Networks of roadside ditches criss-cross every watershed, installed by highway staff to capture road runoff and transfer it to the nearest streams. The impact of these ditches on the stream ecosystems has largely been overlooked until recently. Previous work in New York has demonstrated that the ditch network within each 20 Km2 watershed effectively lengthens the total stream channel length by up to four-fold, captures an average of 22% of the hill runoff within each basin, and shunts it rapidly to the stream where it increases the peak water heights by up to 300% during storms – cumulatively altering natural flow regimes and effectively “re-plumbing” the watershed. Over the past four years, we have been investigating the role of ditches as conduits of dissolved nitrogen and phosphorus from adjacent land uses. Concentration, discharge, and load measurements were taken in the roadside ditch and stream networks within a small (11 km2) agricultural watershed in NYS over four growing seasons, and contrasting dry and wet years. Two study ditches captured surface runoff and tile drainage from ag fields treated with livestock manure; the third drained a small woodlot. To investigate the potential for removal of reactive N as a benign (N2) or GHG, we measured gas fluxes using static chambers in the three ditches under different management strategies and a range of soil conditions.
Results/Conclusions
The ditches were major conduits of dissolved N from manure-spread farm fields to the stream network with concentrations as high as 38 mg/L and daily loads up to 30 kg N/day. Two ditches which transported 5% of the total watershed runoff acted as pollutant plumes, and accounted for 25% of the in-stream nitrogen load during storm events. N2O fluxes were measured in each ditch associated with substrate denitrification, with high variability in space and time. However, given the very high dissolved nitrate loads, in-ditch denitrification is not likely able to remove significant N under any of the conditions studied. Dissolved phosphorus concentrations were negligible in all ditches. We conclude that increased recognition is needed of the critical roles that roadside ditches are playing in stream water quality and ecosystem health. Engagement of highway staff, farmers and local governments must all be involved in water resource discussions if roadside management practices maintained for the past century are to be improved.