How to simulate pollutant partitioning in coastal waters

Product(s): OpenFlows FLOOD
Version(s): 10.03.XX.XX
Area:  Modeling

Problem

How to simulate pollutant partitioning in coastal waters?

Solution

With OpenFlows FLOOD you can simulate dispersion of different types pollutants (including heavy metals) in coastal waters using the MOHID Water numerical engine. A pre-requisite to model these types of processes is that the hydrodynamic and fine (cohesive sediment) transport model must already be implemented, calibrated and validated

The pollutant dispersion model assumes that contaminants (heavy metals, metalloids, pesticides or hydrocarbons), occur generally in the aquatic environment in two distinct forms: dissolved or adsorbed on to particulate matter (typically fine cohesive sediments). The dissolved phase is transported by currents and the particulate phase follows the transport patterns of particulate matter (currents, settling, deposition and erosion).The partition between the dissolved and particulate phases is controlled by a partition coefficient (which determines how much contaminant can adsorb on to sediments and how much stays dissolved). If the phases are not in equilibrium they will tend to that equilibrium at a defined adsorption/desorption rate. The partition coefficient and adsorption/desorption rate depend on many different factors (type of contaminant, physical, chemical and biological composition of the particulate matter, pH, temperature, salinity, etc.). In the model, it is assumed the that partition coefficient and adsorption/desorption are constant (defined by the user based on literature or lab/field experimental data). 

There are 2 main ways of simulating this type of dispersion, Eulerian (using the partitioning module in the WaterProperties configuration file) or Lagrangian (using Lagrangian particle tracking module together with the partitioning module). 

The Eulerian methodology is described below: 

In the WaterProperties configuration file 2 additional water properties must be defined: dissolved contaminant and particulate contaminant.

For both properties activate the following options:

  1. DISCHARGES                        : 1 (then in the Discharges module, the discharge parameters must be set (e.g. position of the discharge, flow, concentration of dissolved and particulate phases, etc.))
  2. PARTITION                             : 1
  3. PARTITION_FRACTION        : 0.3 (if for the dissolved phase the value of PARTITION FRACTION is 0.3, then for the particulate phase the value must be 0.7 - this means that, in equilibrium, 70% of the pollutant is found in the particulate phase). 
  4. PARTITION_RATE                  : 5e-5 (kinetical rate at which the adsorption/desorption systems tends to equilibrium - units is 1/seconds)
  5. PARTITION_COUPLE             : particulate contaminant (for the dissolved contaminant property) and PARTITION_COUPLE            : dissolved contaminant (for the particulate contaminant property

For the particulate property don't forget to set the keywords related with settling (VERTICAL_MOVEMENT        : 1 - and add the property to the FreeVerticalMovement module) and erosion/deposition fluxes (BOTTOM_FLUXES        : 1 - add the property in the InterfaceSedimentWater module to keep track of how much particulate contaminant is deposited in the bottom sediments, how much is eroded, etc.)

 

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