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Morning Sun Inc.

Morning Sun Inc. has installed two settling ponds to treat stormwater runoff coming from a large development property currently under construction. The company has been discharging sediment laden runoff with dissolved Chloride concentrations of 224 mg/L into Pond 1 which is currently completely lined with clay. They plan on uncapping (removing the clay) along a sand dike constructed between Pond 1 and Pond 2 in order to filter the water before it discharges to an adjacent stream.
They have asked us to analyze their system and answer three primary groups of questions:
1. What is the maximum allowable flow rate (ft3/day) from the construction site in order to keep Pond 1 from over-flowing (Pond 1 will be full when they open the dike)? What will the groundwater flux be? 6847.51ft^3/day
2. How long, on average (days), will water and contaminants from Pond 1 take to reach Pond 2? 49.41 days
3. What is the mass flux (kg/day) of the contamination? How much contaminant mass will pass from Pond 1 to Pond 2 over a period of a week? Assuming the water in Pond 2 was contaminant-free at the beginning, what is the maximum concentration (mg/L) possible in Pond 2 after that week? After a month?
Mass flux: 0.054kg/day
How much: 0.378kg
Max week: 0.0029mg/L
Max month: 0.013mg/L
Construction plans suggest:
• The two settling ponds were dug in 13 ft of sand with their beds set on low permeability clay.
• A clay cap, which is mostly impermeable, will be left on top of the dike, extending up from the same elevation as the Pond 2 outlet.
• The plan specifications suggest the sand dike filter consists of material with a median grain size (D50) of 0.22 mm (fine sand) with 10% silt
Additionally, our surveyors have been on site and provided the following information:
• The sand dike separating Pond 1 and Pond 2 will be 168 ft wide by 1014 ft long.
• Pond 1 is 703 ft long and Pond 2 is 567 ft long.
• The maximum elevation of Pond 1 is 658.43 ft.
• The outlet of Pond 2 is at an elevation of 653.37 ft.
Please address the following issues and prepare a memo with your findings by midnight, October 22:
1. Provide a sketched conceptual model for the report as part of your site description. Make sure all dimensions are clearly labeled. No need
2. Provide a description of your mathematic model (equations and concept sketch for equations) as part of your methods. Where did the main equation come from originally? Where did you get your porosity and hydraulic conductivity information? No need
3. Provide any assumptions you are making in your calculations:
a. Are you assuming saturated or unsaturated conditions?
b. Are you assuming the flow is turbulent or laminar?
c. Are you assuming steady or unsteady (changing over time) flow from the outfall?
d. Are you assuming the flow is 1-dimensional, 2-dimensional, or 3-dimensional?
e. Are you making assumptions about changes to the system over time?
f. Are you making assumptions about the contaminants? What are they?
4. Provide answers for the primary questions. Hydraulic conductivity estimates are often off by an order of magnitude? How much do your results change if you vary the hydraulic conductivity in a meaningful way? How much do they change if you change porosity by +/- 10% or the hydraulic gradient by +/- 20% (reasonable errors)?
5. Discuss results with respect to the sensitivity analysis you performed and the assumptions you made. 1) How well do your assumptions represent reality? 2) How accurately have you determined the parameters of the model? Did you get your values from tables? Could you have done anything to improve your accuracy? 3) What about your assumptions with the contaminants?
6. Provide a final verdict on results and suggest next steps. Will the flow rate likely keep up with runoff rates? What might you want to do to cover your and Morning Son’s behind?

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