Challenges and a Strategy for Agricultural BMP Monitoring and Remediation of Nitrate Contamination in Unconsolidated Aquifers

Abstract

Nitrate in groundwater and surface water is among the most common contamination problems in the world. Efforts to reduce the loss of excess nutrients at the regional scale currently focus on Best Management Practices (BMPs) designed to facilitate the optimal use of fertilizers, adjusted for specific site conditions. Performance monitoring of regional BMPs has proven to be problematic owing to the large areal scales, the inherent heterogeneity of nutrient mobility on landscapes and in the subsurface, and the extended time lag between implementation and overall improvements in groundwater quality. In order to shorten the time for useful assessments of progress and for the beneficial results to be realized, a strategy is proposed here with specific application to unconsolidated granular aquifers. First, a novel approach for quantifying BMP performance in the short term is demonstrated involving the monitoring of transient nitrate storage in the unsaturated zone. Secondly, a temporary remediation method based on in situ denitrification is implemented for reducing nitrate levels in public supply wells in the interim period until the full influence of the regional nutrient management BMPs is realized. The remediation approach incorporates a passive component that necessitates enhanced site characterization methods including high definition assessments of aquifer properties and groundwater velocity. The preliminary findings from a series of field investigations conducted between 2004 and 2011 at a site in southern Ontario indicate that this combined two‐step strategy may be capable of providing short‐term quantifiable evidence of BMP performance and also of attenuating nitrate to desirable levels during the time lag period between implementation of the BMPs and the arrival of their desired beneficial effects on the quality of public well water supplies. This may avoid the need to construct permanent above‐ground water treatment facilities that could eventually become redundant.

Review: the environmental status and implications of the nitrate time lag in Europe and North America

“…These results indicate that tu was still ongoing, and that the effects of changed management practices in 1997 were yet to be observed at groundwater level, although decreases in NO3could be observed in the upper layers of the soil. Other studies (Bekeris, 2007;Rudolph et al, 2015) have reported observing similar peaks in NO3between 2 and 3 m below ground level in multi-layered soil profiles.…”

Integrated Deep Soil N and Groundwater Isotope Investigation of N Sources Captured by Municipal Wells

“…Sousa et al () point out “the unsaturated zone acts as storage for contaminants that are gradually released to the water table.” They report that while lag times can be estimated with numerical models, these are at best rough estimates because of the variability in hydraulic conductivity. Rudolph et al () indicated that it will likely take decades to observe the beneficial effects of best management practices (BMPs) on groundwater receptors beneath agricultural fields with deep soils in Ontario, Canada.…”

Decadal variations in groundwater quality: A legacy from nitrate leaching and denitrification by pyrite in a sandy aquifer

“…The 2006–2009 data in the upper part of T4 displays imperfection in the transient NO 3 input function used to feed the model simulation, which was derived directly from the data in Figure A. Figure A represents relatively few flow paths and hence will not fully represent the wide range of conditions in nutrient uptake, climate, soil properties, N application, and so forth, which may control NO 3 leaching from the root zone in the agricultural recharge area [ Rudolph et al ., ].…”