# Turbidity in Source Water 

## Introduction

The characteristics of turbidity in surface water supplies are a
function of many factors. Watershed features, such as geology, human
development (i.e., agricultural uses or urban development), topography,
vegetation, and precipitation events can all greatly influence raw water
turbidity. In addition, reservoirs and ponds can often dampen the impact
of increased turbidity events by acting as points in a stream or river
where particles can settle before being drawn into the intake of a
treatment plant. Wells and infiltration galleries along streams or
rivers can also reduce the impact of turbidity increases in streams by
their use of a natural aquifer as a filter. This chapter will discuss
the turbidity in surface water and ground water under the direct
influence (GWUDI) of surface water, and other source water
characteristics as they relate to turbidity in raw water supplies.

## 9.2 Occurrence of Turbidity in Surface Water Supplies {#occurrence-of-turbidity-in-surface-water-supplies .unnumbered}

There are many natural processes by which turbidity is created and
conveyed to a raw water intake for a water treatment plant. The
following discussion focuses on the origins of turbidity in surface
water supplies including rivers, lakes and reservoirs.

### Rivers

The largest component comprising the mix of particles creating turbidity
found in rivers is caused by erosion of materials from the contributing
watershed. Turbidity may be created from a wide variety of eroded
materials, including clay, silt, or mineral particles from soils, or
from natural organic matter created by the decay of vegetation.
Particles may capture and hide, or mask, other inorganic and organic
constituents that are present in the watershed.

To help understand the formation of turbidity in rivers, it is important
to understand the natural hydrologic cycle. The diagram shown in Figure
9-1 illustrates the hydrologic cycle. Natural evaporation occurs from
water bodies, such as oceans and lakes, and forms clouds, which then
condense into precipitation. As precipitation falls to the earth, it
first infiltrates the soil and replaces soil moisture and eventually
recharges ground water aquifers. Runoff occurs when the rate of
precipitation exceeds the rate of water infiltrating the soil. As runoff
flows over the land surface, the water can cause soil and other
materials to erode, which results in increased turbidity. Runoff then
collects in streams and rivers that flow back into water bodies such as
lakes and oceans. From there, the hydrologic cycle begins again.

![](media/image1.wmf){width="3.407638888888889in" height="3.3in"}

Source: AWWA, 1990.

[]{#__RefHeading___Toc448538398 .anchor}Figure 9-1. The Natural
Hydrologic Cycle

Thus, during precipitation events, river turbidities are higher than
during periods between precipitation events when the ground water may be
supplying nearly all flow to the stream.

Watersheds have great influence on the resulting stream turbidity during
a precipitation event. Figure 9-2 illustrates the various components of
a watershed. Watersheds can be very large, such as the Mississippi River
drainage basin, which drains most of the central United States. A large
drainage basin usually consists of runoff that has large amounts of
turbidity from the various sources of urban and agricultural runoff and
the differing soil types present. Smaller watersheds may be highly
urbanized and create high turbidity runoff. However, areas which have
been protected from development can produce lower levels of turbidity
even during runoff events.

Mountainous watersheds generally contain turbidity particles that are
largely colloidal rock matter. In glacial areas, the grinding of
mountain glaciers often produces rock particles that add a blue or green
color to the mountain rivers. Mountain watersheds that contain little
development usually have much lower turbidities than those that may be
from an agricultural area in the plains or seacoast areas. Although low
in turbidity, these type of watersheds may still be contaminated by
*Giardia* and *Cryptosporidium* and other enteric viruses due to
wildlife.

Rivers in the plains or coastal areas generally have watersheds which
have farming and urban runoff that add turbidity from topsoil and
organic matter. The topsoil particles are largely clay, and can hide or
carry many different types of contaminants which are a concern to water
utilities. These trapped contaminants can include agricultural chemicals
and compounds such as fertilizers and pesticides.

![](media/image2.wmf){width="2.0388888888888888in"
height="3.0229166666666667in"}

Source: AWWA, 1990.

