A Field Investigation of Matrix Diffusion Under Variably-Saturated Conditions: Implications for Contaminant Transport in the Vadose Zone of the Eastern Snake River Plain, Idaho
Abstract
We present results from a field-scale infiltration tracer test involving two non-reactive solutes with different free-water diffusion coefficients to demonstrate that matrix diffusion is an important transport process in the variably-saturated, fractured basalt of the Eastern Snake River Plain (ESRP) vadose zone. Although diffusive mass transfer between a variety of flowing and stagnant fluid domains (matrix diffusion) has been postulated as part of the conceptual model of flow and transport in the ESRP basalt this is the first study to demonstrate this process experimentally. The shallow subsurface of the ESRP is unsaturated except where contacts between units of contrasting permeability give rise to areas of local saturation (perched water). Tracer breakthrough curves from wells completed at the base of the surficial alluvium did not exhibit a matrix diffusion signature, but breakthrough curves in two wells completed at the base of the basalt layer that underlies the alluvium had clear matrix diffusion signatures. Based on multiple lines of evidence, we believe this matrix diffusion occurred near the base of the basalt layer where water was perched and diverted horizontally through a rubble zone in the basalt directly above a low-permeability sedimentary interbed. Tracer breakthrough curves obtained from a suction lysimeter installed near the bottom of the interbed and almost directly below one of the basalt wells displayed a matrix diffusion signature that was consistent with the signature in the basalt. The matrix diffusion parameters deduced from analyses of the breakthrough curves in the two basalt wells and the lysimeter were in good agreement with each other, suggesting that these parameters may be broadly applicable to fractured basalts within the shallow ESRP vadose zone.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.H21B0826D
- Keywords:
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- 1832 Groundwater transport;
- 1875 Vadose zone