Environmental Engineering Reference
In-Depth Information
in streams and rivers is affected primarily by the geology of the area through
which the water flows. Streams that run through areas with granite bedrock
tend to have lower conductivity because granite is composed of more inert
materials that do not ionize (dissolve into ionic components) when washed
into the water. On the other hand, streams that run through areas with clay
soils tend to have higher conductivity, because of the presence of materials
that ionize when washed into the water. Groundwater inflows can have the
same effects, depending on the bedrock they flow through.
Discharges to streams can change the conductivity depending on their
make-up. A failing sewage system would raise the conductivity because of
the presence of chloride, phosphate, and nitrate; an oil spill would lower con-
ductivity. The basic unit of measurement of conductivity is the mho or sie-
mens. Conductivity is measured in micromhos per centimeter (µmho/cm) or
microsiemens per centimeter (µS/cm). Distilled water has a conductivity in
the range of 0.5 to 3 µmho/cm. The conductivity of rivers in the United States
generally ranges from 50 to 1500 µmho/cm. Studies of inland freshwaters
indicate that streams supporting good mixed fisheries have a range between
150 and 500 µmho/cm. Conductivity outside this range could indicate that
the water is not suitable for certain species of fish or macroinvertebrates.
Industrial waters can range as high as 10,000 µmho/cm.
Sampling and equipment Considerations
Conductivity is useful as a general measure of stream water quality. Each
stream tends to have a relatively constant range of conductivity that, once
established, can be used as a baseline for comparison with regular conduc-
tivity measurements. Significant changes in conductivity could indicate
that a discharge or some other source of pollution has entered a stream.
Conductivity is measured with a probe and a meter. Voltage is applied
between two electrodes in a probe immersed in the sample water. The drop
in voltage caused by the resistance of the water is used to calculate the con-
ductivity per centimeter. The meter converts the probe measurement to
micromhos per centimeter (µmho/cm) and displays the result for the user.
Note:
Some conductivity meters can also be used to test for total dis-
solved solids and salinity. The total dissolved solids concentration in
milligrams per liter (mg/L) can also be calculated by multiplying the
conductivity result by a factor between 0.55 and 0.9, which is empirically
determined (see Method 2510 in
Standard Methods
).
Suitable conductivity meters cost about $350. Meters in this price range
should also measure temperature and automatically compensate for tem-
perature in the conductivity reading. Conductivity can be measured in the
field or the lab. In most cases, collecting samples in the field and taking them
to a lab for testing is probably better. In this way, several teams can collect