[]{#__RefHeading___Toc448538399 .anchor}Figure 9-2. Typical Watershed

One agricultural practice of particular concern is confined animal
feeding operations, such as feed lots. This type of development can
produce runoff with high concentrations of organic material and
nutrients. In addition, many microorganisms, including *Giardia* and
*Cryptosporidium* which are known to be present in animal wastes, can be
carried downstream.

Because of the high percentage of impermeable surfaces in urban areas,
the magnitude of runoff often increases leading to the increased levels
of turbidity and related contaminants. In addition to eroded soils,
runoff can include domestic and industrial wastes, fertilizers, road
de-icing salts, overflows from combined sewers, eroded material from
construction sites, and contaminants from roadways and parking lots.
Significant industrial development can result in high levels of toxic
materials, and power generating facilities can cause elevated water
temperatures.

Figure 9-3 shows the possible sources of contaminants that can comprise
or be found in turbidity particles.

![](media/image3.wmf){width="6.029861111111111in"
height="7.718055555555556in"}

Source: Hammer, 1997.

[]{#__RefHeading___Toc448538400 .anchor}Figure 9-3. Sources of
Contaminants in Raw Water Supplies

Organic materials reaching the rivers from both undeveloped and
urbanized land can serve as a food source for bacteria and other
microorganisms. Inorganic nutrients such as nitrogen and phosphorus from
wastewater and agricultural runoff can provide nutrients to stimulate
the growth of algae, resulting in increased turbidity during low flow
times.

The turbidity in streams and rivers is a constantly changing phenomenon.
During dry periods, when no precipitation occurs, turbidity levels
usually drop to a somewhat stable value for the stream. A precipitation
event in the watershed can then bring additional suspended material into
the stream and greatly increase the turbidity. Generally, the more
intense the precipitation event, the higher the turbidity values
experienced in the stream. In addition, turbidity levels are typically
found to be higher further downstream in a watershed due to the amount
contributed from upstream, the variety of contributing factors it
contains, and biological growth that accumulates in the stream as the
water moves through the basin.

##### Low Turbidity in Rivers {#low-turbidity-in-rivers .unnumbered}

Low turbidity streams and rivers (less that 20 NTU) are those which are
usually located at the upper reaches of a undeveloped watershed. These
watersheds include high mountain areas, as well as those watersheds with
little or no development. Characteristics of these watersheds generally
include:

-   Little or no development;

-   No agricultural activity;

-   Heavy natural vegetation along streambanks; and

-   Little streambank erosion.

Although these streams are usually low in turbidity, Total Organic
Carbon (TOC) levels can be high during runoff events resulting in an
increase in turbidity from biological growth or the presence of
significant natural color. In addition, higher mountain streams with low
turbidities are often low in alkalinity due to the lack of natural
buffering materials in the water. This makes the treatment of these
waters subject to swings in pH with the addition of coagulants. A pH
adjustment chemical and the resulting increase in alkalinity is usually
needed to achieve good coagulation and produce stabilized water.

##### High Turbidity in Rivers {#high-turbidity-in-rivers .unnumbered}

High turbidity streams and rivers tend to be located in watersheds which
have erodible soils and/or significant agricultural farming activity.
They can also be streams which receive runoff from urban and
industrialized areas. Large rivers such as the Missouri, Ohio and
Mississippi, have consistently high levels of turbidity in the lower
reaches of its watersheds.

