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Kim ey>>> Horn Page 26 The National Primary Drinking Water Regulations (NPDWRs) establish monitoring and other requirements for municipalities to achieve compliance with maximum contaminant levels based on locational running annual averages (LRAA) for total trihalomethanes (TTHMs) and haloacetic (HAAS) and maximum residual disinfectant residuals. The USEPA determined that regulating these two groups of DBPs would yield an overall reduction in all DBPs and set MCLs of 80 ppb and 60 ppb for TTHMs and HAA5s, respectively. Operational Evaluation Levels (OELs) are also established to provide greater level of guidance for water purveyors through putting higher weight towards the most recent quarter of DBP concentrations. These requirements are defined under the Stage 1 and Stage 2 Disinfection Byproducts Rule. The intent of this rule is to improve protection of public health through reducing exposure to disinfection byproducts, which may cause liver, kidney or central nervous system issues. DBP formation is a function of total organic carbon (TOC) and bromide concentrations, disinfectant utilized and water age. Drinking water systems utilize disinfection to inactivate, or "kill," viruses or organisms to protect customers from waterborne diseases and pathogens. IRCU utilizes source water from the upper Floridan Aquifer (UFA) for treatment, which is relatively free of TOC. The source water has hardness and alkalinity that is helpful in stabilizing the finished water, but also has bromide (ranging from 1-3 mg/L) which is a precursor to DBP formation. The membrane softening system rejects bromide similar to calcium, allowing for reduction in bromide species, but the pre-treated raw water bypass flows utilized for stabilization yield bromide concentrations, albeit minimal, in the finished water resulting in DBP formation. In reviewing the 2016-2021 DBP compliance sampling data, bromide species DBPs attribute to high percentage of the overall HAA5 and TTHM concentrations. Previous recommendations have been made, and implemented, to reduce blend water flows, thereby lowering the overall bromide concentrations in the product water and yielding reduced DBP formation. As discussed previously, there is an ongoing project to increase membrane softening capacity and reduce blend flows at Oslo to approximately 17%. The loss in alkalinity through reduced blend flow rate is to be offset through operational changes to increase carbonic acid solution and caustic feed for post-treatment. This change in blend percentage should be mimicked at Hobart to maintain consistent water quality throughout the system. The following graphs display the LRAA and OEL values from 2016 — 2021 for the six (6) distribution sites for TTHMs and HAA5s.