LAKES AND RESERVOIRS - Water Quality Engineering in Natural Systems - page 274

The real source is located at (0 m, 0 m) and the image
sources are located at (0 m, ±45 n m), where n designates
the image pair. Equation (7.94) yields the following
results

continually release nutrients into the water column by
diffusion, even if aerobic conditions are present at the
sediment-water interface. Phosphorus is typically the
limiting nutrient affecting eutrophication, and measures
to control eutrophication usually fall into the following
categories: (1) control of point sources, (2) control of
nonpoint sources, (3) limitation of phosphorus content
in the lake, (4) limitation of internal loading, and (5)
limitation of algal development in the lake, without
changing the phosphorus budget.
In some cases, eutrophication is a localized problem
within a larger lake due to high concentrations that
might be associated with a local river inflow. In such
cases, mechanical mixing devices to redistribute nutri-
ents within the lake might be sufficient to control
eutrophication.

x i y i c i

i (m) (m) (mg/L)

1 0 0 214.1

2 0 90 4.9

3 0 −90 4.9

4 0 180 0.0

5 0 0.0

−180

Total 224.5

Based on these results, it is apparent that one pair of
image sources are sufficient to account for the finite
width of the lake and that the expected steady-state
concentration 30 m downstream of the source is approx-
imately 225 mg/L.

7.6.1.1 Control of Point Sources. Point sources of
phosphorus typically consist of domestic wastewater
discharges, which usually contain 5-10 mg/L P and 20-
40 mg/L N. Advanced wastewater treatment with
precipitation, sedimentation, and/or filtration are con-
ventional approaches for phosphorus removal from
domestic wastewater.

7.6 MANAGEMENT AND RESTORATION

The best shoreline for a lake is a natural one. Natural
shoreline protection, such as native species, woody
growth, and scrub-shrub species, provides habitat for
terrestrial and aquatic species, and also provides a buffer
to filter pollutants as they get washed over the vegeta-
tion during rainfall runoff events. The capability of
natural vegetation to withstand shoreline erosion is also
an important consideration since the root system of
natural vegetation tends to give the soil a supporting
structure that holds the shoreline together. This support
does not exist with shallow-rooted vegetation such as
grass lawns, which are considered by some to be the
worst lake shoreline. Ideally, lakes should be surrounded
by an undisturbed buffer; however, the desirability of
lakefront properties is often given a higher priority.

7.6.1.2 Control of Nonpoint Sources. Nonpoint
sources of phosphorus are commonly associated with
agricultural operations. Other nonpoint sources of
phosphorus include wastewater from populated areas
not connected to a sewage system (e.g., septic tanks),
effluents from isolated farmhouses, water birds, and
open-air bathing facilities.
Best management practices (BMPs) are typically
used to control nonpoint sources of nutrients. Control
measures in agricultural areas include spreading of
animal manure in a way that is compatible with plant
growth and lake protection, evergreen strips along trib-
utaries, and the use of crops that are compatible with
the ground slope, condition of the soil, and the distance
of the crop area from the lake. For example, maize
planted on sloping fields is particularly disadvantageous
because catastrophic erosion can take place during
heavy rainfall.

7.6.1 Control of Eutrophication

The consequences of eutrophication are weed-choked
shallow areas, algal blooms, oxygen-depleted deep
waters, degradation of potable water supplies, limita-
tions on recreational water use, degraded fisheries,
reduced storage capacity, and disruption of downstream
biological communities. The prevention and control of
eutrophication is an important issue of concern in most
lakes, particularly those that are sources of drinking
water. Most techniques to control eutrophication are
designed to manage the inflow of nutrients and sedi-
ment into the lake, and it is widely recognized that sedi-
ment deposited on the bottom of lakes can play a
decisive role as a nutrient source. Benthic sediments can

7.6.1.3 Chemical Treatments for Phosphorus. Alu-
minum salts such as aluminum sulfate (alum, Al 2 (SO 4 ) 3 )
and sodium aluminate (Na 2 Al 2 O 4 ) or ferrous chloride
(FeCl 2 ) have a strong affinity to sorb inorganic phospho-
rus and remove phosphorus-containing particulate
matter from the water column as part of the floc that
forms. The result, after the floc settles, is not only a
reduction of phosphorus availability but also a substan-
tial increase in water clarity. Adverse effects may occur
if the dosage of alum is too high. Particularly in softer