Multi-scale surface-groundwater interactions: Processes and Implications
Site-based investigations of stream-subsurface interactions normally focus on a limited range of spatial scales - typically either very shallow subsurface flows in the hyporheic zone, or much larger scale surface- groundwater interactions - but subsurface flows are linked across this entire continuum. Broad, multi-scale surface-groundwater interactions produce complex patterns in porewater flows, and interfacial fluxes do not average in a simple fashion because of the competitive effects of flows induced at different scales. For example, reach-scale stream-groundwater interactions produce sequences of gaining and losing reaches that can either suppress or enhance local-scale hyporheic exchange. Many individual topographic features also produce long power-law tails in surface residence time distributions, and the duration of these tails is greatly extended by interactions over a wide range of spatial scales. Simultaneous sediment transport and landscape evolution further complicates the analysis of porewater flow dynamics in rivers. Finally, inhomogeneity in important biogeochemical processes, particularly microbial processes that are stimulated near the sediment- water interface, leads to a great degree of non-linearity in chemical transformation rates in stream channels. This high degree of complexity in fluvial systems requires that careful approaches be used to extend local observations of hyporheic exchange and associated nutrient, carbon, and contaminant transformations to larger spatial scales. It is important to recognize that conventional advection-dispersion models are not expected to apply, and instead anomalous transport models must be used. Unfortunately, no generally applicable model is available for stream-groundwater interactions at the present time. Alternative approaches for modeling conservative and reactive transport will be discussed, and a strategy articulated for coping with the complexity of coupled surface-subsurface dynamics in fluvial systems.
Separating In-Channel and Hyporheic Transient Storage Processes in River Networks - A Path Toward Improved Quantification of Stream-Groundwater Interactions
Slow moving, transient storage zones of streams and rivers have been identified as critical habitats and
locations of enhanced biogeochemical processing of stream and groundwater borne nutrients and pollutants.
Stream tracer experiments provide a useful approach to characterizing the transport and fate of dissolved
stream loads. However, the retardation of stream water and associated dissolved solutes is rarely discerned
between in-channel, surface transient storage zones (STS; e.g., eddies, sides of pools, etc.), and hyporheic
transient storage (HTS). These two storage zone types are likely to have very different conditions, with STS
having sun light and potentially abundant dissolved oxygen, and HTS being dark and with potentially strong
reducing conditions. In this presentation we will present the development and application of two versions of a
2-storage zone solute transport model that account for STS and HTS exchange separately. One incorporates a
competing storage zone design, the other a nested arrangement of storage zones. Comparing simulations of
both models, the nested-storage zone model indicates much longer storage residence times than the
competing storage zone model. Further, we have applied the competing model to several reaches along the
Ipswich River network, a 5th order coastal watershed in northern Massachusetts, USA. Our findings from the
Ipswich studies indicate that both the size of STS and HTS normalized to the main channel, and the mean
residence times in these zones increase with increasing stream size at baseflow conditions. The application
of these 2-storage zone approaches will enhance our ability to quantify hydrologic exchange processes and
their influence on zone-specific biogeochemical processes across stream networks.
The application of sampling corers and passive samplers for dissolved organic matter to assess groundwater surface water interactions in the hyporheic zone of low-order streams
Small streams can serve as model systems for understanding the interactions between groundwater and surface water. Comparatively, small streams with permanent and intermittent hydroperiods can effectively illustrate the differences in nutrient exchange across the sediment-water interface. For example, when a streambed is dry, characteristics of groundwater are easily distinguishable from surface waters. Once pulsed with surface water resulting from precipitation on the surrounding watershed, surface water and groundwater interact spatially and temporally. This may have unpredictable consequences for nutrient cycling, such as the amount and type of dissolved organic matter (DOM), and the populations of microbes that inhabit the interstices of sediments at the groundwater - surface water interface (also known as the hyporheic zone). Here we present a multidisciplinary approach to characterizing seasonal DOM dynamics in the hyporheic zone of two small streams in southern Ontario. We selected two streams with contrasting hydroperiods (one permanent, one intermittent), and installed a network of steel sampling corers 1-metre into the subsurface. Between 2006 and 2008, we placed passive samplers to collect DOM and autoclaved sediment for microbial colonization at specified depths in the corers. We collected and replaced the samplers and sediment on a monthly basis. Isolated and extracted DOM was analyzed using nuclear magnetic resonance (NMR) spectroscopy and a series of molecular-fingerprinting techniques were used to characterize microbial community composition. We show how molecular structures of DOM varied at different depths in the hyporheic zone, and across several seasons. This agreed with evidence that sediment microbial communities shifted predictably throughout the seasons, among sites and with depth. Our multiple-site characterization provides useful insights into the biogeochemical interactivity across the sediment-water interface. We conclude that regular field sampling of the hyporheic zone using sampling corers, combined with multi-disciplinary molecular techniques, are extremely valuable in understanding seasonal biogeochemical dynamics in small streams.
