Testing mixing models of old and young groundwater in a tropical lowland rain forest with environmental tracers
Article first published online: 29 APR 2010
Copyright 2010 by the American Geophysical Union.
Water Resources Research
Volume 46, Issue 4, April 2010
How to Cite
2010), Testing mixing models of old and young groundwater in a tropical lowland rain forest with environmental tracers, Water Resour. Res., 46, W04518, doi:10.1029/2009WR008341., , , and (
- Issue published online: 29 APR 2010
- Article first published online: 29 APR 2010
- Manuscript Accepted: 16 DEC 2009
- Manuscript Revised: 15 NOV 2009
- Manuscript Received: 1 JUL 2009
- groundwater hydrology;
- groundwater/surface water interaction;
- groundwater quality
 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.