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Groundwater Water Quality
Water Quality
Groundwater Water Quality
The groundwater object allows you to model Salinity with a layered approach (Layered Salt) when the groundwater object is using the Head Based Groundwater Grid method in the Solution Type category.
Following is a brief description of the groundwater solution equations when using a head based solution. Flows to/from the groundwater object are either input, provided across a link or computed. When computed based on head, the computation uses previous timestep information. Following is the mass balance equation for connected groundwater objects.
(11.1)    
(11.2)    
Figure 15.6 (see “solveGWMB_givenPreviousElevations” in Objects and Methods) illustrates the existing groundwater object, including all of these mass balance components and typical links that are created.
Conceptual Overview of Salinity Modeling
The shallow groundwater is not always well mixed in terms of salinity. In some areas, the upper portion of the aquifer interacts quickly with the surface, but further down, the interaction is slower. This process can be modeled as two fully mixed layers within the groundwater object.
The current implementation of the ground water quality model allows you to model salinity with the Layered Salt method.
Figure 11.1 shows the groundwater object with two layers in terms of flow and volume slots. Figure 11.2 shows the two layers in terms of salinity slots. All slots and links shown in Figure 15.6 (see “solveGWMB_givenPreviousElevations” in Objects and Methods) are still present; the object just has additional slots and links for salinity modeling.
Figure 11.1  Slots on the groundwater object to model flow and volume with two layers
Figure 11.2  Slots on the groundwater object to model salinity with two layers
In this approach, the thickness of the upper layer is constant. Each mass balance component except Evaporation and Evapotranspiration have an equivalent Salt Concentration component. They are shown in red in Figure 11.1. On the groundwater object, the Flow Upstream/Downstream/Left/Right, Inflow From Surface Water, Pumped Flow and Percolation could be either negative or positive.When the flow is coming into the groundwater object, the salt concentration comes from the linked slot or be specified (input or rules). If the flow is out of the groundwater object, the concentration is computed by the groundwater object. When the groundwater object is defining the concentration, the previous values are used to avoid iteration problems.
Following are assumptions used in this computation:
• Inflow from Surface Water is added to (or removed from) the upper layer.
• Specified Inflow is added to either layer based on a user specified proportion.
• Evaporation is removed from the upper layer and no salt is removed.
• Evapotranspiration is removed from the upper layer and no salt is removed.
• Percolation is removed from (or added to) the lower layer.
• Pumped Flow is removed from (or added to) either layer as specified by the user.
Following is a conceptual overview of the computation to determine storage and salt concentration in each layer. See “Dispatch Method - Solve 2 Layer Salt” for details on this computation.
1. Determine the layer storages and the proportion based on the capacity of the upper layer.
2. Compute the lateral flow and salt flux from each layer based on the storage proportion on the given object or adjacent objects.
3. Compute pumped flow proportion and salt flux.
4. Compute inflow from surface water salt flux.
5. Compute specified inflow salt flux.
6. Compute percolation salt flux.
7. Compute the intermediate layer storage and concentration based on the computed incoming and outgoing components.
8. By comparing the intermediate and final storage and salt concentration, transfer water and salt between the two layers.
9. Compute the salt concentration of each layer.
10. Set salt mass slots if method is selected.
Note:  This conceptual design is for two layers. It can be reduced to one layer by specifying a large Upper Thickness.
User Methods
Following are the user-selectable methods on the Groundwater object that model water quality.
Groundwater Water Quality
This category is used to specify the constituents and approach for water quality modeling
* None
No water quality is modeled and no slots are added.
* Layered Salt
The following slots are added based on the selected method in the Lateral Link Direction category. These slots represent salt and flow in each linked direction for both the upper and lower layer.
Storage Slots
The following storage slots are added for all methods in the Lateral Link Direction category.
Storage Upper
Type: Series Slot
Units: Volume
Description: Storage in the upper layer.
Information:  
I/O: Output only
Storage Lower
Type: Series Slot
Units: Volume
Description: Storage in the lower layer.
Information:  
I/O: Output only
Storage Capacity Upper
Type: Scalar
Units: Volume
Description: The volume of water that can be stored in the upper layer when full
Information: This value is computed at beginning of run (read-only).
I/O: Output only
Storage Proportion
Type: Series Slot
Units: No Units
Description: The proportion of the total storage that is in the upper layer
Information:
I/O: Output only
Storage Proportion Previous
Type: Series Slot
Units: No Units
Description: The proportion of the total storage that is in the upper layer
Information: The Storage Proportion of the previous timestep.
