Why are stream and ditch sediment important to water quality?
Best management strategies implemented in most watersheds to reduce phosphorus (P) loads to surface waters have been successful, however, internal P loading within streams and ditches may still provide P to overlying water. Phosphorus retention and release by sediments is important for understanding sediment P status and buffering capacity and for determining the potential environmental fate of sediment bound P in flowing water systems.
What did we do?
Eight headwater streams and drainage ditches within Eagle Creek Watershed in central Indiana were selected to evaluate soluble P (SP). Stream and drainage ditch water and sediment were collected monthly from 8 selected locations within the Eagle Creek watershed in central Indiana for two consecutive years to estimate if there were any seasonal and/or land use trends. Sediments and water were analyzed for soluble P, and 24-hour P isotherms were performed to determine the P sorption capacity and to calculate the equilibrium P concentration (EPC0). The relationship between EPC0 and SP in the water column allows for the prediction of the potential for sediments to either release P to or retain P from the water column.
What have we learned?
Surface water P concentrations varied seasonally and were consistently greater during summer (P<0.05). Surface water SP concentrations increased with the percentage of land classified as urban (P<0.0001). Generally, we observed lower P concentrations in sediment during summer and greater P concentrations during winter and spring. We also observed greater P concentrations in areas that had a greater percentage of land used for agriculture and in some cases, sub-catchment area influenced the P content that was observed. Sediment EPC0 concentrations were not related to water column SP, however, when sediments were separated as ‘sinks’(r = 0.49) or ‘sources’(r = 0.65), a strong correlation was found between sediment EPC0 and water column SP (P<0.0001).
Information from this study will assist managers and planners in targeting areas with the greatest potential for loss of P from sediments to overlying water. These results will also assist in improving nutrient criteria thresholds for the watershed.
Candiss O. Williams, Research Soil Scientist, USDA NRCS Kellogg National Soil Survey Laboratory & Research Candiss.Williams@lin.usda.gov
Brad Joern, Professor, Department of Agronomy, Purdue University Douglas R. Smith, Research Soil Scientist, USDA ARS Grassland, Soil, and Water Research Laboratory
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