Environmental Engineering Reference
In-Depth Information
Sampling and equipment Considerations
Total solids are important to measure in areas where discharges from sew-
age treatment plants, industrial plants, or extensive crop irrigation may occur.
In particular, streams and rivers in arid regions where water is scarce and
evaporation is high tend to have higher concentrations of solids and are more
readily affected by the human introduction of solids from land use activities.
Total solids measurements can be useful as an indicator of the effects of runoff
from construction, agricultural practices, logging activities, sewage treatment
plant discharges, and other sources. As with turbidity, concentrations often
increase sharply during rainfall, especially in developed watersheds. They
can also rise sharply during dry weather if earth-disturbing activities occur
in or near the stream without erosion control practices in place. Regular moni-
toring of total solids can help detect trends that might indicate increasing ero-
sion in developing watersheds. Total solids are closely related to stream flow
and velocity and should be correlated with these factors. Any change in total
solids over time should be measured at the same site at the same flow.
Total solids are measured by weighing the amount of solids present in a
known volume of sample. This is accomplished by weighing a beaker, filling
it with a known volume, evaporating the water in an oven, and completely
drying the residue, then weighing the beaker with the residue. The total sol-
ids concentration is equal to the difference between the weight of the beaker
with the residue and the weight of the beaker without it. Because the residue
is so light in weight, the lab must have a balance that is sensitive to weights
in the range of 0.0001 g. Balances of this type are called
analytical
or
Mettler
balances
, and they are expensive (around $3000). The technique requires that
the beakers be kept in a desiccator—a sealed glass container that contains
material that absorbs moisture and ensures that the weighing is not biased
by water condensing on the beaker. Some desiccants change color to indicate
moisture content. The measurement of total solids cannot be done in the
field. Samples must be collected using clean glass or plastic bottles or Whirl-
Pak
®
bags and taken to a laboratory where the test can be run.
Conductivity
Conductivity is a measure of the ability of water to pass an electrical current.
Conductivity in water is affected by the presence of inorganic dissolved solids
such as chloride, nitrate, sulfate, and phosphate anions (ions that carry a neg-
ative charge) or sodium, magnesium, calcium, iron, and aluminum cations
(ions that carry a positive charge). Organic compounds such as oil, phenol,
alcohol, and sugar do not conduct electrical current very well and therefore
have a low conductivity when in water. Conductivity is also affected by tem-
perature—the warmer the water, the higher the conductivity. Conductivity