I/O: Output only
Storage Proportion Upstream, Downstream, Left and/or Right Previous
Type: Series Slot
Units: No Units
Description: The proportion of storage in the upper layer on the adjacent objects
Information: These are four distinct slots as specified by the selected method in the Lateral Link Direction category. There is a slot for each connected object.
I/O: Output only
Upper Thickness
Type: Scalar
Units: Length
Description: The thickness of the upper layer
Information:  
I/O: Required Input
Flow Slots
The following slots are added based on the Lateral Link Direction.
Flow Upper Upstream, Downstream, Right, and/or Left
Type: Series
Units: Flow
Description: Lateral flow out of the object from the upper layer.
Information: These are four distinct slots as specified by the selected method in the Lateral Link Direction category. There is a slot for each connected object.
I/O: Output only
Type: Not linkable
Flow Lower Upstream, Downstream Right, and/or Left
Type: Series
Units: Flow
Description: Lateral flow out of the object from the lower layer.
Information: These are four distinct slots as specified by the selected method in the Lateral Link Direction category. There is a slot for each connected object.
I/O: Output only
Storage and Lateral Flow Salt Slots
Following are salt mass and concentration slots associated with storage in the upper and lower layers and lateral flows.
Salt Conc Upper
Type: Series
Units: Concentration
Description: Concentration of the well mixed upper layer
Information: Initial value is required
I/O: Output only
Salt Conc Upper Previous
Type: Series
Units: Concentration
Description: The Salt Conc of the upper layer at the previous timestep
Information:  
I/O: Output only
Salt Conc Upper Upstream, Downstream, Left and/or Right Previous
Type: Series Slot
Units: Concentration
Description: The salt concentration in the upper layer on adjacent objects
Information: These are four distinct slots as specified by the selected method in the Lateral Link Direction category. There is a slot for each connected object.
I/O: Output only
Salt Conc Lower
Type: Series
Units: Concentration
Description: Concentration of the well mixed Lower layer
Information: Initial value is required
I/O: Output only
Salt Conc Lower Previous
Type: Series
Units: Concentration
Description: Concentration of the well mixed Lower layer at the previous timestep
Information:  
I/O: Output only
Salt Conc Lower Upstream, Downstream, Left and/or Right Previous
Type: Series Slot
Units: Concentration
Description: The salt concentration in the Lower layer on adjacent objects
Information: These are actually 4 separate slots as specified by the selected method in the Lateral Link Direction category. There is a slot for each connected object.
I/O: Output only
Specified Inflow Slots
Following are salt slots associated with the Specified Inflow.
Specified Inflow Salt Concentration
Type: Series
Units: Concentration
Description: The salt concentration of the specified inflow slot.
I/O: Specified or output. If the Specified Inflow is negative or zero (out of the object), the salt concentration must be output. If the Specified Inflow is positive (into the object), then the salt concentration must be input or set by a rule.
Specified Inflow Salt Mass
Type: Agg Series
Units: Mass
Description: The salt mass associated with the specified inflow slot.
Information: There are three columns for: Total, Upper, and Lower salt mass into the object.
I/O: Output only
Specified Inflow Proportion
Type: Series Slot with Periodic Input
Units: No Units
Description: The proportion of the Specified Inflow that enters the upper layer.
Information: This value must be specified.
I/O: Input or Rules
Inflow from Surface Water Slots
Following are salt slots associated with Inflow from Surface Water. Surface water always enters/leaves the upper layer.
Inflow from Surface Salt Concentration
Type: Multi
Units: Concentration
Description: This slot transfers the salt concentration to connected surface sources.
Information: There should be one link/column per column in the Inflow from Surface Water slot (or an error is posted at run start). The first column is the sum column, which is not relevant as these are concentrations not mass.
I/O: Output typically
Inflow From Surface Salt Mass
Type: Series
Units: Mass
Description: This slot tracks the total mass (through all surface water links) that is transferred into (+) or out (-) of the surface of the groundwater object.
Information:  
I/O: Output only
Percolation Slots
Percolation always enters/leaves the lower layer. Following are salt slots associated with Deep Percolation. They are automatically added when the Head Based Percolation method (Deep Percolation category) and Layered Salt are selected.
Percolation Salt Concentration
Type: Series
Units: Concentration
Description: This slot transfers the salt concentration to the deep aquifer.
Information:  
I/O: Output only
Percolation Salt Mass
Type: Series
Units: Mass
Description: This slot displays the salt flus to the deep aquifer.
