The improvement of techniques to determine Predicted Environmental Concentrations for risk assessment of produced water discharges, has resulted in the developed of a facility-specific environmental transport fate and bio-impact model. The model combines physical dilution and dispersion with a geochemical model predicting sediment-water partitioning, volatilisation and degradation processes. The resulting exposure concentrations are then used in the bio-impact model which predicts bio-uptake and consequent toxic effects.
Considerable efforts have been made in validating the model at every predictive stage with laboratory and field studies. Data from these validation trials is presented in this paper together with a view on the application of the modelling techniques in offshore produced water risk assessment.
Production waters discharge from offshore production facilities has been an accepted practice for many years but the environmental fate and impact of these waters is now coming under increased scrutiny. Although improvements in wastewater engineering have increased the removal of suspended solids and hydrocarbon fractions, this is against a background of increasing water volumes within aging reservoirs. In addition, advances in analytical chemistry allow detection of trace waste water components, some of which may have potential adverse impacts. Increasingly, risk assessment is being applied to aid the regulation and management of discharges. In Europe this has been focused through the pan-European CHARM project. Although the approaches used in CHARM provide a good basis for hazard assessment, risk analysis is strongly hampered by simplified approaches to the determination of Predicted Environmental Concentrations (PECs).
Production waters present considerable challenges when considering their fate and effects in the marine environment. In addition to the hydrocarbon fraction, these waters also contain residues from many speciality chemicals used in the production process. Before waters are even discharged, complex reactions between the various waste stream components take place making quantification of exactly what has been discharged a difficult problem. Following discharge, the waters are subject to mixing and dispersion in a highly dynamic physical environment and chemical transformation through a range of degradation processes. Although degradation can aid the removal process, it results in the formation of metabolites which in themselves can be more toxic that the precursor compound. The effect of produced waters on biota is further complicated by the diversity of species, different uptake mechanisms and variable uptake rates, and a multitude of acute and chronic toxic effects.
Against this background, the prediction of fate and impact might seem an impossible task. This paper describes experimental and modelling approaches which have allowed PEC determination by a combination of theoretical and empirical approaches.
Two underlying principles have been applied in the approach to PEC determination. The first is to treat the overall process in stages. In this way individual components can be validated and a measure of the confidence in predictions obtained. The approach also allows useful information to be obtained at every stage which can contribute to understanding of weaknesses in the overall system. An example of this approach is in the modelling and validation of exposure concentrations as a precursor to uptake and bio-impact modelling.