download pdf - Institut für Umweltphysik - Ruprecht-Karls-Universität ...

122 CHAPTER 4. AQUATIC SYSTEMS
4.1.2 Dating young groundwater in the North China Plain
Christoph von Rohden (participating scientists: Andreas Kreuzer, Werner Aeschbach-Hertig, Chen
Zongyu (IHEG), Rolf Kipfer (IGMR))
Abstract A large aquifer system in the alluvial North China Plain was sampled for groundwater
along a transect. One of the sampling campaigns was intended to extract information about the
recharge and the residence times of young groundwaters. The SF6– and 3 H- 3 He dating techniques
were used, and the applicability of SF6 as dating tool was investigated.
SF 6 -conc [fmol/l]
3 H-conc. [TU]
3.0
2.5 (12°C)
2.0
1.5
1.0
0.5
25
20
15
10
5
0
0 10 20 30 40 50 60 70 80 90 100 110
distance [km]
20
15
10
5
SF 6 -age [yr]
equality
0
0 5 10 15 20 25 30 35
3 3
H- He-age [yr]
Figure 4.3: Left panel: SF6-concentrations along sampling transect. The dashed grey line indicates
the equilibrium concentration with recent atmosphere at 12 ◦ C (top). Bottom: 3 H-concentrations. A
clear transition from young post-bomb to older waters can be identified. Red symbols denote samples,
which should not contain SF6 due to the lack of 3 H (≥50 y). The large scatter and the level
SF6-concentrations, particularly in 3 H-free samples, imply natural (terrigenic) sources of SF6. Right
panel: Comparison of calculated apparent 3 H- 3 He– and SF6-ages. SF6-ages are clearly underestimated,
strongly limiting the use of SF6 as dating method.
Background Groundwater is the dominant water
source for municipal, urban and agricultural
use in the densely populated region (≈200 Mio
people) of the North China Plain (NCP). Concerning
a sustainable management, the growing
water demand in this semiarid region becomes
more and more problematic. Quantitative age information
of young groundwater in the recharge
area is crucial in this context.
Methods and results Besides the well established
3 H- 3 He-method, that in general gives reliable
results, we tested the use of SF6 as dating
tool. Both methods draw on the variable atmospheric
input to the hydrosphere within the last
≈40 years. The sampling took place from the Taihang
mountains in the west near Shijiazhuang to
the centre of the NCP.
The results suggest that the unconfined (upper)
aquifer is — at least in parts — recently
recharged. However, the model ages calculated
from the SF6-measurements are to a large extent
not consistent with the 3 H- 3 He ages. Many SF6samples
have a distinct excess, most prominently
in samples without tritium. Natural sources of
SF6 must be discussed as explanation, which is
uncommon for fluvial or alluvial deposits. The
applicability of SF6 as — at least exclusive — dating
tool for young groundwater in this region must
therefore be put into question.
Based on the reliable 3 H- 3 He data we interpret the
distribution of the apparent tracer ages against
the background of extensive irrigation to be a result
of the recharge possibly influenced by strong
groundwater use (depression cones) in the urban
area of Shijiazhuang.
Funding The work was supported by the German
Research Foundation (DFG).
Main publication The study was presented at
the IAH conference in Beijing (von Rohden et al.
[2006]) and is in preparation for publication.