Research papersA tracer-based method for classifying groundwater dependence in boreal headwater streams
Introduction
Headwater streams have a large effect on downstream hydrological and geochemical processes and ecological functions (Freeman et al., 2007, Finn et al., 2011). Groundwater (GW) is generally a major contributing factor to maintaining the baseflow of headwater streams (Sophocleous, 2002, Winter, 2007) and has specific geochemical, physical, and biological characteristics that differ from surface water (SW) (Bertrand et al., 2012). Therefore, any changes in GW discharge to headwater streams can have a major impact on stream water quality and volume, with the most pronounced effects occurring in areas that are highly influenced by GW. In order to reduce the impacts on groundwater-dependent ecosystems (GDEs), tools are needed for their classification, management, and protection (EC, 2006, Richardson et al., 2011, Rohde et al., 2017). However, GDE classification of headwater streams is not straightforward because standard procedures are lacking and the GW-SW transition zones and ecotones can vary temporally and seasonally.
Dynamic environmental factors govern the healthy functioning of freshwater ecosystems and are categorized as: flow patterns, sediment and organic matter inputs, temperature and light penetration, chemical and nutrient conditions, and plant and animal assemblages (Younger, 2006). In GW-dependent stream ecosystems, many organisms rely on conditions sustained by GW. These conditions are i) discharge volume, ii) stable thermal regime, and iii) water quality (Bertrand et al., 2012). During sensitive summer and winter low-flow periods in particular, GW input to headwater streams provides important refuges maintaining stable discharge (Younger, 2006) and thermal conditions in these streams (Dugdale et al., 2013, Snyder et al., 2015). As GW-dominated streams provide more stable conditions for stream ecosystems than SW-dominated streams (Webb et al., 2008), these ecosystems will become even more important in supporting thermal refuges in a changing climate. However, GW-dominated headwater streams can be sensitive to local anthropogenic actions such as agriculture, drainage for forestry, and GW abstraction (Ramchunder et al., 2012, Rossi et al., 2012, Rossi et al., 2014, Saarinen et al., 2013, Eskelinen et al., 2016), which can lower GW levels, alter GW discharge patterns to headwater streams, and change water quality and the ecology of streams (Poff and Zimmerman, 2010). This emphasizes the need to classify these ecosystems, in order to better protect and manage them and the connected GW systems.
Use of environmental tracers, such as stable water isotopes and water chemistry, is an efficient and flexible method to study dynamic and spatially varying GW-SW interactions in water courses and streams (Leibundgut et al., 2009, Bertrand et al., 2014). However, past tracer approaches have often focused on only one location in a stream (Kendall and Coplen, 2001, Litt et al., 2015, Soulsby et al., 2015, Niinikoski et al., 2016) or on intensive sampling for a rather short period (Klaus and McDonnell, 2013). Thus, these studies cannot give a full spatial and temporal picture of GW dependence in the stream continuum. Recent studies highlight the importance of spatially dense sampling to determine stream water chemistry (Zimmer et al., 2013) and stable water isotopes (Singh et al., 2016). The specific geochemistry of different landscape units in the catchment can alter the chemical composition of stream water (Blumstock et al., 2015). A study by Zimmer et al. (2013) suggests that GW contributions from distinct soil types control the spatial similarities found in stream water chemistry in varying flow conditions. The catchment structures (i.e. catchment “forms”, the hydrogeological setup of the catchment) also cause small-scale differences in baseflow stable water isotopes (Singh et al., 2016). In general, variations in stream water quality can result from changes in mixing proportions of GW and SW, and spatial or temporal changes in water quality, which can complicate interpretation of tracer data (Kirchner, 2016a, Kirchner, 2016b).
