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Use of chlorine-36 to determine regional-scale aquifer dispersivity, eastern Snake River Plain aquifer, Idaho/USA

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

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, , , ,
DOI: 10.1016/S0168-583X(00)00216-0

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Abstract

Chlorine-36 (36Cl) derived from processed nuclear waste that was disposed at the US Department of Energy's Idaho National Engineering and Environmental Laboratory (INEEL) through a deep injection well in 1958, was detected 24-28 yr later in groundwater monitoring wells approximately 26 km downgradient from the source. Groundwater samples covering the period 1966-1995 were selected from the US Geological Survey's archived-sample library at the INEEL and analyzed for 36Cl by accelerator mass spectrometry (AMS). The smaller 36Cl peak concentrations in water from the far-field monitoring wells relative to the input suggest that aquifer dispersivity may be large. However, the sharpness of the 1958 disposal peak of 36Cl is matched by the measured 36Cl concentrations in water from these wells. This implies that a small aquifer dispersivity may be attributed to preferential groundwater flowpaths. Assuming that tracer arrival times at monitoring wells are controlled by preferential flow, a 1-D system-response model was used to estimate dispersivity by comparing the shape of predicted 36Cl-concentration curves to the shape of 36Cl-concentration curves measured in water from these observation wells. The comparisons suggest that a 1-D dispersivity of 5 m provides the best fit to the tracer data. Previous work using a 2-D equivalent porous-media model concluded that longitudinal dispersivity (equivalent to 1-D dispersivity in our model) was 90 m (Ackerman, 1991). A 90 m dispersivity value eliminates the 1958 disposal peak in our model output curves. The implications of the arrival of 36Cl at downgradient monitoring wells are important for three reasons: (1) the arrival times and associated 36Cl concentrations provide quantitative constraints on residence times, velocities, and dispersivities in the aquifer; (2) they help to refine our working hypotheses of groundwater flow in this aquifer and (3) they may suggest a means of estimating the distribution of preferential flowpaths in the aquifer. ?? 2000 Elsevier Science B.V. All rights reserved.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Use of chlorine-36 to determine regional-scale aquifer dispersivity, eastern Snake River Plain aquifer, Idaho/USA
Series title:
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
DOI:
10.1016/S0168-583X(00)00216-0
Volume
172
Issue:
1-4
Year Published:
2000
Language:
English
Publisher:
Elsevier
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
First page:
679
Last page:
687
Number of Pages:
9