For the cities of Abbotsford, Bellingham, and Langley, evenness increased with distance from the city center. The opposite was true for Hope, which approached zero with distance from city center. Seven of the eleven cities experienced peak evenness km from the city center. Governing bodies and stakeholders may have differing — and sometimes conflicting - agendas.
Ecological Indicators
Determining where and how government agencies are working together and identifying areas where cross-border management strategies and policies vary is the first step to creating holistic approaches for managing trans-boundary systems. Understanding the historical and political background of an area helps to inform land managers of which practices are most effective for maintaining a healthy and productive landscape.
An important component of this broader challenge of managing cross-border systems is mapping such areas in consistent and comparable way Pardington and Cardille, Geodata are routinely collected by a multitude of government and private agencies at differing scales, using various classification schemes, and for varying purposes. This creates many challenges concerning obtaining continuous, uniform, accurate data for a system.
Landscape pattern analysis is one way to compare across regions and 31 jurisdictions, identify how landscape changes are affecting landscape patterns, and in turn, how aquatic resources might be impacted. Landscape evenness increases as urban land increases and agricultural intensifies in the region Landscape evenness increased in all cities over the year period examined, driven primarily by gains in urban and losses in agricultural lands. Additionally, the Canadian town of Hope was the only city where urban land cover decreased.
Agricultural land was lost in all cities except in Abbotsford where it increased slightly. The greatest decreases in agricultural land occurred in the five US cities. While agricultural land decreased over time surrounding most cities, according to the farm census, total production remained the same or increased, indicating the agricultural activities may be intensifying on remaining agricultural lands USDA , Ministry of Agriculture Land use policies zoning, master plans, growth boundaries help to determine urban form and its impact.
For example, in the greater Seattle area, growth management efforts to increase housing densities within growth boundaries has had unintended consequences, encouraging low density housing sprawl in rural and wildland areas just beyond those planned growth boundaries Robinson et al Land parcels falling within the ALR are limited to farm uses according to Agricultural Commissions bylaws.
The ALR likely prevents or at least slows the rate of agricultural conversion to urban in the Canadian portion of the study area. As such, understanding how policies impact landscape patterns and identifying where landscape changes are occurring can help to better focus planning efforts to reduce negative consequences of LULC change on aquatic ecosystems. Identifying changes in landscape pattern helps focus land planning to improve water quality Landscape patterns serve as useful indicators of overall water quality.
The percentage of land in forest and non-forest cover as well as the density of paved roads are among the strongest predictors of overall water quality Wu and Sun , Hunsaker and Levine , Swank and Bolstad Additionally, strong links between land cover and water quality changes associated with major storm events suggests that even small changes in land cover have important implications for water quality Swank and Bolstad , Larson and Grimm , Janke et al The spatial organization of land cover, measured by contagion and dominance, may also have a bearing on water quality Hunsaker and Levine , Moreno-Mateos, , Kelting et al Hence, spatiotemporal changes in landscapes may therefore have important implications for water quality.
Groundwater is an important source of drinking water for the eleven cities in this study. Furthermore these aquifers are shallow, unconfined aquifers highly vulnerable to contamination.
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As such, understanding patterns of development and land conversion, and linking these changes to implications for water quality, can help planning and regulatory bodies focus efforts on critical 33 locations that may be negatively impacted by LULC. For instance, increases in urban areas have led to an increase in food demand which has resulted in agriculture intensification within the region. This agricultural intensification has increased nitrate contamination of transboundary water sources in the region causing an international dispute over water quality.
Mapping changing landscape patterns can help identify where land use is most likely to be intensified, or conversely, where it will likely remain stable or change in only trivial ways. In turn, this information can be used to create indicators of potential contamination and used in conjunction with water quality data to map and model areas vulnerable to contamination.
In a world of limited resources, such localized targeting may be as or even more effective than broad regulations intended to protect water quality Wear et al My results showed that in most cities in the Greater ASA area, urbanization is encroaching upon agricultural land use, which in turn is encroaching upon forested lands.
Furthermore, patterns of urbanization and agricultural intensification along the urban-to-rural gradient can be well captured by landscape metrics even surrounding smaller cities undergoing major landscape transitions. I showed how landscape change is spatially heterogeneous and often occurs differentially along the urban to rural gradient. Additionally, these results provide important knowledge base for land use managers in the region.
While this 34 chapter provides important background context of LULC in the Greater ASA region, the deeper implications of these changes for water quality will be further explored in subsequent chapters. An Approach for Transboundary Aquifer Monitoring 3. Increasing use of synthetic and organic fertilizers, disposal of waste particularly from animal-based agriculture , and changes in landscape patterns are key factors responsible for the progressive increase in nitrate concentrations in groundwater over the last 30 years Townsend and Howarth Adverse effects may also be possible below WHO guidelines, however; long-term exposure to nitrate in community water supplies as low as 2 - 4 mg N L-1 has shown possible links to bladder and ovarian cancer Weyer et al Trans-boundary aquifers are particularly vulnerable to contamination.
While nearly river basins traverse international boundaries Transboundary Water Assessment Programme , twice as many aquifers span international political boundaries IGRAC Cross-border aquifers are the primary source of freshwater on almost every continent, yet the number 36 of international agreements for transboundary rivers and lakes vastly outnumber those of transboundary aquifers Eckstein and Eckstein Long water residence time, large storage capacity, physical inaccessibility for remediation, and lack of regulations make many aquifers challenging to manage, especially in cross-border settings Foster and Chilton Furthermore, the spatio-temporal scale of data collection and monitoring conducted by different countries may not be compatible nor shared among jurisdictions.
