form the main diffuse sources of the NO3−
that is leached to groundwater. However, NO3−
is also released from exercise yards and manure
storage facilities, which represent important
potential point sources of contamination. The
problem of groundwater contamination by nitrates
appears to be worsening at the end of the 20th
century. Elevated concentrations of nitrate (greater
than 2 mg/L) in drinking water have been associated
with adverse health effects. Ingestion of nitrate
by infants can cause low oxygen levels in the
blood, a potentially fatal condition known as
methemoglobinemia or “blue baby” disorder.
High nitrate and nitrite levels can also cause
methemoglobinemia in livestock and other animals.
If excessive amounts of phosphorus and nitrates
are added to the water, algae and aquatic plants
can be produced in large quantities. When these
algae die, bacteria decompose them, and use up
oxygen. This process is called eutrophication.
Dissolved oxygen concentrations can drop too low
for fish to breathe, leading to fish kills.