Assessing the use of 3H-3He dating to determine the subsurface transit time of cave drip waters.
(3)H-(3)He measurements constitute a well-established method for the determination of the residence time of young groundwater. However, this method has rarely been applied to karstified aquifers and in particular to drip water in caves, despite the importance of the information which may be obtained. Besides the determination of transfer times of climate signals from the atmosphere through the epikarst to speleothems as climate archives, (3)H-(3)He together with Ne, Ar, Kr, Xe data may also help to give new insights into the local hydrogeology, e.g. the possible existence of a perched aquifer above a cave. In order to check the applicability of (3)H-(3)He dating to cave drips, we collected drip water samples from three adjacent caves in northwestern Germany during several campaigns. The noble gas data were evaluated by inverse modelling to obtain recharge temperature and excess air, supporting the calculation of the tritiogenic (3)He and hence the (3)H-(3)He age. Although atmospheric noble gases were often found to be close to equilibrium with the cave atmosphere, several drip water samples yielded an elevated (3)He/(4)He ratio, providing evidence for the accumulation of (3)He from the decay of (3)H. No significant contribution of radiogenic (4)He was found, corresponding to the low residence times mostly in the range of one to three years. Despite complications during sampling, conditions of a perched aquifer could be confirmed by replicate samples at one drip site. Here, the excess air indicator ΔNe was about 10 %, comparable to typical values found in aquifers in mid-latitudes. The mean (3)H-(3)He age of 2.1 years at this site presumably refers to the residence time in the perched aquifer and is lower than the entire transit time of 3.4 years estimated from the tritium data.
Kluge
Tobias
T
Institute of Environmental Physics, Heidelberg University, Heidelberg, Germany. tobias.kluge@yale.edu
Wieser
Martin
M
Aeschbach-Hertig
Werner
W
D001272:Atmosphere / Q000737:chemistry; D002910:Chronology as Topic; D002980:Climate; D019015:Geologic Sediments; D006371:Helium / Q000032:analysis*; D007554:Isotopes / Q000032:analysis; D018570:Risk Assessment; D013696:Temperature; D014316:Tritium / Q000032:analysis*; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000032:analysis*; D014881:Water Supply / Q000032:analysis* / Q000592:standards
D007554:Isotopes; D014874:Water Pollutants, Chemical; D014316:Tritium; D006371:Helium