Dynamics of POPs Distribution in Sea Water

MSC-E Technical Note 1/2001

B.Strukov

ABSTRACT

The integration of global economics and a great advance in the development of information technologies during recent decade motivated the emergence of various international organizations, which objective is the evaluation and monitoring of global processes in the natural environment. The monitoring of persistent organic pollutant (POP) dispersion in the atmosphere, soil, water on the global scale is one of the directions of these organization activities.

The availability of modern powerful computers allow to devise models of POP global dynamics with a high degree of detalization in space and time. Within the framework of the EMEP, Meteorological Synthesizing Centre-East has developed a three-dimensional model of POP dynamics in the atmosphere, soil, vegetation and marine environment within the geographical scope of Europe, the North Atlantic and European Arctic with horizontal resolution about 150x150 km². This model is under operation.

Main task of this work is to improve the model description of POP distribution in the marine environment.

The transport of pollutants in the sea basin of the EMEP region as well as POP fluxes through the sea surface are described by the sea module of the model. The marine environment in POP transport problems is characterized by the capability to accumulate a large share of a pollutant with its subsequent transfer to other media [Wania, 1998].

The characteristic time of POP degradation in sea water can be as much as decades.

The sea module considers processes of POP three-dimensional turbulent diffusion, advection with dynamic fields of sea currents, POP degradation and a pollutant flux through the sea surface.

Model calculations with the sea module [Strukov et al., 2000] qualitatively correctly describe POP concentration field in the marine environment and are in reasonable agreement (within an order of magnitude) with measurement data. In the module, however, it is assumed phase an organic pollutant is in the dissolved state. It should be mentioned that in the marine environment more than 90% of POPs can be associated with particulate matter and aggregated to dissolved organic compounds [Schulz-Bull, 1998]. Between POP bound molecules and POP in the solution the dynamic redistribution takes place. As a first approximation it may be assumed that this distribution is in equilibrium, i.e. POP concentration associated with particulate matter and dissolved organic compounds is proportional to POP concentration in the dissolved phase.

At a sufficiently large share of POPs associated with sinking particles the export of a pollutant out of the grid together with particles can essentially affect the mass balance in the water medium. POP molecules associated with particles and dissolved organic compounds of a large molecular mass do not take part in the exchange process ot the water-air surface thereby altering the parameters of POP flux through the sea surface.

Particle suspension in sea water consists of micro-organisms (phytoplankton, bacterio-plankton, micro-zooplankton), organic (products of plankton vital activity), inorganic particles connected with organic material (particles deposited from the atmosphere, products of denudation processes).

The highest concentration of particles and, consequently, the largest POP share on organic carriers are characteristic of the upper oceanic layers, shelf and productive zones, i.e. of zones essentially affecting POP concentration dynamics in the model (oceanic upper layers). In regions with high particle concentrations and intensive POP fluxes from the atmosphere to the ocean together with particles, a great quantity of pollutants reaches the oceanic bottom, that may appreciably affect vital activity of benthos organisms.

Concentrations of micro-organisms, organic particles and dissolved organic compounds in sea water are determined by a scheme of plankton evolution in a biotope. Their adequate description at different meteorological and ecological conditions is possible by mathematical models for marine biogeocenosis.

The model developed consists of two parts: the first describing the dynamics of POP carriers and the second simulating on this basis the transport of POPs themselves.

The first submodel operates with a simplified system of plankton evolution in a biotope. It describes the dynamics of primary producer - phytoplankton, which concentration depends on fields of biogens and solar radiation intensity. In its turn phytoplankton evolution affects the concentration dynamics of bacterioplankton, detritus and dissolved organic compounds. Zooplankton dynamics is not considered since it is less important for POP transport. Concentration of all the fields considered are calculated with allowance made for three-dimensional advection and three-dimensional turbulent diffusion. Organic particle distributions also depend upon source intensities located in shelf zones and lower oceanic layers, when the lower boundary of the mixed layer reaches the bottom.

The field of inorganic particles incoming from the atmosphere, shelf zones and bottom sources (if mixing is acting throughout the water depth) is determined by advection, turbulent diffusion and sedimentation.

Tentative calculations show that the introduction of mechanisms of pollutant partitioning with the dissolved phase and the phase associated with particles and dissolved organic compounds can inflict an appreciable impact on pollutant concentration dynamics. With the availability of particles sedimented to the bottom there appears an additional mechanism of POP downward transport. Spatial distribution of POP concentrations vary due to a considerable difference in degradation rates of pollutants associated with carriers in comparison with that of a dissolved substance.

Seasonal variations of POP concentrations in sea water also undergo appreciable changes. In the majority of regions phytoplankton concentration is essentially increased during spring months ("spring blossom"). It leads to the increase of bacterioplancton, detritus and dissolved organic matter. These changes define annual concentration variations of POPs associated with carriers and POP dissolved phase.

For further refinement of the description of POP concentration fields in the marine environment within the considered domain the emphasis should be placed on the following:
- refinement of dynamic models, parametric coefficients for phytoplankton and bacterioplancton in the calculation region,
- description of biogen field dynamics,
- integration of microzooplankton to the dynamics model for better description of biogeocenosis in the considered biotope,
- parametrization of POP fluxes on the bottom of the water basin,
- more detailed description of particle and dissolved organic compound sources such as riverine runoff and tidal events,
- refinement of the content of suspended particles and dissolved organic compounds considered in redistribution of POP between dissolved and bounded states.

After refinement the model should be used both for more detailed investigation of POP cycling in the marine environment and for preparing fields of POP carriers (particulates and dissolved organics) to be used in operational POP models.