7.47 Use the library or Internet to research one dead zone located in the United States
and one located overseas, such as the Baltic Sea, northern Adriatic Sea, Yellow Sea, or
Gulf of Thailand. Write a two-page report discussing environmental, social, and
economic issues associated with the dead zones. What management solutions would you
propose to reverse the dead zones?
Solution:
Students’ responses will vary.
7.48 Go to EPA’s Region 9 web page that is devoted to Lake Tahoe
(http://www.epa.gov/region9/water/watershed/tahoe/). Lake Tahoe (California and
Nevada) can be differentiated into deep water and near shore zones. EPA reports that “in
addition to being a scenic and ecological treasure, the Lake Tahoe Basin is one of the
outstanding recreational resources of the United States. The communities and the
economy in the Lake Tahoe Basin depend on the protection and restoration of its
stunning natural beauty and diverse recreational opportunities in the region.” (a) List the
States which EPA’s Region 9 serves. (b) Lake Tahoe is listed under Clean Water Act
Section 303(d) as impaired by input of what 3 pollutants? (c) What one pollutant has the
greatest impact on the lake’s deep water quality as measured by water transparency? (d)
what two additional pollutants play an important role regarding water quality of the near
shore zone?
Solution:
7.49 EPA reports that “in addition to being a scenic and ecological treasure, the Lake
Tahoe Basin is one of the outstanding recreational resources of the United States. Go to
the following web site to obtain Secchi depth data at Lake Tahoe
(http://terc.ucdavis.edu/research/SecchiData.pdf). (a) Describe how a Secchi depth
measurement is performed. (b) Produce one properly labeled and captioned graph that
provides the summer average, winter average, and annual average Secchi depth of Lake
Tahoe from 1968 – 2011. Use this figure to answer the question, has water quality in the
lake improved since 1968?
Solution:
0
20
1968 1978 1988 1998 2008
Year
7.50 Go to the web site of the Mississippi River/Gulf of Mexico Watershed Nutrient
(Hypoxia) Task Force. (http://water.epa.gov/type/watersheds/named/msbasin/index.cfm).
On this web site, EPA reports that “hypoxia can be caused by a variety of factors,
including excess nutrients, primarily nitrogen and phosphorus, and waterbody
stratification due to saline or temperature gradients.” Under the Hypoxia 101 web site,
hypoxic waters have dissolved oxygen concentrations of less than 2–3mg/L.
(http://water.epa.gov/type/watersheds/named/msbasin/hypoxia101.cfm). (a) List the four
sources of nutrients that impact this water body.
Solution:
7.51 (a) What was the size (in square miles) of the 2012 Gulf of Mexico hypoxic zone
reported by NOAA scientists on July 27, 2012? (b) Was this larger or smaller than in
2011? (c) What caused the change in the dead zone from 2011 to 2012?
Solution:
7.52 List the three most common contributors to wetland loss in the U.S.
Solution:
7.53 Using Figure 7.29, estimate the nitrogen loading (lbs N/square mile) to a urban
watershed if the watershed protects (a) 5 percent of its wetlands and (b) 15 percent of its
wetlands.
Solution:
7.54 Two groundwater wells are located 100 m apart in permeable sand and gravel. The
water level in well 1 is 50 m below the surface, and in well 2 the water level is 75 m
below the surface. The hydraulic conductivity is 1 m/day, and porosity is 0.60. What is:
(a) the Darcy velocity; (b) the true velocity of the groundwater flowing between wells;
and (c) the period it takes water to travel between the two wells, in days?
Solution:
7.55 The hydraulic gradient of groundwater in a certain location is 2 m/100 m. Here,
groundwater flows through sand, with a hydraulic conductivity equal to 40 m/day and a
porosity of 0.5. An oil spill has caused the pollution of the groundwater in a small region
beneath an industrial site. How long would it take the polluted water from that location to
reach a drinking-water well located 100 m downgradient? Assume no retardation of the
pollutant’s movement.
Solution:
7.56 An underground storage tank has discharged diesel fuel into groundwater. A
drinking-water well is located 200 m downgradient from the fuel spill. To ensure the
safety of the drinking-water supply, a monitoring well is drilled halfway between the
drinking-water well and the fuel spill. The difference in hydraulic head between the
drinking-water well and the monitoring well is 40 cm (with the head in the monitoring
well higher). If the porosity is 39 percent and hydraulic conductivity is 45 m/day, how
long after the contaminated water reaches the monitoring well would it reach the
drinking-water well? Assume the pollutants move at the same speed as the groundwater.
Solution:
7.57 Spills of organic chemicals that contact the ground sometimes reach the
groundwater table, where they are then carried downgradient with the groundwater flow.
The rate at which they are transported with the groundwater is decreased by sorption to
the solids in the groundwater aquifer. The contaminated groundwater can reach wells,
which is dangerous if the water is used for drinking. (a) For a soil having ρb of 2.3 g/cm3
and a porosity of 0.3, and for which the percent organic carbon equals 2 percent,
determine the retardation factors of trichloroethylene
( )
ow
log 2.42K=
,
hexachlorobenzene
( )
ow
log 5.80K=
, and dichloromethane
( )
ow
log 1.31K=
. (b) Which
compound would be transported farthest, second farthest, and least far with the
groundwater if these chemicals entered the same aquifer?
Solution:
a. You will use equation 7.23 to solve for the retardation factor, Rf. However, first you