####### *Rapidly Changing Turbidity Case Study* {#rapidly-changing-turbidity-case-study .unnumbered}

*There are many rivers which experience rapid changes to turbidity in
response to precipitation events. One such example of rapidly changing
turbidity involves the Metropolitan Utilities District (MUD) of Omaha,
Nebraska. The Florence Water Treatment Plant in Omaha is located on the
Missouri River. In the spring, the combination of snowmelt and rainwater
runoff, can cause turbidity levels in the Missouri River to rise
rapidly. The upstream drainage area is largely agricultural with several
urban centers. The Florence Water Treatment Plant receives water
directly from the Missouri River through a surface intake. The plant has
pre-sedimentation using cationic polymer followed by split-treatment
with lime softening and alum. Disinfection and filtration complete the
treatment.*

*During the period between March 5th and March 9th of 1995, the plant
experienced a*

*severe increase in the raw water turbidity associated with spring
precipitation. Turbidity values increased from under 100 NTU on March
5th to over 1,000 NTU on March 8th. A graph of the turbidity increases
is shown on Figure 9-4. While the Florence Plant has experienced similar
episodes in the past, the 1995 event was one of the largest swings in
turbidity due to runoff in recent years.*

*During the four day time period, the plant operators were able to
manage the increasing turbidity levels by increasing chemical dosages
while monitoring raw water quality parameters such as color, hardness,
and turbidity. Over the years, the plant operators have noticed the raw
water color will begin to increase and hardness will begin to drop prior
to the arrival of the high turbidity water from snow melts or
precipitation events. The increased color results from the suspension of
decayed organic matter in the spring runoff. The reduction in hardness
experienced is typical of runoff.*

*By observing color and hardness during a runoff event, the plant
adjusted polymers and coagulants slightly ahead of the turbidity event
so that optimal treatment could be maintained through the plant. In
fact, no appreciable increase in turbidity from the pre-sedimentation
basins were recorded throughout the event. The quality of the finished
water effluent was unimpacted.*

![](media/image4.wmf)

###  {#section .unnumbered}

### High turbidity streams and rivers are usually subject to large increases in turbidity as the result of precipitation events. Larger amounts of precipitation cause erosion by bringing topsoil and decaying organic matter into the rivers. Increases in turbidities can sometimes occur within short periods of time as large flushes of precipitation runoff pass through the watershed. {#high-turbidity-streams-and-rivers-are-usually-subject-to-large-increases-in-turbidity-as-the-result-of-precipitation-events.-larger-amounts-of-precipitation-cause-erosion-by-bringing-topsoil-and-decaying-organic-matter-into-the-rivers.-increases-in-turbidities-can-sometimes-occur-within-short-periods-of-time-as-large-flushes-of-precipitation-runoff-pass-through-the-watershed. .unnumbered}

### Lakes and Reservoirs

Lakes and reservoirs capture water either naturally or through man-made
impoundments through the construction of dams on streams or rivers.
Because of the quiescent nature in a reservoir, turbidity levels are
generally lower than streams or rivers due to the settling of particles.

##### Low Turbidity in Lakes and Reservoirs {#low-turbidity-in-lakes-and-reservoirs .unnumbered}

In general, larger reservoirs or lakes have lower turbidity levels. For
example, the Great Lakes usually have turbidity levels below 100 NTU,
whereas rivers can have turbidities reaching over 1000 NTU. Lakes and
reservoirs provide longer detention times, allowing for adequate
settling of the larger turbidity particles and suspended solids. Of
course, intakes near a river inlet to the lake may be subject to greater
swings in turbidity since they may experience carryover of the river
turbidity into the lake before settling can occur.

Low turbidity levels can also be found in smaller reservoirs receiving
drainage from higher quality streams serving non-agricultural watersheds
such as those found in mountainous areas. These reservoirs have
watersheds which have streams carrying low sediment loads due to a great
deal of snow melt runoff. They usually produce relatively stable sources
of supply with little swings in turbidity. However, because they are
small they sometimes do not have the capacity to deal with the extremes
caused by severe intense precipitation or destruction of vegetation by
fire. Such was the case experienced by the Denver Water Department in
the spring of 1996.