Thus, the complexity of coordinating among international agencies working across multiple jurisdictions has also likely exacerbated the long-term difficulties in addressing groundwater nitrate. Understanding how land use and land cover LULC patterns impact groundwater systems is a critical first step towards mitigating nitrate contamination. One approach is through easily measured landscape indicators. Landscape indicators quantify the amount and arrangement of land cover such as percent agriculture and percent forest cover and the physical structure of vegetation on the land surface Meyer and Turner They allow for an affordable, broad-brush approach to characterizing the landscape and classifying potential LULC impacts.
A long-standing, well-developed body of research examines the correlations between landscape indicators and aquatic ecosystems Gergel et al , Allan , Johnson and Host However, the strength of quantitative predictions can vary greatly depending upon the indicator used and the region in which it is applied. A plethora of research has examined landscape indicators of surface waters Hale et al , Mallin et al However, despite clear connections between surface and groundwater 37 systems, few studies have examined landscape indicators within the context of monitoring groundwater and aquifers Gurdak and Qi , Keeler and Polasky Development of landscape indicators linked to land use and incorporating topography and geology can help identify and potentially explain mechanisms and processes acting above and below the land surface Sophocleous and are especially relevant to unconfined aquifers which have no overlying impervious rock layer and are therefore susceptible to contamination.
Other methods such as such as mass balance nutrient modelling can be expensive and data intensive; however, landscape indicators are a rapid and relatively affordable way to assess likely groundwater contamination. Thus, understanding landscape indicators that relate to groundwater in unconfined aquifers can improve our understanding of terrestrial groundwater interactions. Long-term monitoring which began in the early s detected nitrate concentrations in exceedance of WHO standards Wassenaar et al , Mitchell et al Decades after being first identified, elevated nitrate concentrations have remained a persistent trans-boundary dilemma for the USA and Canada Chesnaux et al , Zebarth et al The complexities of this problem have challenged the many managers, farmers and policy makers who have initiated a wide variety of nutrient management strategies, with little apparent success in reducing nitrate concentrations in the aquifer.
Within this context, I ask two primary questions: Are there temporal trends in nitrate concentrations over time? How well do landscape indicators help explain patterns of groundwater nitrate concentrations? To accomplish this, I first tested for statistical trends in nitrate concentrations over time. I then statistically linked agriculturally-focused landscape indicators to nitrate concentrations measured in ASA monitoring wells along the US-Canada border.
Based on previous studies, I expect 38 elevated nitrate concentrations in areas with large amounts of livestock and berry production Lockhart et al , Wassenaar In addition, three subsidiary objectives were examined to determine: To do so, I calculated landscape indicators within terrestrial zones of influence for differently-sized radii surrounding each well and accounted for directionality of subsurface flow. Finally, I compared analyses of landscape indicators and groundwater nitrate concentrations collected at ASA monitoring wells across the US-Canada border.
I hypothesize that incorporating distance and directional flow into landscape indicators will increase predictive power and improve our understanding of the factors contributing to high nitrate concentrations throughout the ASA. I further foresee challenges arising from this comparative cross-border approach as geodata and monitoring techniques differ between countries. The aquifer supplies drinking water for , residents of Canada in the city of Abbotsford and the township of Langley Chesnaux et al as well as nearly 10, people in the United States towns of Sumas, Lynden, Ferndale, Everson, Nooksack, and scattered rural areas.
The unconfined, highly permeable sand and gravel aquifer is recharged primarily by direct precipitation Fraser Valley Soil Nutrient Study and consists of mostly coarse-grained sediments of glaciofluvial drift 39 origin. Such loose, or unconfined, soil strata provide minimal filtration for contaminants USGS Mean groundwater age is approximately 20 years old while models suggest a mean travel time of 6. Red raspberry is the predominant agricultural crop in the region followed by significant areas of forage grass and pasture Zebarth et al Because shallow groundwater is younger and therefore more likely to reflect more recent landscape practices than deeper groundwater , only shallow wells with mid-screen depths less than 10m below the mean height of the water table were used.
Prior to June , ECCC sampled monthly; however to reduce program costs, they began quarterly sampling thereafter i. For Canadian wells, mean, median, minimum, and maximum nitrate were determined on a quarterly and annual basis. Many US wells had only one sample collected between Geospatial Data To assess contemporary land use and land cover, a variety of geospatial datasets were consolidated from US and Canadian sources Table 3.
Together these datasets provided the best continuous coverage for the Canadian portion of the ASA region. To create a seamless cross-border mosaic, these datasets were harmonized to commensurate resolutions, formats, and classification schemes representing LULC categories consistent across the region. Comparison of characteristics of geodatasets from the US and Canada used in this chapter. Approach for improving concordance among these datasets is explained further in the text. To determine linkages between LULC and nitrate concentrations, contemporary land use patterns e.
In addition to the landscape indicators in Table 3. Due to build-up of root pathogens and viruses in soil, raspberry fields are typically renovated uprooted and replanted anew every years. Renovations include removal of root balls and old canes followed by tillage. A critical component of renovations, aside from planting of new raspberry plants, is the addition of soil amendments typically poultry manure to increase soil nutrients.
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Renovations occur in the fall and the following spring manure is applied to bare fields and new raspberries are planted Forge It is during this application of manure in the spring when fields are bare that it is hypothesized intense leaching occurs Staver and Brinsfield In order to capture the dynamic nature of this regionally-abundant crop, Google Earth imagery was used to photo-interpret past renovations of raspberry fields i. Further distinction was made among berry types: Berry production has been associated with high fertilizer use in the region.
