Solute concentrations in stream water vary with discharge in patterns that
record complex feedbacks between hydrologic and biogeochemical processes. In
a comparison of three shale-underlain headwater catchments located in
Pennsylvania, USA (the forested Shale Hills Critical Zone Observatory), and
Wales, UK (the peatland-dominated Upper Hafren and forest-dominated Upper
Hore catchments in the Plynlimon forest), dissimilar concentration–discharge (
Streams are regularly monitored to evaluate watershed geochemistry, ecosystem health, and suitability for human use. However, streams integrate hydrologic and biogeochemical processes over varied spatial and temporal scales, making it difficult to determine both the sources and flow paths of solutes. While many researchers examine short-term to long-term element variability in stream water, it has remained difficult to derive generalized models quantifying solute concentration–discharge behavior (Fisher et al., 2004; Sivapalan, 2005; Zimmer et al., 2012). Flow paths may dictate stream chemistry by controlling fluid residence times and chemical equilibration of flowing water with soil minerals within catchments (Maher, 2011). Therefore, it is necessary to understand how heterogeneous flow paths through distinct chemical sources within a catchment influence observed solute concentration patterns within streams.
When the discharge of a stream (
Behavior differences amongst individual solutes in the stream have been linked to variability in solute concentrations within a catchment; in other words, discrete zones of element mobilization within soils and sediments can lead to pulses of solute transport into a stream (McClain et al., 2003; Andrews et al., 2011). This effect is furthermore affected by changes in hydrologic connectivity, defined as the water-mediated transfer of constituents between water sources (Pringle, 2001), within a catchment during rainfall events. Stream chemistry can vary during storm events as dominant water inputs to the stream shift from groundwater and riparian zones during base flow to hillslope runoff at high flow as pore waters stored in upland soils become increasingly connected to the stream (McGlynn and McDonnell, 2003a). Throughout this paper, groundwater is defined as water that is stored in catchment soils and bedrock below the water table, and pore water is defined as water that is present in the pores of unsaturated soil in the vadose zone. Upland soils become hydrologically connected to the stream when soil layers become water-saturated, promoting downslope flow within the unsaturated zone. As a result, concentrations of solutes that are stored preferentially in the riparian zone, e.g., dissolved organic carbon (DOC) released from soil organic matter (SOM), peak in the stream prior to discharge or with rising discharge during storm events (McGlynn and McDonnell, 2003b; Hood et al., 2006). Variability in organic carbon dynamics across different landscape units can subsequently control metal export from headwater catchments and downstream hydrochemistry (Köhler et al., 2014).
Many previous studies examine single catchments and/or catchments that were
developed on multiple lithologies (e.g., Johnson et al., 1969; Krám et
al., 1997; Brown et al., 1999; Likens and Buso, 2006; Godsey et al., 2009),
making the interpretation of solute behaviors difficult at best. When
mono-lithologic catchments are compared, insights into other factors that
influence the response of stream chemistry to discharge (e.g., biota,
climatic) can be developed. To elucidate controls on stream chemistry not
primarily driven by lithology, we examined
Water chemistry was compared for three sites: (1) the Susquehanna Shale Hills Critical Zone Observatory (Shale Hills) in central Pennsylvania, USA, and (2) the Upper Hore and (3) Upper Hafren subcatchments in the Plynlimon experimental forest in Wales, UK (Fig. 1). The Shale Hills and Plynlimon forests are underlain almost exclusively by Fe-rich, organic-poor, Silurian-aged shale formations that are stratigraphically equivalent. Although these headwater catchments vary by size and location, their similar lithologies and extensive hydrogeochemical characterization (e.g., Kirby et al., 1991; Neal et al., 1997, 2011, 2013a, b; Shand et al., 2005a, b; Jin et al., 2010; Brantley et al., 2013a–j; Dere et al., 2013) allow for development of a unifying theory on factors controlling concentration–discharge behavior.
Map views of the Susquehanna Shale Hill Critical Zone Observatory (Shale Hills, PA, USA; left) and Plynlimon (Wales, UK; right) catchments. Symbols mark locations of precipitation (black square), stream water (red circle), pore water (black triangle), and groundwater (green circle) samplers. Shading delineates major landscape features that are organic rich or organic poor in each catchment: swale (brown) versus planar (light green) slopes at Shale Hills or peat (tan) versus forested (dark green) regions at Plynlimon. Notably, the most organic-rich soils are in lowlands in Shale Hills but uplands in Plynlimon; consequently, inputs from organic-rich soils dominate stream flow under low-flow conditions in Shale Hills but high-flow conditions in Plynlimon.