Information:  
I/O: Output only
Deep Aquifer Salt Conc
Type: Scalar
Units: Concentration
Description: This slot represents the deep aquifer salt concentration
Information: If Percolation Salt Concentration is not linked, this slot must have a value.
I/O: Required Input
Pumped Flow Slots
Following are salt slots associated with pumped flow. They are automatically added when the Input Pumped Flow method (Groundwater Pumping category) and Layered Salt are selected. Pumped flow can come out of either or both layers.
Pumped Flow Proportion
Type: Series
Units: No Units
Description: The proportion of pumped flow that comes out of the upper layer.
Information:
I/O: Input, Rules, or Output
Pumped Flow Salt Concentration
Type: Multi Slot
Units: Concentration
Description: The well mixed salt concentration of the water coming out of both upper and lower layer
Information: When there are multiple links to this slot, the subslots will be shown. The first column is the sum column, which is not relevant as these are concentrations not mass.
I/O: Output only
Pumped Flow Salt Mass
Type: Series
Units: Concentration
Description: The well mixed salt mass of the water coming out of both upper and lower layer
Information:  
I/O: Output only
Show Salt Mass and Flux Category
The category, Show Salt Mass and Flux, has methods that are used to show salt mass values when desired.
For each Lateral Link Direction, there are two methods available.
* None
No salt mass slots are shown
* Salt Mass Slots (No Linked Objects), Upstream, Downstream, Right and Left Slots
Note:  Method selection in this category is optional and does NOT affect the final salt concentrations. The purpose of these methods is for debugging and viewing salt mass values. Select a new method when you want to see mass values during debugging. Select the None method when you are finished. This saves some space in terms of model files size.
The following slots are added:
Salt Mass
Type: Agg Series Slot
Units: Mass
Description: Salt Mass on this groundwater object. The Agg Series has four columns: Total salt mass, Upper layer salt mass, lower layer salt mass, and salt mass flux upper to lower layer.
Information:  
I/O: Output only
Salt Mass Flux Upstream, Downstream, Left and/or Right
Type: Agg Series Slot
Units: Mass
Description: Salt mass flux out of this groundwater object in the direction specified. The Agg Series has three columns: Total flux, upper layer flux, and lower layer flux.
Information: These are actually 4 separate slots as specified by the selected method in the Lateral Link Direction category. There is a slot for each connected object.
I/O: Output only
Solution / Dispatching
Following is a description of the beginning of run and dispatching behavior when Layered Salt is selected.
Beginning of Water Quality Run
At the beginning of run, the scalar value for the Storage Capacity Upper is computed as follows:
(11.3)    
On the initial timestep, the upper and lower storage, and proportion are computed as shown in Equation 11.4 through Equation 11.9. Also, the Salt Conc Upper and Salt Conc Lower are checked for valid values on the initial timestep.
* Dispatch Method - Solve 2 Layer Salt
Required Knowns
• Storage
• Inflow From Surface Water
• Elevation
• Elevation Previous Left/Right/Upstream/Downstream (depending on linked directions)
• Salt Conc Upper Previous
• Salt Conc Lower Previous
Required Unknowns
• Salt Conc Upper
• Salt Conc Lower
In addition, the method require any salt concentrations that are provided across links to be known or the dispatch method exits and waits for them to be known. When not provided across links, the previous values of many slots must be known.
At each water quality dispatch, the following computations are performed.
Note:  The object has already solved for flows, storages, and elevations so all mass balance values are known.
Determine Upper and Lower Storage
The storage in each layer is computed as follows.
If Storage(t) < Storage Capacity Upper
(11.4)    
(11.5)    
Else
(11.6)    
(11.7)    
Then the storage proportion is computed as follows:
(11.8)    
The same value is set on the Storage Proportion Previous slot at the next timestep.
Note:  The current timestep value was set during a previous timestep’s solution. It is not the Storage Proportion at a previous timestep, but a separate slot called Storage Proportion Previous. This is done to enable the object to consider solving at the next timestep.
(11.9)    
Compute Lateral Upper and Lower Flow and Salt Flux
The flow in or out of each layer is proportional to the relative proportion of storage in each layer. Each side of the object is computed similarly, for example, the right side would be as follows:
If Groundwater Flow Right(t) <= 0 (flow is out of this object, use this object’s values)
(11.10)    
(11.11)    
(11.12)    
(11.13)    
ELSE (Flow is into this object, use adjacent object’s values)
(11.14)    
(11.15)    
(11.16)    
(11.17)    
Repeat this for each direction: Left, Upstream, and Downstream.