Berry field renovations Total Area of Renovations m2 Newly-planted and replanted raspberry fields m2 since Raspberries are fertilized heavily within the first 3 years of planting with declines in fertilizer application in subsequent years. Manure is a source of nitrate. Throughout the ASA, the main source of nitrate to the aquifer is likely attributed to agriculture Zebarth et al.
Large gravel mining operations in the ASA may contribute to surface and groundwater pollution. Impervious surfaces are not included in this category as they are included in urban. Denitrification in wetlands potentially reduces nitrate loading to surface and subsurface waters. Additional Co-variates Water Table Height m Mean height of the water table As water tables rise, increase leaching of nitrate to groundwater may be expected.
Depth of Mid-Screen Depth below water table m The mid-screen refers to the location in the well where water enters. The depth of this mid-screen determines depth below the average height of the water for water sampling. I used mean depth of screen casing from Depth is related to the movement of groundwater and is associated with the age of groundwater. Hence, it can be expected that the deeper the mid-screen, the older the water being collected.
Because groundwater moves both laterally and vertically in an aquifer USGS , radii of different sizes were evaluated to capture potential lateral water movement. Zones of 1 km radii were also evaluated but due to lack of statistical significance were not included here.
I further incorporated a unique approach using upstream semi-circular zones of influence corresponding to the predominately-southwesterly direction of groundwater flow Figure 3. While the zones of influence do not address residence time or aquifer volume both of which may be important to groundwater nitrate concentrations , they do help further explore directionality as well as the extent of landscape influence.
Statistical Analysis First, to examine long-term trends in nitrate concentrations, I used Mann-Kendall MK tests to detect either monotonic upward or downward trends over time from A monotonic trend upward or downward means that a variable consistently increases or decreases over time, yet the trend may or may not be linear. A positive MK score indicates an increase with time whereas a negative indicates the opposite.
Though Mann-Kendall tests can be 45 computed for a time series with missing values, performance will be adversely affected. Therefore, missing values were interpolated using the mean value of the two years prior and two years after any missing quarterly samples. Measurements collected in January replaced missing values in December Wells with more than 3 missing values throughout the time series were excluded. Long-term trend analysis was only conducted for Canadian wells as comparable long-term nitrate measurements were not available for US wells.
Trend significance was mapped for each well. My second approach examined connections between nitrate concentrations and landscape indicators. For wells located in Canada, measures of central tendency mean, median as well as minimum and maximum nitrate were determined annually as well as quarterly March, June, September, and December. For US wells only sampled once within the study period , a single nitrate sample value was used as the response variable.
Backward stepwise regression was used to determine which landscape variables Table 3. My goal was to seek the best models consisting of no more than independent variables to avoid model over-fitting. To accomplish this, the least significant variables in each model were sequentially omitted in the interest of parsimony as judged by their significance levels, partial R2 values, and AIC scores.
Akaike information criterion AIC aids model selection by evaluating the relative quality or goodness of-fit of different statistical models and helps identify and penalize models which are over-fit with too many additional variables Burnham and Anderson Models with the lowest AIC score, given a similar number of independent variables, indicate the highest quality model. Additional variables were warranted in a model only if they lowered the AIC by at least two points. Shapiro-Wilk tests were used to assess normality of independent and dependent variables and landscape indicators were transformed as needed using the arcsine square root transformation.
Nitrate concentrations in nine wells decreased from Figure 3. Two wells demonstrated a significant increasing trend over time Figure 3. The remaining four wells demonstrated no significant trends. Recall that trends could not be evaluated for US wells because long-term nitrate measurements were not available. In contrast, the lone nitrate sample available for each US well ranging from 0.
Proportion of raspberries as well as forage and pasture land are important predictors of groundwater nitrate concentrations A total of 12 models were created 4 scales of influence x 3 response variables: The best model for each response variable is shown in bold Table 3. Regardless of zone radii size, direction, or jurisdiction, the proportion of raspberries and forage and pasture most strongly and consistently explained groundwater nitrate concentrations.
Proportion of mixed berries was positively associated with nitrate values in one model with a partial R2 value of 0. In contrast, proportion of raspberries was weakly negatively associated with nitrate in half of the models, with partial R2 values ranging from 0. Proportion of forest and water table height were negatively associated with nitrate in one of the models each, with partial R2 values of 0.
For all other models, changing the size of the zone of influence did not significantly change R2 values. My results show nitrate concentrations were strongly correlated e. The strength of these correlations supports the premise that land use affects groundwater quality in aquifers overlain by highly permeable materials. Previous studies using an allocation model for groundwater nutrient loads based on land cover classes, found similar results determining that average nitrate concentrations were highest beneath cropped fields and residential areas Schilling et al Other studies have found higher nitrogen concentrations in groundwater in watersheds dominated by agricultural as compared to forestry-dominated catchments Lawniczak et al For models of annual median nitrate, mixed berries were strongly and blueberries weakly positively correlated with nitrate while raspberries demonstrated a negative relationship.
In contrast, on the US-side of the aquifer where groundwater data were largely lacking, raspberries were positively correlated with nitrate, albeit weakly. Raspberries were the only berry type on the US-side to demonstrate a significant relationship of any kind with nitrate. Reduced sampling effort frequency of observations collected on the US side may have also played a role and certainly would not represent temporal trends. Spatial scale of measurements An emphasis on LULC in circular areas surrounding water table wells is a simple and effective method for correlating land use and water quality Barringer et al I examined two spatial extents by measuring landscape indicators within and m distances surrounding wells.
