Water Quality Processes

The 1D Water Quality Solver is capable of modelling a range of water quality variables and processes simultaneously. These include:

1. Conservative pollutants
2. Decaying pollutants
3. Coliforms
4. Salt
5. Water temperature
6. Sediment
7. Oxygen balance
8. Water/Sediment oxygen interactions
9. Phytoplankton
10. Macrophytes
11. Benthic algae
12. pH

The 1D Water Quality Solver is written in a modular fashion so that not all the processes need to be studied at once. However, the river or estuary environment is the environment in which some of these variables and processes interact. It therefore is not possible to run some modules without including other features. For instance, phytoplankton growth interacts with dissolved oxygen. In this overview the different modules will be briefly described. The dependency of the different modules on one another will also be explained.

For an in-depth view of the modelling methods and mathematical relationships between the water quality variables and pollutants please refer to Water Quality Definitions.

Conservative pollutant

The transport of non-decaying pollutants such as cadmium can be simulated in the 1D Water Quality Solver. The pollutant is assumed not to interact with other pollutants or variables in the model. This module can be run independently of the other modules.

Decaying pollutants and coliforms

Coliforms and some other pollutants decay by natural processes. The transport and decay of such pollutants can be simulated by the 1D Water Quality Solver. These modules may be run independently of the other modules.

Salt

The transport of salt is particularly important in estuaries. The longitudinal variation of salinity and hence density will affect the flow regime and the longitudinal dispersion coefficient. In addition, the amount of oxygen which may be dissolved in salt water is significantly less than that in fresh water. Therefore salinity variations are important when predicting dissolved oxygen levels in estuaries. Salt may be modelled as a constant background value or in a similar manner to the conservative pollutant (see 2.2.1). Simulated salinity values are used to determine the longitudinal dispersion coefficient. This module may be run independently of the other modules. The importance of salt in an estuary means that it is strongly recommended that salt is included in any estuary water quality model.

Temperature

Most of the biochemical and biological processes simulated in the 1D Water Quality Solver are temperature dependent, i.e. temperature affects the reaction rates of the processes. Temperature may be simulated as a constant background value or as a transportable variable allowed to vary in time and space. This may be of use in an estuary model if the difference between sea and river water temperatures is significant or seasonal temperature variations need to be accounted for. This module may be run independently of the other modules though it will usually be used in conjunction with the dissolved oxygen, phytoplankton, benthic algae and macrophyte modules.

Sediment

Suspended solids or mud are transported in the same way as conservative pollutants or salt. In addition to advection mud is allowed to settle on to the river bed where it can consolidate or be eroded back into the water column depending on the flow regime. The trapping of pore water in the bed layers is also simulated. This module may be run independently of the other modules. If the interaction of oxygen balance and sediment or phytoplankton modules is needed then this module must be included in the simulation.

Dissolved Oxygen

Oxygen balance in the 1D Water Quality Solver is governed by the transport, dispersion and interaction of dissolved and particulate organic matter and oxidizable nitrogen. The dissolved oxygen module includes only the dissolved variables, i.e. fast dissolved BOD, slow dissolved BOD, fast organic nitrogen, slow organic nitrogen, ammoniacal nitrogen, nitrites, nitrates and dissolved oxygen. Dissolved oxygen enters the model by reaeration at the water surface and from inputs and is consumed by the decay of BOD and the nitrification of ammoniacal nitrogen to form nitrites and nitrates. Ammoniacal nitrogen is formed by the decay of organic nitrogen. At low dissolved oxygen levels, denitrification is simulated as BOD consumes oxygen from oxidised nitrogen. If no nitrates or nitrites are available for denitrification then oxygen demand is satisfied by the reduction of sulphates. The reaction rates simulated in this module of 1D Water Quality Solver are temperature dependent. This module must be run in conjunction with the temperature module. The effect of salinity on saturated dissolved oxygen concentrations means that it is also essential to run this module in conjunction with the salt module for all estuary or salt water applications.

