The Enhanced Coagulation requirements in the Stage 1 Disinfectants and Disinfection Byproducts (D/DBP) Rule were written to take into account variations in raw water quality nationally and, specifically, the difficulty in removing total organic carbon (TOC) from high alkalinity, low TOC waters. These are exactly the types of water treated by each of four surface water treatment plants operated by the Indianapolis Water Company (IWC). Jar tests conducted on source water from one IWC treatment plant in 1996 and 1998 had indicated that the water contained TOC that was "not amenable to enhanced coagulation" as defined in the Stage 1 D/DBP Rule. An evaluation of full-scale plant data was conducted and additional jar tests were run to expand these findings to other IWC treatment plants and build a database for a potential waiver application. However, the findings from these evaluations indicate that demonstration of compliance for some IWC facilities may be more difficult than expected. Due to the inconsistent quality of the source water, the TOC removal requirement determined from jar testing varied significantly. For three series of jar tests conducted within a one-month period on the supply water for one plant, the resulting Step 2 alternate minimum TOC removal requirement ranged from 11% to 32%. The TOC removal in the plant likewise varied significantly during this test period. However, on the day of each jar test, the TOC removal in the plant exceeded the alternate TOC removal requirement determined from the jar test. The implication is that given the source water quality, the treatment plants are achieving the goals of the Enhanced Coagulation requirements. However, demonstrating compliance could be problematic and will likely have to depend on specific procedures negotiated with the State. An alternative minimum TOC removal value, established as allowed in the regulation, may only be appropriate for the given day of the jar test. Compliance determinations made based on this alternative minimum TOC removal value could result in inappropriate conclusions regarding compliance status. Includes 3 references, tables, figures.