R.H. Fitzgerald, P.E.
Groundwater contamination by salt is not exclusively a coastal environmental issue related to seawater intrusion. Salt, either applied directly, or in a mixture with sand, has long been used to de-ice roadways in northern portions of the United States during winter months. Operations involving salt storage, mixing, and loading are done at facilities throughout these states. Commonly, portions of these facilities remain uncovered and such exposure to the environment can result in the transport of salt into surface water and groundwater.
Operation of a salt handling facility in Indiana resulted in the contamination of the aquifer below the facility with high levels of sodium and chloride. Many facility operations occurred without shelter from rain and snow. The main receptor near this facility is a municipal water supply wellfield. The wellfield is located approximately 1,700 feet north of the site (refer to Fig. 1). Groundwater monitoring has confirmed the movement of salt at the base of the aquifer (approximately 120 feet below ground surface) toward the wellfield. Groundwater modeling was done at this site to aid in the design of remedial alternatives.
Initial model calibration involved the use of a single-phase flow model. The flow model was calibrated to steady-state and transient conditions. A 2-phase, sharp-interface model was then used to simulate the historical migration over a 30 year period of a dense salt plume at the base of the aquifer beneath the facility. Concurrently, a solute transport model was linked to the transient simulation. The solute transport model was used to simulate the dispersive movement of salt from the dense phase plume into the surrounding aquifer.
Model results compared favorably with field-measured salt concentrations at monitoring wells in the vicinity of the facility and wellfield. The models were then used to screen remedial alternatives involving groundwater pumping and surface flushing. The screening process involved estimation of remedial well concentrations and clean-up times.