Date of Award

7-2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Hydrogeology

Advisor

Falta, Ronald

Committee Member

Molz , Fred

Committee Member

Moysey , Stephen

Abstract

Groundwater transport models are commonly used at sites with groundwater contamination. Models are used to study the extent of the contamination and the possible risks it poses. Contaminant transport models use properties of the aquifer and the solute to simulate how the solute moves through the aquifer. Screening level models are often used as a first modeling approach at sites to get an idea of the general trends of contaminant transport. They offer an efficient approach to modeling, usually requiring fewer input parameters than other models.
This study uses a screening level model to describe tritium transport for the second Macro-Dispersion Experiment (MADE). The MADE site, at Columbus Air Force Base, Mississippi, is situated at an aquifer with a high degree of heterogeneity in hydraulic conductivity. The variability in hydraulic conductivity at the site produces non-classical tritium transport. Ideally, the tritium tracer should have behaved as an instantaneous source with a tracer pulse traveling with groundwater flow. However, the actual distribution of tritium by the end of the sampling period shows that the bulk of the tracer stays within 20 m of the injection site with a low-concentration leading edge extending almost 300 m downgradient.
There have been a variety of attempts to model the tritium distribution for the MADE-2 experiment. A traditional advection-dispersion model with an instantaneous point-source fails to capture the actual tritium distribution. Others have used dual-domain mass transfer numerical models to capture the tritium distribution.
An alternate approach is to treat the area near the injection site as a source zone, where the discharge concentration leaving the source is proportional to the tritium mass remaining in this zone. This study uses REMChlor, an analytical model available through the US EPA. REMChlor uses a source-plume concept to describe solute transport at sites that have non-aqueous phase liquid (NAPL) contamination.
REMChlor was used for this modeling approach with the source depletion term set to 1 (Γ= 1) to simulate exponential depletion of the mass in the source zone. This produces a 1 to 1 relationship between source mass and discharge. The ability of this model to reproduce the tritium distributions is largely dependent on the size of the area defined as the source zone. This approach reproduces key features of the observed tritium distributions such as the plume length and the mass distribution along the centerline of the site.

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