Nitrate Removal in Stream Riparian Zones: The Last Fifteen Years
Anthropogenic loadings of nitrate from agricultural fertilizer use and deforestation can result in levels deleterious to stream ecosystems and downstream receiving water bodies. Riparian zones represent perhaps our most effective management tool in mitigating these elevated stream nitrate levels. In many settings these interfaces between the terrestrial and the aquatic have been shown to efficiently remove elevated nitrate loadings through denitrification and/or plant uptake. However, it was realized early that some riparian zones are not effective nitrate removers, and the relative importance of plant uptake versus denitrification was unclear. The uncertainty that existed 15 years ago fueled a plethora of studies on nitrate removal in stream riparian zones. This talk will highlight the most important findings of this research over the last decade and a half. Notably, the detailed description of hydrological flowpaths into and through riparian zones to the downstream environment has gone a long way to explaining the ineffectiveness of some riparian zones. Furthermore, the use of 15-N isotope tracers and field and lab incubation studies have aided in quantifying the importance of the denitrification pathway of removal. Patterns of terminal electron donors and acceptors and the importance of the intersection of the nitrate-elevated water with a source of bioavailable organic carbon, including deeply buried carbon, were a series of key achievements. Somewhat surprisingly, it has been shown that hydrogeology/landscape setting has a greater control on nitrate removal that climate/geography, as indicated by studies in different hydrogeologic settings in southern Ontario and uniform settings across a climatic gradient in Europe. Finally, the integration and generalization of these findings to the watershed-regional scale has aided in the transfer of knowledge from the scientist to the manager.
Water Table Dynamics of a Rocky Mountain Riparian Area
Riparian areas in mountain valleys serve as collection points for local precipitation, hillslope runoff, deeper groundwater, and channel water. Little is known about how complex hydrological interactions among these water sources govern riparian water table dynamics, particularly on an event basis partly owing to a lack of high frequency spatial and temporal data. Herein I describe the magnitude and rate of change of groundwater storage in a 1.3 km2 Canadian Rocky Mountain peat riparian area. Weekly manual measurement of hydraulic heads in a network of 51 water table wells during the summers of 2006 and 2007 showed large temporal and spatial variations in well response. A near constant increase in the spatial heterogeneity of the water table was observed as the riparian area dried. Cluster analysis and principle components analysis were performed on these weekly data to objectively classify the riparian area into spatial response units. Results were classification of the standpipes into five distinct water table regimes. One well representing each water table regime was outfitted with a sensor in 2008 that measured hourly head, which was used to characterize temporal dynamics of water table response. In spring, snowmelt runoff combined with an ice lens 20-30 cm below the ground surface led to consistently high water tables throughout the riparian area. In summer, the water table fell throughout the riparian in response to declining hillslope inputs and increased evaporative demand, but rates of decline were highly variable among the water table regimes. Chloride concentrations suggest variability reflects differences in the degree to which the water table regimes are influenced by stream stage, hillslope inputs, and proximity to beaver dams. Water table regime responses to rain events were flashy, with dramatic rises and falls (up to 20 cm) in short periods of time (<30 h), suggesting the unsaturated soil was near saturation. The stream was considerably more responsive to the storm events in spring than summer, probably because the water table rose above the ground surface during spring storms which generated saturation overland flow. Results from this study have implications for riparian biogeochemical dynamics and export and plant community composition.
Stream-Aquifer Exchange of Water and Nitrogen Along a Beaver-Dammed Stream Draining a Rocky Mountain Valley
Dynamic exchange of water across the stream-riparian zone interface is important in increasing stream water transit time through basins and enhancing redox-sensitive biogeochemical reactions that influence downstream water quality and ecosystem health. Such exchange may be enhanced by beaver dams, which are common throughout lower-order streams in North America and Europe. We investigated lateral exchanges of water and nitrogen along a beaver dammed, second-order stream draining a ∼1.3 km2 Canadian Rocky Mountain peat valley bottom. Measurements of hydraulic heads and chloride concentrations from a network of 80 water table wells were used to identify areas of stream water and groundwater mixing in the riparian zone, and their spatiotemporal dynamics in summer 2008. Stream stage was found to be the greatest factor affecting lateral movement of channel water into the riparian zone. Channel water flowed laterally into the riparian area upstream of the dams and back to the channel downstream of the dams. Little stream-aquifer exchange was found where dams were not present except during an overbank flood. Nitrate and DON concentrations were similar across the riparian area (P>0.05), regardless of whether the water was classified as groundwater, stream water or mixed water. In contrast, ammonium and DOC concentrations were significantly higher in the wells classified as groundwater or mixed water than those classified as stream water. Potential mass flux calculations show the riparian area immediately downstream of the beaver dam was a source of ammonium and nitrate to the stream, and a sink along the rest of the reach. DON shows similar trends with the exception of a net potential influx immediately upstream of the beaver dam. This work will aid in the understanding of stream-aquifer exchange and nitrogen cycling in riparian areas, and the effects that beaver have on these processes.
