Constituents:Simulating Reactive Constituents in GSSHA

Reactive constituent transport can be simulated on the overland flow plane, the channels including reservoirs, and in the soil. Most commonly, constituents will be simulated in both the overland flow plane and in the channels. It is possible to simulate constituents in the channels alone, if a point source of contaminants is introduced to the channel. Simulation of constituents in the soil column requires that constituents are simulated on the overland flow plane as well. A description of the methods is described in detail in Downer 2009 WQ TN and Downer and Byrd (2007): TMDL TN

11.1.1 - Simulation of Constituents on the Overland Flow Plane

Constituents are simulated on the overland flow plane by including the OV_CON_TRANS card in the project file. The only way to specify parameter values for overland constituent transport is in the MAPPING_TABLE file. Details of the required MAPPING_TABLE inputs are specified in Section 12. The inputs required depend on the type of constituents selected for simulation. Constituents can be simple (first order) or NSM (full nutrient cycle).

Two types of reactive constituent transport are available in GSSHA. Constituents can be simulated as simple first order reactants with specified uptake rates from the soil and specified decay rates. The nutrient cycle can also be simulated with the Nutrient Simulation Model (NSM) (Johnson and Gerald, 2008). In either case, the overall simulation methods within the GSSHA model are the same. Only the rates of mass absorption and decay are different. It is therefore possible to simulate nutrients as simple constituents, as well as simulating them with the full nutrient cycle. It is up to the user to determine the appropriate level of chemical kinetics for the problem to be solved. More on both of these options is provided in subsequent sections.

In addition to reactions and transformations, contaminants on the overland flow plane are gained and lost due to:

1. Addition by rainfall
2. Uptake from the soil surface
3. Exchange with soil
4. Infiltration
5. Exchange with channels
6. Exchange with groundwater
7. Addition by point sources
8. Exchange with reservoirs

More detail on the methods used in GSSHA are provided in Downer and Byrd (2007): TMDL TN

11.1.1.1 - Addition by Rainfall

The concentration (g m³) of each constituent being simulated is constant throughout the simulation. The rainfall concentration is specified in the MAPPING_TABLE file. Rainfall inputs are added directly to the overland flow plane, where they may either infiltrate, or pond and produce contaminated runoff.

11.1.1.2 - Uptake from the Soil Surface

Contaminants on the overland flow plane can be considered in one of two ways. They can be considered to be laying on the soil surface, or they can be considered to be mixed in the soil column. The default is that contaminants are present on the soil surface. The amount of contaminants (Kg) is specified for each cell in the MAPPING_TABLE file. Then, the uptake coefficient (K) (m d^-1), specified for simple constituents in the MAPPING_TABLE file and calculated for NSM, is used to move the contaminants into the overland flow based on the concentration deficit (solubility of the constituent and the concentration in solution). For simple constituents these parameters are specified in the MAPPING_TABLE file. For nutrients, these calculations are performed by the NSM. The flux (F) (g s^-1) is computed as

F=KA(Cmax -C)/86400.0

Where Cmax is the maximum concentration (g m^-3) of the contaminant (solubility), C is the concentration of contaminant in the ponded surface water, and A is the area of the computational grid cell (m²). The value 86400.0 converts the reaction rate (K) into (m s^-1);

11.1.1.3 - Exchange with Soils

Optionally, contaminants distributed on the overland flow plane at the beginning of the simulation may be mixed into the soil column by including the SOIL_CONTAM card in the project file. Currently, only the Green and Ampt with redistribution INF_REDIST (GAR) model of infiltration (Ogden and Saghafian, 1997) and the two layer soil moisture model (Downer, 2007) can be used to simulate constituents in the soil column. When simulating constituents in the soil column the mass of contaminants (Kg) specified in the MAPPING_TABLE file is distributed over a specified mixing depth in the soil column. The mixing layer depth (m) is specified with the MIXING_LAYER_DEPTH card which specifies an optional third layer in the soil column in addition to the SOIL_MOISTURE_DEPTH and the TOP_LAYER_DEPTH.

