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Dating degassed groundwater with 3 H/ 3 He
Author(s) -
Visser Ate,
Broers Hans Peter,
Bierkens Marc F. P.
Publication year - 2007
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2006wr005847
Subject(s) - groundwater , noble gas , nitrate , fractionation , groundwater flow , aquifer , solubility , diffusion , environmental science , environmental chemistry , hydrology (agriculture) , geology , soil science , chemistry , thermodynamics , physics , geotechnical engineering , organic chemistry
The production of gases in groundwater under contaminated locations by geochemical and biological processes is not uncommon. Degassing of these gases from groundwater and repartitioning of noble gases between water and gas phase distorts groundwater dating by 3 H/ 3 He. We observed noble gas concentrations below atmospheric equilibrium in 20 out of 34 groundwater samples from agriculturally polluted sandy areas in the Netherlands. From the absence of nitrate in degassed samples, we conclude that denitrification causes degassing. The 22 Ne/ 20 Ne ratios show that degassing had attained solubility equilibrium and had not caused isotopic fractionation by diffusion. To correct for the loss of tritiogenic 3 He due to degassing, we present a single‐step equilibrium degassing model. We use the total dissolved gas pressure at the monitoring screen to estimate the depth and timing of degassing, which is essential to estimate travel times from degassed samples. By propagating the uncertainties in the underlying measurements and assumptions through the travel time calculations, we found a travel time uncertainty of 3 years (a). We therefore conclude that 3 H/ 3 He dating can produce valuable information on groundwater flow even at sites with strong degassing.