advances in canadian research coupling hydrology and water quality, 2003-2007

Canadian Water Resources Journal Vol. 34(2): 187–194 (2009) © 2009 Canadian Water Resources AssociationRevue canadienne des ressources hydriques

Brian A. Branfireun1 and Merrin L. Macrae2

1 Department of Geography, University of Toronto at Mississauga, Mississauga, ON L6L 3Z12 Department of Geography, University of Waterloo, Waterloo, ON N2L 3G1

Submitted September 2007; accepted February 2008. Written comments on this paper will be accepted until December 2009.

Advances in Canadian Research Coupling Hydrology and

Water Quality, 2003-2007

Brian A. Branfireun and Merrin L. Macrae

Abstract: Canada is a country of considerable fresh water resources, with a mature community of hydrologists whose research is concerned with the transport and fate of nutrients and other pollutants that impact water quality, either from a human use or ecosystem perspective. This work has led to important advances in understanding that have contributed to regulatory action with sometimes global impact. However, water quality issues persist, largely due to increased natural resource extraction, land-use change, climate change and the identification of emerging contaminants of concern. This paper highlights important advances by Canadian researchers coupling hydrology and water quality over the period 2003-2007.

Résumé : Le Canada est un pays dont les ressources en eau douce sont considérables et qui dispose d’une collectivité mûre d’hydrologues qui mènent des recherches sur le transport et sur le sort des matières nutritives et polluantes influant sur la qualité de l’eau, tant du point de vue de la présence humaine que de l’écosystème. Ces travaux ont mené à d’importants progrès, au chapitre de la compréhension, qui ont contribué à des mesures réglementaires ayant parfois une incidence mondiale. Toutefois, les problèmes de qualité de l’eau persistent, en grande partie en raison de l’extraction accrue des ressources naturelles, d’un changement d’affectation des terres, du changement climatique et de l’identification de nouveaux contaminants suspects. Le présent article souligne les importants progrès accomplis par les chercheurs du Canada en matière de couplage entre l’hydrologie et la qualité de l’eau au cours de la période allant de 2003 à 2007.

Introduction

Nearly 10% of the landmass of Canada is covered by fresh water; close to 10% of that total comprises the Canadian portions of the Great Lakes, shared with the United States of America. Although approximately 60% of Canada’s freshwater drains to the Arctic

watershed, 85% of the country’s population lives along the southern border with the USA; the majority in the region of the lower Great Lakes (Erie and Ontario). Unregulated land-use conversion, and municipal and industrial discharges throughout the 20th century, led to profound and highly visible water quality degradation episodes (e.g., eutrophication due to excess

188 Canadian Water Resources Journal/Revue canadienne des ressources hydriques

© 2009 Canadian Water Resources Association

phosphorus; Nicholls et al., 2001; Schindler, 2006) and contamination of fish and piscivorous birds with metals and persistent organic pollutants (Shear, 2006). Extensive monitoring programs and regulations to control the release of priority chemicals and nutrients from critical discharge points emerged in the latter part of the century.

This monitoring of fresh water quality in the Great Lakes and other Canadian inland waters has led to important scientific breakthroughs (e.g., Schindler, 1987), and subsequent regulatory actions controlling the release of problematic nutrients and chemicals into the freshwater environment. Although the regulation of largely point-source discharges has resulted in regional improvements in water quality, increasingly intensive extraction of natural resources (e.g., forest harvesting; Westbrook and Devito, 2004), continued poor regulation of agricultural practices (Macrae et al., 2007b) and climate change (Eimers et al., 2003) are contributing to the degradation of surface water quality across the country. Emerging contaminants of concern, such as new persistent organics, pharmaceuticals and endocrine-disrupting compounds, further complicate this picture. Given their direct impact on human health, they may lend renewed support to the monitoring and regulation model for water quality management.

For many nutrients and priority pollutants, understanding controls on water quality requires that society move beyond the simple management of sources and consider the numerous processes that govern the delivery, transformation and transport of nutrients and chemicals of concern. Importantly, many nutrients and other chemicals are dispersed widely over the landscape, either by historical or contemporary atmospheric deposition (e.g., NO3

-; SO42-; Hg), or by anthropogenic

activities such as agricultural application of P and N. To further complicate this issue, perturbations to either the natural or built environment by human influence can produce unpredictable changes in the fate and transport processes affecting solutes of concern.