Increasing the zone of influence improved model fit for two of the three 54 response variables. One might reasonably expect m zones of influence to improve model fit because landscape activities immediately surrounding groundwater wells might influence recorded nitrate concentrations in wells more than activities further away, however my results indicated the converse.
One explanation is that the well screen depth of the wells sampled is sufficiently below the water table to capture water originally infiltrating further up-gradient than m Zebarth et al The m scale of measurement slightly improved model fit for annual median nitrate, however.
The radius size used in the literature has varied greatly, but is an important consideration. For larger radii, land within its perimeter contributes proportionally less water to a well, potentially weakening correlations between groundwater quality and land use. However, a smaller radius, approaching the size of the minimum mapping unit, might unduly influence derived landscape indicators via exclusion of important up-gradient features, susceptibility to localized positional errors in mapped features, or misidentification of fine-scale features within the zone.
Thus, selection of an appropriate zone of influence is important to maximizing the correctness of an association between land use and groundwater quality McLay et al Similar to my findings, other studies have found significant positive relationships between shallow aquifer nitrate concentrations and land use in estimated recharge zones within a m radius of wells Keeler and Polasky , Nolan et al , Kolpin , McLay et al My incorporation of berry field renovations associated with greater fertilizer application was a unique component of my landscape indicator approach and helps improve mechanistic understanding of potential N sources.
No prior studies have explicitly examined the number or areal extent of field renovations in this way, despite being a suspected source of nitrate. That field renovations showed a strongly significant positive correlation with annual median nitrate concentrations is consistent with renovated fields being heavily amended with poultry manure before being replanted with raspberries. As a result, information regarding N loading is largely unavailable for this region. Instead, crop types within a certain distance of wells was my proxy for N loading.
Although knowledge of crop type at a location helps indicate where inorganic and organic fertilizer is likely to be applied, it does not indicate the rate of actual application. The rate and timing of N fertilizer application vary based on regional and local factors including crop type, tillage practice, crop rotation, and irrigation practices and from farmer to farmer. The relationship between these regional and local factors along with nitrogen use efficiency results in residual soil 56 nitrogen being susceptible to leaching into the aquifer after heavy periods of precipitation, despite the lack of direct measurements.
Additional factors Factors not included in this study may help further explain nitrate in the aquifer. Additional sources of nitrate to groundwater may include septic tanks, lawn fertilizers, and domestic animals in residential areas Nolan et al Nitrate concentrations in groundwater on Nantucket Island, Massachusetts, increased with number of septic tanks and percentage of high-density residential and agricultural land and decreased with percentage of forest and undeveloped land Gardner and Vogel However, in my study region, studies indicate minimal influence of septic tanks Robertson et al Geological factors such as soil drainage type help explain nitrate concentrations Nolan et al Future research should take into account livestock production both poultry production present in Canada and dairy production present in the US across the aquifer.
Additionally, incorporating past land use into the models would help to capture lag-effects and account for legacy nitrate present across the aquifer. In the next chapter, I build off these results and address the persistence of landscape legacies in impacting measured groundwater nitrate concentrations. Lack of consistent data for evaluating transboundary systems is a global problem One major challenge of cross-border monitoring can be a lack of consistent data among governing jurisdictions.
For example, sampling frequency of groundwater nitrate varies greatly between the US and Canada. The US has monitoring 57 stations in Whatcom County, WA, managed by a myriad of individual landowners, private and governmental agencies. This inconsistent frequency of sampling makes it challenging to compare long-term groundwater nitrate concentrations between the US and Canada. Further exacerbating cross-border monitoring is the lack of consistency in monitoring within each country.
As mentioned, ECCC began domestic well monitoring in the s, and started the dedicated network program in the late s. When the program started, samples were collected monthly. To cut down on program costs, quarterly sampling was implemented in June This lack of temporal uniformity introduces challenges when statistically analyzing long-term trends, as does singular sampling, as available for the USA portion.
A lack of consistent transboundary information of relevance to managing groundwater is an issue of global proportions. The political dimension of water becomes increasing important when shared across national boundaries and can be a potential source of conflict Mylopoulos and Kolokytha In contrast, shared water resources can also provide opportunities for discourse leading to cooperation leading to joint management and monitoring.
In addition, a lack of integrated approaches and legal agreements as well as administrative shortcomings, make transboundary cooperation and management difficult Rahaman and Varis , Katerere et al Countries monitoring the same aquatic system may have different goals and thus different intended applications for their data collection. In order for transboundary water systems to be adequately managed, joint 58 approaches and techniques for monitoring should be further developed.
Approaches such as landscape indicators, which can integrate existing data from disparate jurisdictions, can potentially play an important role in this integration. Increasing use of synthetic and organic fertilizers, disposal of waste particularly from animal farming , and changes in landscape patterns are key factors responsible for the progressive increase in nitrate concentrations in groundwater over the last 30 years Townsend and Howarth N fertilizer and manure loading is an important component of assessing potential nitrate contamination Keeler and Polasky , Nolan and Hitt , Nolan et al Within the same aquifer, I found nitrate concentrations were increasing and decreasing from at different wells.
As such, sharing evidence with farmers as well as policy makers on the crop types and land use practices most statistically linked to nitrate concentrations 59 is important an important part of finding solutions. Landscape indicators can act as a proxy for N loading and allow for an affordable, broad-brush approach to characterizing the landscape and classifying potential LULC impacts, thus helping to break down the complexity of coordinating among international agencies and helping to address some of the difficulties associated with managing and monitoring groundwater nitrate.
