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94 Chapter Three

careful measurement of pH in the field with no loss To illustrate carbonate dissolution pathways in
of carbon dioxide by exsolution from the sample. a limestone aquifer, Fig. 3.17 shows the results of
In the groundwater environment, Pco is invariably chemical modelling of recharge to the Chalk aquifer
2
−4
>10 atmospheres and again it is noticed that HCO − 3 in north Norfolk, eastern England. In this area, the
2−
rather than CO is the dominant ionic species of DIC Chalk aquifer is covered by glacial deposits including
3
in groundwater. In Box 3.5, the carbonate chemistry outwash sands and gravel and two types of till deposits
of a limestone aquifer in the west of England is that are distinguished by the mixture of contained
described to illustrate the changes in the distribution clay, sand and carbonate fractions. The evolution of
of carbonate species along a groundwater flowpath. the carbonate chemistry for this aquifer system can
The chemical evolution of the carbonate system be modelled for both open- and closed-systems as
can be considered as occurring under ‘open’ or demonstrated by Hiscock (1993). Two shallow well
‘closed’ conditions. As shown in Fig. 3.15, under open waters in the glacial deposits that are both under-
conditions of carbonate dissolution, a constant Pco saturated with respect to calcite represent starting
2
is maintained while under ‘closed’ conditions, car- conditions for the two models. Using the computer
bonate dissolution occurs without replenishment of program mix2 (Plummer et al. 1975) small increments
CO . Using the theory outlined above, it is possible to of carbonate are added to the groundwaters until sat-
2
model paths of chemical evolution for groundwater uration is reached. The initial and final chemical com-
dissolving carbonate. Steps along the paths are com- positions of the well waters are given in Table 3.5 and
puted by hypothetically dissolving small amounts of the evolution paths followed in each case are shown
carbonate material (calcite or dolomite) for a given in Fig. 3.17. The agreement between the modelled
temperature and starting condition for Pco until the chemistry and the actual Chalk groundwaters sam-
2
water becomes saturated (lines 2 and 3 at 15°C in Fig. pled from beneath the glacial deposits demonstrates
3.16). Lower temperatures will shift the saturation that calcite saturation is achieved under open-system
line to higher solubilities, while higher temperatures conditions. The distribution of points about the phase
will result in saturation at lower solubilities. boundary in Fig. 3.17 is, as explained by Langmuir
It is noticeable in Fig. 3.16 that, for closed-system (1971), the result of either variations in the soil Pco
2
dissolution, the pH values at saturation are higher at the time of groundwater recharge or changes
−
and the HCO concentrations lower. In reality, very in groundwater chemistry that affect Pco . For the
3 2
small quantities of calcite and dolomite exert a strong Chalk aquifer in north Norfolk, the main reason
influence on the carbonate chemistry of groundwater is variations in soil Pco . In areas of sandy till cover,
2
flowing through the soil zone such that open-system recharge entering soils depleted in carbonate attain
conditions typically dominate, resulting in a pH calcite saturation for lower values of soil Pco , in the
2
invariably between 7 and 8. range 10 −2.5 to 10 −2.0 atmospheres. By contrast, the

BO X
Carbonate chemistry of the Jurassic limestones of the Cotswolds, England
3.5

The Jurassic limestones of the Cotswolds, England, form two major bands within the limestones, as well as the presence of several
hydrogeological units in the Upper Thames catchment: (i) lime- faults, gives rise to numerous springs. Flow through the limestones
stones in the Inferior Oolite Series; and (ii) limestones in the Great is considered to be dominantly through a small number of fissures.
Oolite Series. These aquifer units are separated by the Fullers Earth At certain horizons a component of intergranular movement may be
clay which, in this area, acts as an aquitard. The groundwater level significant, but on the whole the limestone matrix has a very low
contours shown in Fig. 1 indicate a regional groundwater flow direc- intrinsic intergranular permeability due to its normally well-
tion to the south-east although with many local variations superim- cemented and massive lithology (Morgan-Jones & Eggboro 1981).
posed upon it. In the upper reaches, river valleys are often eroded to A survey of the hydrochemistry of boreholes and springs between
expose the underlying Lias clays. The presence of these clays 1976 and 1979 is reported by Morgan-Jones and Eggboro (1981).
together with the Fullers Earth clay, intercalated clay and marl The hydrochemical profiles shown in Fig. 2 are for the selected