Water Resources--Office of Water Quality
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Three methods for field measurement of turbidity are described in this
section: the nephelometric method or "turbidimetric determination," using a
cuvette-based turbidimeter (6.7.3.A);
"determination by submersible sensor" using a multiparameter water-quality
instrument with a turbidity probe (6.7.3.B); and
the absorptometric determination, using a spectrophotometer (6.7.3.C). Procedures are similar for use of
turbidity instruments in surface water and ground water, although some
applications may differ, as described below.
||Turbidity is time sensitive--Measure sample turbidity
on site to avoid biased values that can result from (1) biodegradation,
settling, or sorption of particulates in the sample; or (2) precipitation
of humic acids and minerals (carbonates and hydroxides, for example) caused
by changes in sample pH during transport and holding.|
| ||Biased or erroneous readings can
result from unmatched cell orientation, colored sample solutions, gas
bubbles, condensation, and scratched or dirty sample cells (see Technical
Note). Condensation on the sample cell commonly occurs on hot days when
humidity is high.|
TECHNICAL NOTE: Causes of
low-biased readings include particulate settling or sorption on container
surfaces, biodegradation, and sample solutions with true color (color from
dissolved substances that absorb light--some instruments are designed with
optics to eliminate bias from color). High-biased or false turbidity
readings can be caused by the presence of condensation and finely-divided
air or other gas bubbles in the sample or on the cell or probe surface, and
scratches, fingerprints, or dirt on the surface of the sample cell or
Collect samples for turbidity measurement or make in situ measurements
using either discharge-weighted, pumped-sample, or grab-sample procedures,
as appropriate for site characteristics and for study objectives (see NFM 6.0).
| ||If taking
discrete samples from a churn splitter or other sample-compositing device,
remove samples for turbidity measurement when the water volume in the
compositor is near maximum.|
| ||Verify the turbidity determination by measuring
turbidity on two or more samples, if samples are removed from a compositing
device or collected as grab samples from the surface-water body. Collect
turbidity sample directly into the cuvette for immediate measurement or
into a clean amber glass bottle for short-term storage.|
| ||If turbidity is measured in situ, take three or more
sequential turbidity readings, until readings stabilize to within ±10
percent (see NFM 6.0).|
Turbidity in ground water generally is less than 5 NTU. Natural
ground-water turbidity of up to 19 NTU has been reported for some
environmental settings (Nightingale and Bianchi, 1977; Strausberg, 1983;
Puls and Powell, 1992). Contaminated ground-water systems, however, can
have considerably higher turbidity (Wells and others, 1989; Gschwend and
others, 1990; Puls and Powell, 1992; Backhus and others, 1993).
For discrete-sample measurement using a turbidimeter or
||During well development--Monitor turbidity caused by
well installation, recording consecutive measurements to document decreases
in turbidity as development proceeds.|
| ||During well purging--Monitor
changes in turbidity by taking sequential readings until purging criteria
are met (NFM 6.0). The final stabilized
turbidity value should be equal to or less than the value recorded at the
end of well development. A decrease in turbidity values during purging
indicates mitigation of subsurface disturbance caused by well installation
and by deployment of data-collection equipment in the well.|
| ||Report the median of the final five or more
sequential measurements that meet the ±10-percent criterion for
For turbidimeter measurement using a flowthrough
- Pump the ground-water sample directly from the sample
discharge line into a precleaned glass or polyethylene sample-collection
- Bailers are not recommended for collecting turbidity
samples, as bailer deployment can cause turbidity.
- Do not collect the discharge passing through the flowthrough chamber in
which pH, conductivity, or other field-measurement sensors are
- Split the sample flow from the well between the turbidimeter and
the flowthrough chamber used for other field measurements, as illustrated
in figure 6.7-2 (parallel lines are not needed if field measurements are
made using a downhole or other in situ method, or when discrete samples are
split from a composite). The turbidimeter requires greater flow
velocity than is appropriate for the flowthrough chamber.
- Position the sample-line split to the turbidimeter/debubbler system in
front of (closer to the well) the flowthrough chamber to avoid sediment in
the flowthrough chamber. (The higher velocity flow required through the
turbidimeter can result in mobilizing sediment--see Technical Note.)
- Set up the debubbler plumbing to maintain a constant head and constant
velocity through the turbidimeter's flowthrough cell.
- To construct a debubbler, use a short length of rigid plastic tubing
with one perpendicular tee through which sample enters, another tee at the
top end (the atmospheric vent), and hose clamps to secure the tubing. The
diameter of the tubing and fittings needed for the debubbler is
proportional to the rate at which sample flows through the turbidimeter.
Referring to figure 6.7-2:
- Water entering debubbler at "A" must exit at both "B" and "C."
- Flow exiting at the top ("C") must be greater than the flow exiting at
the bottom ("B").
- The tubing extending from the debubbler bottom ("B") to the
turbidimeter will probably need a smaller diameter than the top tubing to
ensure a minimum velocity of 0.46 to 0.61 meters per second (1 1/2 to 2
feet per second).