Structures

When simulating oxygen balance additional reaeration may be generated at structures such as weirs or sluices. This can be simulated in the 1D Water Quality Solver by specifying an additional reaeration parameter for each structure. This is done in the structure module. This module is used in conjunction with the dissolved oxygen module.

Oxygen with Sediment

In the 1D Water Quality Solver, the oxygen balance processes can interact with the sediment processes. In addition to the variables simulated in the dissolved oxygen module, fast particulate BOD and slow particulate BOD are also simulated. Particulate BOD exerts an additional oxygen demand on the water column and can settle onto or be eroded from the river bed. Dissolved variables such as nitrates can be trapped in the pore water. The oxygen balance processes described in the dissolved oxygen module are also simulated in the bed by the oxygen with sediment module. The 1D Water Quality Solver calculates the sediment oxygen demand exerted on the overlying water. This module must be run in conjunction with the dissolved oxygen module.

Phytoplankton

Primary productivity (growth) is the process by which organic material is synthesised from inorganic compounds. This is undertaken by plants and algae which take up nutrients from the water column in the form of nitrates and phosphates and using energy from sunlight grow through photosynthesis. Oxygen is produced as a by-product. The nutrients are released back into the water column by the process of respiration and the decay of detritus. The detritus is simulated as particulate matter which can settle onto and be eroded from the bed as it decays. These processes are simulated in the phytoplankton module of the 1D Water Quality Solver which simulates algae as a single representative species. This module must be run in conjunction with the oxygen with sediment and the solar radiation module.

Macrophytes

Macrophytes are large plants which are rooted in the bed of the river. They take up nutrients through their roots from the pore water. As with phytoplankton their growth is limited by sunlight and nutrient concentrations. This module must be run in conjunction with the phytoplankton module.

Benthic Algae

Some algae live on the bed of the river. Their primary productivity is simulated in the benthic algae module. All the processes simulated by the phytoplankton module apply to benthic algae except that the algae are assumed to live on the bed material. They therefore may be taken up into suspension when the bed is eroded and then be advected before settling back on to the bed in another part of the river system. This module must be run in conjunction with the phytoplankton module.

Solar radiation

Primary productivity of phytoplankton, macrophytes and benthic algae is dependent on the presence of sunlight. The amount of sunlight is represented by solar radiation. The solar radiation is used to calculate the light limitation factor for primary productivity of phytoplankton, macrophytes and benthic algae. This module is required by the phytoplankton module.

Silicate

The phytoplankton, macrophyte and benthic algae modules all assume that nitrates and phosphates are the most important nutrients for primary productivity and are the ones that will in general limit growth. Silicates are important in the growth of diatoms as their cell walls are impregnated with silicon. If it is thought likely that silicate will act as a limiting nutrient for primary productivity then it should be included in the simulation via the silicate module. The silicate module must be run in conjunction with the phytoplankton module.

Adsorbed phosphorus

Phosphorus can be adsorbed on to the surface of sediment particles. Phytoplankton, macrophytes and benthic algae can not easily access this form of phosphorus. Dissolved phosphorus in the form of orthophosphate is the most readily available form for uptake by algae and plants. The 1D Water Quality Solver can simulate the adsorption of phosphate on to sediment particles and its desorption back to the dissolved state. This process can play an important role in limiting the supply of phosphate to plants and algae. This module must be run in conjunction with the phytoplankton module.

pH

The pH indicates the acidity or alkalinity of the water body. Within the 1D Water Quality Solver, it is used to calculate the proportion of ammoniacal nitrogen that is unionised. This proportion is determined from a function of pH and temperature. This module must be run in conjunction with the dissolved oxygen module.

Figure 1 summarises the dependency of the different modules to one another. For example, in order to simulate benthic algae you also need to model phytoplankton, solar radiation, oxygen with sediment, sediment, dissolved oxygen and temperature. Alternatively, if you need to model pH you will need to model dissolved oxygen and temperature, while the modelling of salt does not require the simulation of any other process.

Figure 1. Dependency of the different modules of the 1D Water Quality Solver (e.g. this graph shows that the Benthic Algae module is dependent on the Phytoplankton module which in its turn is dependent on the Oxygen with Sediment module etc).