As a result of the rupture of a storage tank, 10 m3 of liquid waste containing 100 kg of dissolved arsenic rapidly infiltrated into a shallow, unconfined sandy aquifer in which the flow is horizontal. The average groundwater velocity in the aquifer is 0.5 m/day, the dispersivity is 0.1m, and the coefficient of molecular diffusion is 2x10-10 m2/s. As the contaminated zone moves through the aquifer, the arsenic does not undergo significant adsorption or precipitation.
A. Estimate the maximum arsenic concentration after the contaminant cloud has moved a distance of 500 m. What will be the approximate dimensions of the cloud? Assume that the leakage from the storage tank can be approximated as a point source and that the aquifer can be treated as a homogeneous medium with uniform flow.
B. Develop a MAGNET model to confirm your results using an analytical approach.
MAGNET/Modeling Hints:
- Use ‘Synthetic mode’ in MAGNET to create a model domain.
- Use the ‘Distance Display’ capabilities when editing geometric features and/or the ‘Ruler’ tool to properly represent distances between model features.
- Represent the rivers as constant head zone features.