Constituents in the soil partition between the soil matrix and the pore water are based on the chemical properties, the soil properties, and the soil moisture. Constituent uptake into water ponded on the overland flow plane occurs due to the uptake rate (K) and the concentration difference between the soil pore water volume and the overland flow plane. As the concentration gradient may be in either direction, the flux may also be in either direction, i.e. the dispersive flux may be into the soil, acting as a sink for the overland plane. How constituents are treated in the soil column are discussed in detail below.

During simulations uptake, decay, and movement between layers will change the concentration in the surface soil layer, as described above. The concentration of materials in the surface soil layer can be held static by using the SOIL_STATIC_CONC card in the project file. This might be desirable when either the concentration in the soil is expected to held constant by addition of more constituent, such as N and P addition due to fertilizer. Furthermore, fluxes between soil layers can be halted by using the SOIL_NOFLUX card. This might be desirable to include if the material in the top layer is being flushed out at an excessive rate and reducing the surface soil layer concentration too rapidly. This option may also be desirable to use if exfiltration is occurring and an excessive amount of constituent is being added to the overland flow plane.

11.1.1.4 - Infiltration

Some or all of the water ponded on the land surface may infiltrate, removing contaminants. Water that infiltrates is assumed to contain the same concentration of dissolved contaminants as the ponded water.

11.1.1.5 - Exchange with Channels

In general the overland flow plane acts as the primary source of contaminants to the channel, and this is a sink. For cases where the OVERBANK card is specified in the project file, along with channel routing, the stream may overflow and add water, as well as constituents, back to the overland flow plane.

11.1.1.6 - Groundwater

The overland flow plane interacts with the groundwater in two possible ways. If the groundwater table is high enough, water may spill out onto the overland flow plane as exfiltration. If the SOIL_CONTAM option is not specified, water spilling back on the overland flow plane has the specified groundwater concentration for that cell. If the SOIL_CONTAM card is in the project file, the concentration will be calculated as part of the soil constituent transport routine, as descibed below. Constituents seeping out of the soil column into the groundwater are accounted for but do not affect the static groundwater concentration for the cell. If exfiltration is occurring, water will enter the soil column from the bottom with concentration specified for the groundwater in that cell. Water seeping onto the surface will also have this concentration. In some cases this action may lead to an excess of constituent being added to the land surface. In that case, using the SOIL_NOFLUX card will stop the addition of constituent onto the land surface from groundwater seepage.

11.1.1.7 - Point Sources

Point sources may be input into any cell in the watershed. Point sources are defined in the OV_POINT_SOURCE file. The OV_POINT_SOURCE file contains the number of points (N) and the i and j location (or link and node), discharge rate, a flag if the location is for a channel (1) or the grid (0), Q (m³ s^-1), and concentrations, C (mg L^-1), for all constituents of each point source as shown below. In the example below, there are N point sources and M constituents.

[# Point sources (N)] [cell i/link point source 1] [cell j/node point source 1] [is_channel] [Q1] [C1,1] [C1,2] [C1,3] ... [C1,M] [cell i/link point source 2] [cell j/node point source 2] [is_channel] [Q2] [C2,1] [C2,2] [C2,3] ... [C2,M] ... [cell i/link point source N] [cell j/node point source N] [is_channel] [Q3] [CN,1] [CN,2] [CN,3] ... [CN,M]

Values in the table are separated by spaces.

11.1.1.8 - Exchange with Reservoirs

Reservoirs in the channel network are also present within the overland flow plane (Downer et al., 2008). Water and constituents may be lost to a reservoir by either flowing into the reservoir, or by the reservoir rising and taking over the overland flow cell. Water and constituents may also flow from the reservoir back onto the overland flow plane. This results in a source for the overland cells adjacent to the rising reservoir.

11.1.2 - Simulation of Constituents in the Channel Network

Simulation of constituents in the channel network is specified by including the CHAN_CON_TRANS card in the project file. Typically, the source for contaminants in the channel is derived from inputs from the overland flow plane. Contaminants may also be added to the channel from groundwater exchange. As contaminants are not currently simulated in the groundwater, static values of contaminant concentration are specified for the groundwater. Contaminants may also be added to the channels as point sources. If CHAN_CON_TRANS is specified in the project file without OV_CON_TRANS transport will be computed only in the channel network. In this case, the only possible sources are point sources.

