An Introduction to Environmental Chemistry

The Onyx River in Antarctica plots in the top corner of Fig. 5.8 as a pure

chloride (sea-salt-dominated) system. As this river evolves and starts to weather

rocks in the catchment, it picks up alkalinity moving on a trajectory toward the

bottom right corner (weathering dominated). Upland systems heavily impacted

by acid mine drainage or acid rain plot toward the bottom left corner of Fig. 5.8,

their anion chemistry is dominated by sulphate with little or no alkalinity present.

Again as these systems mature and acquire weathering products including alka-

linity, they too move in trajectories toward the bottom right corner (weathering

dominated). As noted earlier, HCO 3 - alkalinity is the dominant anionic compo-

nent of most mature rivers and groundwaters, explaining their position at the

bottom right of Fig. 5.8. The only exception is the Rio Grande that plots well

away from most mature rivers. Although this river has the highest HCO 3 -

alkalinity of those plotted on Fig. 5.8 (3.0 mmol l-^1 HCO 3 - ; Table 5.2), the

160 Chapter Five

+

(Sulphuric acid)

SO 4 2–

10 20 30 40 50 60 70 80 90

90

80

70

60

50

40

30

20

10

90

80

70

60

50

40

30

20

10

+

+ +

Rio Grande

ALK

(Weathering)

Limestone

groundwater

Schist

groundwater

Global average river

Yare

Orinoco

Onyx (mouth)

Onyx (near source)

Mackenzie

Granite groundwater

Ganges

% Cl

% SO –

2– 4

Onyx

(source)

Cl–

(Sea-salt)

Woods Lake

Sagamore Lake Panther

1 Lake

2

3

% Alkalinity

+

Rivers

Global average river

Groundwater

Acid mine drainage

Adirondak Lakes (acid deposition)

Fig. 5.8Ternary plot of alkalinity, chloride and sulphate data to trace weathering, sea-salt

and sulphuric acid (acid deposition/mine drainage) inputs to river- and groundwaters. River

and groundwater data are from Tables 5.1–5.3 and Box 5.1. River Yare (eastern England) is

representative of a river system developed on limestone (chalk) bedrock. The Adirondak Lakes

data (from Galloway et al. 1983) represent response of nearby lakes to acid deposition. Woods

Lake has pH of 4.7 and zero alkalinity; it cannot buffer effects of acid input. As alkalinity

increases, due to increased carbonate weathering in the bedrock, the effects of acid deposition

are progressively buffered (Sagamore (pH 5.6) to Panther Lake (pH 6.2)). Acid mine drainage

(AMD) data (from Herlihy et al. 1990) are all averaged from streams in the eastern USA

where (1) are strongly impacted acidic streams, (2) are strongly impacted non-acidic streams

and (3) are weakly impacted non-acidic streams.