Identifying OSTDS Vulnerability Under Sea‐Level Rise Using GIS‐Based Multi‐Criteria Evaluation: Effects of Groundwater Response

ABSTRACT Coastal, low‐lying Onsite Sewage Treatment and Disposal Systems (OSTDS) are increasingly vulnerable to sea‐level rise (SLR) and groundwater inundation. This study developed three spatially explicit GIS‐based multi‐criteria evaluation frameworks: Bathtub‐Current, Bathtub‐Future under 2 ft (0.6 m) of SLR and MODFLOW‐Future under 2 ft (0.6 m) of SLR, which assessed OSTDS vulnerability for current and future SLR and how alternative representations of SLR‐modeled groundwater response affect vulnerability classification and prioritization by integrating SLR responses, depth to water table, saltwater intrusion, recurrent flooding, storm surge and OSTDS density using Delphi elicitation, pairwise comparison weighting and Weighted Linear Combination. At 2 ft of SLR, the bathtub approach reproduced 86% of the MODFLOW inundation extent. The three vulnerability surfaces showed strong agreement, with Pearson correlations of 0.966–0.994 and RMSE values of 0.036–0.077, although individual OSTDS classes differed. At least one framework classified 1961 OSTDS as highly vulnerable, while all three frameworks classified 577 OSTDS as highly vulnerable, among the 56,080 OSTDS evaluated. Results show hydrostatic modeling is an acceptable screening tool for regional vulnerability assessments; however, it is not interchangeable with groundwater modeling or field verification. The framework provides a screening tool with a tiered approach to prioritize monitoring and resource allocation for at‐risk OSTDS.