There is a broadly emerging awareness of the need to couple terrestrial and aquatic compartments to understand the controls on water quality exerted by complex landscapes at a range of spatial and temporal scales (Grimm et al., 2003). Within the discipline of hydrology, the controls exerted by hydrology on biogeochemical processing and chemical transport have been conceptually recognized for some time.

Process-oriented research coupling hydrological and biogeochemical processes has emerged as a unique focus in the Canadian hydrological community. The extraordinary challenge of linking the physical and hydrological characteristics of the catchment to the delivery of non-point source nutrients and other contaminants requires such an approach.

This paper provides a discussion of recent progress in research in Canada over the past four years (2003-2007) that couples hydrology and water quality. Although this topical area has not been separately addressed in previous reviews of advances in Canadian hydrology, aspects of water quality have been touched upon in reviews on wetland (Price et al., 2005) and forest (Buttle et al., 2005) systems. This overview concentrates exclusively on recent Canadian work relevant to water quality issues that is based on a process-oriented hydrological framework and is intended to shed light on works or topical areas that have made important breakthroughs or are defining new research trajectories. This is not an exhaustive review and is in no way intended to be a reflection of the full depth and breadth of research on hydrology and water quality in Canada. Engineering, geotechnical, limnological and toxicological approaches to water quality management, remediation, and environmental (biogeo)chemistry that are not explicitly coupled to water flows in the landscape are considered beyond the scope of this discussion, but are by no means to be considered less important.

With these caveats in place, some of the most novel and groundbreaking research efforts in recent years have focused on: 1) hydrological and biogeochemical interactions between uplands (whole catchment; hillslope) and lowlands (riparian zones; wetlands), and 2) the implications of environmental change (land-use; climate) on hydrological and biogeochemical processes that govern water quality. Much of this work has in turn focused on nutrient cycling, including N, P, and C, and other biologically-significant chemicals including SO4

2- and Hg.

Coupling Catchment Hydrology and Water Quality

The current state of knowledge concerning hydrological controls on the flushing of labile nutrients (NO3

-, DON

Branfireun and Macrae 189


and DOC) at the hillslope and headwater catchment scale is equivocal, largely because of frequently contradictory field evidence (Weiler and McDonnell, 2006). In an effort to look beyond the complexities of individual hillslopes and focus on the first-order controls, Weiler and McDonnell (2006) explored the problem in a modelling framework, based on an initial numerical hydrological modelling exercise driven by “collective field intelligence” (Weiler and McDonnell, 2004). These hydrological “virtual experiments” led to the testing of nutrient flushing hypotheses in subsequent work. They suggest that this approach can lead to greater progress in understanding the fundamental controls on nutrient mobilization and transport processes at the hillslope scale than attempting to reconcile the range of outcomes from field studies, scale from the plot to hillslope, and/or incorporate site-specific ecotonal boundaries. They conclude that antecedent wetness and slope geometry exert significant control over the flushing of DOC and N at the hillslope scale. Importantly, the outcomes of the virtual experiments also showed that, when present, a riparian zone was the primary source of mobilized nutrients and these zones are the key to understanding nutrient dynamics at the catchment scale. This contention is conceptually parallel with the work of Gergel (2005) who also argues that there is a considerable spatial sensitivity in nutrient loadings as a function of the arrangement of sources and sinks in the landscape. Richardson et al. (2007) explore this sensitivity and demonstrate that the spatial patterns of discrete “hot spots” of nutrient reduction are governed by first (global) and second

(local) order variations in subsurface topography which influence hydrologic residence time in sediments of varying thickness and depth, with variable saturated hydraulic conductivity. Although their results are consistent with many empirical observations, Weiler and McDonnell (2006) acknowledge many factors that may change the observed behaviour, including soil depth, riparian zone width, and most importantly denitrification, which can have a profound impact on the source or sink function of the riparian zone, but was not included in their analysis.

