MIGRATION OF A CONSERVATIVE CONTAMINANT: INFILTRATION, FRACTURE CONDUITS, AND RE-EMERGENCE OF CHLORIDE IN WELLS AND STREAMS

The hydrologic behavior of road salt can be used as a model for understanding pathways for contaminants in groundwater. Past work has indicated a strong bias in chloride sampling results in residential wells downslope and proximal upslope from the road where salt is seeded in the winter months. This ongoing analysis proceeds with the hypothesis that fracture orientation in bedrock, if steep in dip and with an optimal strike direction further enhances the conductivity of the salt solute.

The results, shown graphically, demonstrate that 61 to 77% of the asymmetric downslope flow is due to fracture conductivity. In intermittent streams which cross regularly salted roads, the conductivity changes observed downstream of the road show a marked increase. This is particularly notable in dry summer months or drought conditions when the groundwater base flow component of the streams is greater. Measurement of small tributaries and springs along these streams point to long term sequestration near the roads and gradual release of the contaminant as base flow. The effect can continue for miles downstream.