produced after thirty minutes and for up to seven days thereafter (herein designated THMFP) appeared to be associated either with particulate matter in the raw water or with larger molecular weight organic substances which were not well adsorbed by the carbon. There were no discernible direct correlations between THMFP and the turbidity, color, TOC, chlorophyll-a concentration and algal populations in the raw water. Runoff from a rainstorm late in the period of study resulted in increases in raw water color, turbidity, and TOC concentrations, but it was impossible to determine which of these factors was responsible for the increased raw water THMFP that occurred at the same time. / Master of Science