Changes in Hydraulic Gradient, Hyporheic Exchange, and Patterns of Nutrient Concentration between Dry and Wet Season Flows for a Tropical Mountain Stream
Mountain streams are a common source in Central America for community water supplies (CWS). These streams become dewatered by the CWS during dry season low flows, with potential impacts on hydraulic gradients, hyporheic exchange flow, terrestrial-aquatic linkages, and nutrient dynamics, which may ultimately affect aquatic and riparian micro-ecosystems. We are presenting preliminary results of a study conducted in Buena Vista, a village in Yoro, Honduras where the mountain stream was instrumented and manipulated to measure impacts of a CWS. Piezometric head and stream water levels were taken at 7 cross-sections along 30 m of step-pool stream, and water quality samples were retrieved from 48 pairs of riparian and stream piezometers and monitoring wells. We computed vertical hydraulic gradients, zones of hyporheic upwelling and downwelling, and nutrient patterns, and their change with streamflow. Streamflow ranged from 30 L/s in the wet-season high flow to about 2 L/s in the dry-season low flow, and were dewatered to about 1 L/s. A HEC- RAS water-surface profile model was calibrated to observed stages to establish gradients along the entire reach, and river head was then input as a boundary condition into a MODFLOW groundwater model to examine patterns of hyporheic exchange. Changes in hydraulic gradients and fluxes are compared with baseline conditions during the dry season low flow without dewatering. Noticeable changes in hydraulic gradient occurred between high and low flows, but changes in low flow to dewatered flow were negligible. Lengths and location of hyporheic upwelling and downwelling zones shifted slightly with changes in flow, but again the dewatering had a minor impact. Concentrations of nitrate, sulfate, chloride, fluoride and dissolved oxygen were detected in the hyporheic zone, the stream water, and adjacent ground water. We are exploring mixing models to assess the extent to which hyporheic exchange migrated to and from the creek to adjacent riparian zones.
Inputs of Dissolved Oxygen to the Grand River in the Glen Morris Area: The role of groundwater and riparian zones
Low levels of dissolved oxygen (O2) have been reported in middle and lower reaches of the Grand River in Southern Ontario, threatening water quality for human consumption and the survival of sensitive species, including fish. Some sections of the Grand River are fed by groundwater discharge from the regional aquifer and riparian zones are characteristic features in these areas. This study aims to evaluate the role of groundwater discharge on dissolved oxygen (O2) cycling in the River. The research is part of a comprehensive long term study examining the O2 cycle in the Grand River, a river which is heavily impacted by urban and agriculture activities. Sampling of domestic wells and springs allowed characterization of the water quality of the bedrock aquifer and its connection with the riparian zone and the river sediment. Piezometers and mini-profilers installed in the riparian zone and river sediments were sampled for O2, NO3, DOC, δ18O- O2 and other indicators of hydraulic and redox conditions, with the purpose of assessing processes that control O2 concentrations in the riparian zone, the riparian/river and at the river sediments. The hydrogeological and chemical data reveal that seasonal fluctuations of the regional aquifer and flooding events in the riparian zone are main factors controlling the O2 and fluctuations in other chemical species in the riparian zone. The O2 and δ18O- O2 patterns indicate the O2 is consumed in some parts of the riparian zone by redox processes such as oxidation of DOC along the groundwater flow system. The effect of the riparian zone on the O2 in the river is more clearly observed in the near shore areas. The distinct inverse correlations between δ18O-O2 and O2 concentration observed in the river sediments show the processes that control the O2 in the river sediments are different than the ones in the riparian zones. O2 and δ18O- O2 patterns could be a reflection of groundwater discharge to the river without interaction with the riparian zone. The other possibility is that during downwelling events, the O2 in the river affects the O2 in the river sediments. A third possibility is that the patterns result from a combination of both. This study illustrates that the combined use of hydrogeological, geochemical and isotopic tools can provide valuable information for evaluating the interaction between regional aquifers, riparian zones and surface water in areas of groundwater discharge and its effect in the O2 cycle in the river.
Water Fluxes Across the Interfaces of Perched Wetland Basins on the Boreal Plain
Perched pond-wetland systems appear to be common on many heterogeneous glacial deposits of the Boreal Plain in Alberta. These systems provide important habitat for Boreal plant and animal communities, and may be vital sources of water for adjacent upland vegetation. The study site is situated on heterogeneous glacial deposits with subdued topography within a sub-humid climate, and is characterized by numerous wetlands surrounded by aspen uplands. The unsaturated nature of the uplands results in laterally discontinuous perched water tables and ponds, and ubiquitous interfaces between local and regional groundwater, surface- water, upland and atmosphere regimes. Detailed hydrometric data were collected, over two hydrologic years, along transects from both a shallow pond and a peatland to their adjacent uplands. These data, combined with simulations using a fully coupled numerical model (HydroGeoSphere), were used to quantify fluxes across the interfaces and changes in storage within different hydrologic compartments. Perched conditions result from the presence of a laterally extensive, low-permeability confining layer underlying coarser-grained (i.e., silt and sand) deposits of variable thickness. The depth to the confining layer in conjunction with atmospheric fluxes controls the spatial distribution of soil-water storage potential, and thus the magnitude and spatial distribution of water fluxes. There is little hydrologic connectivity between adjacent wetlands or between wetlands and the regional groundwater flow systems. Consequently, vertical water fluxes dominate due to the sub-humid climate, available soil-water storage, and isolation from regional groundwater systems.