- we assume that the dissolved carbon is 14C free
- under closed consitions, half of the C in 2HCO3- has
an activity of 100pmC, half is zero, under open conditions carbon
constantly exchanges with the soil CO2 and ao
remains 100pmC
- Chemical and isotopic evolution in recharge zone: (Fig) (Fig) Open and closed system
conditions
- fractionation between the individual carbon species is important
for understanding this evolution (Fig)
(Tab & Fig)
- under fully closed conditions the increase in d13C
is solely due to mixing, under open conditions d13C
is controlled by the soil gas and the isotopec fractionation
Statistical approach
characteristic (empirical) values for q:
q |
aquifer |
0.65-0.75 |
karst |
0.75-0.90 |
sediments
with fine-grained carbonate |
0.90-1.00 |
crystalline
rocks |
a value often used in the literature is q=0.85 (Vogel, 1970)
Alkalinity correction (Tamers, 1975)
proposed for groudnwater systems in which calcite is dissolved under
closed system conditions:
q=(mH2CO3+0.5mHCO3-)/(mH2CO3+mHCO3-)
(m being the concentration of the species)
this results into q=0.5 for many systems
Chemical mass balance correction
again, proposed for closed system conditions:
q = mDICrech/mDICfinal
mDIC
rech is the
14C active part in the
recharge area, mDIC
final is the totla DIC at some point
along the flow line
d13C mixing model (Pearson,
1965; Pearson and Hanshaw, 1979)
assumes that any cahnge in
14C will also be reflected in
13C:
q = (d13CDIC
- d13Ccarb)/(d13Csoil - d13Ccarb)
(DIC: measured, carb: carbonate dissolved (0‰), soil: soil air (-23‰)
this simple model is also only strictly valid for closed conditions and
does not take into account fractionation effects (low pH environments)
Matrix exchange
more complex models taking into account the chemical and isotopic
evolution have been developed (e.g. Fontes and Garnier, 1979)
Additional complications
- matrix diffusion (=> loss of 14C)
- sulphate reduction (14C dilution)
- incorporation of geogenic CO2 (14C dilution)
- methanogenesis, 2CH2O => CO2 + CH4
(14C dilution)
Modeling 14C ages with NETPATH
- traves the geochemical and isotopic evolution using and
integrated mass balance approach
<>constraints include: C, S, Ca, Mg, Na, Cl, 34S, redox, with
controlling phases such as dolomite, calcite, gypsum, aquifer organic
matter, CO2, cation exchange, halite and pyrite
The Bunter sandstone aquifer in the UK
An example of a aquifer that was dated by radiocarbon is the Bunter
Sandstone aquifer in the UK:
- downgradient evolution of carbon isotopes in groundwaters of the
Bunter Sandstone (Fig)
- stable isoptopes as function of radiocarbon age (Fig)