FIELD APPLICATION OF A STEADY-STATE MASS-BALANCE MODEL FOR HYDROPHOBIC ORGANIC-CHEMICALS IN AN ESTUARINE SYSTEM

A one-dimensional, steady-state mass balance model was applied to desc ribe instream and sediment concentrations of four hydrophobic organic chemicals in a discharge canal and receiving water bayou. The chemical s examined were hexachlorobenzene, hexachlorobutadiene, hexachloroetha ne, 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 implementati on of national sediment quality criteria (SQC). The modeling effort re lied upon ambient monitoring data that were collected for purposes oth er than supporting a modeling effort. Given data uncertainties and ass umptions in the modeling framework, model results were reasonably cons istent with observations in the receiving water bayou. There were larg e discrepancies between model results and observed sediment concentrat ions in the discharge canal. These discrepancies are likely caused by an undocumented source of chemicals to the canal sediments, due to his torical landfilling of wastes; and/or canal sediment concentrations be ing in temporal disequilibrium, due to historically higher chemical lo ading. The merits of steady-state versus time-variable models for desc ribing sediment quality are compared. Steady-state models are most app ropriate for effluent permitting purposes, where the objective is to d etermine the long-term relationship between wastewater loads and resul ting sediment concentrations. A time-variable model framework will be required to establish model credibility for situations where the stead y-state assumption is violated, although the required information on h istorical chemical loads is often unavailable.