The influence of alum concentration, pH and the molecular weight of organic substances present in a water were evaluated in regards to the removal of trihalomethane (THM) precursors from a surface water supply, Peak Creek, in southwestern Virginia. Water was coagulated with various doses of alum with the pH uncontrolled and with the pH controlled at 5.2 - 5.3. Raw water and treated waters selected for efficient turbidity and optimum total organic carbon (TOC) removals were concentrated 400X using vacuum evaporation at a temperature of 40° C. Concentrated waters were fractionated in regards to molecular weight (MW) with columns packed with Sephadex G-25 and G-75 gels. The fractionated samples were chlorinated for 96 hours and THM production was determined. A preliminary investigation explored the effect of coagulant alum concentration in regards to removal turbidity, TOC and THM precursors from VPI Duck Pond water. A second preliminary investigation involved determining the MW chromatographs of raw and treated Claytor Lake and Peak Creek waters concentrated to different degrees. The results indicated that THM precursors and heavier MW organic substances were preferentially removed from water by alum coagulation; that conducting alum coagulation at pH 5.2 - 5.3 induced more effective turbidity, TOC and THM precursor removals; and that TOC concentration was a more effective surrogate parameter than turbidity for determining the removal of THM precursors. / Master of Science