An Introduction to Environmental Chemistry

process is illustrated in Fig. 5.9 using data from Esthwaite, a lake in northwest

England. In March the water column is well mixed and oxygen concentrations

are uniformly high, around 350-400mmol l-^1 at all depths. By May, however, the

water column is stratifying (warmer above 7 metres water depth, cooler below)

and oxygen concentrations begin to fall in water depths below 8 metres. By the

end of summer a large area of very low oxygen concentrations (< 10 mmol l-^1 ) has

formed below 8 metres depth. In the autumn the stratification is destroyed by

cooling and by strong winds that mix oxygenated surface waters with the deep

water.

The rate of oxygen consumption usually increases as the supply of organic

matter increases, either due to enhanced photosynthesis in the surface waters, or

due to direct discharge of organic waste, for example sewage. The Thames estuary

(UK), Chesapeake Bay (USA) (see Section 6.2.4) and the Baltic Sea (see Section

6.8.1) are all examples of water bodies affected by low oxygen concentrations, and

similar processes occur in groundwater when oxygen consumption exceeds supply.

Once oxygen has been used up, bacteria use alternative oxidizing agents (elec-

tron acceptors, see Box 4.3) to consume organic matter. These alternative oxi-

dants are used in an order that depends on energy yields (see Table 4.7). Nitrate

reduction (denitrification) is energetically favourable to bacteria, but is often

limited in natural freshwaters by low nitrate concentrations. Anthropogenic

inputs, however, have resulted in increased nitrate concentrations in rivers and

groundwater (Section 5.5.1), increasing the availability of nitrate for bacterial

reduction. One of the byproducts of denitrification is nitrous oxide (N 2 O; Fig.

5.12), a powerful greenhouse gas which is increasing in concentration, probably

because of human perturbation of the nitrogen cycle.

Iron (Fe) and manganese (Mn), both potential electron acceptors, are common

as insoluble Fe(III) and Mn(IV) oxides. In reducing environments (at about the

162 Chapter Five

0

2

12

14

M AMJ JAOS

10

8

4

6

(b) Dissolved oxygen (mmol l–1)

Depth (m)

400

350

300

400 400

350

250

(^20010)
100
300
300
250
250
0
2
12
14
M AMJ JAOS
10
8
4
6
(a) Temperature (°C)
Depth (m) 6
8
12
10
14
16
16
(^2018)
16
16
10
12
14
Fig. 5.9Variation in (a) temperature and (b) dissolved oxygen for lakewater (Esthwaite in
northwest England) between March and October 1971. Modified from Heany et al. (1986).
© 1986. This material is used by permission of John Wiley & Sons, Inc.