My work, creating proxies for nitrate loading to groundwater, provides an important new approach which is transportable to other regions facing similar challenges. Particularly problematic are historical land-use practices with lasting impacts on contemporary water quality Harding et. For example, hazardous waste disposal from industrial activities, acid mine drainage from mining, and chemical leaks from decades old underground storage tanks can have long-term repercussions on aquatic ecosystems Bhaduri et.
Agricultural operations are among the primary sources of nonpoint source NPS pollution to aquatic systems. Excess nutrients from fertilizer and manure, particularly excess nitrogen N and phosphorus P , have had enormous consequences to freshwater systems globally. The Chesapeake Bay, the Mississippi, inland and coastal waterways of Florida, and the Great Lakes are among many systems with persistent histories of water quality degradation and eutrophication Dale et. Despite our knowledge of landscape legacy effects on surface waters Sharpley et.
Decades of intensive agriculture has led to groundwater nitrate contamination worldwide. Due to the high mobility of nitrate NO3- , groundwater is particularly susceptible to contamination from leaching, especially in shallow unconfined aquifers underlying agricultural lands. Potential health effects of nitrate contamination in drinking water include blue baby syndrome and increased cancer risk Weyer et. Additionally, environmental N-loading can contribute to loss of habitat in aquatic systems 61 Johnson et.
Since the s, increased fertilizer use, in combination with nitrogen fixation by crops, mineralization of animal manure, and various other sources has resulted in increased release of N into the environment Puckett et. In the last 60 years, the use of industrially fixed N in the form of fertilizer increased fold in the United States Puckett et. The movement of contaminants such as nitrogen in groundwater adds a layer of complexity to understanding the interaction between land use practices and water quality. Though nitrate is highly mobile and quickly leaches to groundwater, the residence time of water in aquifers is typically several orders of magnitude higher than in lakes and wetlands Philips et al Nitrogen in groundwater moves laterally as well vertically and as such can take years or decades to move deep into an aquifer.
Shallow groundwater closer to the surface is younger and more likely reflects recent landscape practices than deeper water. In the US and Canada, shallow wells are typically used for agricultural irrigation, whereas deeper wells are used for private water supplies, and the deepest wells are used for public water supplies. Eventually, as nitrate moves deeper into an aquifer, impacts to drinking water supplies can occur. The variable travel-time of nitrate through groundwater systems creates a lag-time between when nitrogen is first applied onto the land surface and when it is captured by groundwater monitoring wells at various depths.
As such, understanding impacts of land use and land cover LULC change on groundwater systems is a critical first step in managing nitrate. Landscape indicators are one approach to quantifying and understanding the impacts of LULC change on aquatic systems. Landscape indicators quantify the amount and arrangement of 62 land cover such as percent agriculture and percent forest cover on the land surface Meyer and Turner and are a cost-effective approach to characterizing regional change. A plethora of research has examined landscape indicators of surface waters Hale et.
However, development of richer detail in landscape indicators that potentially identify and explain mechanisms of water quality pollution, both above and belowground, is needed Sophocleous Long-term monitoring since the early s detected nitrate concentrations in exceedance of WHO standards Wassenaar et al , Mitchell et al and persistent elevated nitrate concentrations have remained a problem for decades.
In recent decades, while the amount of total N applied over the aquifer has changed little, the source has changed substantially from inorganic fertilizer to manure. Furthermore, export of N from the region has declined leading to a surplus over the aquifer Zebarth et al, The many complexities of this problem have challenged managers, farmers, and policy makers in both the USA and Canada who have initiated a wide variety of nutrient management strategies - with little apparent success - in reducing overall nitrate concentrations in the aquifer.
This perceived lack of success may be in part a result of the time lags between the implementation of management practices and the residence time of nitrate in aquifers. Thus, 63 approaches to help understand water quality legacies across jurisdictional boundaries are particularly critical. Cross-border landscape indicators are particularly appealing in this context. Within this context, I ask two questions to understand the historical dynamics of land cover change and its potential impact on nitrate concentrations in the ASA aquifer: To accomplish this, I examined trends in groundwater nitrate concentrations at monitoring wells across the aquifer.
Then, I assessed temporal changes in a suite of landscape indicators using historical aerial imagery since the s. I hypothesized that correlations between present day nitrate concentrations and historic LULC and may be important because of lag effects. The aquifer supplies drinking water for nearly , residents of Canada in the city of Abbotsford and the township of Langley as well as 10, people in the United States towns of Sumas, Lynden, Ferndale, Everson, Nooksack, and scattered rural areas Chesnaux et.
The unconfined, highly permeable sand and gravel aquifer lies within the agriculturally productive Fraser-Whatcom Valley and is recharged primarily by direct precipitation Fraser Valley Soil Nutrient Study Over the last four decades, there has been a shift in land use from dairy production to more raspberry, blueberry, and poultry production. Mean groundwater age is approximately 20 years and models predict an average of 6. Groundwater Nitrate Concentrations I examined nitrate concentrations in 22 groundwater wells in Canada. Prior to June , ECCC sampled monthly; however to reduce program costs, they began quarterly 65 sampling thereafter i.
March, June, September, December. Mean, median, minimum and maximum nitrate were determine on both a quarterly and an annual basis. To account for the depth of each well and its impact on the age of the water collected, I calculated the mean depth of mid-screen for each well Figure 4. Geospatial Data To characterize land cover change over time, I compiled and integrated a diverse suite of remotely sensed imagery, with special emphasis on characterizing agricultural features thought to influence nitrate concentrations.
In order to do this, I trained and collaborated with an assistant to aid in the collection, georeferencing, photo-interpretation, and digitizing processes which took several months. The photo acquisition process involved amassing hard copy aerial photographs circa from Dr.
