The Efficiency of a 2.7 acre Farm Pond in Reducing Nutrient and Sediment Loading from a 68.2 acre Watershed

Abstract

Significant amounts of sediment and nutrients from agricultural watersheds are removed from storm runoff by farm ponds. The purpose of this study was to determine the efficiency of a 2.7 acre farm pond in reducing sediment and nutrient loading from a 68.2 acre dairy farm watershed in northcentral Wisconsin for the period 1966-1979. A gauging station was constructed at the pond inflow to monitor flow and water quality runoff data continuously in 1979. Fourteen years of overland flow to the pond was simulated with the Non-Point Source Pollution (NPS) model. Observed average sediment and nutrient concentrations for snowmelt and all other runoff in 1979 were used to calculate total sediment and nutrients input over the fourteen year life of the pond. Total sediment and nutrient trapped in the pond was quantified by analysis of sediment samples during the winter of 1980. Trapping efficiencies based on sediment sampling were found to be 100% for sediment, 12.4% for total phosphorus and 11.9% for total nitrogen. If calculated on an inflow/outflow basis, efficiencies decrease for sediment to 95.2% and increase for total phosphorus and total nitrogen to about 80% and 60%, respectively. Average yearly sediment loading was found to be approximately 370 pounds per acre, well below the average gross erosion estimates for cropland for the Big Eau Pleine River watershed of 4600 lbs/acre and somewhat below a 460 - 920 lbs/acre value that assumes a 10 - 20% delivery ratio. The use of grassed waterways and diversions on a major portion of the watershed probably contributed to reduced sediment loading. Average yearly nutrient loading of 2.27 lbs/acre for total phosphorus and 10.2 lbs/acre for total nitrogen were greater by factors of 2.5 to 10 times reported values for the Big Eau Pleine watershed. These greater loadings were attributed to a greater density of animals. Dairying operations on the study area watershed are believed to contribute to overall high nutrient loading for the watershed. Grab samples of runoff from barnyard or manured areas were found to contain greater than 100 times the concentration of sediment and nutrients as compared to cropland. Systematic snow sampling on the watershed provided data that attributed high nutrient loading in snowmelt runoff to spreading of manure in the winter.