Mass balance modeling to elucidate historical and continuing sources of dioxin into an urban estuary

• Mass balance spreadsheet model developed in Excel.
• Integrates spatially and temporally dioxin data from RMA2–WASP models.
• Extensive 2002–2005 dioxin dataset for calibration (tissue, sediment and water).
• Goal to evaluate loading from runoff, effluent, air deposition, sediment resuspension.
• >60% of loading attributed to sediment; other loads 1 order of magnitude smaller.

Polychlorinated dibenzo-p-dioxins and dibenzofurans (dioxins) are typically found in sediment, water and tissue as in the case of the Houston Ship Channel and Upper Galveston Bay (HSC–UGB) in Texas studied in this research. While hydrodynamic and fate and transport models are important to understand dioxin distribution in the various media, it is difficult to assimilate modeling results into a decision framework without appropriate tools that can aid in the interpretation of the simulated data. This paper presents the development of a mass-balance modeling tool linked to RMA2 and WASP models of the HSC–UGB system for 2002–2005. The mass-balance tool was used to aggregate modeling results spatially and temporally and estimate the relative contribution of sediments to dioxin loading into the Channel in comparison to runoff, deposition, and permitted effluent discharges. The total sediment associated-dioxin load into the system calculated using the mass balance model was 2.34 × 107 ng d−1 (almost 86% of the toxic equivalent load), and the re-deposited load to the sediment from the water column was 1.48 × 107 ng-TEQ d−1, such that 8.6 × 106 ng-TEQ d−1 or approximately 69% of the average daily dioxin flux is transported between model segments as sediment. The external loads to the system contribute approximately 3.83 × 106 ng-TEQ d−1, a value that is an order of magnitude smaller when compared to the contribution from sediment. These findings point to the need for sediment remediation strategies that take into account the spatial locations within the system that serve as sediment sources to dioxin in the water column.