Quantification of Groundwater Nitrate Loading after Raspberry Field Renovation Using High‐Resolution Passive Diffusion Sampling

The Abbotsford‐Sumas Aquifer is a permeable, unconfined aquifer in British Columbia, Canada, where raspberry ( Rubus idaeus L.) production is an important source of groundwater NO 3 contamination. Renovation of raspberry fields (i.e., canes chopped, soil tilled and fumigated, and spring manure application prior to replanting), which typically occurs every 6 to 10 yr in response to decreased crop vigor, has been suggested as a possible cause of significant interannual variation in groundwater NO 3 concentrations. This study used high‐resolution passive diffusion sampling to quantify the magnitude and timing of NO 3 loading to shallow groundwater from a commercial raspberry field during a 6‐yr (2009–2015) monitoring period after crop renovation. After renovation, the annual NO 3 loading increased from ∼95 kg N ha −1 in Year 1 to ∼245 kg N ha −1 in Year 2 and decreased to ∼85 kg N ha −1 in Year 3. The average annual NO 3 loading from Years 4 to 6 (72 kg N ha −1 ) was assumed to reflect annual loading without a renovation effect, and the increased loading during Years 1 to 3 was attributed to renovation. Renovation contributed an estimated 33 to 23% of total groundwater NO 3 from this field for a 6‐ to 10‐yr renovation cycle. Most of the NO 3 loading associated with renovation occurred in Year 2 and was attributed to the manure application. The increased NO 3 loading after renovation likely contributes to the spatial and temporally varying NO 3 patterns observed in the aquifer. Reducing manure applications during renovation and decreasing renovation frequency have the potential to decrease the groundwater NO 3 concentration. Core Ideas We estimated NO 3 flux using vertically high‐resolution groundwater sampling and Darcy flux. Loading was estimated under raspberry field for 6 yr after stand renovation. Renovation frequency is an important control on long‐term NO 3 concentration. An estimated one‐third to one‐quarter of groundwater NO 3 was from renovation in a 6‐ to 10‐yr cycle. Manure was potentially the primary source of additional NO 3 loading during renovation.