Recent research undertaken in Canada has clearly demonstrated that denitrification in riparian zones is a first-order process governing the supply of nitrate to surface waters in a range of hydrological settings and cannot be ignored. Through the direct investigation of

an impressive array of riparian zones across a continuum of hydrogeologic settings in southern Ontario, Vidon and Hill (2004a) is the first study to examine in detail the interacting effect of upland aquifer size, topography and lithology on riparian hydrology. They suggest that their classification of riparian hydrologic types can be used in most glacial till and outwash landscapes to predict the relative degree of hydrologic connectivity between upland and riparian zone. In subsequent work at the same field locations, Vidon and Hill (2004b) found that denitrification was the primary mechanism for nitrate removal from groundwater in riparian zones at all sites across the continuum of hydrogeologic settings investigated. Topography, permeable sediment depth and sediment texture were found to influence the linkages between groundwater flow paths and the supplies of electron donors and acceptors that affect the location of highest denitrifying activity. Work by Mitchell and Branfireun (2005) supports this finding through their discussion of the important linkages between hydrological flowpaths and reduction-oxidation reactions.

Considerable insights can be made when knowledge about hydrological and biogeochemical processes are merged, even at the conceptual level. Combining the hydrological properties of a range of riparian zone types with information on denitrification processes, Vidon and Hill (2006) propose a conceptual framework for riparian zone hydrological functioning and nitrate removal efficiency that provides guidance on riparian zone width requirements for 90% nitrate removal. The conceptual model was supported by independent data from a range of riparian sites elsewhere in the world.

Whereas topography was identified as a first-order control on solute transport by the work discussed above, Devito et al. (2005), working in the western Boreal Plain, suggest that topographically-based hydrologic response units may not be applicable in many landscapes, indicating that geologic and climatic controls must be considered, possibly even ahead of topography. Although this work does not specifically address water quality, it has important implications for solute transport in that it challenges our common conceptual model that considers topography the default first-order control on water and solute transfer.

The hydrology community in Canada is generating fundamental scientific insights into the coupling of watershed hydrological and biogeochemical processes.



It is important to note that many of these new insights are then directed at practical and applied research questions to improve the scientific understanding of the effects of natural and anthropogenic changes to the landscape and climate on water quality.

Implications of Environmental Change on Water Quality Canadian hydrologists have made significant progress evaluating the impacts of land use and climate change on the hydrological controls on nutrient fate and transport. With respect to land use, particular attention has been paid to two areas that are of greatest concern to the national socioeconomic landscape: forestry practices and intensive agriculture.

Significant work has recently emerged evaluating the impacts of forest disturbance (clearcutting, fire) on the transport and fate of nutrients (Chanasyk et al., 2003; Foster et al., 2005; Macrae et al., 2005; 2006; Prepas et al., 2003; Smith et al., 2003) and metals such as mercury (Kelly et al., 2006) in surface and subsurface water. It is clear that management practices, regional climate and subsurface geology can modify the impacts of forest harvesting on water quality, and that factors controlling water quality differ in eastern and western sections of the boreal forest. Increases in phosphorus loading to surface water following fires and clearcutting (Chanasyk et al., 2003; Prepas et al., 2003) have been observed in the Western Boreal Plain where soils are rich in phosphorus. In contrast, Macrae et al. (2005; 2006) found that clearcutting had little effect on soil moisture and soil and ground water nutrient pools in the Western Boreal Plain. In more eastern sections of the boreal forest, Westbrook et al. (2006) did not observe any difference in the mobility of dissolved organic nitrogen following forest harvesting. Westbrook et al. (2006) and Westbrook and Devito (2004) did, however, observe large differences in nitrogen transformations between clearcut and forested areas, but suggested there was a low potential for N export to receiving surface waters because of the large capacity for nitrogen consumption in the system. Collectively, these works illustrate the complexity of the systems under study and highlight the need to better understand coupled hydrological and biogeochemical process in these landscapes.

Work has also emerged that examines the impacts of different forest management practices on water quality. Foster et al. (2005) observed that partial cutting was superior to clear-cutting for improving water quality in the Turkey Lakes Watershed in central Ontario. Nitschke (2005) found that forest harvesting and wildfire variably affected the water chemistry of headwater systems, where nutrient and ion concentrations in runoff differed both in magnitude and direction of change following these disturbances.