Shale Hills is an 8 ha forested headwater catchment nested within the larger
Susquehanna River basin in Pennsylvania, USA. Shale Hills contains primarily
Inceptisol soils developed from shale residuum or colluvium of the Rose Hill Formation, which is dominantly comprised of clay minerals and quartz
(Lin et al., 2006; Jin et al., 2010). Small areas of
Ultisols are present near the stream (Lin et al., 2006). The regional mean
annual temperature (MAT) is 10
Soil profile descriptions and associated soil organic carbon (SOC) (% wt) and dissolved organic carbon (DOC) (mM) averages.
Water samples from Shale Hills were collected approximately daily from the stream outlet (2008–2010) and biweekly from soil lysimeters (2006–2011) from March to early December each year (Table 2). Detailed methods and results of chemical analyses, including isotopic variation and concentrations of major ions and DOC, have been reported elsewhere (Jin et al., 2011; Andrews et al., 2011; Brantley et al., 2013a–j). Aluminum concentrations in the stream were consistently below detection limits; thus, Al data were not examined for Shale Hills. Daily discharge rates were estimated from continuous discharge measurements integrated over 10 min intervals from the stream weir at the catchment's outlet (Duffy, 2012). Soil water was collected from suction lysimeters installed in the soil at 10 cm depth increments from 10 to 50 cm depth in the south planar valley floor (SPVF) and from 10 to 90 cm depth in the south swale valley floor (SSVF). The groundwater was sampled from a 2.8 m deep well located 80 m upstream from the weir. Major cation (2000–2011; NADP, 2011) and trace element concentrations (Herndon et al., 2015) have been reported for precipitation samples collected from NADP sites PA-15 and PA-42. Vegetation chemistry was previously determined for green leaf and leaf litter samples collected throughout summer and autumn seasons, respectively, in the Shale Hills catchment (Herndon et al., 2015).
Element concentrations (
The Plynlimon forest is a 682 ha watershed located at the headwater of the
River Severn, 20 km from the west coast of Wales (Reynolds et al., 1997).
MAT is 7.2
We focus here on two adjacent headwater catchments within the Plynlimon watershed: the Upper Hore and the Upper Hafren (Fig. 1). The Upper Hore (162 ha) is predominantly forested with periodically saturated, organic-rich Stagnopodzol soils and uplands that are dominated by grass and saturated peat soils (Kirby et al., 1991). In contrast, the Upper Hafren (122 ha) is dominated by heath and peat soils, with waterlogged and organic-rich peaty gley soils located in riparian areas (Kirby et al., 1991). Generally, the main flow paths in both catchments are approximately orthogonal to the valley direction, with highly fractured shallow bedrock providing an important pathway and storage for water throughout the catchments, especially under base flow conditions (Haria and Shand, 2004; Shand et al., 2005a, b; Shand et al., 2007). Shallow and deeper groundwater appear to be poorly connected but some mixing does occur (Haria and Shand, 2004; Shand et al., 2005b; Shand et al., 2007). Flow in organic horizons, however, tends to be largely lateral rather than vertical, providing minimal water–rock interaction in peat-dominated portions of the catchment and increasing contribution to streams during high-flow conditions (Shand et al., 2009).
Stream chemistry data for the Upper Hore and Upper Hafren catchments were
collected throughout the year for all years between 1983 and 2005, and 1990
and 2010, respectively (Neal et al., 2013a, b). Due to extensive tree-cutting
in the Upper Hore in 2005, data collected from 2005 to 2010 were evaluated
separately to examine the influence of tree removal on
Solute concentrations (
To analyze stream chemistry under different flow regimes, stream water
discharge (
Linear regressions were fit to log-transformed
Log-
Molar ratios of major divalent (Ca : Mg) versus univalent
(K : Na) cations are plotted on the left and the molar ratios of Mn (mmol) to
Mg (mol) versus dissolved organic carbon (mmol L
To investigate sources of solutes mobilized to the stream, element ratios in the stream under different flow regimes were compared to element ratios in pore waters, precipitation, groundwater, and leaves (where available). Element ratios have been used in other studies to link river chemistry to end-member reservoirs (e.g., Gaillardet et al., 1999). Molar ratios of divalent cations (Ca : Mg) were compared to univalent cations (K : Na) in each reservoir to understand how elements exhibiting non-chemostasis (Ca, K) vary relative to chemostatic elements (Mg, Na). To further explore the association of certain non-chemostatic solutes with organic C, molar ratios of Mn (a non-chemostatic element) to Mg were compared to DOC concentrations. Average DOC concentrations were used to define soil waters as organic rich or organic poor, as discussed in Sect. 3.2. In Shale Hills, green leaves were used to represent the most organic-rich end-member since pore waters could not be sampled from the thin O horizon.
