Groundwater Monitoring to Support Development of BMPs for Groundwater Protection: The Abbotsford‐Sumas Aquifer Case Study

Abstract

The Abbotsford‐Sumas Aquifer is arguably the most studied case in Canada of groundwater nitrate contamination associated with agricultural production. Underlying some of the most productive agricultural land in Canada, this highly vulnerable trans‐boundary aquifer provides a unique case study on the opportunities and challenges of addressing water quality issues. A groundwater monitoring program initiated in the early 1990s has been important in tracking spatial and temporal variation in groundwater nitrate concentration. However, small land parcels with spatially and temporally variable land use and management practices and sub‐horizontal flow in this highly permeable sand and gravel aquifer make it difficult to relate groundwater monitoring results to specific agricultural practices. Other approaches pointed to the historical over‐application of N relative to crop requirement (primarily as manure used to increase soil organic matter during replanting but also as a nutrient source during production). Despite changes in agricultural practices, and programs aimed at raising grower awareness, no appreciable change in average groundwater nitrate concentration has occurred over the monitoring period. On individual land parcels, nitrate contamination may be reduced through development and adoption of an integrated suite of beneficial management practices (BMPs) to improve N fertilization, irrigation and alley vegetation management, and in particular to eliminate application of any organic soil amendment such as untreated manure in which the N has not been stabilized (e.g., by composting). However, the substantial N imbalance on a regional scale, and the lack of an effective on‐going consultative process among stakeholders, remain major barriers to the development, demonstration and adoption of BMPs.

A Field‐Scale Approach to Estimate Nitrate Loading to Groundwater

“…The risk of NO 3 contamination is also enhanced in sandy soils with limited water‐holding capacity, high water inputs as rainfall and/or irrigation (Power and Schepers, 1989; Strebel et al, 1989), and where oxic groundwater does not support denitrification (Rivett et al, 2008). Despite considerable efforts to develop improved agricultural practices (Raun and Schepers, 2008), groundwater NO 3 concentrations have exceeded the drinking water guidelines for human health for decades in some jurisdictions (Exner et al, 2014; Zebarth et al, 2015), with increasingly important implications for public water supply management.…”

“…The quantification of groundwater NO 3 loading associated with a specific agricultural field (i.e., set of management practices) so that water quality improvements can be objectively assessed is a major challenge (O'Donnell, 2012; Zebarth et al, 2015). In many cases, soil zone techniques to measure NO 3 leaching from the root zone are commonly used on small (<0.05 ha) experimental plots where groundwater zone monitoring is not tractable.…”

“…This coarse sand and gravel aquifer is highly vulnerable to NO 3 contamination due to a thin and permeable surficial soil layer, high annual precipitation, and intensive agricultural production (Liebscher et al, 1992). Nitrate isotope composition (Wassenaar, 1995), soil N budgets (Zebarth et al, 2015), and elevated dissolved O 2 levels (Cox and Kahle, 1999) suggest that significant denitrification does not occur in the aquifer, and that the groundwater NO 3 originated primarily from poultry manure, and to a lesser extent from mineral fertilizer sources.…”

Use of δ15N and δ18O Values for Nitrate Source Identification under Irrigated Crops: A Cautionary Vadose Zone Tale

“…Intensive agricultural cropping (i.e., raspberry, blueberry [ Vaccinium corymbosum L.]) and poultry production are important land uses over this aquifer. Associated land application of synthetic fertilizers and manure has resulted in a history of elevated groundwater nitrate (Zebarth et al, 1998; Zebarth et al, 2015). A 1993 regional study of ASA groundwater using nitrate isotopes reported that animal manure was the primary source of groundwater nitrate (Wassenaar, 1995).…”

Effect of soil acidification on nitrification in soil

“…That understanding of the effects of soil pH on the nitrification process has important economic implications, given evidence that the blueberry crop commonly has greater N uptake (Poonnachit and Darnell 2004) and greater plant growth (Peterson et al 1988) when N is supplied in NH ' 4 form compared with NO ( 3 form. The effects of soil acidification on nitrification also have important environmental implications because many of the fields being converted to blueberry are located over the transboundary AbbotsfordÁSumas Aquifer, an important source of potable water that is highly susceptible to NO ( 3 contamination (Zebarth et al 2015).…”