| Rising groundwater nitrate concentrations associated with the increased use of synthetic and organic agricultural fertilizers in recent decades have prompted the adoption of agricultural best management practices (BMPs), which may include a targeted reduction in nutrient application. This study was prompted by the lack of published research about the effectiveness of these agricultural BMPs, particularly in cases where the thickness and geologic composition of the unsaturated zone prevent short-term effects from being observed in the groundwater quality. The monitoring of nitrate mass load through the unsaturated zone below agricultural land was therefore proposed as a novel technique to assess the effect of agricultural BMPs. The objectives of the study were to: develop field techniques and apply computational models for the quantification of nitrate mass flux below active agricultural land operating under a BMP; scale the point mass flux results to a nitrate mass load across the agricultural parcel; and assess the resulting nitrate mass load measurements as indicators to evaluate the effect of the BMP. The study was conducted in the vicinity of the Thornton Well Field in the County of Oxford, where the nitrogen application rate on a 73-hectare parcel of land was reduced by 20 to 50% relative to historical rates in an effort to curb increasing groundwater nitrate concentrations in the municipal supply wells.; At eight topographically and geologically diverse locations across the study site, soil water content profiles, soil temperature profiles and groundwater quality were regularly monitored, and several rounds of geologic cores were collected for analysis of bulk soil nitrate and an applied bromide tracer. Meteorological parameters were also continuously measured on-site. The field data were applied in several analytical techniques for estimating recharge, including the calculation of unsaturated zone tracer movement, an unsaturated zone water balance, and an overall water balance based on an empirical equation for evapotranspiration. Two numerical models designed to simulate flow in the unsaturated zone (Simultaneous Heat and Water, SHAW, and Hydrologic Evaluation of Landfill Performance, HELP) were also used in order to refine the recharge estimates. The recharge rate at each measurement location was then combined with unsaturated zone nitrate data to quantify nitrate mass flux. Upscaling of the flux values to field-scale mass load was based mainly on topography, geology and field observations such as occasional surface water ponding.; The calculation of stored nitrate mass in the shallow subsurface (i.e., the upper two to three metres) showed some correlation to changes in surface nitrogen application, with the greatest decreases in stored mass (up to 80%) observed at locations underlain by sand where there was a switch from a corn-soybean-wheat rotation to a grass crop. In contrast, the calculation of nitrate mass load suggested that the post-BMP value (4.1 t NO3-N/yr below the area on which BMPs were implemented) was greater than the pre-BMP value (2.2 t NO3-N/yr). However, the calculation of nitrate mass load was limited by several factors, including a lack of nitrate concentration data from the deep unsaturated zone and an above-average (by 30%) annual precipitation rate; as a result, the findings suggesting an increase in nitrate mass load in response to decreasing nutrient inputs should be interpreted with caution. Across the study site, the nitrate mass flux through the portion of the unsaturated zone assumed to have been affected by the BMPs ranged from 3.4 to 13.2 g/yr/m 2, indicating that some areas of the study site are more critical than others in terms of their contribution to groundwater nitrate.; Continued monitoring of nitrate mass load and stored nitrate mass in the unsaturated zone is recommended to determine whether further benefits from the BMPs are observed as the measurement period lengthens and the... |