Na and Mg behaved near chemostatically in all catchments (Fig. 2; Table 4),
while Si and K were only chemostatic at Shale Hills. A subset of
non-chemostatic solutes exhibited similar trends to DOC; however, trends
were opposite between Shale Hills and Plynlimon. Specifically, when
Slopes of regression lines fit to
In the Shale Hills stream, higher concentrations of stream solutes were
observed during the dry summer season (June through September) relative to
the wetter spring and autumn (Fig. S2). While concentrations of the
chemostatic elements increased only slightly (
In the Upper Hore where trees were harvested, solute concentrations and
We examined the chemical composition of soil pore waters in order to
investigate sources of solutes to the stream. Pore waters in each catchment
were categorized into distinct chemical pools based on DOC concentrations
(Table 2): “organic-rich” waters were defined by average DOC > 1 mM,
while all other waters were “organic poor”. At Shale Hills, pore
waters collected from the A horizon (10 cm) of the swale (SSVF) were
organic rich (1.28
At Shale Hills, concentrations of the non-chemostatic solutes Mn, Fe, and Ca
showed evidence of DOC-related behavior. For example, Mn and Fe were
positively correlated with DOC across all pore waters (
Like Shale Hills, concentrations of the chemostatic elements Na and Mg were
spatially homogeneous in pore waters at Plynlimon amongst the different
soils (RSD
Element ratios in stream water under low-, moderate-, and high-flow regimes
were compared to element ratios in pore waters, precipitation, and
groundwater (Fig. 3). At Shale Hills, stream chemistry was most similar to
pore waters from organic-rich soils and green leaves at low flow and
approached values for pore waters from organic-poor soils at high flow.
Ratios of
In contrast to this behavior at Shale Hills, stream chemistries in the Upper
Hore and Upper Hafren catchments were most similar to organic-poor sources
(precipitation, groundwater) at low-flow and organic-rich sources (soil pore
waters) at high flow (Fig. 3c–f; Fig. S6). Values of
Finally, we explored how chemical heterogeneity in soil pore waters
influenced concentration–discharge relationships in the streams.
Specifically, we evaluated solute heterogeneity due to redistribution by
vegetation as the ratio of solute concentrations in “organic-rich” to
“organic-poor” pore waters. As previously defined, these pore waters were
collected from A versus B horizons at Shale Hills, and organic versus
mineral soils in the Upper Hafren and Upper Hore. The slope of the
concentration–discharge plot (
At Shale Hills, elements concentrated in the organic-rich pore waters were
diluted rapidly in the stream with increasing discharge, consistent with
increasing inputs of water from mineral soils as the planar hillslope soils
become saturated during storms (Qu and Duffy, 2007). This trend is
documented in Fig. 4a where the concentration ratios for organic-rich
versus organic-poor soil waters were negatively correlated with respect to
No significant correlation (
The degree of non-chemostatic behavior for a solute in
stream water, denoted by
Cross-site comparison of the Shale Hills and Plynlimon headwater catchments revealed that the behaviors of non-chemostatic solutes were controlled by the spatial variability of those elements in soil waters and the distribution of DOC. Conversely, chemostatic solutes were homogeneously distributed in pore waters across the catchments. In the following sections, we discuss how the landscape distribution of chemically distinct pools and the connectivity between organic-rich soils and the stream control how concentrations vary with discharge. We contend that the behavior of certain elements are non-chemostatic in these systems due to their association with organic matter. The distribution of soil organic matter across landscapes is in turn influenced by climate (e.g., SOM generally increases with increasing moisture and decreasing temperatures on large geographic scales) and geomorphology (e.g., organic matter accumulates in depressed areas such as swales on small geographic scales).
