Geology Reference
In-Depth Information
obtain water from the system's reserves). The reference springs allow the
adjustment (linear, or exponential, in Mediterranean regions with a non-
linear relationship between effective rainfall and elevation) of the elevation-
18 O relationship. δ 18 O indeed varies with temperature (low→warm→not
very negative, high→cold→highly negative).
This nomogram allows the estimation of the average recharge elevation
for the aquifer's reserves.
6.2.3 Water-soil gas-rock exchanges: example of the Doubs
valley deep karst (Celle-Jeanton et al., 2003; Mudry et al., 2002)
The infiltration caused by a precipitation event dissolves carbon
dioxide in the soil, which is indicated in our climate by a δ 13 C of -22‰.
This CO 2 corrodes solide carbonates, in which the δ 13 C is around 0‰.
Stoichiometrically, a molecule of CO 2 attacks a molecule of carbonate to
give two molecules of HCO 3 - , therefore marked with the average of -11‰.
Water transiting rapidly in carbonate rock, isolated from the soil atmosphere
(saturated zone), will retain its original mark (Figure 80). Water stored in
the unsaturated zone (perched aquifer, epikarst aquifer), will exchange
carbon with the soil atmosphere and will therefore be depleted with respect
to the initial -11‰. On the other hand, water redissolving carbonate after
having precipitated it into the aquifer (very slow transit) will be enriched
in 13 C. Figure 80 shows these three types of water in the Doubs valley karsts
upstream of Besançon.
Figure 80 “Deep” karst of the Doubs valley: circulation paths.
Search WWH ::




Custom Search