Chemostatic concentration–discharge behaviour observed in a headwater catchment underlain with discontinuous permafrost

Abstract

Concentration-discharge dynamics were evaluated in a small ($ 2.25 km 2 ) headwater catchment underlain with discontinuous permafrost on the Seward Peninsula of western Alaska. A large storm, during which 48 mm of rain fell over a 24-h period, enabled the evaluation of solute concentration-discharge response to a sizeable hydrological event, while water stable isotopes enabled an appraisal of the contributions of event water. Under normal catchment conditions, chemostatic behaviour was observed for solutes typically derived from mineral weathering (e.g. calcium, magnesium, sodium and silica). The chemostatic behaviour observed for most solutes under normal catchment conditions indicated that catchment storage and residence times are sufficiently long for many solute generating reactions to approach equilibrium. Following the storm however, most solutes exhibited dilutive and highly variable behaviour. This likely indicated the exceedance of a discharge threshold where chemostatic behaviour could no longer be maintained for most solutes. Dissolved organic carbon and silica were the only solutes monitored to exhibit chemostatic behaviour during all time periods.

Water‐rock interactions drive chemostasis

“…The relatively fast cation-exchange reactions, compared to timescales for clay precipitation, make Ca 2+ more susceptible to changes associated with discharge, which we propose as the cause of slightly lower, but still chemostatic, C-Q slopes of these divalent cations compared to Al 3+ and SiO 2(aq) . This weak chemostasis for divalent cations, relative to Al 3+ and SiO 2(aq) , has been observed in other catchments (Conroy et al, 2022;Godsey et al, 2009Godsey et al, , 2019Herndon et al, 2015;Kim et al, 2017) but is rarely attributed to cation exchange.…”

“…However, despite the large number of C-Q studies, few investigate how the geochemical processes that shape groundwater chemistry evolve along flowpaths before contributing to streamflow. For example, many C-Q studies have report chemostasis for both SiO 2(aq) and/or Al 3+ as well as still chemostatic but slightly more negative C-Q slopes for Ca 2+ and/or Mg 2+ for a variety of catchments (Conroy et al, 2022;Godsey et al, 2009Godsey et al, , 2019Herndon et al, 2015;Kim et al, 2017), highlighting the potential for geochemical reactions to impact individual solutes differently. Here, we investigate the coupled geochemical reactions that happen along groundwater flowpaths, including (1) primary mineral dissolution leading to (2) clay precipitation and (3) subsequent cation-exchange reactions.…”

Multi‐year high‐frequency sampling provides new runoff and biogeochemical insights in a discontinuous permafrost watershed

“…While the above conceptual model was developed and refined over several decades, there remains limited longer‐term multi‐year data sets that combine flows with water quality and DOC, particularly in permafrost‐underlain watersheds (Conroy et al, 2022; Gandois et al, 2021; Koch et al, 2021; Shogren et al, 2019). In addition, how catchment spatial structure influences these flow‐chemistry patterns is less clear, as riparian and hillslope contributions in cold environments have not been clearly defined.…”

“…This high dispersion can be introduced by multiple factors including: (1) source and transport limitations (Benettin et al, 2017); (2) in‐stream biogeochemical transformations disconnected from hydrological changes (Bieroza & Heathwaite, 2015; Moatar et al, 2017); and (3) long‐term and seasonal variations (Hirsch, 2014; Zhang et al, 2016). Monitoring of C–Q patterns over seasons and a range of flow conditions have been explored using high‐frequency methods across temperate to snow‐dominated catchments (Andrea et al, 2006; Bende‐Mischl, Verburg and Cresswell, 2013; Vaughan et al, 2017; Burns et al, 2019; Knapp et al, 2020), although implementation in northern environments is less common (Conroy et al, 2022; Gandois et al, 2021; Koch et al, 2021; Shogren et al, 2021). Increased resolution monitoring is particularly important in high latitude environments where seasonality combined with a heterogeneous soil profile exerts considerable control over stream solutes and flow volume.…”

Catchment concentration–discharge relationships across temporal scales: A review

“…A. Rose et al, 2018;Thompson et al, 2011) Pairing δD and δ 18 O with C-Q 1 2 years of 30-min continuous data from automated field laboratory (Knapp et al, 2020) Discrete 2 2 years of sub-weekly frequency automated grab samples (Conroy et al, 2022) 3 3 years of sub-monthly frequency grab samples (F. Liu et al, 2017) Spectral analysis 1 >20 years of daily data (Cheng et al, 2021) Cross-scale 2 5 years of hourly data (Heathwaite & Bieroza, 2021) 3 ≥2 years of 15-min data (Jiang et al, 2020;Wenng et al, 2021) Fractal scaling 1 2 years of 7-h data and long-term weekly data (Kirchner & Neal, 2013) Cross-scale 2 ≥2 years of 15-min data (Hansen & Singh, 2018;Jiang et al, 2020) 3 >3 years of daily grab sample data (Aubert et al, 2014) C-Q moving window 1 >5 years of daily data (Fazekas et al, 2021;Zimmer et al, 2019) Cross-scale 2 ≥2 years of 15-min data (Fazekas et al, 2020;Wymore et al, 2021) Dynamic time warping & clustering methods 1 1 year of daily concentrations averaged across ≥1 year(s) of data (Bolotin et al, 2023) Cross-scale 2 >3 years of subhourly sensor data (Javed et al, 2021) Note: "Discrete" refers to approaches focused on single time scales; we note that this does not mean these analyses cannot be done at multiple time scales and compared. Thus, comparisons across time scales using discrete methods are indirect.…”