This study evaluated the use of nanofiltration (NF) membranes to reduce trihalomethane formation potential (THMFP) in drinking water sources along a hydric system. Ten NF membranes were tested on pre-chlorinated water and showed high rejection of THM precursors, though permeability varied. NF270, ESNA1LF2 and TFC-SR100 membranes were selected for further study based on conductivity rejection capabilities. Water reservoirs significantly increased THM precursor levels. Smaller conductivity rejection by ESNA1LF2 resulted in lower THMFP reduction, demonstrating the relationship between conductivity rejection and disinfection byproduct reduction by NF membranes.