Environmental Engineering Reference
In-Depth Information
the amount of light penetrating the water, which reduces photosynthesis
and the production of DO. Suspended materials can clog fish gills, reduc-
ing resistance to disease in fish, lowering growth rates, and affecting egg
and larval development. As the particles settle, they can blanket the stream
bottom (especially in slower waters) and smother fish eggs and benthic mac-
roinvertebrates. Sources of turbidity include
• Soil erosion
• Waste discharge
• Urban runoff
• Eroding stream banks
• Large numbers of bottom feeders (such as carp), which stir up bot-
tom sediments
• Excessive algal growth
Sampling and equipment Considerations
Turbidity can be useful as an indicator of the effects of runoff from construc-
tion, agricultural practices, logging activity, discharges, and other sources.
Turbidity often increases sharply during rainfall, especially in developed
watersheds, which typically have relatively high proportions of impervious
surfaces. The flow of stormwater runoff from impervious surfaces rapidly
increases stream velocity, which increases the erosion rates of streambanks
and channels. Turbidity can also rise sharply during dry weather if earth-
disturbing activities occur in or near a stream without erosion control prac-
tices in place.
Regular monitoring of turbidity can help detect trends that might indicate
increasing erosion in developing watersheds; however, turbidity is closely
related to stream flow and velocity and should be correlated with these fac-
tors. Comparisons of the change in turbidity over time, therefore, should be
made at the same point at the same flow.
Turbidity is not a measure of the amount of suspended solids present or
the rate of sedimentation of a stream, as it represents only the amount of
light that is scattered by suspended particles. Measurement of total solids is
a more direct measurement of the amount of material suspended and dis-
solved in water. Turbidity is generally measured by using a turbidity meter.
Volunteer programs may also take samples to a lab for analysis. Another
approach is to measure transparency (an integrated measure of light scatter-
ing and absorption) instead of turbidity. Water clarity or transparency can
be measured using a
Secchi disk
(see
Figure 10.4
) or transparency tube. The
Secchi disk can only be used in deep, slow-moving rivers; the transparency
tube (a comparatively new development) is gaining acceptance around the
country but is not yet in wide use.