Compute Upper and Lower Pumped Flow and Salt Flux
Next, compute the amount of pumped flow that is coming out of each layer.
If Pumped Flow Proportion is input or set by a rule, its value is used. Otherwise, the Pumped Flow Proportion is set to the Storage Proportion Previous value.
Note:  Other methods could be developed to specify this proportion.
(11.18)    
(11.19)    
Compute the mass coming out of each layer, as follows:
(11.20)    
(11.21)    
Assuming the pumped flow is fully mixed, then the concentration can be computed as follows:
(11.22)    
This value is necessary to link to other objects.
Compute Upper and Lower Specified Inflows
Compute the amount of Specified Inflow flow that is entering each layer, as follows:
(11.23)    
(11.24)    
If the flow is into this object (Specified Inflow is positive), use the specified Salt Concentration value, as follows:
(11.25)    
(11.26)    
Otherwise, flow is out of this object (negative or zero Specified Inflow) use the previous timestep salt concentrations:
(11.27)    
(11.28)    
In this case, also set the Specified Inflow Salt Concentration to the weighted average of the upper/lower mass and flows.
Compute Inflow from Surface Water Salt Flux
Inflow from Surface Water enters or leaves the upper layer. Inflow from Surface Water is a multi-slot and each column can be either negative or positive. Thus, the following computation must be performed on each column, i, of the slot. There must be an associated linked salt concentration for each flow, as follows:
(11.29)    tempSaltMass = 0 (this is a local variable used to track the sum)
For Each column, i, of the Inflow From Surface Water multi slot
IF Inflow From Surface Wateri(t) <= 0 (flow is out of this object, use this object’s upper layer)
(11.30)    
(11.31)    
ELSE (Flow is into this object, use linked object’s values)
(11.32)    
End IF
(11.33)    tempSaltMass = tempSaltMass + Salt Mass For This Column
End FOR
(11.34)    Inflow From Surface Salt Mass (t) = tempSaltMass
Compute Percolation Salt Flux
Deep percolation enters or leaves the lower layer.
If Percolation(t) > 0 (flow is out of this object, use this object’s lower layer)
(11.35)    
(11.36)    
Else (Flow is into this object, use specified concentrations.
A value for deep aquifer salt conc must be given)
(11.37)    
End if
Compute Temporary Storage and Concentration
Although the final storage from the groundwater mass balance solution is known, it is not known how flows in and out of the upper and lower layer are included. A temporary storage and concentration is computed for each layer which are then used to compute the flux between layers. First, compute the upper layer intermediate storage and salt concentration, as follows:
(11.38)    
(11.39)    
Then, compute the lower layer intermediate storage and salt concentration, as follows:
(11.40)    
(11.41)    
Transfer Water and Salt Up or Down and Compute Layer Concentration
Now that the intermediate storage and salt concentrations have been computed, the method computes how the water and salt transfers between the two layers to maintain a constant thickness of the upper layer. This computation uses the actual storage in each layer and the intermediate values computed previously; see “Compute Temporary Storage and Concentration”.
If Storage Upper Intermediate(t) >= Storage Upper(t)
Transfer water from the upper to the lower layer
(11.42)    
(11.43)    
(11.44)    
(11.45)    
Else If Storage Upper Intermediate(t) < Storage Upper(t) and Storage Lower Intermediate(t) > 0
If Storage Lower Intermediate(t) >= Storage Upper(t) - Storage Upper Intermediate(t)
(11.46)    
Else
(11.47)    
End If
(11.48)    
(11.49)    
(11.50)    
End If
Finally, as preparation for the next timestep’s dispatch, the previous salt concentrations are set at t plus one.
(11.51)    
(11.52)    
Set Salt Mass and Flux
If the None method is selected in the Show Salt Mass and Flux category, the dispatch method is complete.
Otherwise, when a Show Salt Mass and Flux method is selected, then the salt mass slots are set. There is one agg series slot for each Lateral Link Direction. Each agg series mass slot contains a column for the Total, Upper, and Lower layers. There is also one agg series slot for the object itself. In addition, to Total, Upper, and Lower columns for Salt Mass, it also shows the Salt Mass Flux Upper to Lower. Each column of these agg series slots is set to the appropriate value as computed above.
The Solve 2 Layer Salt dispatch method is complete.
 
Revised: 06/03/2019