##### High Turbidity in Lakes and Reservoirs {#high-turbidity-in-lakes-and-reservoirs .unnumbered}

High turbidity events in reservoirs usually occur in smaller reservoirs
or lakes that receive water from a agricultural watershed or urban
drainage area. Larger reservoirs also experience high turbidity as a
result of water quality changes during annual thermal changes in the
lake, and may experience high turbidity events associated with severe
flooding. During periods of heavy watershed runoff, smaller reservoirs
are usually not large enough to effectively settle out the turbidity
particles before they reach the intake location of a water treatment
plant. In addition, if a reservoir or lake is shallow, wave action
created by high winds can stir up sediments from the bottom and
re-suspend particles.

Reservoir turbidity events can also be caused by seasonal turnover of
the thermal stratification levels which form in the reservoirs. In deep
reservoirs (i.e., those over 20 feet) de-stratification often occurs in
the fall when the upper levels of the reservoir become cooler and
denser, drop to the lower levels, and destroy the stratification. This
"overturning" effect can happen quickly and may bring anoxic water that
is nutrient-rich from the lower depths to the surface where algae is
present. Sudden algal blooms can then severely raise turbidity levels.
The resulting impacts on raw water supplies can be minimized through the
construction of intake structures with multiple level draw-off points.

####### *Lake Reservoir Turbidity Case Study* {#lake-reservoir-turbidity-case-study .unnumbered}

*A forest fire in the watershed area of Buffalo Creek near Denver has
caused some long-lasting impacts on the tributary area turbidities. The
Foothills Water Treatment Plant receives water from this watershed via a
storage reservoir in Watertown Canyon. In May 1996, the Buffalo Creek
watershed experienced a forest fire which destroyed many trees and
ground vegetation. Prior to the fire, the highest turbidities observed
were around 40 NTU (Denver Water). Usually, the turbidities from the
reservoir were less than 20 NTU.*

*During and after the fire, increased erosion has caused the turbidity
to reach as much as 400 NTU during peak runoff events. Figure 9-5 shows
a comparison of the maximum average day turbidities before and after the
fire. The watershed is still recovering from the fire and so these
higher turbidities may still occur during precipitation events. Since
the fire, Denver Water Department has installed a turbidimeter on the
intake tower at the Strontious Springs Dam, as well as instituting an
observation system of spotters who live in Buffalo Creek that will
inform the Department when heavy rainfalls are occurring so that
appropriate flow and chemical adjustments can be made at the downstream
plant so that finished water quality is not effected.*

![](media/image5.wmf){width="5.653472222222222in"
height="3.217361111111111in"}

## 9.3 Ground Water Under the Direct Influence (GWUDI) {#ground-water-under-the-direct-influence-gwudi .unnumbered}

GWUDI is also a water supply source that can be subject to changes in
turbidity. These sources usually consist of a well or infiltration
gallery located adjacent to or under a stream, river or lake. However,
wells that are located significant distances away from surface water
sources can also be influenced by surface water runoff due to geologic
conditions that transport water quickly from the ground surface to the
well.

The classification of a water source as GWUDI is usually the result of a
Microscopic Particulate Analysis (MPA) test that examines the source for
certain surface water organisms in the water supply. The presence of
these organisms is an indication that the supply is influenced by
surface water. In addition, if raw water turbidities or temperatures
rise and fall with the source water this can also be an indicator of a
connection between the surface water source and the well.

The degree to which these types of raw water sources are subject to
changes in turbidity are usually a function of how far the well is from
the surface water source and the type of linking aquifer. Wells in very
close proximity to a river with a coarse gravel aquifer may mirror the
turbidity changes in the river very closely. On the other hand, wells
that are several hundred feet from a surface water source in a tight
sandy aquifer may not experience noticeable changes in turbidity in
response to a surface water turbidity event.

## Additional Watershed Considerations

Water facilities should also be aware of the nature of their watershed
and any contamination sources. Some sources of contamination may be
present which are not a concern under normal circumstances, but may
become a problem during a large precipitation event. For example, some
U.S. cities have combined sanitary and storm sewers which can result in
surface water contamination during large rain events.

