One of the wake-up calls for the environmental movement in the United States was
ID: 115967 • Letter: O
Question
One of the wake-up calls for the environmental movement in the United States was the polluted condition of Lake Erie during the 1960’s and 1970’s. It particularly entered the national consciousness when the hydrocarbon-laden Cuyahoga River – a major tributary that flows through Cleveland, Ohio – would periodically catch on fire. The picture below is of one such incident on November 3, 1952.
The smallest of the Great Lakes (surface area = 25.7 x 109 m2 , average depth = 19 m), Lake Erie is bordered to the north and south by some of the most productive farmland in Canada and Ohio, and along its shores are some of America’s biggest industrial cities. The main ecological problem that arose with Lake Erie was its burgeoning hypoxic or “dead zone” of oxygen depletion. This resulted from the high biological oxygen demand (BOD) of the adjacent land uses compared to its relatively small volume. Use the table below to answer the following questions.
Assume that each person generates 2.0 kg of biologically active waste per day, agricultural practices result in 0.5 kg of biologically active waste per m2 of farmland each year, the BOD decay constant is 0.11 day-1 , and that Lake Erie is wellmixed. Tributary System Flow Rate Principal BOD Source INFLOW Cuyahoga River 28.3 m3 /sec Cleveland, OH (1.8 million people) Detroit River 5380.2 m3 /sec Detroit, MI (3.9 million people) Maumee River 141.6 m3 /sec Toledo, OH (0.5 million people) Sandusky River + River Basin + Huron River + Grand River 368.1 m3 /sec (together) 84.5 x 109 m2 of farmland Buffalo River 28.3 m3 /sec Buffalo, NY (1.1 million people) OUTFLOW Niagara River 5946.5 m3 /sec
(a) What is the detention time of Lake Erie? Hint: The volume of a lake is equal to its area x its average depth
(b) Calculate the BOD concentration (kg/m3 ) in each of the five tributary systems – assume no decay. Hint: The BOD is placed into the rivers. To find the concentration, we need to figure out how much mass is placed into a certain volume. Here the volume is all of the water rushing by in a day. 2nd Hint: Recall that 1000 L = 1 m3
(c) Draw a diagram of Lake Erie as a CMFR for BOD input, output and decay. Assume that all of these wastes make it into their respective waterways just before they feed into the lake.
(d) Write out the mass balance equation for the model.
(e) What is the concentration of BOD that enters the Niagara River? Express your final answer in a unit of mg/L.
Explanation / Answer
a) the retention time of the lake is capacity storage = area * depth
= (25.7 x 109)*19
=488.7 c
now total out flow or discharege is 5946.5 so Tr= reservoir volume/annual discharge,
=488.7x 109/5946.5 = 0.08218 x 109 m3 /sec
b)
BOD, mg/l = (Initial DO-Final DO) x BOD per day
= (5946.5-5380.2 ) * 0.5
= 283.15 mg/l
d) Water level and chemical composition in lakes are controlled by the input and output. We will examine the mass balance of lakes in the context of the hydrologic cycle.
IN - OUT = S (storage change) IN = P + IS + IG + other terms (sewer outlet, snow drift, etc.) OUT = E + OS + OG + other terms (pumping, etc.) S = Change in the volume of water
e)
BOD, mg/l = (Initial DO-Final DO) x BOD per year
=(5946.5-5380.2 ) * 2
= 1132.6 mg/L
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.