An Introduction to Environmental Chemistry

The Chemistry of Continental Waters 153

pH

0 0.5 1 1.5 2 2.5 3

Alkalinity (meq l–1)

0

2

4

6

8

10

Ca

2+

(mmol l

–1

)

Ca2+

pH

1.0

Siliceous mudrock Limestone

Bog source Stream flow

0.8

0.6

0.4

0.2

Fig. 5.6Relationship between pH, alkalinity and dissolved calcium ions in stream waters in
the Malham Tarn area of northern England, flowing from bog on siliceous mudrock to
limestone. Note that pH is buffered around 8 once limestone weathering begins. Data from
Woof and Jackson (1988).

CO 32 - (Fig. 5.5). These species react to maintain the pH within relatively narrow
limits. This is known as buffering the pH and the principles, using worked exam-
ples, are demonstrated in Box. 5.3. The relationship between pH, CaCO 3 weath-
ering and alkalinity is nicely illustrated by real data (Fig. 5.6) from a small stream
in North Yorkshire (UK). The stream begins as drainage from an organic peat
bog. The initial pH is about 4 and hence the alkalinity is zero (Fig. 5.6). The
stream then flows from the bog over siliceous mudrocks with very limited poten-
tial for weathering such that the water chemistry changes little. After this the
stream flows onto limestone, the acidic water reacting rapidly with the CaCO 3
to release Ca^2 +and HCO 3 - ions:

eqn. 5.12

In less than 100 m of distance flowing on the limestone the pH rises sharply to
values of 6 or 7 and it continues to increase to a value of about 8 at which it sta-
bilizes due to the buffering action of the alkalinity (Fig. 5.6). The alkalinity is
strongly buffering the system at values above 2 meq l-^1 where the relationship
between pH and alkalinity is asymptotic (Fig. 5.6). The relationship between Ca^2 +

CaCO 323 ()sa++H CO()qªCa()^2 aq+ 2 HCO 3 - ()aq