Field Application of a Steady-State Mass Balance Model for Hydrophobic Organic Chemicals in an Estuarine System

Abstract

A one-dimensional, steady-state mass balance model was applied to describe instream and sediment concentrations of four hydrophobic organic chemicals in a discharge canal and receiving water bayou. The chemicals examined were hexachlorobenzene, hexachlorobutadiene, hexachloroethane, and 1,2,4-trichlorobenzene. The objective of the study was to test a mass balance modeling approach for relating point source effluents to resulting sediment concentration, in support of future implementation of national sediment quality criteria (SQC). The modeling effort relied upon ambient monitoring data that were collected for purposes other than supporting a modeling effort. Given data uncertainties and assumptions in the modeling framework, model results were reasonably consistent with observations in the receiving water bayou. There were large discrepancies between model results and observed sediment concentrations in the discharge canal. These discrepancies are likely caused by an undocumented source of chemicals to the canal sediments, due to historical landfilling of wastes; and/or canal sediment concentrations being in temporal disequilibrium, due to historically higher chemical loading. The merits of steady-state versus time-variable models for describing sediment quality are compared. Steady-state models are most appropriate for effluent permitting purposes, where the objective is to determine the long-term relationship jbetween wastewater loads and resulting sediment concentrations. A time-variable model framework will be required to establish model credibility for situations where the steady-state assumption is violated, although the required information on historical chemical loads is often unavailable.