SEASONAL CHANGES IN CONDUCTIVITY DUE TO ROAD SALT IMPACTED RUNOFF AND RAIN EVENTS AT A DRAINAGE POND IN ALLENDALE, MI

Michigan, like many Midwestern states, applies road salt during winter. Salt can affect runoff and enter the surrounding watershed, potentially impacting aquatic systems. While other studies have examined road salt impacts at a regional scale, we have monitored a small, one-acre site in more detail. Of interest is how runoff affects water in our hydraulically connected system and what factors contribute to sudden changes in conductivity. At Allendale K-8 Public Schools, runoff from the campus is discharged via stormwater pipe into the southern end of a 1/3 acre man-made pond that is on average 1.3 ft deep. A creek on the northern end of the pond acts as the sole outlet and discharges into a small stream (Sevey) that flows through the campus. In 2021, we used data loggers to measure conductivity of the water at four locations: the stormwater pipe, pond, exit creek and Sevey stream.

During winter, in January and February, five peak conductivity events were detected in the stormwater pipe and correlated to periods of temperatures above freezing. The conductivity increased by a factor of 5 during the largest event (400 – 2000 uS/cm), equivalent to a peak chloride concentration of ~500 mg/l. Conductivity measured in the pond and exit stream during the same interval varied between 250 – 350 uS/cm and highlighted the dilutional influence of the pond. Runoff from a larger area as monitored in the Sevey stream showed a ~2.5x increase in conductivity (325 – 800 uS/cm) during one period of above freezing temperatures.

Conversely, stormwater runoff from rain during the summer resulted in sudden decreases in conductivity. The stormwater pipe outlet varied between 450 and 40 uS/cm with conductivity lows during rain events roughly proportional to the magnitude of the rain event. Unlike the winter, the range of conductivity in the pond and exit creek mirrored that of the pipe, probably the result of larger discharges of water into the pond during summer rain events. Due to high variability in our calculations of residence time for the pond, we determined that this concept is not applicable to a system this small.

Our study highlights the usefulness of high resolution conductivity measurements to monitor water quality changes and flow dynamics during the winter and summer months, along with the dilutional abilities of ponds during runoff events.