A practical GIS-based hazard assessment framework for water quality in stormwater systems

A practical 4-stage-3-model framework is introduced that models and maps pollutant concentrations in, and potential hazards of, municipal stormwater point-source releases to surface water bodies. The framework consists of a stormwater model, a stormwater quality model, geographic information systems (GIS) mapping models, and a hazard assessment procedure based on event mean concentrations (EMC) and fuzzy logic. This framework was applied to project both outfall concentrations and environmental hazards of a degradable point-source pollutant, monochloramine, in stormwater effluents released to the North Saskatchewan River, Alberta, Canada, under different weather conditions. Monochloramine loads were over the permissible limits under dry weather flows throughout the basin, and only a design storm of a 10-year return period produced monochloramine concentrations below regulation discharge limits. An EMC-based hazard map was developed that revealed 25% of the study area to lie within the moderate to high-risk zones. EMC-based hazards were driven primarily by proximity to the system outlet, total pollutant mass and system layout. Next, a generalized, fuzzy-based monochloramine hazard map was developed, using monochloramine decay-inducing factors such as annual rainfall, land-use type, EMC values, ground slope, decay rates, property assessment values, drainage system proximity and density of impervious areas. This map showed 54% of the basin at moderate to high risk. The maps developed using the proposed 4-stage-3-model framework can help system operators to improve management of stormwater pollutants without time- and labor-intensive, case-by-case calculations and to focus mitigation measures on releases from specific "hot spot" locations in the system. The framework can be generalized to other chemical substances that impair stormwater quality through either point or non-point source pollution. (C) 2019 Elsevier Ltd. All rights reserved.