This research documents the seasonal variation of de-icing agent contamination by measuring the specific conductivity of runoff and groundwater from an infiltration basin that serves a highway drainage system in southeast Massachusetts. The order of magnitude of the winter maxima of the runoff and groundwater specific conductivities over a 9 year period of record is 10 mS cm−1. The groundwater specific conductivity decreases by 1–2 orders of magnitude to its fall minimum, which is an order of magnitude higher than the fall minimum of the runoff. This warm weather source behaviour implies slow, seasonal dissolution of de-icing agent solids deposited in the infiltration basin during the winter. A completely mixed reactor idealization of the basin models this postulate as a coupled balance of dissolved and solid de-icing agents. The data suggest that 13% of the de-icing agent solids applied to the highway deposits in the basin, where it dissolves into the infiltrating groundwater with a first order decay rate of 0.0035 day−1.