In-Situ TROLL 9500 Operators Manual - Manual (Page 120)

115
Multi-Parameter
Water Quality TROLL ®
TROLL 9500 Operator’s Manual 0095110 rev. 007 01/09
17 NITRATE
Unpolluted groundwater 0.05 ppm N
Surface water 0.1 to 20 ppm N
Waste water ~ 30 ppm N
Drinking water standard (EPA) 10 ppm N
Typical Nitrate values
WHAT IS NITRATE ?
Nitrogen is an essential nutrient for plants and animals. It exists in the
environment in many different forms, constantly being replenished
as part of the nitrogen cycle. Nitrate (NO
3
– ) is one form of nitrogen in
the ecosystem that is very soluble in water. Nitrate enters the water
system when runoff from rainfall or irrigation washes through soils
that contain nitrate. The nitrate dissolves in the water and is carried to
nearby streams and lakes. It also permeates downward into the soil
where it may enter underground aquifers. The concentration of nitro-
gen in a body of water depends mostly upon the land cover and soil
type. Highest concentrations are associated with shallow groundwater
and agricultural use of the land. Agriculture is a large contributor to
the pollution of surface water and groundwater because of the use
of fertilizers that contain nitrate. Also, densely populated livestock
produce large quantities of manure that can be changed into nitrate
upon decay.
Nitrate remains in surface water until it is consumed as a nutrient by
plants or other organisms. Surface streams have nitrate concentra-
tions ranging from 0.1 to 20 ppm N. Levels as high as 30 ppm N are
FOUND
WHY MEASURE NITRATE?
High nitrate levels in drinking water are associated with health
problems. Nitrate is reduced to nitrite in the digestive system, where
it may then oxidize iron in the hemoglobin molecule of red blood cells
to form methemoglobin. Methemoglobin lacks the capacity to bind and
carry oxygen to tissues. As a result of this health risk, the EPA has
established 10 ppm N as the maximum limit of nitrate in drinking water
for the USA.
High levels of nitrate in lakes can lead to a process called eutrophi-
cation. Nitrates are nutrients for aquatic plants and algae, causing
overproduction when present in excessive levels. This accelerated
growth or bloom can eventually lead to a number of negative impacts
on the aquatic environment such as a reduction in dissolved oxygen,
which can lead to the death of fish and other aquatic life. Reduction of
sunlight to submerged aquatic vegetation due to increased coverage
on the surface causes a corresponding reduction in photosynthesis
and eventual death.
THE NITRATE SENSOR
The In-Situ sensor is an ion-selective electrode (ISE) that is selective
for the nitrate ion (NO
3
– ). It is a double-junction combination ISE with
a silver/silver-chloride reference half-cell, PVC membrane, and refer-
ence electrolyte gel. It measures the concentration in parts per million
of nitrate ion in solution (calculated as nitrogen, ppm as N).
The Nitrogen Cycle