Modulation of Riverine Concentration‐Discharge Relationships by Changes in the Shape of the Water Transit Time Distribution

Torres, Mark A.; Baronas, J. Jotautas

doi:10.1029/2020gb006694

Cited by 20 publications

(16 citation statements)

References 113 publications

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“…Low VC stratifies the flows via different depths, leading to distinct chemistry in older waters in deeper zones and in younger waters in shallow soils (Jin et al., 2014; Richardson et al., 2020; Sullivan et al., 2016). Large shallow and deep water contrast and non‐chemostatic (dilution and flushing) patterns in these hillslopes suggest that chemical heterogeneity is required for non‐chemostatic patterns, which is similarly observed in other studies (Torres & Baronas, 2021). This may appear surprising but align with reactive transport principles: Concentration gradients do develop even in idealized, homogeneous systems, as water continues to interact with minerals and dissolve out solutes along its flow paths, although mineral heterogeneity may intensify concentration gradients.…”

Section: Discussionsupporting

confidence: 83%

“…In large hillslopes with long flow paths, long MTTs can also drive weathering to equilibrium such that the control of VC diminishes. The Damköhler coefficient that explains CQ relationships of geogenic solutes (Ibarra et al., 2016; Maher & Chamberlain, 2014; Torres & Baronas, 2021; Wymore et al., 2017) indicate that long transit times lead to chemical equilibrium and chemostatic patterns. It implies that the dependence of weathering rates on VC and MTT will weaken as spatial scales increase.…”

Section: Discussionmentioning

confidence: 99%

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Vertical Connectivity Regulates Water Transit Time and Chemical Weathering at the Hillslope Scale

Xiao

Brantley

2021

Water Resources Research

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How does hillslope structure (e.g., hillslope shape and permeability variation) regulate its hydro‐geochemical functioning (flow paths, solute export, chemical weathering)? Numerical reactive transport experiments and particle tracking were used to answer this question. Results underscore the first‐order control of permeability variations (with depth) on vertical connectivity (VC), defined as the fraction of water flowing into streams from below the soil zone. Where permeability decreases sharply and VC is low, >95% of water flows through the top 6 m of the subsurface, barely interacting with reactive rock at depth. High VC also elongates mean transit times (MTTs) and weathering rates. VC however is less of an influence under arid climates where long transit times drive weathering to equilibrium. The results lead to three working hypotheses that can be further tested. H1: The permeability variations with depth influence MTTs of stream water more strongly than hillslope shapes; hillslope shapes instead influence the younger fraction of stream water more. H2: High VC arising from high permeability at depths enhances weathering by promoting deeper water penetration and water‐rock interactions; the influence of VC weakens under arid climates and larger hillslopes with longer MTTs. H3: VC regulates chemical contrasts between shallow and deep waters (Cratio) and solute export patterns encapsulated in the power law slope b of concentration‐discharge (CQ) relationships. Higher VC leads to similar shallow versus deep water chemistry (Cratio ∼1) and more chemostatic CQ patterns. Although supporting data already exist, these hypotheses can be further tested with carefully designed, co‐located modeling and measurements of soil, rock, and waters. Broadly, the importance of hillslope subsurface structure (e.g., permeability variation) indicate it is essential in regulating earth surface hydrogeochemical response to changing climate and human activities.

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Section: Discussionsupporting

confidence: 83%

Section: Discussionmentioning

confidence: 99%

Vertical Connectivity Regulates Water Transit Time and Chemical Weathering at the Hillslope Scale

Xiao

Brantley

2021

Water Resources Research

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“…Previous studies have reported non‐monotonic behavior for Si (Torres & Baronas, 2021), sediments and nutrients (Underwood et al., 2017), and a variety of other solutes such as metals (Moatar et al., 2017; Zhi et al., 2019). The concentrations of DOC and NO 3 at the two study sites here decrease with depth and exhibit flushing CQ relationships.…”

Section: Discussionmentioning

confidence: 97%

“…The deeper subsurface contains reactive minerals such as carbonate (W‐9) as well as silicate and pyrite‐containing shale (Hafren). In addition, groundwater is typically much older than shallow water, which allows for longer reaction time with parent rocks (Maher, 2011; Torres & Baronas, 2021). In other words, prolonged exposure to parent rocks and longer transit times in groundwater give rise to higher concentrations of weathering‐derived solutes at depth and dilution CQ relationships.…”

