Linking landscape indicators to groundwater nitrate concentrations in a transboundary aquifer

Groundwater aquifers provide nearly half the freshwater used in drinking and cooking. However, in the last century, massive transformations of landscapes have produced enduring impacts on natural resources such as groundwater. Excess nitrate contamination of groundwater is a growing health concern, particularly in agricultural regions. Despite its importance, very few studies have quantitatively linked land use land cover (LULC) and groundwater nitrate concentrations. Furthermore, understanding the impacts of LULC on transnational water resources is especially challenging as multi-jurisdictional data disparities and inconsistencies can complicate monitoring efforts. Here, I developed a suite of innovative long-term monitoring approaches and evaluated their utility in a well-studied transnational aquifer where elevated groundwater nitrate concentrations are of concern. My overall objective was to develop approaches for examining LULC impacts to groundwater via two primary components. First, I used remote sensing to examine two decades of LULC change surrounding 11 groundwater dependent cities. Second, I created more localized landscape indicators and evaluated their correspondence to long-term trends in groundwater nitrate concentrations. I examined two nested spatial extents spanning the US-Canada border including: small cities throughout the Greater Abbotsford-Sumas Aquifer region as well as the confined extent of the Abbotsford-Sumas Aquifer (ASA) proper. I integrated a unique combination of historical photography, transnational satellite imagery, and groundwater monitoring wells spanning four decades. Throughout the larger region, I found that landscape evenness increased over time driven by greater forest losses in Canada and greater losses of agricultural land in the USA. Within the localized ASA, I determined that groundwater nitrate concentrations could be explained using landscape features measured within the vicinity of wells. Landscape indicators such as the proportional area of berries, raspberry fields undergoing renovations, as well as forage/pasture were particularly useful. I further determined that long-term trends in nitrate were best explained by historical landscape indicators from two decades prior (as opposed to contemporaneous indicators). Very few studies have examined LULC emphasizing transboundary aquifers and even fewer have quantitatively linked groundwater nitrate concentrations to land use practices. Thus, this work demonstrates a valuable, consistent monitoring approach that is transportable to other regions facing similar challenges.Forestry, Faculty ofGraduat