Page 114 - Hydrogeology Principles and Practice

HYDC03 12/5/05 5:36 PM Page 97

Chemical hydrogeology 97

BO X
Continued
3.5

Fig. 3 Cross-plots of calcite saturation
index versus dolomite saturation index
for (a) borehole and (b) spring samples
in the Jurassic limestone aquifers. The
numbered samples shown are those
located in Fig. 1. After Morgan-Jones and
Eggboro (1981).

Values of pH of about 7.5 were recorded for all unconfined limestones contain small amounts of fluorite (CaF ) but this mineral
2
groundwater samples. With the onset of confined conditions, pH has a very low solubility (K at 25°C = 10 −10.4 ) and the majority
sp
−
−
values increase as a result of changes in HCO concentration and of Ca-rich waters have low F values. Morgan-Jones and Eggboro
3
−
+
ion exchange reactions lead to an increase in Na and decrease (1981) considered that rainwater is the principal source of F in the
in Ca 2+ concentrations (Fig. 2). The removal of Ca 2+ causes the unconfined aquifer areas. A maximum value of 9.8 mg L −1 was
groundwater to dissolve more carbonate material, but without the recorded at sample location 10 in the confined Great Oolite aquifer.
replenishment of CO in the closed-system conditions that prevail, Here, the increase is related to the onset of ion exchange and to the
2
+
the pH begins to rise as H ions are consumed in the calcite dissolu- availability and solubility of fluorite within the limestone. The equi-
−
2+
tion reaction (eq. 3.5). For example, the confined groundwater librium activity of F is dependent on the activity of Ca as defined
−
+
2+
of sample 8 has a pH of 9.23 and Ca , HCO and Na concentra- by the equilibrium constant:
3
−1
tions of 4, 295 and 137 mg L , respectively, compared with the
+
− 2
2
unconfined sample 2 which has corresponding values of 7.31 (pH) [ Ca ][ ] eq. 1
F
−
2+
+
−1
and 100 (Ca ), 246 (HCO ) and 6.9 (Na )mgL . K CaF 2 = [ CaF ]
3
Of the minor ions, fluoride shows considerable enrichment 2
with increasing confinement of the groundwaters. The majority of Hence, the decline in the activity of Ca 2+ with increasing ion
−
−
−1
spring samples have F concentrations of <100 mgL (Fig. 2). Most exchange allows the F activity to increase.