WRESTLING WITH THE VADOSE ZONE! QUANTIFYING TIME LAGS IN AGRICULTURAL BMP PERFORMANCE

Elevated levels of nutrients in groundwater worldwide have been associated with leaching of excess fertilizers from agricultural land use practices. Considered a regional, non-point contaminant source, the fate and transport of agricultural nutrients such as nitrate leaching from farm lands is complicated by surface and subsurface heterogeneity, diversity in land use practices, transformation processes and climatic seasonality. One approach to mitigating this type of groundwater contamination is through the adoption of nutrient management strategies on croplands that are designed to minimize the leaching losses below the root zone. These are often referred to as nutrient Best Management Practices (BMPs).

Although these BMPs have been implemented on agricultural landscapes around the world, there is a paucity of science-based evidence that can be used to quantify their effectiveness, due in part to the long time lags between the initiation of the BMP and the subsequent impact on the receptor of concern, such as a public supply well. Surface applied nutrients must first migrate through the unsaturated zone, which can significantly influence the timing and distribution of the nutrient species arriving at the water table. In many cases, performance assessment relies heavily on predictive simulations.

In this presentation, the role of the vadose zone in influencing the fate and transport of nitrate will be examined within the context of quantifying the effectiveness of regional-scale nutrient BMPs. Challenges related to field-based estimations of BMP performance will be discussed. A simple method will be presented, based on relative advective travel times in both the vadose and saturated zones, to determine whether it is necessary to incorporate the vadose zone processes into the modeling endeavour when estimating BMP response times. In cases where vadose zone flow and transport are deemed crucial for the predictive modeling, the transport parameters such as the assigned dispersivity value can considerably influence the simulated residence times of nitrate in the vadose zone. Conventional modeling approaches may lead to significant underestimations of vadose zone travel times. Applications from actual field sites will be used for illustrative purposes.