At first glance, it may appear contradictory that concentrations of non-chemostatic elements in the streams at Shale Hills and Plynlimon trend in opposite directions with increasing discharge; however, the discrepancy can be explained by differences in the distributions of organic-rich source waters in each system. Similar to bioactive elements identified by Stallard and Murphy (2013), we attribute non-chemostatic concentration–discharge behavior to changing water flow through organic-rich soil matrices; however, we also observe that organic-rich sources and flow paths vary between the catchments (Fig. 1).
At Shale Hills, meteoric water passes through the thin organic horizon and organic-rich A horizon (< 15 cm deep) and is transported along the horizon interfaces to the stream via preferential flow paths (Lin et al., 2006; Jin et al., 2011; Thomas et al., 2013). The stream receives water from organic-rich swales and surface soils during dry periods, and water inputs from organic-poor hillslope soils increase as the catchment saturates (Qu and Duffy, 2007; Andrews et al., 2011). Consequently, we observed that stream water chemistry was similar to organic-rich soil waters at low flow and organic-poor soil waters at high flow (Fig. 3). Solutes derived largely from organic-rich soils exhibited greater variability over different flow regimes due to their high spatial variability in soil pore water. Increasingly negative slopes for non-chemostatic elements at high discharge (Fig. 2b) may reflect the transition in hydrologic connectivity and hillslope inputs to the stream. Stream chemistry did not reflect inputs from groundwater during dry periods, consistent with a previous finding that the water table drops to > 2 m below the stream bed during late summer (Thomas et al., 2013).
In the grass-dominated Upper Hafren, which contains peat soils that
experience minimal water–rock interaction (Kirby et al., 1991),
concentrations of chemostatic elements in the soils never deviated far from
an average precipitation signal (Fig. 4). In contrast, concentrations of
non-chemostatic elements were not driven by precipitation, and we propose
that pore-water concentrations of these elements are regulated by
vegetation. During the drier growing season, certain non-chemostatic
elements may be depleted from soil pore water and accumulated in vegetation,
leading to lower concentrations in the stream. Indeed, it is
well established that seasonal uptake by vegetation regulates concentrations
of nutrient elements in stream water (e.g., Johnson et al., 1969; Vitousek,
1977; Mulholland, 1992). Warming and drying of the surface peat during this
time increases microbial decomposition, thereby increasing mobility of
elements that accumulate in vegetation by releasing them from storage in
organic matter (Kirby et al., 1991). According to this conceptual model,
once transpiration decreases and flow increases through the soil in autumn,
concentrations of these elements increase in the stream because (1)
transpiration is reduced and the soil water is no longer being depleted, (2)
the surface peat is flushed of elements that have accumulated, providing
elements in addition to precipitation. As observed at the Upper Hafren and
Upper Hore, concentrations of non-chemostatic elements begin to increase in
the stream as discharge increases following low flow in the summer (Figs. S3 and S4).
This effect may be especially prominent in the peat regions
since the grass vegetation decomposes annually with little above-ground
storage (i.e., peat is leaky with respect to nutrients), and anoxic
conditions limit complete conversion of SOM to CO
In the spruce-forested Upper Hore, long-term storage of nutrient elements in
above-ground biomass is expected to deplete soil pore waters of elements
without the flushing effect due to rapid turnover observed in the Upper
Hafren (Reynolds et al., 2000). Instead, the positive
concentration–discharge slopes in the Upper Hore result from flushing of
upland peat soils at high-flow conditions (Neal et al., 1990). These effects
can be observed by comparing pre- and post-harvest concentration–discharge
slopes in the Upper Hore. Tree harvest impacted stream concentrations and
Values of
Previous studies have hypothesized that hydrologic connectivity within landscapes (McGlynn and McDonnell, 2003a; Hood et al., 2006; Clow and Mast 2010) and/or interactions between soil moisture and mineral reactive surface area (Godsey et al., 2009; Clow and Mast 2010) can explain concentration–discharge relationships across multiple catchments. Our results contribute to the understanding of solute behavior by highlighting the importance of hydrologic connectivity across landscapes and at mineral surfaces. At both Shale Hills and Plynlimon, the distribution of soil organic matter and its hydrologic connection to the stream governed non-chemostatic concentration discharge behavior of several solutes (Ca, K, Mn, Fe and Al), a process similarly invoked to explain stream DOC behavior in storm events (McGlynn and McDonnell, 2003b). Our results highlight the need to include or enhance reactive transport modules in spatially distributed watershed-scale hydrologic models such as TOPMODEL (Beven and Kirkby, 1979), the Penn State Integrated Hydrologic Model (PIHM; Qu and Duffy, 2007), and the Regional Hydro-Ecological Simulation System (RHESSys; Band et al., 1991). Specifically, combining reactive transport modeling (RTM) with the ability of spatially distributed models to simulate soil moisture, temperature, and water fluxes at variable depths across geomorphic features (e.g., swales vs. planar slopes) will allow researchers to elucidate specific-flow water paths and transit times and better test drivers of chemostasis (cation exchange) and dynamics of mobile vs. immobile water. RT-Flux-PIHM (reactive transport flux - Penn State Integrated Hydrological Model) is one model under development (Duffy et al., 2014) that will provide this platform, but it is imperative to cross-compare outputs from various models in order to reach a consensus.
