Mapping Water Quality Contamination Legacies Using Long-term Landscape Indicators in a Transboundary Aquifer

Historical land-use and land cover (LULC) can impact water quality for decades and is among the factors responsible for elevated nitrate (NO3-) in many groundwater systems. Understanding the persistent "echo" effect of land-use is essential to create policies to improve water quality. To address this, we examined the Abbotsford-Sumas Aquifer, a source of drinking water to 100,000+ people in British Columbia and Washington. We asked two questions: 1) Are long-term groundwater NO3-concentrations changing over time and space? 2) What is the relative importance of historical versus contemporary LULC in explaining groundwater NO3-concentrations? First, NO3- concentrations in 16 shallow groundwater wells were examined using seasonal Mann-Kendall tests to detect long-term trends. We determined that over 50% of wells show significant, albeit contrasting, trends. For our second question, landscape indicators were calculated at each well and correlated with monthly NO3-concentrations. To create landscape indicators, a series of historical aerial photos (1974-2011) were scanned and georeferenced in ArcGIS. Using heads-up digitizing, we identified and quantified fine-scale features with suspected mechanistic links to NO3-sources within immediate zones of influence surrounding wells. Backward stepwise regression identified parsimonious statistical models that best predicted NO3- concentrations using landscape indicators and several co-variates. Preliminary results show the area of historical raspberry renovations was positively correlated with mean NO3- concentrations. This work helps develop affordable monitoring approaches for understanding the persistent legacy effects of land-use.