PHOSPHORUS LOSSES IN SURFACE RUNOFF AND SUBSURFACE DRAINAGE WATERS ON TWO AGRICULTURAL FIELDS IN QUEBEC

Abstract

Concentrations of phosphorus (P) above the water quality guideline of 0.03 mg L-1 arecommonly found in rivers draining agricultural lands in Quebec. This gives rise toeutrophication, which has become quite problematic in some parts of Quebec. Agricultural nonpointsource pollution is the dominant source of this P. While the links between P losses, surfacerunoff, erosion and manure management are reasonably well understood, there is far lessinformation available on the movement of P via subsurface tile drainage systems. Since themajority of intensively managed agricultural lands in Quebec are subsurface drained, thisrepresents a major knowledge gap. Two field sites, located in the Pike River Watershed ofSouthwestern Quebec, which drains into Lake Champlain, have been instrumented to measureand sample surface runoff and tile drainage waters. Monitoring has been ongoing since October2000. Results are reported here for the six site-years of data.

Subsurface drainage was the dominant pathway by which water left the fields. On average, tiledrainage accounted for 81% of the total annual drainage. Surface runoff was responsible formajority of the annual P losses. It accounted for 60% of the annual total, which was on average1.09 kg/ha. The flow weighted average annual P concentration in subsurface drainage watersranged between 0.06 mg L-1 and 0.37 mg L-1. The flow weighted average annual P concentrationin surface runoff waters ranged between 0.20 mg L-1 and 2.15 mg L-1. On average, Pconcentrations in surface runoff were 10.9 times higher than those found in subsurface drainagewaters, indicating that subsurface drainage may reduce annual P losses. These results alsoreveal that soil test P and percent P saturation are, on their own, inadequate indicators ofpotential for P pollution.