Rise and fall of road salt contamination of water-supply springs

A storage pile of de-icing agent consisting principally of sodium chloride was placed in the recharge area of two springs, and remained there for 2 years. Water flow is through fractures in rocks with low matrix permeability, along a hydraulic gradient developed along fracture zones. Salt contamination in the springs was noticed about 1 year after the salt was placed. When the salt was removed 1 year later, chloride concentrations in the springs exceeded 500 mg/L. Monitoring for the following 5 years showed salt contamination rising for the first year, but receding to normal background after 5 years. Chloride to sodium ratios of the spring waters indicated that some sodium was initially sequestered, probably by ion exchange on clay minerals, in the early part of the monitoring period, and released during the latter part; thereby extending the period of contamination.