CEE 577

28 March 2006

MID-TERM EXAM

 

Closed book, 1 sheet of notes allowed.

Answer 2 of the following 3 questions. Please state any additional assumptions you made, and show all work.

 

 

 

I.                    (50%) Whole Hog creek flows through and area of the North Carolina Piedmont that is heavily populated with pig farms. The creek discharges into a lake used for recreational purposes. Concern over phosphorus loading of the lake has led to a short-term study of nitrogen imputs during storm events. Data from this brief, 8-day survey period is shown below.

 

a.       Determine the total phosphorus concentration for each day using the standard log-log model

b.      Estimate the mean mass loading of phosphorus (in kg/d) into the reservoir based on these values.

 

Day

Flow (m3/s)

Total Phosphorus Concentration (g/L)

1

0.2

 

2

0.5

12

3

4.2

 

4

13.1

55

5

2.2

 

6

8.3

 

7

2.3

 

8

0.9

 

 

 

 

II.                 (50%) Lake Noir is an urban surface water that has received inputs of lead for many years. Efforts to clean up the lake resulted in the termination of nearly all point loads of lead as of 1990. Since that time lake levels have dropped due to decay (0.00005 d-1) and hydraulic flushing. Unfortunately, Senator Porkbarrel succeeded in getting his brother's button cell battery business exempted from the lead ban. The amount of lead discharged is directly related to the number of batteries produced. Due to changes in the battery industry, production of this particular type has dropped off since the business began (see table below). During this time lead waste has been constant at 800 g Pb per battery produced. The output data below were collected for last year (2005) and projected into the future. Its also important to know that the Porkbarrel button cell battery company had just moved to the shores of Lake Noir and began discharging on January 1, 2005. Also, as of this date, the lake concentration of lead was 0.13 g/L. The lake has a volume of 1.25 million cubic meters, a surface area of 0.3 million square meters, and an outflow of 800 cubic meters per day. Ignore any temperature effects or sorption to sediment.

 

 

Porkbarrel Battery Co. Data

Year

Output (batteries/yr)

2005

5,800,000

2010

5,100,000

2015

4,400,000

2020

3,700,000

2025

3,000,000

 

 

 

  1. Calculate the expected lead concentration at the beginning of the year 2020.
  2. Determine the year when the maximum Pb concentration will drop below 0.05 g/L.

 

 

 

III. (50%) On a separate sheet of paper, answer any five (5) of the following questions.

 

A.                 Describe the difference between mechanistic and empirical modeling

B.                 Is a first order reaction always faster than a zero order reaction? Explain.

C.                 Sketch out a qualitative concentration vs distance (downstream) profile for a river with a point discharge at x=0 and a distributed source that runs from x=3 to x=6 miles. Show qualitatively how increases in the 1st order decay rate for a pollutant changes this profile.

D.                 Explain the relationship between Secchi-disk depth and lake trophic state. Why is there such a relationship?

E.                  Describe 3 different methods for determining stream velocity.

F.                  Describe 2 different ways of measuring drainage basin area

G.                 Describe the factors that determine re-aeration in rivers, and contrast this with the factors that determine re-aeration in lakes. In your description, relate micro-scale processes (molecules) to macro-scale (bulk water or air)