1. Inorganic Carbon-14 Matt Baillie 3/25/04 HWR696T

2. Outline Production of 14 C Variance through time of 14 C production How to get 14 C into groundwater Complications and corrections Conclusions

3. Production in the atmosphere 14 C produced through secondary spallation reactions between neutrons and 14 N atoms 14 C atoms then quickly combine with O 2 to form 14 CO 2 Subsurface production unimportant due to CO 2 in soil From (Taylor, 2000)

4. Temporal production variance Variation in production of 14 C in the atmosphere dependent on cosmic ray flux, which is in turn dependent on solar activity, geomagnetic field, etc. Atmospheric production can be calibrated using dendrochronology, as well as U-Th dating of corals Industrial age burning of fossil fuels has put a huge amount of “ dead ” carbon into the atmosphere, diluting atmospheric 14 C Atmospheric testing of nuclear weapons increased (up to double) the 14 C in the atmosphere  Now approaching previous levels due to moratorium on atmospheric testing, as well as 14 CO 2 going mostly into the oceans

5. Temporal production variance

7. Getting 14 C into groundwater 14 CO 2 incorporated into plants through photosynthesis, undergoing depletion 14 C is passed from plants to soil, and becomes slightly enriched due to the diffusion of 12 CO 2 into the atmosphere Soil CO 2 levels are 10-100 times greater than atmospheric CO 2 levels, so absolute amounts of 14 C are much higher in the soil than in the atmosphere

8. Getting 14 C into groundwater In open system conditions (contact with the soil), 14 C is replenished, and remains slightly enriched from soil levels In closed system conditions, 14 C is no longer replenished by the soil, and begins to decay away

9. Getting 14 C into groundwater

10. Once the 14 C is in closed system conditions and assuming no other processes affect it subsequently, the groundwater can be dated using the equation: where t is the mean residence time of the groundwater, a t is the activity of the 14 C at the time of sampling, and a 0 is the initial activity of 14 C

11. Complications What was the initial 14 C activity in the atmosphere when the groundwater entered closed system conditions? Carbonate dissolution introduces “ dead ” carbon into the groundwater, taking 14 C-active carbon out of the groundwater Matrix diffusion of 14 C into dead-end pores decreases 14 C in groundwater Reduction of organics by sulphate adds 14 C-free carbon to the groundwater Geogenic (mantle/deep crust) 14 C-free CO 2 Methanogenesis introduces “ dead ” carbon

12. Corrections To correct the calculated 14 C age, apply a correction factor, q:

13. Corrections Initial activity can be determined through the variations in atmospheric 14 C through time

14. Corrections Matrix diffusion: correction based on matrix porosity and fissure porosity in a dual- porosity aquifer Sulphate reduction: stoichiometric correction Geogenic CO 2 : δ 13 C correction Methanogenesis: δ 13 C and stoichiometric correction

15. Corrections For carbonate dissolution, correction factors are more complicated, and there are therefore several different correction models that can be applied  Statistical correction  Alkalinity correction  Chemical mass-balance correction  δ 13 C mixing (δ 13 C model)  Fontes-Garnier model

16. Carbonate corrections Statistical correction  Simple geometric correction based on the type of aquifer system: 0.65-0.75 for karst systems 0.75-0.90 for sediments with fine-grained carbonate such as loess 0.90-1.00 for crystalline rocks (from Vogel, 1970)  Can be estimated by:for any given recharge area  Limited in usefulness to waters found near the recharge area

17. Carbonate corrections Alkalinity correction  Correction based on the initial and final DIC concentrations (from Tamers, 1975)  Assumes fully closed system conditions, with no exchange between the groundwater and the soil CO 2 during dissolution  Model is of “ limited interest ” (Clark and Fritz, 1997)

18. Carbonate corrections Chemical mass-balance correction  Closed-system model, with dissolution below the water table and no exchange with soil CO 2  Estimated by:  With mDIC rech being estimable from the pH of the recharge area, and: mDIC final = mDIC rech +[mCa 2+ +mMg 2+ -mSO 4 2- +1/2(mNa + +mK + -mCl - )]  Only useful in geochemically simple systems with no carbonate loss from the groundwater

19. Carbonate corrections δ 13 C mixing (δ 13 C model)  Uses 13 C as a tracer, useful in open and closed systems.  First introduced by Pearson (1965) and Pearson and Hanshaw (1970), later modified to work at higher pH (7.5-10):  Enrichment factor chosen for the soil greatly affects groundwater age, and is based on pH in the recharge area; assumes that this pH was the same when the groundwater was originally recharged

20. Carbonate corrections Fontes-Garnier model (1979; 1981)  Calculates q based on both chemistry and δ 13 C values of groundwater  Uses Ca and Mg concentrations as a proxy for carbonate dissolution, as well as δ 13 C to partition the carbon into DIC that has exchanged with soil CO 2 and that which has not  Does not take into account DIC sources aside from carbonate dissolution and soil CO 2 exchange

21. Conclusions Inorganic 14 C is a useful tool for determining mean residence time of groundwater IF:  Initial 14 C activity is known  Recharge conditions can be determined  Conditions within the aquifer are somewhat known (in relation to carbonate dissolution)  Groundwater is not too old for the method to be useful (for all practical purposes, water must be at most 30,000 years in residence (Clark and Fritz, 1997))