Color and grey scale aerial photographs captured in 1: Ground control points were selected approximately per image to minimize residuals RMS error and ensure no residuals exceeded 4. In total, geoprocessing and the subsequent digitizing of photos explained next took about two and a half months. Within a m radius of each groundwater monitoring station, we manually delineated land cover using six broad categories: When a polygon of 25 m2 consisted of more than one land cover type, majority rule was used to assign the cover type to the dominant land cover class.
It should be noted that there were a few instances in which we did not want to lose important class features i. I also delineated raspberry fields in and using ArcMap Raspberry fields had previously been identified and delineated on the images and ground verified during the same decade by Dr. Hans Schreier and his colleagues.
However, a complete lack of any source of independent historical ground verification information circa thwarted any attempt at rigorous identification of raspberry fields on images. Landscape indicators calculated within the m wedge-shaped zone of influence surrounding each groundwater monitoring station using geodata described in Table 1. Landscape Indicator Units Description Potential Relationship to Groundwater Nitrate Agriculture Proportion of Zone Area used for the production of annual crops such as corn, soybeans, vegetables, as well as areas of grasses, legumes, or grass-legume mixtures planted for livestock grazing or the production of seed or hay crops.
This class also includes all land being actively tilled. Agriculture is main source of nitrate over the aquifer Zebarth , Chesnaux Vegetation Proportion of Zone Areas dominated by trees deciduous, evergreen, and mixed Natural vegetation buffers impacts of nitrate contamination. Bare Land Proportion of Zone Areas of bedrock, gravel pits, and other accumulations of earthen material.
Large gravel mining operations in the ASA may influence groundwater recharge rates. Impervious surfaces not included in this category but included in urban. Developed Land Proportion of Zone Mixture of constructed materials and vegetation e. Anthropogenic activities increase impervious surfaces and decrease vegetation cover which can impact aquifer recharge rates.
Streams and ponds may be surface-groundwater recharge zones and may lower nitrate levels. Hedgerows Proportion of Zone Hedge or wild shrubs and trees, typically bordering a road or field. These do not include raspberries and blueberries but may include wild blackberries growing in the region.
Hedgerows absorb some nitrate running off fields thus reducing leaching through the soil into the groundwater. Average water level calculated from Shallow groundwater likely represents contemporary land use practices while deeper groundwater likely represents past land use. Thus, I quantified landscape indicators over multiple time steps within terrestrial zones of influence surrounding each well.
Landscape indicators Table 4. Notably, due to limitations in aerial photo coverage among years, only a subset of 14 wells could be examined via this land cover change approach. Despite not addressing residence times or aquifer volume which may be important in predicting nitrate concentrations , these zones helped spatially delimit potential contributing areas and incorporate known directions of flow. Landscape indicators within variable-sized radii m, m were evaluated to identify which extent produced the strongest correlations between land use and nitrate concentrations.
Due to a lack of statistical significance at some extents i. Statistical Analysis First, to examine long-term trends in nitrate concentrations, I performed Mann-Kendall MK tests to detect monotonic upward or downward trends over time A monotonic upward or downward trend indicates a consistent increase or decrease through time, which may or may not be linear. A positive MK score indicates an increase with time whereas a negative MK score indicates the opposite. Because the performance of MK tests is adversely affected when evaluating a time series with missing values, I interpolated missing values using means from two years before and after any missing quarterly samples.
Lastly, I mapped wells according to the significance and direction of trends. For nitrate measurements in and , I determined measures of central tendency mean, median as well as minimum and maximum nitrate on both an annual and quarterly basis. First, I determined if nitrate concentrations measured at a given point in time and , were better explained by contemporaneous or prior lagged landscape indicators , , using backward stepwise regression. My goal was to seek the best parsimonious models with no more than independent variables and avoid model over-fitting as indicated by inflated R2 values and Akaike information criterion AIC scores.
AIC aids model selection by evaluating the relative quality or goodness-of-fit of different statistical models by identifying and penalizing those which are over-fit containing too many additional variables Burnham and Anderson Models with 74 the lowest AIC given a similar number of independent variables, indicate higher quality models. An additional model variable was warranted only if it lowered AIC by at least two points.
Shapiro-Wilk tests assessed the normality of independent and dependent variables and landscape indicators e. Seven of the eleven monitoring wells had decreasing trends in nitrate concentrations from Figure 4. Four wells demonstrated a significant increasing trend over the same period Figure 4.
The remaining eleven wells demonstrated no significant trends. I also graphed boxplots showing the median, first and third quartile of nitrate levels for individual wells Figure 4. Historical landscape indicators better explained nitrate than contemporary landscape indicators Overall, the best models used lagged landscape indicators measured prior to the year of the nitrate measurement.
I created eight models and the best model for each is in bold Table 4. I further explain model results in subsequent sections. The utility of models using historical indicators was made clear in an examination of nitrate concentrations from both and Table 4. Firstly, nitrate concentrations in were best explained by historical landscape indicators whereas land cover in did not explain any of the variance in nitrate Table 4.
Given the similar R2 and AIC values for models with and land cover, no one best model was chosen and instead both models were deemed equivalent. This result does not necessarily indicate declining nitrate, but rather that the magnitude of change in nitrate trends both positive and negative diminished over time. For my first objective, I sought to understand if long-term nitrate concentration were changing over time and space. For my second objective, I sought to understand the relative importance of historical versus contemporary LULC in explaining groundwater nitrate concentrations.
