Excessive nitrate leaching from crop production has been linked to elevated nitrate levels in groundwater. Beneficial Management Practices (BMPs) have been implemented to reduce nitrate leaching, but monitoring rarely detected improvements in underlying groundwater quality. A study (2011-2016) was conducted in a potato-grain-forages rotation field in Prince Edward Island (PEI), Canada, to understand the processes that govern this lack of a positive response. The field consists of fine sandy loam soil and is underlain by 7–9 m of glacial till, which overlies the regional fractured ‘‘red-bed” sandstone aquifer. The water table is generally located in the till close to the till/bedrock interface. Field treatments included one field zone taken out of production in 2011 with the remaining zones kept under a conventional potato rotation. Taking potato land out of production simulated an extreme scenario of nitrate reduction as a BMP and allowed an assessment of the effects of soil, weather and geology on nitrate transport without adding confounding factors related to management practices. Multilevel piezometers were installed to monitor the responses of groundwater nitrate level to the contrast treatments. Soil sampling detected a significant amount of nitrate after potato harvest in fall 2011. This nitrate was detected in the tile drains in late fall 2011, in the overburden at a depth of ~3.5 m in December 2012 by coring and in the aquifer (peak concentration) in early spring 2014 by the multilevel systems. These data suggested that the nitrate migration followed a uniform flow path in the till and the ongoing release of nitrate from the vadose zone created a persistent nitrate plume in the underlying aquifer. This finding implies that agricultural vadose zones can store a significant amount of nitrate with a substantial delay in release to the groundwater.