Highway deicing salt runoff events and major ion concentrations along a small urban stream

Highway deicing activities can influence the quality of waters draining urb an areas that experience multiple winter season freeze/thaw cycles. However , because of the flashy hydrology of smaller urban streams, and the unpredi ctable nature of deicing runoff, these events are difficult to fully docume nt by traditional monitoring approaches. The frequency, duration, intensity and downstream attenuation of highway deicing salt runoff events were capt ured by remote continuous flow and conductivity monitoring, in combination with dry and wet weather grab sampling, at four stations along a three-kilo meter-long unculverted reach of an urban Pennsylvania stream, Nine Mile Run . Base flow dry weather conductivity values along Nine Mile Run averaged 1,23 2 umhos/cm and could drop as low as 61 umhos/cm following summer storms. Ho wever, at a major storm sewer discharge, which drains a highly urbanized an d almost completely culverted subbasin of 9.8 km(2), short duration winter thaw peak conductivity values as high as 32,457 umhos/cm were documented. W hile such peaks tended to occur during very minor winter flow pulses, and w ere highly attenuated by channel storage at downstream stations, the shock loads of salt experienced were sufficiently elevated to cause concern about osmoregulatory stress to freshwater organisms. Even during base dry weathe r summer flow conditions, the major ion composition of the waters of Nine M ile Run was dominated by sodium chloride, rather than calcium sulfate, as o ccurs in nearby drainages.