The objective of the project is to estimate the environmental risk of produced water discharges, emphasising long term exposure to low concentrations of chemicals, exposure to time variable concentrations and exposure to mixtures of chemicals.
A three year programme was set up for the development of the model, carried out as a joint research project between Norwegian oil companies (Statoil, Norsk Hydro, Elf, Agip) and several research institutes (Sintef, TNO, Rogaland Research).
The DREAM-model is based on a dynamic and 3 dimensional dispersion module, capable of modelling the spatial transport of the individual components in the discharge plume over time. Processes as (bio-)degradation, volatilization and adsorption are taken into account.
In reality exposure concentrations are far from constant. To make a refined risk assessment (focussing on actual instead of potential risk) risk assessment models should take this variability into account. In DREAM, risk calculations are based on body burdens, dynamically calculated using uptake and elimination kinetics. Subsequently, body burdens are compared with the critical body burden to identify the expected environmental impact.
The figures below (Figures 1, 2, 3 and 4) show the principles of risk assessment in the DREAM-model, following the DEB-Theory of Kooijman (1996) for survival. For sublethal endpoints, comparable models can be applied. The top figures are based on (continuous) exposure to a constant concentration of a chemical, while the bottom figures are based on time variable exposure. The relationship between (external) exposure concentration (Ce) and time is depicted in Figure 1. Using uptake and elimination kinetics, the external exposure concentration can be transformed into internal concentration (BB: Body Burden) as a function of time (Figure 2). Comparison of the BB with an internal threshold concentration for survival (CBB: Critical Body Burden) yields the probability for an organism to die at a specific moment in time, indicated by the Hazard Rate (HR) (Figure 3). It is assumed that below this internal threshold concentration no effects will occur. Finally, the HR determines survival as a function of time (Figure 4).
Environmental risk in DREAM is defined as ‘probability of sublethal adverse effects on representative species of the ecosystem’. This effectively means that effect data on sublethal effects as growth and reproduction will be used to calibrate the model. Furthermore, DREAM estimates the effects on a large number of simulated individuals to obtain a probabilistic expression of environmental risk.
Preliminary calculations with a prototype of the risk model (implemented in ModelMaker®) have indicated that the methodology as described in this poster is very suitable to be applied for dynamic risk assessment. At this moment, the risk model is being integrated with the dispersion model for fully dynamic risk calculations. The greatest challenge is the availability of effect data to calibrate the model. Where currently used risk assessment models are usually based on NOEC and EC50 data, the DREAM-model requires time-related dose response curves. A major objective of the DREAM project is therefore to generate these data for chemical compounds relevant for produced water discharges.