Environmental Engineering Reference
In-Depth Information
phosphorus, nitrates in excess amounts can accelerate eutrophication, caus-
ing dramatic increases in aquatic plant growth and changes in the types of
plants and animals that live in the stream. This, in turn, affects dissolved
oxygen, temperature, and other indicators. Excess nitrates can cause hypoxia
(low levels of dissolved oxygen) and can become toxic to warm-blooded ani-
mals at higher concentrations (10 mg/L or higher) under certain conditions.
The natural level of ammonia or nitrate in surface water is typically low (less
than 1 mg/L), but in the effluent of wastewater treatment plants it can range
up to 30 mg/L. Sources of nitrates include wastewater treatment plants, run-
off from fertilized lawns and cropland, failing on-site septic systems, runoff
from animal manure storage areas, and industrial discharges that contain
corrosion inhibitors.
Sampling and equipment Considerations
Nitrates from land sources end up in rivers and streams more quickly than
other nutrients such as phosphorus because they dissolve in water more read-
ily than phosphorus, which has an attraction for soil particles. As a result,
nitrates serve as a better indicator of the possibility of a source of sewage or
manure pollution during dry weather. Water that is polluted with nitrogen-
rich organic matter might show low nitrates. Decomposition of the organic
matter lowers the dissolved oxygen level, which in turn slows the rate at which
ammonia is oxidized to nitrite (NO
2
) and then to nitrate (NO
3
). Under such
circumstances, monitoring for nitrites or ammonia (considerably more toxic
to aquatic life than nitrate) might also be necessary. (See
Standard Methods
,
Section 4500-NH
3
and Section 4500-NH
2
, for appropriate nitrite methods).
Water samples to be tested for nitrate should be collected in glass or polyethyl-
ene containers that have been prepared by using Method B (presented earlier).
Most monitoring programs usually use two methods for nitrate testing:
the cadmium reduction method and the nitrate electrode method. The more
commonly used cadmium reduction method produces a color reaction mea-
sured either by comparison to a color wheel or by use of a spectrophotometer.
A few programs also use a nitrate electrode, which can measure in the range
of 0 to 100 mg/L nitrate. A newer colorimetric immunoassay technique for
nitrate screening is also now available.
Cadmium Reduction Method
The
cadmium reduction method
is a colorimetric method that involves contact
of the nitrate in the sample with cadmium particles, which cause nitrates to
be converted to nitrites. The nitrites then react with another reagent to form
a red color, in proportional intensity to the original amount of nitrate. The
color is measured either by comparison to a color wheel with a scale in mil-
ligrams per liter that increases with the increase in color hue or by use of an