- The atmospheric vent should be located at the highest point in the
debubbler system to prevent siphoning.
NOTE: Backpressure must not be allowed in a flowthrough
chamber containing pH or dissolved-oxygen sensors, and the line to the
flowthrough chamber must be disconnected or bypassed until any appreciable
volume of sediment clears from the line. Water should never
discharge from the atmospheric vent.
The nephelometric method for making turbidimetric determinations that is
described in this section requires a photoelectric turbidimeter that meets
USEPA specifications.5 This method is applicable in the range
of turbidity from 0 to 40 NTU without dilution, and from about 40 to 1,000
NTU with dilution. The method has been tested for drinking and process
water and yields real values in NTU.
Check the turbidimeter against a standard before measuring
- Warm up the turbidimeter according to the manufacturer's
- Rinse a clean, dry, scratch-free, index-marked cell with the turbidity
standard selected at the NTU within the range of interest.
- Shake and pour standard into the sample cell to the fill mark and dry
the cell exterior with a lint-free cloth.
- Follow manufacturer's instructions for readout of turbidity value and
record the NTU of the standard used and the turbidity value measured in the
turbidimeter calibration log.
- Determine the required reading for the turbidity standard from the
calibration curve for the instrument's range and adjust the calibration to
the required NTU reading.
- Measure sample turbidity as soon as sample is collected (see Technical Note).
Turbidity should be measured immediately. However, if
temporary storage of samples becomes necessary, collect samples in clean
amber glass bottles, keep out of sunlight, and keep chilled at or below
4°C to prevent biodegradation of solids. The holding time must not
exceed 24 hours (American Society for Testing and Materials,
Measurement of sample with turbidity less than 40
For measurement of sample with turbidity exceeding 40
- After instrument standardization check, empty the cell of turbidity
standard and field rinse a freshly cleaned cell with the sample to be
tested. Change gloves.
- Measurement of discrete sample (skip to step 3 for
flowthrough cell measurement):
- Shake the sample vigorously to completely disperse the solids. Allow
air bubbles to disappear before filling sample cell.
- Pour the sample into a sample cell to the line marked (to the neck if
there is no line). Do not touch cell walls with fingers.
- Remove condensation from the cell with a clean, soft, lint-free cloth
or tissue. If condensation continues, apply a thin coating of silicon oil
on the outside of the cell about every third time the cell is wiped dry of
- Orient the cell with standard in the turbidimeter. Go to step
- If using an instrument with an internal flowthrough
- Orient the cell in the cell chamber of the turbidity instrument.
- Pump a steady stream of sample in-line from the sample source.
- Use a constant flow rate through the turbidity instrument.
- Flow to the turbidimeter must be sufficient to keep particulates
suspended (1 1/2 to 2 feet per second).
- Check periodically for condensation on flow cell--remove any moisture
from cell using soft, lint-free wipe. If necessary, wipe cell walls with
two drops of silicon oil and a lint-free wipe. If available, try a gas
sweep of the flowthrough cell compartment using dry nitrogen gas.
- Make sure that the flow rate of the gas does not exceed the
rate recommended by the manufacturer.
- Filter the gas to remove particulates and moisture--use a filter that
includes desiccant (particulates or moisture in the gas stream can cause
additional variability in the turbidity readings).
- Eliminate air bubbles in sample before measurement using a debubbler
- Determine the measured NTU value of the sample either by reading
turbidity directly from the instrument scale or by using the instrument
value and calibration curve, as is appropriate for the instrument being
used (see Technical Note).
- Record three to five separate readings at regularly spaced
- Report the median of the last three or more sequential values that fall
within ±10 percent.
NOTE: When using the 0.2-NTU scale only, you may need to
subtract a correction factor from the reading to correct for stray light.
The Hach Company reports the correction for the 0.2-NTU scale to be on the
order of 0.04 NTU for the Hach 2100PTM. The
stray-light correction is determined by reading turbidity from an empty
instrument (without cuvette).
- Quality control.
- Repeat discrete sample measurement on two additional samples and check
that they fall within the ± 10-percent criterion. Report the value of
the first if two samples are measured, or report the median if three or
more samples are measured.
- Using a clean sample cell, repeat the procedure, substituting
turbidity-free water to run a blank. Run the blank either before or after
the sample, following manufacturer's instructions.
- Obtain discrete sample.
- Dilute the sample with one or more equal volumes of turbidity-free
water until turbidity is less than 40 NTU after mixing and degassing.
- For 100- and 1,000-NTU ranges only--place the cell riser into the cell
holder before the sample cell. This decreases the length of the light path
in order to improve the linearity of measurements. Do not use the
cell riser for the lower NTU ranges.
- Follow procedures for samples with turbidity less than 40 NTU.
- Based on the dilution factor and original sample volume, compute the
turbidity of the original sample (see 6.7.2,
"Calibration for Turbidimeter," steps 11and 12):
- Add volume of dilution water (in mL) to sample volume (in mL).
- Multiply by NTU of diluted sample.
- Divide by the volume of sample (in mL) that was diluted.
