Assessing the impact of Stormwater runoff from I-59 to a headwater stream in the Valley-and-Ridge Appalachians

Stormwater runoff from urban areas has been shown to have constituents that cause adverse impacts to aquatic ecosystems. However most studies have focused either on characterizing runoff water quality parameters directly at the source or the effectiveness of various pollutant removal alternatives. This work presents an ongoing investigation on the impact of stormwater runoff from an interstate in the perspective of water quality parameters measured within the receiving stream. The stream is perennial and located in the headwaters of a largely rural watershed with a catchment area of 7 mi² in a region within the Valley-and-Ridge Appalachians in Alabama. Soils in the watershed are generally fine (clay/silts), with some exposed rock, and fast response of overland flows due to rainfall events was noticed. Interstate I-59 (average traffic flow at the site of 1230 vehicles/hour) intercepts the watershed and the stream at different points. Water quality sampling stations and hydrological parameters have been collected at selected points in the watershed. Depth-discharge relationships were developed for four cross sections in the watershed, and base flow was determine to be 3.0 ft³/s in the stream. Field results show that flow rate contribution caused by the interstate ranged from an average of 15 ft³/s during moderate rain events to up to 200-900 ft³/s in major rain events. Water quality parameters selected for this investigation included: nitrogen and phosphorous species; dissolved oxygen; total suspended solids and total solids; pH; turbidity; among others. Nutrient levels sampled so far have been low, with nitrate concentrations ranging from 0.02 – 0.2 mg/L and total phosphorous from 0.1 – 0.8 mg/L. TSS and turbidity levels ranged from 0.2 – 500 mg/L and 0.3 to 242 NTU respectively. TDS levels are very low and have been shown to drop and recover during a rain event indicating that the roadway runoff is not a source of ions to the stream. The differences in these measurements between stations located upstream and downstream of the interstate were less than 1% of the absolute value in all measurements. This may be an indication that the predominant pollutant sources in the watershed are not the interstate, but development further upstream. It is hoped that this research results will help to improve in SWMM modeling results in representing roadway impacts in watersheds within the same physiographic province.