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Radiogenic 4 He as a conservative tracer in buried‐valley aquifers
Author(s) -
Van der Hoven Stephen J.,
Wright R. Erik,
Carstens David A.,
Hackley Keith C.
Publication year - 2005
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 - 10.1029/2004wr003857
Subject(s) - aquifer , hydrogeology , groundwater , tracer , geology , hydrology (agriculture) , radiogenic nuclide , groundwater flow , aquifer properties , water well , soil science , environmental science , geochemistry , groundwater recharge , geotechnical engineering , physics , mantle (geology) , nuclear physics
The accumulation of 4 He in groundwater can be a powerful tool in hydrogeologic investigations. However, the use of 4 He often suffers from disagreement or uncertainty related to in situ and external sources of 4 He. In situ sources are quantified by several methods, while external sources are often treated as calibration parameters in modeling. We present data from direct laboratory measurements of 4 He release from sediments and field data of dissolved 4 He in the Mahomet Aquifer, a well‐studied buried‐valley aquifer in central Illinois. The laboratory‐derived accumulation rates (0.13–0.91 μcm 3 STP kg water −1 yr −1 ) are 1–2 orders of magnitude greater than the accumulation rates based on the U and Th concentrations of the sediments (0.004–0.009 μcm 3 STP kg water −1 yr −1 ). The direct measurement of accumulation rates are more consistent with dissolved concentrations of 4 He in the groundwater. We suggest that the direct measurement method is applicable in a variety of hydrogeologic settings. The patterns of accumulation of 4 He are consistent with the conceptual model of flow in the aquifer based on hydraulic and geochemical evidence and show areas where in situ production and external sources of 4 He are dominant. In the southwestern part of the study area, Ne concentrations are less than atmospheric solubility, indicating gases have been lost from the groundwater. Available evidence indicates that the gases are lost as groundwater passes by pockets of CH 4 in glacial deposits overlying the aquifer. However, the external flux from the underlying bedrock appears to dominate the accumulation of radiogenic 4 He in the aquifer in the southwestern part of the study area, and the loss or gain of helium as groundwater passes through the overlying sediments is minor in comparison.