Modellling the benthic impact of coastal discharges
Marlow, David R.
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The discharge of effluent into coastal waters can lead to environmental degradation, and proactive management of this practice is therefore desirable. This requires a capacity to predict environmental impacts. Benthic, impacts can be detected in the redox state of the upper part of the sediment column. A mechanistic model has therefore been developed that describes vertical gradients of sedimentary redox intensity (Eh). The model is essentially a one dimensional multi-component reaction-transport model. Bioturbation is represented using a simple Fickian analogue, and irrigation is modelled as a non-local exchange process. Degradation of organic matter is characterised as six sequential metabolic reactions. Monod formulations combined with an inhibition scheme are used to model the utilisation of electron acceptors. 'Various other reactions are also included. The model equations are discretised. using finite differences, and solved for both steady state and transient cases. The redox intensity is calculated from, the predicted distribution of oxidants and reductants using a simple Nerstian approach. Consequently, only a limited interpretation can be placed on the calculated Eh profile. A theoretical investigation of the model response demonstrates that both high data requirements and uncertainty in parameter values impedes the predictive application of the redox model. Hence, whilst the model can describe the development of vertical redox, gradients, and therefore has some diagnostic capacity, it does not represent a viable tool that would aid in the proactive management of coastal resources. An alternative modelling approach is, however, suggested that would use predicted carbon fluxes to give a qualitative expression of benthic impact