Recent tracer modelling approaches have focused particularly on studying the water age distributions in catchments (e.g., Birkel and Soulsby, 2015, Soulsby et al., 2015, Huijgevoort et al., 2016, Ala-aho et al., 2017). These modeling efforts are increasingly being supported by continuous measurement of isotope data (Tweed et al., 2016), which has improved estimates of young water fractions (Stockinger et al., 2016) and thus also estimates of GW fractions in streams. However, at this point high-resolution data cannot be obtained cost-effectively from several locations along the stream, which would be necessary for assessing spatial variations in GW dependence. Although low-resolution isotope data have uncertainties when estimating water fractions during large events (Soulsby et al., 2015), it can be sufficient to assess GW dependence during low flows and small events, which account for the majority of the hydrological year and sustain the ecological functions of streams. Thus, many tracer-based studies discuss GW-SW interactions in streams (e.g., Hagedorn and Whittier, 2015, Scholl et al., 2015, Duvert et al., 2016) but none has proposed a generalized method for water management purposes that could be used to evaluate the GW dependence in headwater streams.
In this study, we combined continuous measurements of discharge, temperature (T), and electrical conductivity (EC) with use of stable water isotopes and other environmental tracers to study the GW-SW transition zones in three boreal streams known to have high proportions of GW from esker aquifers. The esker aquifer systems studied are of a type commonly found in areas covered by the last glaciation and are of great importance in the Northern hemisphere as a source of potable water (Kløve et al., 2017). Typically, headwater streams originate from esker boundaries (esker-peatland interaction). We expected to find a transition zone within which a spring-originated stream turns into a SW-dominated system. By definition, SW is the water in surface storage units (e.g., lakes, streams, rivers, wetlands) and GW is the water underground. However, since our aim was to study the transition and mixing between these in streams, we applied the term SW to water that is already modified by surface processes, i.e., river water or water originating from surrounding peatlands. Our specific objectives were to determine the GW dependence along the stream continuum, study the spatial and temporal changes in mixing zones in the three streams, and develop a tool and guidelines for the use of different environmental tracers in evaluating the GW dependence of streams. This work addresses the following research questions: i) How can headwater stream sections be classified to GDEs? ii) What measurements should be made, and from where, to obtain a sound GDE classification for headwater streams? iii) Is it possible to evaluate the GW dependence of a stream based on a single sampling campaign?
Section snippets
Study sites
The three streams selected for the study are located in Rokua and Viinivaara esker aquifer areas and are 60 km from each other. Eskers are one of the most common aquifer types in deglaciation areas in Finland, Sweden, and Eastern Canada (Britschgi et al., 2009, Nadeau et al., 2015, Berthot et al., 2016, Holmlund et al., 2016, Quillet et al., 2017). The study sites belong to a mid-boreal coniferous forest belt. The Rokua study area represents disturbed ecosystems and Viinivaara contains nearly
Stream discharge and its origin
The discharge in the three streams studied varied temporally and spatially, with a generally increasing trend downstream (Fig. 2, Fig. 3). Baseflow was highest for Siirasoja stream (approximately 15 L s−1 km−2), considerably higher than for Lohioja stream (7.5 L s−1 km−2). The base flow in Mesioja was approximately 1.3 L s−1 km−2, which was only 9% of that in Siirasoja. Siirasoja and Lohioja streams responded similarly to precipitation events and with a similar timing and magnitude of the
The role of groundwater in boreal streams
Groundwater plays an important role in boreal headwater systems as the main source of base flow, providing unique biodiversity due to its cool and stable temperature and supply of good-quality water (Ilmonen et al., 2012, Jyväsjärvi et al., 2015, Lehosmaa et al., 2017). Classification of streams and aquifers is needed to protect these systems and manage water abstraction and land use activities that influence the GW source (Laini et al., 2012, Nadeau et al., 2015, Rossi et al., 2015) or the
Conclusions
Spatial and temporal aspects of the groundwater dependence of three boreal headwater streams were studied using environmental tracers and a novel tool for classification and management of groundwater-dependent stream ecosystems was developed. Our stream tracer index method combines the ecohydrologically important characteristics dominated by groundwater, namely groundwater volume in streams, thermal properties of streams, and stream water quality. As water quality tracers are site-specific,
Declaration of Competing Interest
None.
Acknowledgements
This study was funded by the Academy of Finland (project no. 128377) and Renlund foundation. We thank technicians and trainees at the Water Resources and Environmental Engineering Research Unit for helping with the field measurements. The numerical data on which this study was based are available from the corresponding author.
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