Introduction to Environmental Modeling



Introduction to Environmental Modeling

INTRODUCTION

Modeling is a tool to simulate or recreate reality. An environment model is a tool specifically designed to simulate or recreate the environment or, more specifically, an environmental system. It is often easier and less expensive to work with models compared to the actual system. However, models are valuable only if they are properly constructed and are fed good data; the popular saying "garbage in garbage out" applies to modeling.

Models are generally of two types: static and dynamic. Static models are used to understand the behavior of a system at rest. Economists use static models extensively. Dynamic models allow us to examine a system over time and are used by environmental scientists to examine changes to an ecosystem. Models have three basic components: the underlying science, a mathematical representation of the science, and a solution of the mathematics.

This Problem Set provides you with the opportunity to explain the basic concepts of modeling and use a model to make determinations about an environmental system. It should help you be able to describe several major challenges facing environmental regulators.

TASKS

Stock and flow modeling is the most basic form of dynamic environmental modeling. As shown in Figure PI3.1, an example of a stock and flow is a human population. You have births and immigrants flowing in (inflow), a population (stock), and deaths and emigrants flowing out (outflow].

Inflow

[pic]

Figure P13.1—Simple Stock and Flow Model.

Based on the information below, you will be modeling the level of a particular contaminant in a pond and answering a series of questions based on use of the model.

Scenario:

The Copper Brothers Manufacturing Company is located on the western shore of Valley Pond as shown in Figure P13.2,

[pic]

Figure P13.2—Valley Pond Study Area.

• The pond's volume is 4 x I07 m3 of water.

• The average flow-through rate is 8 X l04 m3/day. That is: (1) the inflow from Little Valley Stream, (2) the water being discharged from the company into the pond, and (3) feeder springs collectively equal the volume of the outflow in Big Valley Stream (i.e., inflows = outflow) at 8 X 104 m3/day.

• The company produces decorative copper art by chemically etching the copper with strong sulfuric acid.

• The plant has a National Pollutant Discharge Elimination System (NPDES) permit issued by the state under the Clean Water Act. The NPDES permit allows the plant to discharge 0.16 tonnes (metric tons) of copper sulfate per day. The plant has an exemplary record of permit compliance. Currently, 25 people are employed at Copper Brothers. The company is the primary employment base for the town of Valley View.

• A family purchased a small camp on the eastern shore of Valley Pond. Over the summer, on numerous occasions, they observed dead fish in their tiny cove near Big Valley Stream. They contacted the State Water Quality Division to file a formal complaint against Copper Brothers.

We need to calculate the steady-state level of copper sulfate in Valley Pond to determine if there is too much in the pond. (That is, how much copper sulfate is in the pond given inflows, outflows, and copper sulfate discharges?) Although the plant is in compliance with their permit, the level of copper sulfate may be too high, biologically, for certain fish species because in the pond, it may be increasing, but it also may be decreasing. This is a function of the accumulation of copper over time.

The rate at which copper sulfate is added to the lake is known (0.16 tonnes—metric tons per day). So, to calculate the steady-state stock of the pollutant, we need to know its residence time in the pond.

We will assume that the pollutant is uniformly mixed in the pond and is highly water-soluble. (As with all models, certain assumptions must be made.) Thus, the residence time of the pollutant is equal to the residence time of the pond water. We can calculate the residence time of the water as:

Residence time: Tw = MW/FW

Tw = residence time of water in the pond

Mw = stock of water: the pond volume

Fw = average, daily flow through rate of the water

1. What is the residence time of the pond water? The steady-state stock of copper sulfate can be calculated based on the following formula:

Steady stock: SCS = FCS*TCS

Scs = steady-state stock of copper sulfate

Fcs= daily discharge amount of copper sulfate

Tcs = residence time of the copper sulfate (see Tw)

2. What is the steady-state stock (load) of copper sulfate in Valley Pond?

3. What is copper sulfate? And what are its likely effects on Valley Pond?

The state's environmental standard for copper sulfate in aquatic systems is 1.98 parts per million. That is, 1.98 parts of copper sulfate per one million parts per water is allowed. We need to calculate the concentration of the copper sulfate in the water. This rather simple calculation requires us to divide the steady-state stock of copper sulfate by the total volume of Valley Pond (then multiply the number by 1 X 106):

Ccs = Scs/Mw

4. What is the concentration of copper sulfate in Valley Pond (expressed in ppm)?

5. Based on the concentration, as a state environmental scientist, what would you communicate to the Chief of the Water Quality Division?

The Copper Brothers Manufacturing Company asserts that if it is prevented from discharging copper sulfate into Valley Pond, it will have to close, which will put all 25 people out of work. The company hires an engineering consultant to investigate manufacturing alternatives. The consultant concludes that Copper Brothers cannot reduce the quantity of copper sulfate it discharges without adversely affecting product quality. The company hires a consulting environmental engineer to present an ecological alternative, which the company proposes to the state. The company proposes to construct a pipeline that will connect Mountain Pond to Valley Pond, which, they argue, would dilute the copper sulfate below any adverse level. Essentially, the project would drain Mountain Pond and increase the volume of Valley Pond. Thus, as designed, the pipeline will increase the volume (but not the flow) of Valley Pond by I million m3 (1 X 1O6 m3). Using this assumption, answer the following:

6. What is the revised residence time of the copper sulfate?

7. What is the revised, predicted steady-state stock (load) of copper sulfate in Valley Pond?

8. What is the recalculated concentration (ppm) of copper sulfate? Will this proposal meet the state's water

quality standard?

9. What are some likely environmental effects (i.e., unintended consequences) of increasing the water volume

of Valley Pond by using Mountain Pond?

10. Is this proposal likely to appease the owners of the camp? Why or why not?

You ask a colleague to review the consultant's report. Your colleague notes that there is a glaring error in the model's assumptions: the consultant did not take into account evaporation! Evaporation will have a significant impact on copper sulfate concentration because evaporating water contains no copper sulfate. Thus, the steady-state concentration of copper sulfate would expect to be significantly higher because one of the possible exit pathways (evaporation) is no longer available. (That is, the flow-through rate and thus the residence time are wrong.) Therefore, the residence time of the copper sulfate is no longer equal to the residence time of the water, but rather the residence time associated only with the outflow of Big Valley Stream. Based on some rough calculations, the total rate at which water exits Valley Pond is: 33% by evaporation and 66% through Big Valley Stream.

11. What is the revised residence time of the copper sulfate without considering the proposed pipeline?

(Remember, the original flow-through rate is reduced by 33%.)

12. What is the revised, predicted steady-state stock (load) of copper sulfate in Valley Pond?

13. What is the recalculated concentration (ppm) of copper sulfate? How does this amount relate to the state's

water quality standard?

The state is concerned that this modeling approach be reevaluated for its ability for accuracy. The state regulators ask for general comments regarding the model.

14. What are some additional inflows and outflows that should be considered?

15. What are some factors that could affect the behavior of water pollutants that could have a significant effect

on biota, which were not incorporated into the model (e.g., review the assumptions)?

16. What are some environmentally less damaging alternatives that the Copper Brothers could employ?

17. As a state environmental regulator, what are some major challenges that you would face in determining

levels of pollution in water bodies using models?

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