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Testing mixing models of old and young groundwater in a tropical lowland rain forest with environmental tracers

Water Resources Research

By:
, , , and
DOI: 10.1029/2009WR008341

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Abstract

We tested three models of mixing between old interbasin groundwater flow (IGF) and young, locally derived groundwater in a lowland rain forest in Costa Rica using a large suite of environmental tracers. We focus on the young fraction of water using the transient tracers CFC-11, CFC-12, CFC-113, SF6, 3H, and bomb 14C. We measured 3He, but 3H/3He dating is generally problematic due to the presence of mantle 3He. Because of their unique concentration histories in the atmosphere, combinations of transient tracers are sensitive not only to subsurface travel times but also to mixing between waters having different travel times. Samples fall into three distinct categories: (1) young waters that plot along a piston flow line, (2) old samples that have near-zero concentrations of the transient tracers, and (3) mixtures of 1 and 2. We have modeled the concentrations of the transient tracers using (1) a binary mixing model (BMM) of old and young water with the young fraction transported via piston flow, (2) an exponential mixing model (EMM) with a distribution of groundwater travel times characterized by a mean value, and (3) an exponential mixing model for the young fraction followed by binary mixing with an old fraction (EMM/BMM). In spite of the mathematical differences in the mixing models, they all lead to a similar conceptual model of young (0 to 10 year) groundwater that is locally derived mixing with old (>1000 years) groundwater that is recharged beyond the surface water boundary of the system.

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Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Testing mixing models of old and young groundwater in a tropical lowland rain forest with environmental tracers
Series title:
Water Resources Research
DOI:
10.1029/2009WR008341
Volume
46
Issue:
4
Year Published:
2010
Language:
English
Publisher:
AGU
Publisher location:
Washington, D.C.
Contributing office(s):
National Research Program - Eastern Branch
Description:
W04518
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Water Resources Research
Country:
Costa Rica