Section: Discussionmentioning

confidence: 99%

Streams as Mirrors: Reading Subsurface Water Chemistry From Stream Chemistry

Stewart

Shanley

Kirchner

et al. 2022

Water Resources Research

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The shallow and deep hypothesis suggests that stream concentration‐discharge (CQ) relationships are shaped by distinct source waters from different depths. Under this hypothesis, baseflows are typically dominated by groundwater and mostly reflect groundwater chemistry, whereas high flows are typically dominated by shallow soil water and mostly reflect soil water chemistry. Aspects of this hypothesis draw on applications like end member mixing analyses and hydrograph separation, yet direct data support for the hypothesis remains scarce. This work tests the shallow and deep hypothesis using co‐located measurements of soil water, groundwater, and streamwater chemistry at two intensively monitored sites, the W‐9 catchment at Sleepers River (Vermont, United States) and the Hafren catchment at Plynlimon (Wales). At both sites, depth profiles of subsurface water chemistry and stream CQ relationships for the 10 solutes analyzed are broadly consistent with the hypothesis. Solutes that are more abundant at depth (e.g., calcium) exhibit dilution patterns (concentration decreases with increasing discharge). Conversely, solutes enriched in shallow soils (e.g., nitrate) generally exhibit flushing patterns (concentration increases with increasing discharge). The hypothesis may hold broadly true for catchments that share such biogeochemical stratifications in the subsurface. Soil water and groundwater chemistries were estimated from high‐ and low‐flow stream chemistries with average relative errors ranging from 24% to 82%. This indicates that streams mirror subsurface waters: stream chemistry can be used to infer scarcely measured subsurface water chemistry, especially where there are distinct shallow and deep end members.

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“…As we infer above, this is likely the result of the distinct secondary phases dominating Ge uptake in each environment (aSi under the glaciers vs. Fe oxyhydroxides or aluminosilicate clays during the experiment and in natural nonglacial environments), which are in turn controlled by the distinct weathering regimes, and in particular the low temperatures, high rates of comminution, and relatively short water residence times in the subglacial environment. While the role that water transit times play in the dynamics of chemical weathering is poorly understood, it has very important implications for our understanding of the relationship between weathering and climate (e.g., Maher and Chamberlain, 2014;Li, 2019;Torres and Baronas, 2021). Therefore, the possibility that dissolved δ 74 Ge might be primarily controlled by rock-water interaction time and may help understand the kinetics of weathering is an intriguing prospect.…”

Section: Implications For Ge Isotope Behavior During Subglacial and Nmentioning

confidence: 99%

Ge/Si and Ge Isotope Fractionation During Glacial and Non-glacial Weathering: Field and Experimental Data From West Greenland

et al. 2021

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Glacial environments offer the opportunity to study the incipient stages of chemical weathering due to the high availability of finely ground sediments, low water temperatures, and typically short rock-water interaction times. In this study we focused on the geochemical behavior of germanium (Ge) in west Greenland, both during subglacial weathering by investigating glacier-fed streams, as well as during a batch reactor experiment by allowing water-sediment interaction for up to 2 years in the laboratory. Sampled in late August 2014, glacial stream Ge and Si concentrations were low, ranging between 12–55 pmol/L and 7–33 µmol/L, respectively (Ge/Si = 0.9–2.2 µmol/mol, similar to parent rock). As reported previously, the dissolved stable Ge isotope ratio (δ74Ge) of the Watson River was 0.86 ± 0.24‰, the lowest among global rivers and streams measured to date. This value was only slightly heavier than the suspended load (0.48 ± 0.23‰), which is likely representative of the bulk parent rock composition. Despite limited Ge/Si and δ74GeGe fractionation, both Ge and Si appear depleted relative to Na during subglacial weathering, which we interpret as the relatively congruent uptake of both phases by amorphous silica (aSi). Continued sediment-water interaction over 470–785 days in the lab produced a large increase in dissolved Si concentrations (up to 130–230 µmol/L), a much smaller increase in dissolved Ge (up to ∼70 pmol/L), resulting in a Ge/Si decrease (to 0.4–0.5 µmol/mol) and a significant increase in δ74Ge (to 1.9–2.2‰). We argue that during the experiment, both Si and Ge are released by the dissolution of previously subglacially formed aSi, and Ge is then incorporated into secondary phases (likely adsorbed to Fe oxyhydroxides), with an associated Δ74Gesecondary−dissolved fractionation factor of −2.15 ± 0.46‰. In summary, we directly demonstrate Ge isotope fractionation during the dissolution-precipitation weathering reactions of natural sediments in the absence of biological Ge and Si uptake, and highlight the significant differences in Ge behavior during subglacial and non-glacial weathering.

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Modulation of Riverine Concentration‐Discharge Relationships by Changes in the Shape of the Water Transit Time Distribution

Cited by 20 publications

References 113 publications

Vertical Connectivity Regulates Water Transit Time and Chemical Weathering at the Hillslope Scale

Vertical Connectivity Regulates Water Transit Time and Chemical Weathering at the Hillslope Scale

Streams as Mirrors: Reading Subsurface Water Chemistry From Stream Chemistry

Ge/Si and Ge Isotope Fractionation During Glacial and Non-glacial Weathering: Field and Experimental Data From West Greenland

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