Transport of constituents within the channel network is calculated with the general 1-D advection-dispersion equation in terms of the mass of constituent (M) equal to the concentration (C) multiplied by the volume (V) with constant dispersion. The details of the equations are described in Downer and Byrd (2007). For the channels, the following sources/sinks are considered in addition to chemical reactions.

1. Exchange with overland flow
2. Exchange with reservoirs
3. Exchange with groundwater
4. Point sources

11.1.2.1 Exchange with Overland Flow

As described above for overland flow, water from the overland flow plane is deposited in the stream network in overland grid cells that contain all or part of a stream node. If the channel spills back onto the overland flow plane, this is treated as sink in the channel calculations. Water can only spill back onto the overland flow plane if the OVERBANK card is included in the project file.

11.1.2.2 Exchange with Reservoirs

As described in Downer et al. (2008) stream networks may contain reservoirs. Water and constituents are lost to the channel in two ways. Water may flow into the reservoir from one or more upstream tributaries. The reservoir may also expand, taking stream nodes or entire reaches. When this occurs, any water and constituents in the overtaken stream node is removed from the channels and added to the reservoir. Discharges from reservoir outlets act as sources to the channel network.

11.1.2.3 Exchange with Groundwater

Channel losses can be simulated whether or not the saturated groundwater surface is included in the simulation. When water seeps into the channel bottom, subsequent loss of constituents occurs as well. When the water table is included in the solution, as either static or varying, exchange can be in either direction. Concentration of constituents is specified for every cell in the groundwater domain. This concentration does not vary in time throughout the simulation. If flow is from the groundwater domain to the channel, water entering the channel is assumed to have the specified groundwater concentrations of constituents. Seepage from the channel to the groundwater is assumed to have the same concentration as the water in the channel node. Additions and subtractions to the groundwater are accounted for but do not affect the specified groundwater concentrations.

11.1.2.4 Point Sources

Point sources may be input into any node in the stream network. Point sources are defined by a constant discharge rate and concentration for each point source.

Point source flows and concentration of contaminants can be input using the CHAN_CON_INPUT table which contains one line with the number of point sources (N) in the file and one line with the node and link numbers, flow, Q (m³ s^-1) , and concentration, C (mg L^-1), for each point source as shown below.

# Point sources (N)
Node #     Link #     Q1     C1 
Node #     Link #     Q2     C2
Etc.
Node #      Link #     QN-1     CN-1

Node #      Link #     QN     CN

11.1.2.4 Defining Paramter Values

Initial concentrations (g m^-3), decay coefficients (d^-1), and dispersion coefficients (m² s^-1) are needed for each node, and are input as uniform values for the entire stream network using the INIT_CHAN_CONC, CHAN_DECAY, and CHAN_DISP_COEF project cards, respectively.

11.1.3 Soil column Transport

Simulation of contaminants in the soil column is selected by including the SOIL_CONTAM card in the project file. For simulations of transport in the soil column, infiltration must be simulated with the GAR infiltration model (INF_REDIST) and soil moisture must be simulated with the simple soil moisture accounting routine. The simple soil moisture accounting routine (Downer, 2008) allows the user to specify up to two soil layers for computations of soil moisture (SOIL_MOIST_DEPTH and TOP_LAYER_DEPTH). Within these layers downward soil water movement is due to gravity. If groundwater is being simulated, the groundwater may rise into the soil column, causing an upward flow of water in the soil column. Soil movement due to capillary pressure is not considered. Infiltration is a source to the top layer. Leakage from the bottom layer is considered a loss. Loss of water, but not constituents, also occurs due to ET, which is taken from both soil moisture layers.

Figure 2 shows the conceptual model of the soil column transport model. Downward fluxes (infiltration, gravity drainage, groundwater recharge) are shown on the left. Upward fluxes (exfiltration, upward groundwater flux) are shown on the right. Diffusive exchange occurs between the top soil layer and the surface water. Exchange between pore water and soil particles occurs in every layer, as does decay and transformations.

