Download CE5504 – Surface Water Quality Modeling

CE5504 – Surface Water Quality Modeling
AQUATOX Assignment 2. Phosphorus Cycling
Objectives
1. To develop an understanding of the treatment of nutrient cycling through detritus.
2. To explore the behavior of the phosphorus cycle.
3. To gain experience in evaluating model behavior.
A. Create a File for This Assignment
Bring up the AQUATOX software.
Select File and then Open and then the filename of the previous assignment.
Select File and then Save As and then enter a new filename.
Change Study Name to Onondaga Phosphorus.
Delete the State Variable OtherAlg1: [TSS Surrogate].
Under Water Volume, Inflow of Water, select Use Constant Loading.
To be certain that you have the correct (i.e. default) mineralization coefficients, select
Site and then Reload Remin. from DB and then OK (3 times).
This is now a bare-bones Onondaga Lake file, ready for modification for this assignment.
From this point forward, modifications may be made using the Wizard or by making
selections directly from the Study Information screen. The latter approach is perhaps
more efficient when working with an ‘existing’ file such as this one.
B. Detritus and Phosphorus Cycling
Total Soluble P represents the form directly available for assimilation by plants. Source
terms in the Total Soluble P mass balance include external loading, decomposition of
detritus and excretion and respiration by plants and animals. Sink terms include
assimilation by plants and washout. All other phosphorus, both soluble and particulate is
maintained within organisms or the various detrital components. The phosphorus cycle is
illustrated in Figure 5 of the Addendum.
1. Total Soluble P
As a starting point, let’s look at system behavior where there is only an initial condition
and then with an initial condition and an external load.
Select Total Soluble P and set the Initial Condition at 0.1 mg/L.
Run the Control simulation and examine output for Total Soluble P.
Can you explain the system behavior? How could you manipulate model inputs to test
your explanation? Try it.
Another way to understand system dynamics utilizes a capability called Save Biological
Rates. Go to Site and Select Save Biological Rates and then Rate Specifications. Select
Excel File as the output form and then move Total Soluble P over to the Track Rates
column and then OK (twice).
Run the Control simulation again and check the output to make sure that the Total
Soluble P behavior remains the same.
Now open the Rate Specification file using Excel. It should be stored as –
C:\Program Files\AQUATOX21\Output\filename_Rate.xls
Here, you can examine all of the source-sink terms for the Total Soluble P state variable
and determine if the output behavior follows your instincts. When you’re done, close the
Excel file and return the Setup setting to Don’t Save Rates.
Now, add a Constant [Tributary] Load of 0.1 mg/L. Note that the authors’ use of the
word Load here is incorrect. Run the simulation and examine/explain the result. Try it
again for an Initial Condition of 0 mg/L and 0.2 mg/L. Try doubling and halving the
load. Seek to resolve your intuition and the model performance.
2. Detritus
Detritus is organic matter and, in AQUATOX, includes both soluble and dissolved forms.
The detrital pool contains carbon, nitrogen and phosphorus and releases soluble forms of
these materials as it undergoes microbial decomposition. Detritus is classified as being
labile if it readily undergoes decomposition and refractory if it does not undergo
decomposition or if it does so less readily. In AQUATOX, refractory detritus is
colonized by microbes and then slowly releases C, N and P, but the rate may be set to
zero to make it truly refractory.
In AQUATOX, detritus is broken into the dissolved and particulate and labile and
refractory forms, with corresponding organic matter to nutrient ratios and degradation
rates. These may be viewed on the Site, Remineralization screen. Detritus is further
characterized by its location in the system, i.e. in the water column or in the bottom
sediments. Detritus in the sediments eventually becomes buried, where it is no longer
actively involved with ecosystem function. The detritus compartments are illustrated in
Figure 54 of the Technical Documentation.
Before considering a detritus simulation, set the Total Soluble P entries to 0 and Reload
Remin. From DB to insure that the default coefficients are being used. Also confirm that
you are working with a completely mixed system, i.e. there is no option to display both
epilimnion and hypolimnion results. Adjust the temperature screen as needed to achieve
completely-mixed conditions.
Let’s examine the behavior of the detrital pool by setting an Initial Condition for
Suspended and Dissolved Detritus. Select Input as Organic Matter, an Initial Condition
of 10 mg/L and 0% particulate and 0% refractory. Thus we are beginning with 10 mg/L
of dissolved labile organic matter.
Run the Control simulation and examine output for Labile Dissolved Detritus. Can you
explain this behavior? Use the two methods introduced above to test your interpretation.
Examine the sensitivity of the simulation to changes in the labile detritus degradation rate
coefficient.
Now let’s look at the behavior of refractory dissolved detritus. Change the initial
condition for Suspended and Dissolved Detritus to 100% refractory. For a first run, set
the refractory detritus degradation rate coefficient to 0 and examine the output for
Refractory Dissolved Detritus. Resolve the behavior with your understanding of the
system.
When refractory detritus has a non-zero degradation rate coefficient, it is colonized by
bacteria and converted to the labile particulate form which can decompose, settle and
wash out. This behavior is a bit more complex, but quite interesting. For the sake of
simplicity, let’s leave the inflow at zero for these simulations. Set the refractory detritus
degradation rate coefficient to it’s default value (0.04), run Control and examine the
output for Refractory Dissolved, Labile Suspended and Labile Sediment Detritus. Use
the methods developed above to help you confirm (a) that mass balance is conserved and
(b) to understand and explain the behavior of the model output.
There are several more compartments and simulations we could test, but let’s leave those
for more advanced studies and look at the interplay between detritus and phosphorus.
3. Detritus-Phosphorus Interactions
Here we want to look at the recycle of phosphorus as detritus is decomposed. Pay
particular attention to the role of the Organic Matter to Phosphorus ratio and the detritus
decomposition rate coefficients.
Setting the Total Soluble P initial condition and loads to 0 and maintaining zero flow, set
the Dissolved Labile Detritus initial condition to 10 mg/L. Resolve the output, plotting
Total Soluble P, Total P and Dissolved Labile Detritus. What can you learn from the TP
variable? Now try the same simulation with Dissolved Refractory Detritus at 10 mg/L
instead. What in the world is going on here (big questions for big shooters). There are
really two questions: (a) what is the model doing? and (b) is what the model is doing
consistent with our understanding of ecosystem function?