Stream concentrations for most major weathering elements (
The exchangeable cation pool is a likely source of chemostatic elements
during rain events (e.g., Clow and Mast, 2010). The cation exchange capacity
of soils along the planar hillslope at Shale Hills ranges from 35 to
71 meq kg
Although geochemically similar to Mg, K, and Na, the concentration–discharge pattern for Ca (Fig. 2) is non-chemostatic at Shale Hills. The mixing model (Fig. 3a) indicates leaves may be a primary source of Ca to the stream during low discharge. Indeed, these shallow soils are strongly leached of Ca (< 0.16 % wt; Jin et al., 2010), and organic matter may be a relatively large pool of Ca in this system. In contrast to Shale Hills where Ca trends are strongly influenced by organic matter, Ca at Plynlimon may be linked to groundwater, an effect most pronounced in the Upper Hore. Ratios of Ca : Mg trend towards organic-poor sources at low flow, likely due to inputs of Ca-rich groundwater during base flow that is diluted by increasing contribution from soil water at high flow. Although a lack of groundwater data from these two subcatchments limits our ability to directly assess inputs to the stream, groundwater collected from lower elevations in the Plynlimon forest are rich in Ca and Si (Neal et al., 1997).
Similar to Ca at Shale Hills, K limitation may drive its increased
association with organic matter at Plynlimon. Values of
A comparison of three shale-derived catchments, the Shale Hills CZO (Critical Zone Observatory) in central Pennsylvania, USA, and the Upper Hafren and Upper Hore catchments in the Plynlimon forest, Wales, UK, reveals that the concentration–discharge behaviors of elements are strongly impacted by the distribution of organic matter in soils and the hydrologic connectivity of these soils to the stream. At Shale Hills, stream water is derived from organic-rich swales at low flow and then from both swale and planar hillslopes with increasing flow. At Plynlimon, stream water is only dominated by water from organic-rich soils at high flow, and contributions from organic-rich upland soils increased following lower elevation tree harvest in the Upper Hore catchment. Solutes that are limiting nutrients or that are strongly retained by vegetation exhibit non-chemostatic behavior in the stream because they are released to the stream along with dissolved organic carbon. This non-chemostatic behavior is opposite between Plynlimon and Shale Hills due to the different landscape distribution of organic-rich soils. Due to minimal redistribution by vegetation, Na, Mg, and Si are equally concentrated in pore fluids for organic-rich and organic-poor soils, and concentrations of these elements in stream water remain relatively constant. From this, we conclude that the transport of elements associated with organic matter, termed previously as organomarker elements (Hausrath et al., 2009), is strongly controlled by the movement of dissolved organic carbon, leading to a distinct non-chemostatic behavior in stream waters that contrasts with the chemostatic behavior of major weathering elements. Stream chemistry in headwater catchments are variable largely because of the chemical heterogeneities in distribution of organic-rich soils in catchments and how those soils connect to the stream.
E. M. Herndon, P. L. Sullivan, and A. L. Dere analyzed the data. E. M. Herndon prepared the manuscript with contribution from all authors.
The authors thank Adam Wlostowski and two anonymous reviewers for helpful
comments that improved the manuscript. This work was facilitated by NSF
Critical Zone Observatory program grants to C. J. Duffy (EAR 07-25019) and S. L. Brantley (EAR
12-39285, EAR 13-31726). This research was conducted in Penn State's Stone
Valley Forest, which is supported and managed by the Penn State Forestland
Management Office in the College of Agricultural Sciences. Plynlimon data and
support were provided by the Natural Environment Research Council Centre for
Ecology and Hydrology (