Concentration versus streamflow trends of major ions and tritium in headwater streams as indicators of changing water stores

Abstract

Documenting the processes that control the variations in stream geochemistry at different streamflows is important for being able to use chemical tracers to understand catchment functioning. The concentrations of most solutes, including reactive cations (e.g., Na, Ca, K, and Mg) and anions that are primarily derived from precipitation (Cl and Br), in five headwater streams from southeast Australia vary little with streamflow and are close to being chemostatic. By contrast, NO 3 and SO 4 concentrations are higher at high streamflows. There is also a systematic increase of 3 H activities from as low as 1.1 to as high as 2.6 TU with increasing streamflow. The changes in geochemistry cannot be explained solely by increased mineral dissolution at high streamflows or enhanced baseflow driven by hydraulic loading. They are best explained by an increased baseflow input augmented by water mobilized from shallower stores as the catchments wet up. The mean transit times of the water sustaining streamflow varies from 35 to 70 years at low streamflows to <7 years at high streamflows. The use of a range of geochemical tracers, including radioactive isotopes, allows the different possible causes of chemostatic behaviour to be assessed and improves our understanding of catchment functioning. K E Y W O R D Schemostasis, headwaters, mean transit times, river chemistry, tritium | INTRODUCTIONVariations in solute concentrations with streamflow (discharge) in rivers and streams contain vital information on residence times, processes of solute generation, biogeochemical processes, and sources of stream water; thus, documenting the controls on solute geochemistry is vital to understanding catchment functioning (Ameli et al.

Concentration–discharge relationships vary among hydrological events, reflecting differences in event characteristics

“…We estimated both the 2-year cQ behavior (i.e., using all recession data from the snow-free periods in 2017 and 2018) and the individual event-scale cQ behavior by fitting powerlaw relationships between concentration c and discharge Q to the data (Clow and Drever, 1996;Musolff et al, 2015):…”

“…Because cQ relationships can vary between solutes and catchments, they are frequently employed as descriptors for catchment hydrological behavior. The cQ relationships obtained from long-term, low-frequency data are particularly useful for characterizing the average behavior of a catchment (Clow and Drever, 1996;Godsey et al, 2009;Godsey and Kirchner, 2014). However, these long-term cQ relationships provide limited insight into the coupling of streamwater chemistry and discharge on shorter timescales, such as during hydrologic events.…”

The variation and controls of mean transit times in Australian headwater catchments

“…The increase in 3 H activities is potentially caused by the inflow of recent rainfall or the mobilization of shallower water stores (e.g., soil water or perched groundwater from higher in the catchment). The observation that, in most catchments, the 3 H activities of the stream water at the highest streamflows remains below that of rainfall (Figure 4) precludes the simple addition of recent rainfall to water from longer‐lived stores in the catchment (Cartwright et al, 2020). That conclusion is consistent with the observation that the concentrations of the major ions are also significantly higher than rainfall (Cartwright, Atkinson, et al, 2018; Hofmann et al, 2018; Howcroft et al, 2018).…”

“…Duvert et al (2016) interpreted variations in δ 18 O values over high flow events in a catchment with 3 H activities significant below those of modern rainfall as reflecting mixing between water with MTTs of a few weeks and baseflow that had MTTs of a few decades. Elsewhere, however, there is little variability of δ 18 O values in weekly to monthly samples of headwater streams (Cartwright et al, 2020; Hofmann et al, 2018), implying little young water was present at the sampling times. Time‐series of 3 H and stable isotopes in Luxembourg catchments yield similar estimates of time‐variant MTTs via StorAge Selection Functions (Rodriguez et al, 2019).…”

“…The degree of weathering, as well as the interconnectivity and transmissivity of fractures in basement rocks, controls hydraulic conductivities and groundwater velocities (Lachassagne, Wyns, & Dewandel, 2011). Whether stores of water with different residence times (e.g., soil water, water from the regolith, or water in perched aquifer systems) are successively mobilized as the catchment wets up following rainfall also influences MTTs (Cartwright, Morgenstern, & Hofmann, 2020; Hrachowitz et al, 2013; Hrachowitz, Soulsby, Tetzlaff, Dawson, & Malcolm, 2009).…”