Radiocarbon Dating

Evidence for paleowaters

A strong indication that groundwater in many parts of the world can be thousand's to ten thousand's of years old is given by the fact that the 2H and 18O compositions of  groundwater in the same aquifer appear to cluster into groups. The likely explanation for this phenomenon is that groundwaters were recharged under differnt climate regimes.

Groundwater dating with 14C

14C is the leading tool for estimating the age of paleao and fossil groundwaters
carbon for dating can be taken from

Radiocative decay


The simple part of the problem is the radioactive decay (Fig):
t = -8267*ln(at14C/ao14C)

Atmospheric variations

Unfortunately, even the initial 14C activity in the atmsophere has not been constant due to:

The 14C pathway to groundwater in the recharge environment (Fig)

t = -8267*ln(at14C/(q*ao14C))

Corrections for carbonate dissolution

approaches have eveloved over the years in the following sequence:
carbonate evolution of  many groundwaters involves dissolution of soil CO2 and subsequent dissolution of carbonate:
CO2(soil) + H2O + CaCO3  =  Ca2+ + 2HCO3-
  • 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

mDICrech is the 14C active part in the recharge area, mDICfinal 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)

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