The effect of blending different water qualities on distribution system water quality was investigated in a field study. Waters produced from five different treatment systems; aeration (G1), nanofiltration (NF0 (G4), CSF-O3-GAC (coagulation sedimentation filtration-ozone-granular activated carbon (S1), IMS (integrated membrane system)(CSF-NF, or S2) and high pressure RO were blended and distributed to 18 different pilot distribution systems (PDS). G1, G4, and RO source waters were taken from the same groundwater. The RO source was augmented with salts to simulate seawater. The S1and S2 source waters were taken from the same surface water. The PDSs consist of PVC, galvanized, lined ductile iron and cast iron pipes taken from existing distribution systems and had a 5-day HRT. Chloramines were added to meet CT and residual requirements. AOC (assimilable organic carbon), BDOC (biological dissolved organic carbon), HPC (heterotrophic plate count)are measures of suspended biological activity. PEPA (potential of exoproteolytic activity) measures attached film bioactivity. The S2 and G4 AOC are essentially equal, but the RO (reverse osmosis) AOC is reduced and less than other treatments. S1 and S2 AOCs are equal. PDS biostability order is G1