Assessing Potential Nutrient Losses in Tile Drained, Macroporous Soils over an Annual Cycle through Conservative Tracer Tracking
University of Waterloo
In agricultural settings that are subject to moderate to large seasonal temperature variation, such as where this work was conducted in southwestern Ontario, farmers often have a narrow window of time between crop harvest and snowfall for applying liquid manure. Although it is well known that liquid manure application in the fall may have associated risk, limited holding capacity often dictates that liquid manure storage structures are emptied before winter. By applying liquid manure to tile drained fields in the fall, there is increased likelihood that nutrients will become surface water contaminants due to reduced water storage capacity of the soil, minimal evapotranspiration and increased macropore flow. The focus of this research is to quantify the potential nutrient loss to surface water via tile drainage after a fall liquid manure application. Methodology involved the application of a NaBr tracer solution over a 2.3m by 6.1m strip of soil immediately adjacent to a tile drain before irrigating the area for nine hours to replicate the worst case scenario of heavy precipitation immediately after manure application. The tracer was applied in early November 2007 and Br concentrations in the tile discharge were subsequently monitored for 1 year. By September of 2008, Br was not regularly detected in the tile effluent and 16, 2m long soil cores were extracted from a 6m by 6m area parallel to the tile where the tracer had been applied. Soil pore water was sampled from each core in 0.1m increments and analyzed for Br concentration in order to quantify the residual mass of Br in the soil. Tile discharge was primarily measured with an electronic flow meter which was augmented by manual measurements during periods of high flow. Discharge measurements were supplemented by results from an empirical model that was developed to test the relationship between tile discharge and hydraulic head in groundwater monitoring wells at the site. A remarkably good correlation coefficient of .94 was obtained between the model and measured tile discharge values. Mass balance calculations indicate that all of the Br was ultimately accounted for, with approximately 98 percent contained in tile discharge and the remainder retained in the soil profile. Within 21 days of the tracer application 22 percent of the bromide mass had been discharged through the tile with 8 percent of the total mass arriving at the tile in the first 48 hours. By April 1, 2008 the majority of the bromide had been discharged, with approximately one quarter of the total bromide mass discharged during the hydrologic response to an early January winter melt event. Results from this work indicate that the majority of soluble nutrients applied close to tile drains, during fall liquid manure applications, will potentially be transported to surface water prior to the following growing season, and that winter and spring melt events are responsible for most of the soluble nutrient flux between tile drained agricultural land and surface water.