Nitrate concentrations were strongly correlated with lagged land cover yet rarely with contemporary land cover. For models using and nitrate as a response variable, bare land and raspberries demonstrated consistent positive correlations. Additionally, vegetation, agriculture, and developed land were consistently positively correlated with nitrate. Hedgerows were inverse related to nitrate while depth of mid-screen and surface water coverage was negatively correlated with nitrate.
Heterogeneous nitrate trends suggests heterogeneous patterns of nitrate application The nitrate increases seen in some wells coupled with declines in others, suggests several possibilities including: These hotspots may be related to individual farm management strategies as soil types across the ASA are quite homogenous. Thus, variation in fertilizer applications and irrigation at the field-scale may explain differences in nitrate trends across the aquifer. Chapter 3 also suggested the important role of raspberry field renovations, pointing to the impact of soil turnover and organic fertilizer amendments on nitrate concentrations in the ASA Gallagher and Gergel Hedgerows, buffer strips, cover crops, and nutrient management plans are effective best management practices BMPs proven to reduce nitrogen runoff to surface waters Garcia-Diaz et al , Blanco-Canqui et al Although I could not account for the impact of nutrient management plans on individual farms, this would be an important next step and could help to explain heterogeneous trends in nitrate across the aquifer.
The nitrogen cycle is a complex biogeochemical cycle influenced by both biological and physical processes. Thus, generalization about the state of nitrate within an aquifer should be made with great caution. My heterogeneous results further bolster the need for caution in extrapolation of aquifer conditions throughout the larger area based on this sample of wells. Lagged land cover impacts are an important consideration for landscape management While BMPs could potentially reduce groundwater contamination, beneficial results of landscape management could take years or even decades to materialize as leaching rates, atmospheric conditions, and water residency times affect measured aquifer contamination Mulla et.
Benefits could take longer to materialize in groundwater systems due to substantive 84 lag times between fertilizer application on the ground surface and arrival in the water table. Leaching rates can be influenced by geological factors i. Furthermore, it may also take years or decades for initial contamination to even be detected. Furthermore, as per the work of Chapter 3, the inclusion of soil data was not warranted in the models. However, soil organic matter estimates would be a helpful long-term indicator as this would help explain the dynamic nature of agricultural soils. Though yearly historical data is not readily available, moving forward records of land use and crop types would be beneficial for understanding potential nitrate leaching.
Historical air photos have a broad application in exploring landscape legacies of groundwater Though the importance of landscape history on contemporary ecosystems has become increasingly apparent Rhemtulla and Mladenoff , Tomscha and Gergel , Tomscha and Gergel , relatively few studies examine water quality legacies using historical land cover from aerial photography.
My approach is transferable to other regions which likely have aerial photography from the s or s Morgan et. A lack of nitrogen information is complicating management Accurate quantification of nitrate leaching to groundwater is challenging due to the complex interaction between land use practices, on-ground nitrogen loading, groundwater recharge, soil nitrogen dynamics, and soil characteristics.
Several models and frameworks estimate nitrate contamination of groundwater using parameters such as soil type, land use and land cover type as proxies for nitrogen loading Almasri , Narula and Gosain , Bernardo et. Specific nitrogen loading values for my landscape indicators are difficult to determine, likely to be highly variable by region, but should be prioritized in future work. Despite their utility in many data-limited situations, weakness are apparent when validating model predictions of nitrate leaching Fox et.
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This work shows that landscape indicators are a useful tool for creating proxies for N-loading and evaluating potential sources and can supplement and help fill this critical gap. Nutrient runoff from agriculture is issue of global importance Tilman et al and historical land-use can have lasting impacts on present-day water quality Harding et. A lack of accurate information hobbles analysis thus complicating aquifer management in many regions Gleick and Cain , Henriksen et. Such inconsistent trends in nitrate concentrations across the aquifer may indicate temporal variability in N loading as well as heterogeneous time lags operating.
I also found contemporary nitrate concentrations were more strongly correlated with historic than contemporary land cover. This result may indicate longer residence times for nitrate in the aquifer than previously assumed with important implications for nutrient management strategies. Moving forward, management of contemporary landscapes will in turn have repercussions for future water quality Bennett et. Conclusions Nearly transboundary aquifers worldwide provide drinking water and irrigation to millions, contributing to human health and economic development IGRAC, Urbanization and agricultural intensification have contributed considerably to contamination of these groundwater resources as well as many cross-border lakes and watersheds.
There are ten trans-boundary aquifers shared between the USA and Canada and numerous more surface water bodies Rivera, Many of these shared aquatic resources, such as the Great Lakes basin, St. The International Joint Commission, a bi-national organization established by the governments of the United States and Canada under the Boundary Waters Treaty of , has provided a mechanism for cooperative management of the St.
Lawrence Great Lakes and other cross-border waters. Although much has been accomplished in managing surface waters across borders, few policy measures have been implemented to manage shared groundwater resources. Furthermore, managing cross-border groundwater resources has proved challenging as spatial and temporal data collected by different countries are often not compatible nor shared among jurisdictions.
The approaches used to monitor and assess land cover in this dissertation are affordable and easily transportable to transboundary water systems across the US-Canada border and beyond. The overall objective of this dissertation was to develop a framework for understanding linkages between land use and land cover LULC and nitrate trends within the context of transboundary aquifers. I developed several innovative spatial approaches and pushed the temporal boundary of aquifer evaluation via integration of multi-method approaches for long-term assessment.
A number of key findings emerged from three primary data chapters. I 88 conclude by describing these key findings and discuss the limitations of my approach. I also suggest directions for future research which include using future scenario planning to engage communities and combat groundwater contamination from a socio-ecological perspective.
