Louisville Water Company and Hazen and Sawyer are planning a project to clean accumulated sediment out of and re-line a 110 MG reservoir. Louisville Water and Hazen worked to characterize concentrations of PFAS and metals which may dictate disposal of reservoir sediments. Louisville Water employed a probabilistic approach to sampling its reservoir sediments in order to have an adequate level of confidence that the analytical results. The approach to characterizing reservoir sediments is fairly straightforward. The reservoir was gridded into 294, 50 ft2 sections, and each section was assigned a sequential number. Randomly generated numbers correspond to the number and order of the grids to be sampled. Selected grids are excluded if there was a safety concern, impediments to sampling, or if adequate sample material was not attainable. In the case of a grid section being excluded, the next grid section on the random number list is to be sampled. Three core samples were collected around the centroid of 15 randomly selected grid sections. Composite samples were sent to a contract lab (Eurofins) for analysis. Eurofins utilized TCLP to analyze the sediment for RCRA metals. Sediments were also analyzed for PFAS, as was TCLP eluant using EPA Method 1633 to determine leachability of PFAS species. The percentage confidence in the data results corresponds to the number of grid sections randomly sampled. Per Slovin’s Equation (n = N/(1-Ne2), a grid section population of N=294, a 75% confidence level (e=0.05) was implemented in this study only required 15 samples. EPA is ramping up its regulatory approach to PFAS, specifically PFOA and PFOS. EPA’s regulatory actions include its published intention to designate PFOA and PFOS as hazardous substances under CERCLA. In 2024, the EPA also proposed listing nine PFAS species as hazardous constituents under RCRA further cementing the regulatory pressure associated with PFAS-impacted residuals. EPA’s new PFAS hazardous substance designation under CERCLA may have profound consequences for water utilities experiencing even low levels of PFAS in their source water, both in disposal costs and potential long-term liability obligations. Future regulatory obligations regarding water treatment residuals remain uncertain. Adequately characterizing these residuals are important to determining operational and project costs for utilities. Very limited systematic data has been developed for drinking water treatment residuals. This probabilistic pathway to sampling and using TCLP eluant to characterize residuals for RCRA metals and PFAS provides a systematic and predictive tool for utilities to determine regulatory obligations and anticipated operational costs of disposal. We will present an overview of the regulatory developments pertaining to PFAS-impacted waste streams and the applicability of these regulations to treatment residuals to characterize the relative risk to public utilities. This will include an explanation of the sampling methodology and results to inform current disposal availability and future susceptibility of these waste streams to additional hazardous designations. Finally, this presentation will highlight several “what-if” scenarios for utilities who may face uncertainty regarding their current and future waste disposal practices and what increased regulatory pressure on hazardous waste streams could mean for future expenditures.