Generalizing riparian hydrologic function in a heterogeneous landscape, Western Boreal Plain, Alberta, Canada.
The Western Boreal Plain (WBP) eco-region of western Canada is experiencing unprecedented development for forest, oil and gas resources stressing the need to assess the role and relative effectiveness of riparian areas in mitigating the impacts of land use changes on water quantity and quality. We compare findings from local scale transect studies across differing surficial geology at the Utikuma Region Study Area (URSA) to characterize the variability in hydrological and biogeochemical processes of riparian areas located on major landforms and landscape positions typical of the WBP. Within the study region, the recurring role of riparian areas on hydrological linkages from uplands to aquatic systems was removal of soil water and groundwater by vegetation and translocation of water to adjacent hillslopes regardless of groundwater function. Water table depressions at the base of hillslopes were commonly observed. The sub-humid climate and deep and heterogeneous surficial deposits result in minimal upland runoff and complex surface -groundwater interactions. Aquatic-riparian flow reversals and losing conditions (water table gradients from aquatic to upland regions) were common and perched stream, pond and wetland systems were observed in areas of contrasting soil texture. The type and seasonality of flow path and variability in riparian function were related to interactions between sub-humid climate, surficial geologic landforms (texture) and topographic position within these landforms. Riparian functions were highly variable in coarse textured outwash landforms and influenced by regional-scale flow system and seasonal freezing. Riparian interactions on fined grained lacustrine plain landscapes were largely restricted to near surface discharge and recharge flow through. In contrast, on poorly drained and mixed textured moraine landforms, riparian systems were often isolated or interacted with recharge or perched flow systems. Our findings not only point to the need to improve our understanding of how riparian functions vary with geologic landform and groundwater or river network location, but also highlight the importance of vegetation interactions in controlling the water budget. Integrating local and regional interactions have large implications in the development of conceptual frameworks and directing modeling efforts in assessing the mitigating role of riparian areas on land use practices in heterogeneous glacial landscapes.
Assessing the Importance of Hot and Cold Phenomena in Riparian Zones at the Watershed Scale
Many studies have investigated nutrient and to a lesser degree contaminant removal and transformations in riparian systems. These studies have encompassed a variety of hydrogeomorphic and climatic regions and represent a wide range of geographic locations. These studies have demonstrated significant spatial and seasonal variability in riparian zone hydrological and biogeochemical conditions. However, recent research has shown that when certain hydrological and biogeochemical conditions converge, riparian zone functioning can change episodically within seasons and over a variety of spatial scales. This presentation summarizes our current understanding of the variables controlling hot and cold moments in riparian zones, both in terms of transport and biogeochemical transformations. In particular, we discuss the hot/cold moment paradox, i.e. that a riparian zone can serve as both a hot spot for biogeochemical transformations and a cold spot for contaminant transport to streams or vice-versa. We also discuss potential trade-offs associated with riparian zone management and hot/cold moment perception. A case study of how spatial analysis can be used to estimate hot moments and spots of greenhouse gas production at the watershed scale is presented. We propose that the presence of cold spots and cold moments within riparian systems represents opportunities where riparian water quality function might be enhanced.
Nitrate Dynamics In Stream Riparian Zones: Some Gaps In Current Knowledge
Despite major advances in knowledge of riparian zone hydrology and biogeochemistry in the past 30 years, several aspects of nitrogen dynamics are not well understood. The role of riparian vegetation in nitrate removal during groundwater transport is unclear. Data from several riparian zones are used to illustrate how dendrochronology can provide an historical perspective of the response of riparian forests to large groundwater nitrate inputs. Although many riparian zones contain organic-rich deposits at depth, the potential for these sites to become nitrogen sources in response to water table fluctuations has not been studied. Analysis shows that a severe drought produced high rates of N mineralization extending to depths of 75 cm in an Ontario riparian zone and increased groundwater nitrate concentrations during a later water table rise. The presentation also reviews the need for continued monitoring of riparian sites to provide a better understanding of the long-term effects of high nitrate loading on nitrate removal effectiveness.