No abstract

Anthropogenic freshwater salinization is increasing with global change. Rising freshwater salinity threatens ecosystem biodiversity, health, and services via toxicity to organisms and mobilization of nutrients and metals. Brining roads is one major source of freshwater salinization that continues to grow with rising urbanization. While the detrimental effects of salinization in streams are well-documented, high-frequency, temporal patterns in salt transport, particularly during winter road de-icing in mixed land use landscapes, are less understood. To address this knowledge gap, we analyzed high-frequency specific conductance as a proxy for salinity across 114 high-flow events from 2013 to 2018 in an impaired stream draining mixed agriculture−urban land use. The specific conductance was highest in winter (median = 947 μS cm −1 ) and decreased with first-order kinetics up to 90 days after brining (β 1 = −0.003), suggesting lasting impacts of road de-icing on water quality. Although hysteresis patterns suggested a transition from distal to proximal salt sources, they showed no clear correlation of flushing versus dilution to brining events. While seasonal brining increased salinity in receiving streams, unpredictable transport dynamics reduced the efficacy of hysteresis in characterizing salt transport dynamics. Thus, the complexity of mixed land use watersheds requires more spatially and temporally explicit monitoring to characterize stream salinization dynamics.

As water moves carbon through watersheds and across ecosystem boundaries, stream corridors collect and integrate landscape-scale signals of carbon cycling. Streams metabolize organic carbon and emit carbon dioxide (CO 2 ) and methane (CH 4 ) derived from internal metabolism and external sources (Hotchkiss et al., 2015;Stanley et al., 2016). The rates at which streams emit carbon to the atmosphere reflects their importance in local and global carbon cycling. Streams emit CO 2 globally at a rate that surpasses the terrestrial and oceanic carbon sinks (Webb et al., 2018) and emit approximately 5% of yearly global CH 4 to the atmosphere (Flury & Ulseth, 2019;Stanley et al., 2016). As global concentrations of atmospheric greenhouse gases continue to rise, quantifying the relative contributions of different sources and sinks of CO 2 and CH 4 grows ever more important.

<p>Streams are dynamic ecosystems susceptible to frequent and long-term physical and chemical changes. Characterizing how solute concentrations change with hydrology is key to understanding solute sources, fate, and transport. Here we tested how solute concentrations respond to changes in flow in a stream draining a mixed land use catchment in Blacksburg, Virginia, USA. To do this, we measured how various solutes (i.e., DOC, DIN, Cl<sup>-</sup>, Na<sup>+</sup>, Mg<sup>+2</sup>, Ca<sup>+2</sup>, SO<sub>4</sub><sup>-2</sup>, K<sup>+</sup>) changed within and across one baseflow period of 24 hours and three high flow events during summer 2021. Solutes concentration relationship with flow dynamics can result in different responses: (1) enrichment (increase in concentration), (2) dilution (decrease in concentration), or (3) chemostasis (no change in concentration). We found that, overall, solutes responded to changes in flow and the patterns observed for each flow event were variable, resulting in both dilution and enrichment. Discharge (Q) ranged from 0.04 - 3.37 m<sup>3</sup>/s during our 8-week sampling period. Dissolved organic carbon (DOC) and dissolved inorganic nitrogen (DIN) concentrations ranged from 2 - 5.7 and 0.23 - 0.94 mg/L, respectively. While DOC exhibited enrichment with increasing Q, DIN, Cl<sup>-</sup>, Na<sup>+</sup>, Mg<sup>+2</sup>, Ca<sup>+2</sup>, SO<sub>4</sub><sup>-2</sup>, and K<sup>+</sup> were mainly diluted during higher flows. However, during baseflow conditions the relationship between Q and solute concentrations was more pronounced (R<sup>2 </sup>>0.30), particularly for DIN and SO<sub>4</sub><sup>-2 </sup>(dilution), and Cl<sup>-</sup> and Na<sup>+</sup> (enrichment). During higher flows, we did not see a general dilution or enrichment pattern for all solutes but there were solute-specific behaviors which were similar among sampling periods. The differences in Q-solute dynamics among the 4 sampling events supports the enhancement of hydrological connectivity and landscape influence during changes in flow and how it can contribute to changes in solute concentration. Additionally, Q-solute patterns observed highlight the importance of time and sampling frequency to develop a well characterization of solute dynamics during changes in flow. Ongoing work is focused on understanding the directionality and timing of responses to further inform changes in solute concentrations during different flow events. Analyses of solute-specific behavior, timing of peak concentrations, and directionality will broaden our understanding of solute chemical dynamics and the different factors that contribute to the variable responses that have been found.</p>