In agricultural settings, work has been done on the identification of nutrient sources, soil and land management practices, and the effectiveness of riparian zones and constructed wetlands in attenuating nutrients in agricultural runoff. Surface waters in Ontario, Quebec and eastern Canada continue to receive nutrient loads via both diffuse and point sources. Fallow et al. (2007) explored leaching losses in winter and developed a predictive model that provides insight into the environmental factors influencing manure and nutrient application timing. The importance of fertilizer application and season also extends to nutrient loading via drainage tiles, which remain a significant source of nutrients to surface waters in many areas of Canada, even though their impact is recognized internationally. Beauchemin et al. (2003) linked phosphorus loading from drainage tiles in Quebec, Canada to soil phosphorus levels, field management practices and the hydrology of sites. In Ontario, Macrae et al. (2007b) demonstrated strong spatial and temporal variability in tile phosphorus export where most of the annual phosphorus loads from tiles were supplied during short periods of the year, and the largest phosphorus loads originated from tiles under fields receiving manure inputs.

Complementary to the studies of land management practices noted above are studies on the efficiency of landscape units to remove nutrient loads from agricultural runoff. Gottschall et al. (2007) found constructed wetlands were effective at removing nitrogen from agricultural (dairy) wastewater in Ontario. Vidon and Hill (2004b) found that riparian zones effectively removed nitrate from subsurface runoff in many environments in southern Ontario. Kasahara and Hill (2006) explored the effect of constructed riffles and a step on hyporheic exchange flow and chemistry in nitrogen-rich agricultural and urban streams in southern Ontario, and found that they induced hyphoreic exchange and resulted in a nitrogen sink. While each of the above



studies is an important contribution to the scientific literature, collectively they demonstrate that Canadians have been actively working towards improving the management of agricultural landscapes. However, there appears to be a strong regional bias toward the central, temperate part of the country with respect to research in this topical area.

Climate change (temperature and precipitation) has the potential to dramatically affect our ability to predict the hydrological and biogeochemical processes that control water quality in Canada. Not only absolute changes in average temperature and precipitation are important, but the timing and magnitude of rainfall events and changes in antecedent hydrological conditions. This is particularly true for temperate and northern regions where winter snowfall comprises a large portion of the annual water balance. Recent work has emphasized the role of climate-driven hydrological variability (i.e., wet versus dry periods, storm versus base-flow) and season (i.e., snowmelt period) as major controls on the temporal variability in nutrient fate and transport of various chemicals in the environment (Macrae et al., 2007a; Quilbe et al., 2006). In a first order agricultural catchment in Ontario, up to 42% of annual total phosphorus, 61% of annual soluble reactive phosphorus, and 33% of annual nitrate export was associated with only one or two large hydrologic events in a given year (Macrae et al., 2007a).

The role of antecedent conditions in temporal patterns in nutrient transport and fate has also been explored. Recent work has shown that nutrient concentrations in overland flow (Langlois and Mehuys, 2003) and stream discharge (Macrae et al., 2007a) in both agricultural landscapes and the Canadian Shield (Turgeon and Courchesne, 2007) differ under wet and dry conditions. Schiff et al. (2005) observed pulses of SO4

2- from a Canadian Shield wetland following drought. The significance of wet-dry cycles and hydrologic connectivity is an emerging focus in the Canadian and international literature. Eimers et al. (2003) explored the effects of drying and re-wetting as well as increased temperature on SO4

2- release and found that re-wetting of dried peat soils released three to four times more SO4

2- than continuously moist peat. This work builds on previous work by Devito and Hill (1999) that also showed pulses of SO4

2-

upon re-wetting of wetland soils. In an agricultural setting, Macrae et al. (2007a) demonstrated that NO3

-

export was greater upon rewetting following drought

periods. Strack et al. (2007) observed elevated DOC production in experimental and drained peatlands due at least partially to larger water table fluctuations than natural sites. Collectively these works suggest that the coupling of hydrologic change with biogeochemical processes is essential to understanding the impacts of a changing climate on water quality.

Summary

Canadian hydrologic science is making important advances in coupling hydrological and biogeochemical processes at scales that are relevant to landscape management now and in the future. With a particular focus on understanding first-order controls, an awareness of the need to address key issues surrounding transferability of findings among landscapes, and an overarching process-oriented approach, Canadian hydrologists are providing critical information that will help address the vast majority of water quality issues facing the nation and the world. Progress in this area will accelerate with improved communication among the water disciplines (hydrology, limnology, aquatic ecology and toxicology) focussed on the common goal of improving the quality of our water resources.

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