Geology Reference
In-Depth Information
A novel use of phosphate to counter acidification has emerged in
recent years.
150
Traditional remedial methods which have been adopted
include the direct liming of lakes, e.g. in Sweden,
151
or of catchments,
e.g. around Loch Fleet in SW Scotland.
152
Although helpful, such
approaches based on neutralization are costly and usually need to be
repeated at regular intervals. Furthermore, the resulting Ca-rich waters
may turn out to support biota quite different from those found in
natural softwater lakes. An alternative approach has been tried by
Davison and co-workers on Seathwaite Tarn, an upland reservoir in
the English Lake District.
131
Phosphate fertilizer was added to stimulate
primary productivity and thereby increase the assimilation of nitrate.
This generates base according to the equation
106CO
2(g)
þ
138H
2
O
þ
16NO
3
(CH
2
O)
106
(NH
3
)
16
þ
16OH
þ
138O
2(g)
(3.110)
"
As concentrations of nitrate are increasingly high in acid waters, the
addition of modest amounts of phosphate may generate sucient base
to combat acidity without inducing excessive productivity. An increase
in pH of 0.5 and a marked increase in biological productivity at all levels
were observed over the three-year period of the experiment. In the
longer term, additional quantities of base should be generated by the
anoxic decomposition of organic material accumulating on the lake-bed
(through the dissimilative reduction of inorganic oxidants such as
nitrate, sulfate, or iron hydr(oxides) present in the sediments - Equa-
tions (3.100) to (3.103)). If oxygen is the electron acceptor there is no net
gain of base (Equation (3.99)), there is no advantage in adding nitrate
because 1 mol of nitrate is required to generate 1 mol of base, and the
generated base (contributing to alkalinity) should not be confused with
the temporary rise in pH associated with CO
2
consumption, which
affects neither alkalinity nor acidity (see Section 3.2.4.1).
3.3.4.2 Nitrate in Groundwater. Principal sources of nitrate in water
are runoff and drainage from land treated with agricultural fertilizers
and also deposition from the atmosphere as a consequence of NO
x
released from fossil fuel combustion. The nitrogen present in soil
organic matter may also be released as nitrate through microbial action.
Nitrate's role as a nutrient contributes significantly to blooms of algae,
which upon death are decomposed first by aerobic bacteria, thereby
depriving fish and other organisms of oxygen.
59,147
There has long been concern expressed over the presence of nitrate in
drinking water at concentrations exceeding the EC guideline of 50 mg L
1
because of the risk of methaemoglobinaemia (blue baby syndrome). Here,
