Geology Reference
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precipitation of calcite must be occurring during
every early morning in close analogy to the sugges-
tion of Liu et al. (2008).
which can, in turn, be simplified to:
L
c
IAP
(Ca,HCO
3
)
L
Ca(HCO
3
)
2
(4)
Insights into precipitation from conductivity
where IPA
(Ca,HCO
3
)
is the ion activity product of
[Ca
2þ
(aq)
] and [HCO
3(aq
2
]. Usefully, in this form
conductivity is linked to calcite saturation (SI ¼
ln(IAP
(Ca,CO
3
)
=k
sp
), where k
sp
Figure 4 shows electrical conductivity data (mS
cm
21
) measured synchronously with the pH
records in Figure 3. Conductivity of water is con-
trolled by Kohlrausch's Law, which states that
conductivity is related to concentration via:
is
the solubility
product constant for calcite.
We established the relationship between con-
ductivity and IAP
(Ca,HCO
3
)
for Welton spring water
(sterilized by the same procedure as used in all the
described experiments) via sequential dilution at
25 8C (Fig. 9). The relationship between is non-
linear, approximating to L
c
¼ 5.38 10
4
IAP
0.4094
over the IAP range 10
27
to 10
25
. Changing pH (via
decreased [H
(aq)
] and increased [OH
(aq)
] and
[CO
3(aq)
L
c
¼
X
n
v
i
L
i
(2)
i¼0
where L
c
is the conductivity of the solution, v
i
and
L
i
are respectively the molar concentration and
molar conductivity of ionic species i, irrespective
of charge sign. In Welton spring water, which is
taken directly from a chalk aquifer, HCO
3(aq)
22
]), metal/Ca ratios and temperature will
cause drift from this calibration, with pH being of
particular importance due to the substantially
higher molar conductivity of CO
3(aq)
2
rep-
22
compared to
HCO
3(aq
2
. This is reflected in a limiting molar
conductivity (L
0
)
anion and Ca
2þ
resents
(aq)
the
dominant cation, and these are characteristically
present in a 2:1 molar ratio (M. Rogerson, unpub-
lished data). Consequently,
the dominant
22
of 69.5 10
24
m
2
mol
21
compared to 44.5 10
24
m
2
mol
21
for CO
3(aq)
for
for Welton spring
HCO
3(aq
2
(Aguado et al. 2006).
Sterilized flumes (3 and 4) both show fairly
constant conductivity values, with a c.2mScm
21
diurnal variability with maximum values in the
water:
(aq)
] E
Ca
2þ
(aq)
L
c
[HCO
3(aq)
]L
HCO
3(aq)
þ[Ca
2þ
(3)
Fig. 9. Calibration of electrical conductivity against Ion Activity Product of calcium bicarbonate derived from serial
dilution of Welton Beck water.
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