In Chapter 2, I explored urban-rural gradients in order to spatially and quantitatively describe long-term landscape changes surrounding groundwater dependent cities from I quantified landscape composition and configuration for eleven cities in the US and Canada and compared these patterns among cities and across borders. I found that evenness among land cover types increased in all cities indicating that proportional abundances of land cover types became more similar. I determined this was driven by an increase in urban land while forest and agricultural land declined.
Additionally, I found greater forest loss in Canada but greater losses of agricultural lands in the USA. This difference in land conversion could be a direct link to differences in land use policies north and south of the border. This chapter provided important regional context for the analyses tackled in Chapters 3 and 4. In Chapter 3 I highlighted the use of high spatial resolution imagery to assess fine-scale landscape features mechanistically linked to nitrate loading in a transboundary aquifer.
By incorporating areas of raspberry renovations i. As surprisingly few studies have quantitatively linked groundwater nitrate concentrations to land use, land cover, or land use practices, my research provides an important new approach that is transportable to other regions facing similar challenges. Building off the general approach and key results of Chapter 3, I further evaluated longer-term trends in nitrate concentrations over 10 additional years. To account for potential lagged impacts of prior land cover, I also quantified a suite of historical landscape indicators depicting nitrate sources over several decades prior while also taking into account the direction of groundwater flow within the vicinity of each well.
While many studies use Landsat imagery to derive land cover, I used high spatial resolution historical aerial imagery in order to map fine scale features mechanistically linked to nitrate loading, such as hedgerows and raspberries. I found that contemporary nitrate concentrations, as well as long-term trends in nitrate, were much better explained by historical than contemporary landscape indicators. However, using cross-border and historical data for such purposes is not without limitations.
One limitation, the lack of consistent transboundary data sources, is not merely a problem distinct to the Greater ASA region, but is a global challenge. Often, too little attention is given to the quality and content of data Verburg et al, yet is critical to recognize that different sources of data will have different strengths and weaknesses for any particular application.
Comparison and integration of different data sources is often hampered by a myriad of issues including: This is challenging to address as it is often impossible to successful resample and recreate most datasets. To address this limitation in my own study, however, I was able to utilize a long-term nitrate dataset collected and overseen by one agency. Though the sampling frequency changed throughout the course of monitoring, I subsampled the data to bolster consistency of comparisons made over time.
Spatial consistency and scaling bias: To address this issue, I compared among spatial datasets of similar spatial resolution e. And where data sets were not of the same scale I reclassified datasets and harmonized data. Thematic differences and inconsistencies in data sources: Thematic differences and inconsistencies are a non-trivial issue in harmonizing cross-border geospatial datasets. Aggregating several hundred incongruous classes from two different jurisdictions to just a few classes presents an array of challenges.
In order to minimize thematic differences, I used ArcGIS to aggregate, reclassify, and thus harmonize imagery to the specific classes needed for the scope of my work. Despite significant attention to 91 detail in this process, some classes may be inconsistent because of uncertainties in the original classification schemes. Furthermore, I reclassified datasets and grouped classes, in some cases from several hundreds to less than ten, thus increasing map accuracy Olofsson et al Differences between land cover and land use: The relationship between land cover and land use is one of the major challenges for monitoring, modelling, and communicating land change Comber , Verburg et al To address this, I created my own landscape indicators which incorporated land cover, land use, and specific land use practices relevant to my specific environmental questions of interest.
In addition to challenges of transboundary land cover data, the use of historical land cover data adds another set of challenges including confirming accuracy of the images being used. One way to confirm accuracy of digitized historical aerial images is to compare the images to other historical maps. Another method of ensuring accuracy of data, is to discuss land use and landscape changes with older and long-time residence within the study area. Finding additional historical images and interviewing long-term residents can be a difficult and time consuming process, however, this can greatly improve confidence in the datasets being used Aronoff Future scenario planning A major challenge for agriculture in the 21st century is producing adequate amounts of food while simultaneously protecting environmental quality along with the health and livelihoods of rural communities Sutton et al This challenge is especially apparent throughout the ASA region, the undisputed raspberry capital of North America.
Additionally, the region has now also started producing blueberries over the last 20 years. As described in previous chapters, agriculture in the ASA has undergone a deep process of change in recent decades. Reforms to government policies, the volatility of food prices, and the emergence of new driving forces will continue to shape the future of farming activities in the region. Decades after widespread nitrate contamination was first identified in the aquifer, elevated nitrate concentrations remains a persistent trans-boundary water quality concern in the USA and Canada.
Envisioning a positive future for agriculture and water quality requires innovative thinking. While the data used in my dissertation was limited in scope to historical and contemporary time periods, additional research should develop future scenarios as a way to explore even more creative options for managing agricultural-based sources of nitrogen. Scenario planning could help to open this discussion and engage local farmers in envisioning and considering plans for positive futures in the region. Scenarios are extremely well-suited to exploring situations where uncertainty is high and controllability is low Peterson et al Scenarios can also help policy makers decide what needs to happen today in order to achieve goals in an uncertain future.
Therefore, creating scenarios in which these rates stay the same, double, or decline can help to predict the consequences of such LULC changes on groundwater quality and help formulate adequate responses and land use planning strategies. Consideration of several contrasting future scenarios can help policy-makers design plans that seek options that are robust to many futures. Thus, linking scenarios to groundwater models can facilitate transition processes Jarchow et al When authors co-submit and publish a data article in Data in Brief , it appears on ScienceDirect linked to the original research article in this journal.
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