Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: One million year old groundwater in the Sahara revealed by krypton-81
  and chlorine-36   Measurements of 81Kr/Kr in deep groundwater from the Nubian Aquifer (Egypt)
were performed by a new laser-based atom-counting method. 81Kr ages range from
\~2x10^5 to ~1x10^6 yr, correlate with 36Cl/Cl ratios, and are consistent with
lateral flow of groundwater from a recharge area near the Uweinat Uplift in SW
Egypt. Low delta-2H values of the 81Kr-dated groundwater reveal a recurrent
Atlantic moisture source during Pleistocene pluvial periods. These results
indicate that the 81Kr method for dating old groundwater is robust and such
measurements can now be applied to a wide range of hydrologic problems.

21
The categories of this paper are: adap-org
34
The categories of this paper are: chem-ph
22
The categories of this paper are: chao-dyn
4
The categories of this paper are: cond-mat
30
The categories of this paper are: q-alg
2
The categories of this paper are: math
36
The categories of this paper are: cmp-lg
0
The categories of this paper are: hep-ph
16
The categories of this paper are: stat
8
The categories of this paper are: hep-ex
25
The categories of this paper are: patt-sol
17
The categories of this paper are: q-fin
11
The categories of this paper are: hep-lat
12
The categories of this paper are: nucl-th
13
The categories of this paper are: quant-ph
28
The categories of this paper are: alg-geom