Contribution of Streambank Erosion As a Non-Point Source of Phosphorus to Lake Champlain, VT.

Lake Champlain has persistent algal blooms associated with excess phosphorus (P) from the landscape. Streambank erosion is ubiquitous throughout the Champlain Basin with 75% of Vermont stream reaches classified as unstable. The P contribution of streambank erosion has not been well quantified, yet could be a significant source of non-point P. The objectives of this study were to 1) assess the variability in total P (TP) and soil test P (MM-P) for eroding riparian soils; 2) relate TP and MM-P with soil physical and chemical properties, including texture, Al, Ca, and Fe; 3) relate TP and MM-P to landscape parameters, including land use/land cover (LULC), landscape position, soil type and parent material; and 2) quantify the potential P load from eroding streambanks. Soil samples were taken from 76 erosion features to a depth of 90 cm on 4 streams in Chittenden County, Vermont and analyzed for texture, total P, Modified-Morgan’s P, and total aluminum, calcium, iron, and manganese. Landscape parameters were assessed using available spatial databases for LULC, parent material, soil type, landscape position. Mean concentrations of TP and MM-P were similar among the four streams and through depth, but not correlated with each other. A strong relationship existing between Ca and TP in excess of apatite ratios suggests apatite-P is a common P form in these soils. Low MM-P concentrations suggest that eroded streambank may act as sinks rather than a source of P. Landscape parameters including LULC and landscape position correlated with TP and MM-P. Streambank erosion from four streams in Chittenden County contributed a total of 11.2 to 14.1 MT of TP and 37.7 kg of MM-P to corresponding stream corridors. Estimated potential P load from eroding streambanks from 1999-2004 ranged from 16 to 95% of total non-point load depending on stream and estimation method.