Figure – Soil column transport model

11.1.3.1 Distribution of Constituents in the Soil Column

When simulating constituents, a third constituent transport layer may be included in the soil transport calculations by including the MIXING_LAYER_DEPTH card in the project file. The MIXING_LAYER_DEPTH is specified in meters. Specification of this layer further divides the surface soil moisture layer. Any initial amount of constituents distributed on the overland flow plane is assumed to be mixed within the MIXING_LAYER_DEPTH. If this additional layer is not specified in the project file, the initial amount of contaminants is assumed to evenly mixed over the TOP_LAYER_DEPTH, and there will be only two layers in the soil transport model. If only one soil moisture layer is specified with the SOIL_MOIST_DEPTH, then the initial amount of constituents is assumed to be mixed over this single layer, and transport is computed for one soil layer only.

Regardless of the total number of layers, the initial constituent loading specified in the MAPPING_TABLE is assumed to be mixed over the depth of the top layer. This mass of contaminants is distributed between an amount absorbed to the soil and dissolved in the pore water. The distributrition is calculated based on the chemical partition coefficient and the soil moisture as described by Johnson and Gerald (2007). For simple constituents, values of the partition coefficient are specified in the MAPPING_TABLE.

11.1.3.2 Exchange with Surface Water

During the simulation, infiltration acts as a source to the top layer. Advection of water transfers water and constituents to lower layers. Leakage from the bottom layer is a sink for that layer. The concentration of constituent in the advected water depends on the mass of contaminant in the layer, the soil moisture, and the partition coefficient.

Exchange with water ponded on the land surface occurs due to the concentration gradient between the pore water in the top soil layer and the ponded water. The flux (F) (g s^-1) is calculated as:

F=KA(C_ponded-C_soil)/86400.0

Where: C_ponded is the concentration in the water ponded on the soil surface, C_soil is the concentration in the soil pore water, and K is the kinetic rate (m d^-1), and A is the area of the computational grid cell (m²). The value 86400.0 converts K from per day to per second. As can be seen in the equation, the direction of the flux is dependent of the relationship between the concentration of the surface water to the soil pore water volume.

11.1.3.3 Interaction with Groundwater

If the water table is being simulated, the water table may be present in any or all of the soil layers. If the water table is present in a layer, the amount of groundwater in that layer is considered in the calculation of soil moisture in the layer for the purposes of partitioning the constituent between dissolved and attached fractions.

If exfiltration occurs, the groundwater is considered to come into the bottom layer, reach equillibrium condition in that layer and then move upward to the next layer, where it reaches equilibrium before being advected upward to the next layer, and ultimately to the land surface. However, the concentration of water exfiltrating at the land surface is assumed to be at the groundwater concentration, not the surface soil layer concentration. In practice, this approach proves superior to using the calculated surface layer concentration. In cases where exfiltration is causing excessive constituent to be added to the overland flow plane, the SOIL_NO_FLUX card can be used, which will result in no fluxes of constituent between soil layers, including exfiltration.

11.1.4 Reservoir Transport

Reservoirs in the stream network are treated separately from the channel network. Each reservoir is considered as a completely mixed reactor. As described above, reservoirs interact with both the overland flow plane and the channel network. Reservoirs can also interact with the groundwater in the same manner as the channels, where the reservoir water and contaminants can seep to the groundwater, and the groundwater can supply water with static concentrations to reservoirs.

11.1.5 Groundwater Transport

GSSHA does not currently simulate fate and transport in the saturated groundwater. Whenever a water table is simulated in the GSSHA model, a concentration is specified for every grid cell in the watershed. Any flux from the groundwater to any other domain has the static constituent concentration of the groundwater cell that the flux occurs from. Fluxes to and from the groundwater do not affect the groundwater concentrations. This simplified conceptualization of groundwater may not be adequate for simulating conditions where the groundwater exchange is significant and the groundwater concentrations vary with time over the period of the simulation.

GSSHA User's Manual

11 Constituent Transport and Fate

11.1     Simulating Reactive Constituents in GSSHA

11.2     Transport Formulations

11.3     Simple Constituents

11.4     Point and Non-point sources

11.5     Multi-phase transport