During dry periods, combined sewer systems collect wastewater and convey
it to the wastewater treatment plant. However, during periods of high
precipitation these systems also collect storm water which also travels
to the wastewater treatment plant. When extreme precipitation events
occur, wastewater plant can be overloaded by the high combined flows.
This causes sewerage to bypass the plant and undergo only preliminary
treatment prior to discharge to a river. If flows are extremely high,
preliminary treatment may also be bypassed and raw sewage may be
discharged directly to the river. These events, termed combined sewer
overflows (CSOs), can cause increases in river turbidity. However, at
times turbidity levels in the receiving stream may actually drop due to
the large amount of sewage flow, but the levels of bacteria and protozoa
in the raw water supply may rise significantly.

Another similar situation may exist where water treatment plants are
situated immediately downstream of livestock operations. These feeding
operations can be sources for runoff containing elevated levels of
*Giardia* and *Cryptosporidium* during runoff events. Downstream
utilities should be aware of these situations and take the necessary
treatment precautions to ensure adequate treatment.

## Water Sources Occurrence in the United States

![](media/image6.wmf)Turbidity occurrence in raw water supplies is
generally associated with surface water sources or GWUDI. An indication
of the percentage of utilities using surface water sources of raw water
supply was presented in the 1997 *Community Water System Survey* (USEPA,
1997). Figure 9-6 illustrates the percentage of utilities using ground
water versus surface water sources by water system size. These
percentages were developed from the data presented in the 1997 study,
which is a representative sample of systems in the U.S.

The figure indicates that smaller systems typically use ground water
while larger systems typically use surface water. Since turbidity
occurrence is generally greater with surface water sources, larger water
systems may be more concerned with potential difficulties arising from
high turbidity levels.

Source: USEPA, 1997.

[]{#__RefHeading___Toc448538401 .anchor}Figure 9-6. Raw Water Source by
Water System Size

## References

1.  AWWA. 1990. *Water Quality and Treatment.* Fourth Edition.
    McGraw-Hill, New York.

2.  Hammer, M.J. 1977. *Water and Wastewater Technology.* First Edition.
    John Wiley & Sons.

3.  LeChevallier, M.W. and W.D. Norton. 1992. "Examining Relationships
    Between Particle Counts and Giardia, Cryptosporidium, and
    Turbidity." *J. AWWA*. 84(12):54-60.

4.  USEPA. 1997. *Community Water System Survey (CWSS),* *Volumes I and
    II*. Office of Water, Washington, D.C. EPA/815-R-97-001a and -001b.

    THIS PAGE INTENTIONALLY LEFT BLANK

9\. Turbidity in Source Water [9-1](#turbidity-in-source-water)

9.1 Introduction [9-1](#introduction)

9.2 Occurrence of Turbidity in Surface Water Supplies
[9-1](#occurrence-of-turbidity-in-surface-water-supplies)

9.2.1 Rivers [9-1](#rivers)

9.2.2 Lakes and Reservoirs [9-7](#lakes-and-reservoirs)

9.3 Ground Water Under the Direct Influence (GWUDI)
[9-9](#ground-water-under-the-direct-influence-gwudi)

9.4 Additional Watershed Considerations
[9-9](#additional-watershed-considerations)

9.5 Water Sources Occurrence in the United States
[9-10](#water-sources-occurrence-in-the-united-states)

9.6 References [9-11](#references)

Figure 9-1. The Natural Hydrologic Cycle
[9-2](#__RefHeading___Toc448538398)

Figure 9-2. Typical Watershed [9-3](#__RefHeading___Toc448538399)

Figure 9-3. Sources of Contaminants in Raw Water Supplies
[9-4](#__RefHeading___Toc448538400)

Figure 9-4. Turbidity Increase Event on the Missouri River at Omaha 9-6

Figure 9-5. Turbidity Increase Due to Forest Fire in Buffalo Creek 9-8

Figure 9-6. Raw Water Source by Water System Size
[9-10](#__RefHeading___Toc448538401)