Movement of road salt to a small New Hampshire lake

Runoff of road salt from an interstate highway in New Hampshire has led to contamination of a lake and a stream that Hows into the lake, in spite of t he construction of a diversion berm to divert road salt runoff out of the l ake drainage basin. Chloride concentration in the stream has increased by o ver an order of magnitude during the 23 yr since the highway was opened, an d chloride concentration in the lake has tripled. Road salt moves to the la ke primarily via the contaminated stream, which provides 53% of all the chl oride to the lake and only 3% of the total streamflow to the lake. The stre am receives discharge of salty water from leakage through the diversion ber m. Uncontaminated ground water dilutes the stream downstream of the berm. H owever, reversals of gradient during summer months, likely caused by transp iration from deciduous toes, result in flow of contaminated stream water in to the adjacent ground water along the lowest 40-m reach of the stream. Thi s contaminated ground water then discharges into the lake along a 70-m-wide segment of lake shore. Road salt is pervasive in the bedrock between the h ighway and the lake, but was not detected at all of the wells in the glacia l overburden. Of the 500 m of shoreline that could receive discharge of sal y ground water directly from the highway, only a 50-m-long segment appears to be contaminated.