EXAMPLE: If 5 volumes of turbidity-free water
were added to 1 volume of sample, and the diluted sample showed a turbidity
of 30 units, then the turbidity of the original sample is computed as 180
Determination of turbidity using a multiparameter instrument with
submersible sensor-containing sonde is useful for water-quality studies in
which the turbidity data will be used qualitatively and not for regulatory
or compliance purposes. Turbidity sensors for these instruments utilize an
LED with near infrared radiation as the light source and turbidity values
normally are reported as NTU. Current instrumentation of this type is not
approved by the USEPA.
Multiparameter instruments can be used with a flowthrough chamber, instead
of being deployed in situ, for monitoring ground-water field measurements.
If measurements will be made in a flowthrough chamber, the turbidity probe
is part of the sonde bundle that includes other field-measurement sensors
(for example, pH, conductivity, temperature, and dissolved oxygen) and a
separate or parallel setup for turbidity measurement (fig. 6.7-2) is not needed.
Multiparameter instruments with internal batteries and memory can be used
in surface-water studies that require long-term deployment. Guidelines for
long-term instrument deployment falls under the topic of continuous
monitors, and is beyond the scope of this chapter--refer to manufacturer's
instructions and recommendations and to guidance documents for continuous
The following procedures apply to in situ determination and to
determination of turbidity in a flowthrough chamber:
- Calibrate the instrument in the laboratory or office before leaving for
the field site (see 6.7.2).
- At the field site, follow procedures for selection of surface-water and
ground-water sampling locations and for in situ (Procedure A) or
flowthrough-chamber (Procedure B) field measurements, as described in NFM 6.0.
Procedure A: In situ measurement--Immerse the sonde with
turbidity and other field-measurement sensors in the water body.
Procedure B: Flowthrough chamber--Secure chamber cover
over sonde/probe to form an air-tight and water-tight seal. Discharge
first sample aliquot to waste, then open connection to flowthrough chamber
and pump sample from water source to flowthrough chamber according to
instructions in NFM 6.0.3.
- Activate instrument to display turbidity values in real time.
- Agitate the turbidity-containing sonde to remove bubbles from the
optical surface: move sonde up and down or in a circular pattern and (or)
activate wiper mechanism if available.
- Monitor turbidity readings as described for other field measurements in NFM 6.0.
- Allow at least 2 minutes before recording the required number of
- Stability is reached if values for three (for in situ procedure) to
five (for flowthrough-chamber procedure) or more sequential readings,
spaced at regular time increments, are within 10 percent.
- Record turbidity readings on field form and in field notes. Log the
reading into instrument memory, if applicable.
- Surface-water sites--Repeat steps 2-5 for in situ
measurements (Procedure A) at each vertical to be measured.
- Before leaving the field site, clean the sonde with a thorough rinse of
deionized water and replace sonde in the storage vessel.
- Quality control. Check instrument performance
periodically by placing a check standard in the instrument storage vessel
and comparing standard value with the reading displayed.
The absorptometric method described below uses a field spectrophotometer to
provide a relative measure of the sample turbidity. The spectrophotometer
shoots a beam of light through the sample at a specific wavelength and
measures the amount of transmitted light absorbed by solids present in the
sample compared to how much of the transmitted light is absorbed by a
The absorptometric method for a Hach DR/2000TM portable spectrophotometer is described below,
because this is the instrument that currently is in use for most USGS field
work. Check operating instructions if using an instrument of
different make, model, or manufacturer--the position on the dial for
wavelength of turbidity may vary for different instruments.
||This method is not approved by the USEPA. It is a
useful method, for example, if the purpose for the turbidity determination
is as an indicator of ambient or "stabilized" conditions during well
development or purging.|
Spectrophotometric measurement of turbidity yields readings in FTU.
Do not enter absorptometrically derived turbidity values into the
Turbidity values below 50 FTU--the range of most surface water and
ground water--are inaccurate using this method and the procedure
is recommended only as a relative measure of sequential turbidity
- Enter the stored program number for turbidity and rotate the wavelength
dial until the display indicates the wavelength value in nanometers (nm)
for the instrument in use--450 nm for a Hach DR/2000TM, for example.
- Put on gloves. Measure standards on the instrument that bracket the
range anticipated in the sample solution. This step checks the accuracy of
the calibration scales. Change gloves with each change in standard
- Pour 25 mL of deionized water into a clean sample cell for the blank.
Hold the cell by the rim--do not touch the cell wall.
- Place blank sample into cell holder, close the light shield, and press
zero. The display should show "wait" and then "0. FTU turbidity."
- Shake environmental sample vigorously to suspend all solids and allow
air bubbles to dissipate.
- Pour 25 mL of sample into another clean sample cell, holding cell by
the rim (top lip).
- Carefully place sample into cell holder.
- Close the light shield. Press read/enter.
- The display first will show "wait" and then show the turbidity value in
- Record the FTU reading.
5The nephelometric method using a calibrated slit
turbidimeter is not described--refer to American Society for Testing and
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Last Modified: 22JUNE98 ghc