How to simulate pollutant partitioning in OpenFlows FLOOD


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

Problem

How to simulate pollutant partitioning in OpenFlows FLOOD?

Solution

OpenFlows FLOOD can simulate different types of pollutants in different types of conditions and using the 2 different engines (MOHID Land and MOHID Water).

Rivers can be simulated using both engines, it depends on the type of engine (and engine setup). Also depending on the type of engine/module and type of simulation some options are available via the UI (property grid) others are only if the user setups the options using the text file editor. 

MOHID Land can simulate rivers in 1D (using the Drainage Network module) and 2D (RunOff module). In 1D, it can simulate fecal contamination decay (fixed or variable T90), BOD/oxygen, full nutrient cycling (algae, zooplankton, oxygen, organic matter, nutrients, etc). It can also simulate generic first order decay and cohesive (fine) sediment transport. This 1D module is always coupled to the 2D hydraulic engine in MOHID Land (module RunOff. In some cases, it can be also run coupled to the RunOffProperties module). 

MOHID Land RunOff can simulate river hydraulics in 2D also and the RunOffProperties module can compute transport of water constituents in 2D (using the RunOff module 2D hydraulics flow and volumes). It can only simulate simple fine sediment transport and simple partitioning (particulate/dissolved phases of pollutants – same as described in the wiki article referring to partition in coastal waters for the MOHID Water engine). It’s not recommended to use this approach because the RunOff module is optimized to simulate overland flow (runoff) and not river hydraulics, but the solution will still be valid. 

MOHID Water can simulate river flows in 2D and 3D. It can simulate fecal contamination decay (fixed or variable T90), BOD/oxygen, full nutrient cycling (algae, zooplankton, oxygen, organic matter, nutrients, etc). It can also simulate generic first order decay and cohesive (fine) sediment transport, and partitioning (dissolved/particulate phases of hydrocarbons, heavy metals). It can also use the Lagrangian module to simulate contaminants dispersion (fecal contamination, BOD, partitioning, oil spills, etc.).,

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.)