Geology Reference
In-Depth Information
NO
3
is reduced in the baby's stomach to NO
2
, which, on absorption,
reacts with oxyhaemoglobin to form methaemoglobin. There have also
been fears over the reaction of NO
2
with secondary amines from the
breakdown of meat or protein to produce carcinogenic N-nitroso com-
pounds, but there is as yet no clear evidence of a link between stomach
cancer and nitrate in water.
153
Nevertheless, there is growing concern over
the contamination of groundwater by nitrate, for example in regions such
as the Sierra Pelona Basin, California.
154
There the local groundwaters,
the major source of drinking water from private water wells located near
each private residence in the rural communities, often exceed the USEPA
maximum contaminant level for drinking water of 10 mg L
1
(NO
3
-N).
Isotopic investigations, based upon
15
N/
14
N, have confirmed the predo-
minance of anthropogenic, organic human, and/or animal waste and
decay of irrigation-enhanced vegetation rather than natural nitrate
sources. In south Dorset, UK, the cause of the increase in the nitrate
concentrations of the Chalk groundwater from a baseline of 1.04 mg L
1
(NO
3
-N)to6.37mgL
1
has been attributed to increased fertilizer use and
increased livestock numbers, mostly during the final 25 years of the 20th
century.
155
It is feared that nitrate concentrations in both surface and
groundwaters of the Chalk in southern England may approach or exceed
the EC limit of 11.3 mg L
1
(NO
3
-N) for potable supplies in future years.
Nitrate in manure and fertilizer is also now under suspicion as an
endocrine disruptor (see Section. 3.3.5.2.3).
156
3.3.5 Organic Matter and Organic Chemicals in Water
3.3.5.1 BOD and COD. The solubility of oxygen in water in equili-
brium with the atmosphere at 251C is 8.7 mg L
1
. Causes of oxygen
depletion include decomposition of biomass (e.g. algal blooms) and the
presence of oxidizable substances (e.g. sewage, agricultural runoff, factory
e
uents) in the water. The addition of oxidizable pollutants to streams
produces a typical sag in the dissolved oxygen concentration. The degree
of oxygen consumption by microbially mediated oxidation of organic
matter in water is called the Biochemical (or Biological) Oxygen Demand
(BOD) (cf. Equation (3.99)). Another index is the Chemical Oxygen
Demand (COD), which is determined by using the powerful oxidizing
agent, dichromate (Cr
2
O
7
2
), to oxidize organic matter
2Cr
2
O
2
7
þ
3CH
2
O
þ
16H
1
4Cr
31
þ
3CO
2(g)
þ
11H
2
O (3.111)
"
followed by back-titration of excess added dichromate with Fe
21
Cr
2
O
2
7
þ
6Fe
21
þ
14H
1
2Cr
31
þ
6Fe
31
þ
7H
2
O (3.112)
"
