THE EFFECT OF TRANSIENT RECHARGE ON SIMULATED ADVECTIVE TRANSPORT OF EXPLOSIVE COMPOUNDS IN GROUND WATER AT CAMP EDWARDS, CAPE COD, MASSACHUSETTS

Live-fire training activities and munitions disposal at Camp Edwards, Cape Cod, Massachusetts has resulted in the release of explosive compounds into the underlying aquifer. The contaminant of most concern is RDX (Royal Dutch Explosive), a carcinogenic compound that can be transported conservatively in ground water under the oxic conditions characteristic of the aquifer. Ground-water flow in the aquifer, which generally consists of highly permeable glacial sand and gravel, is radially outward from a water table mound located within Camp Edwards towards coastal discharge boundaries beyond the base boundary. Steady-state ground-water-flow models have been used at the site to predict advective-transport paths of contaminants from potential source areas and to identify potential source areas for contaminants detected in the subsurface. These modeling results are used to locate observation wells and soil-sampling grids.

Time-varying recharge can change heads in the aquifer and shift the position of the top of the water-table mound. As a result, hydraulic gradients and ground-water-flow lines in the aquifer can change through time. Transient modeling at the site has shown that ground-water-flow patterns in areas downgradient of the water-table mound and closer to discharge boundaries, where hydraulic gradients are steeper, are not greatly affected by changes in recharge. Ground-water-flow patterns in areas near the top of the water-table mound, where gradients are small, can change with time; particles released from the same location at different times can take different paths. Comparisons of steady-state and transient particle tracks indicate that calibrated steady-state models are sufficient to simulate advective transport in downgradient areas of the site, whereas steady-state models may not accurately simulate advective transport near the top of the water-table mound.