Tracing technique review: Contamination in municipal separate storm sewer systems (MS4s).

Municipal Separate Storm Sewer Systems (MS4s) are designed to route and discharge stormwater runoff directly into local water bodies. However, contamination from sources such as urban runoff and illicit connections conveyed through MS4s poses serious environmental risks, underscoring the need for source tracing research. This paper presents a comprehensive review of research conducted on tracing contamination in MS4s. The review includes a literature review, an analysis of water quality and hydrological data, and an analysis of tracing techniques, with the aim of identifying research gaps and proposing future directions. The findings indicate a growing research interest in MS4-related contamination over the past decade. Various types of water quality and hydrological data are shown to play distinct but critical roles in tracing efforts. Tracing techniques fall into three categories: (1) source identification, providing quantitative insights into source contributions; (2) movement tracking, integrating conventional approaches with computational models to estimate source locations; and (3) risk analysis, assessing current conditions and predicting future risks. Each category of techniques has its strengths and limitations, and their integration may enhance overall tracing effectiveness. This review also highlights key research gaps, including underutilization of water quality and hydrological data, insufficient integration of economic considerations, limited accuracy of tracing techniques, continued reliance on conventional methods, and the absence of a systems-level perspective. Future development should therefore focus on integrating economic analysis and a system-level perspective with more accurate, data-driven tracing techniques to achieve effective and sustainable MS4s pollution control.