Page 147 - Hydrogeology Principles and Practice

HYDC04 12/5/05 5:36 PM Page 130

130 Chapter Four

13
The result of calcite dissolution (eq. 4.11) is that the δ C =+2‰
lithic
13
groundwater would be expected to have a δ C value 13
13
of about −12‰. Away from the soil zone, δ C values δ C biogenic =−26‰.
are less negative than those near to the aquifer out-
From equation 4.13, the fraction of biogenic carbon,
crop as a result of the continuous precipitation and
Q = (−6.3 − (+2))/(−26 − (+2)) = 0.2964 and substitu-
dissolution of carbonate as the water flows through
tion in equation 4.15 gives the following corrected
the aquifer. The isotopic composition of the precipit-
groundwater age:
ated carbonate mineral differs from that of the car-
bonate species in solution. As a result there is isotopic
fractionation between the aqueous phase and solid t =−8267 log (13/100) + 8267 log (0.2964) = 6813.5 a
e
e
c
carbonate-containing mineral phase.
To account for the two sources of inorganic and or, in practical terms, about 7 ka.
organic carbon in the groundwater, let Q equal the Further corrected groundwater ages for the Chalk
proportion of biogenic carbon in the sample. To aquifer of the London Basin are given in Table 4.1 and
determine the proportions of lithic and biogenic car- the results shown as age contours in Fig. 4.8c. The
bon, then the following relationship can be used with oldest groundwaters are found below the Eocene
13
the δ C data: London Clay in the confined aquifer at the centre of
the Basin. Modern groundwaters are present in the
13
13
13
δ C = δ C (Q) + δ C (1 − Q) eq. 4. 12 recharge area on the northern limb of the basin. The
sample biogenic lithic
groundwater becomes progressively older towards
or, rearranged: the centre of the basin with flow induced by pumping
of the aquifer (compare with the map of the Chalk
δ 13 C − δ 13 C potentiometric surface shown in Box 2.4). The upper
Q = 13 sample 13 lithic eq. 4.13 limit for the age measurement is about 25 ka with
δ C biogenic − δ C lithic groundwater at the centre of the Basin known to be
14
older than this limit for the C dating method (Smith
Using the calculated value of Q, the corrected
et al. 1976a).
groundwater age, t , can be found from the dating
c
equation (eq. 4.10) by applying the fraction of bio- In situations where a negative corrected ground-
14
genic carbon to the initial C activity, A , such that: water age is obtained, then the sample has become
0 14
swamped by the modern atmospheric C content
14
which is in excess of 100 pmc. The extra C was con-
A
t =−8267 log eq. 4.14 tributed by the detonation of thermonuclear devices
c e
AQ in the 1950s and early 1960s. In such cases, the
0
groundwater age can be assumed to be modern. This
or, separating terms:
effect and other inherent shortcomings, for example
14
contamination with atmospheric C during field
A
t =−8267 log + 8267 log Q eq. 4.15 sampling and laboratory errors in the measurement

c e e 13 14
A 0 of δ C and C, mean that any groundwater sample
with an age of up to 0.5 ka can be regarded as modern.
As an example, the following information was More advanced approaches to correcting C ages
14
obtained for a groundwater sample from Waltham that account for carbon isotope exchange between
Abbey PS (sample 12, Table 4.1) in the Chalk aquifer soil gas, dissolved carbonate species and mineral car-
of the London Basin: bonate in the unsaturated and saturated zones under
open- and closed-system conditions (Section 3.7) are
14
A = C = 100 pmc included in Mook (1980). However, the extra effort
0 atmospheric
expended may not be justified, especially if mixing
14
A = C = 13 pmc
sample
between two groundwaters is suspected. For exam-
13
δ C =−6.3‰ ple, in the case of the Chalk aquifer discussed above,
sample