Water discharges at sea: How to estimate their real impact?
Authors: Guillerme M. , Tozzolino P. 
Published: December 2007

The approach adopted for assessing actual impact of water discharges at sea draws on the risk assessment process.

Direct effects or impacts are estimated both by acute toxicity tests and by the use of various biomarkers (mussel growth, Erod, Ache).

Future outcome and long-term effects are evaluated by taking into consideration the bioaccumulation and biomagnification processes in the food chain.

A continuous flow system was especially designed for conducting controlled and reproducible long term toxicity and bioaccumulation experiments on marine organisms.

Kinetics of bioaccumulation in the flesh of turbot, of alkyl phenanthrenes and chrysene are given as an example.

Introduction

Exploring for and producing oil and gas involve discharge to sea of a number of residues. These include, amongst others: drilling muds and cuttings which may contain hydrocarbons and surface active chemicals; produced waters at varying degrees of salinity which may contain hydrocarbons and residual treatment products. Accidental discharges (chiefly hydrocarbons) may also occur, with the possibility of substantial consequences on both the Environment and industry.

Non-accidental discharge to sea is controlled and subject to obligations or recommendations issued by governmental bodies or to international conventions (OSPAR). Most of the recommendations were based exclusively on the measurement of quantities discharged and on their hydrocarbon content. The impact of discharge was often appraised only by monitoring of the state of the sea floor and benthic fauna; little was done with respect to water and pelagic fauna.

Present trends, particularly in North Sea countries and as regards discharges of chemicals, are to characterise the substances physically and chemically, to describe the processes involved in their dilution/diffusion in the medium, to test them for toxicity, to evaluate accumulations of residues in the food chain and to assess degradation. However this may be the burning issue, and the ever more topical question which must be answered is: How could we assess the real impact of our discharges on an ecosystem? The reply to this question would enable the oil and gas industry to choose the best practicable environmental solution. It would further allow them to avoid excessive build-up of treatment installations and, above all, to lay down realistic rules and standards for discharge, acceptable by all. So how can we assess the real impact of our discharges? It is in an attempt to answer this question that we have launched several studies and research projects in Norway.

Methodology

The adopted approach draws on the risk assessment process; it is based, firstly, on the evaluation of immediate, toxic effects of discharge and, secondly, on the estimation of medium and long term effects (see fig. 1).

Direct effects are estimated by measuring the acute toxicity of the fluids (OSPARCOM tests for instance) and will, in the future, be estimated by carefully selected biomarkers. As for indirect (or long-term) effects, these are assessed on the basis of the capacity of the constituents in the discharge to be eliminated or accumulated in a food chain. Evaluation of direct effects

Acute toxicity tests. The objective of the toxicity test was to study how test organisms respond to different products or chemicals which are liable to be discharged to the sea. P. 247^