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Wildfires significantly alter hydrological and biogeochemical processes, impacting downstream water quality and posing risks to ecosystems and human communities. Following the 2022 Hermit’s Peak-Calf Canyon (HPCC) wildfire in New Mexico, the largest wildfire recorded in the state of New Mexico, we deployed high-resolution in-situ sensors at three locations along a > 160 km fluvial network to investigate event-scale solute transport dynamics and their environmental drivers. Our objective was to evaluate how post-fire runoff events influenced water quality behavior across spatial (headwaters to mid- and high-order streams) and temporal (event to seasonal) gradients. We found that acute water quality impacts were most severe near the burn area, where turbidity reached ~8,500 FNU and dissolved oxygen fell below regulatory thresholds. These extremes, largely missed by traditional discrete sampling, were strongly driven by storm event size and seasonal variability. In contrast, farther downstream, solute export behavior was better predicted by longer-term indicators such as time since the fire and vegetation recovery metrics. Our analysis reveals distinct spatial shifts in concentration-discharge behavior that depend on the water quality parameter type, event features, and site position in the watershed. These findings highlight the need for longitudinal, high-frequency monitoring to detect and anticipate wildfire-induced water quality risks and inform more adaptive, spatially targeted watershed management strategies. 

The increasing severity and frequency of wildfires in forested watersheds pose significant challenges to water quality management. This study examines the impacts of the 2022 Hermit's Peak-Calf Canyon gigafire, the largest wildfire in New Mexico's history. The wildfire burned over 1,382 km², affecting a key watershed that supplies drinking water to Las Vegas, NM. We conducted a longitudinal assessment of post-fire water quality dynamics across a 170 km fluvial network, analyzing flow, water quality parameters, nutrient and metal concentrations, and mobilization patterns. We found that post-fire nutrient concentrations exceeded pre-fire medians by up to two orders of magnitude. Our analyses revealed solute-specific transport patterns that are difficult to predict with static watershed- or fire-specific characteristics (e.g., burned area and percent severities). ${\text{NH}}_4^{+}$, ${\text{PO}}_3^{-}$, and ${\text{NO}}_2^{-}$were closely and positively associated with discharge and turbidity near the burn perimeter, while ${\text{NO}}_3^{-}$and TON exhibited strong mobilization trends ~170 km downstream. In contrast to nutrients, calcium, magnesium, and manganese levels showed no significant pre- vs. post-fire shifts, while concentrations of trace metals like Cr³⁺, Pb²⁺, Zn²⁺, and Sr²⁺surpassed background levels and public health thresholds. Our findings emphasize the significant propagation of wildfire disturbances over hundreds of kilometers and suggest the need for integrated watershed management strategies, including the management of large-scale flood control mechanisms to mitigate the far-reaching impacts of water quality disturbances post-fire. 

The TIERRAS project is an open-access platform that compiles a database of more than 400 tracer injection experiments in rivers and streams, sourced from previously published studies and reports. It also includes interactive features that allow users to explore, download, and contribute new data. The goal is to provide a centralized and accessible repository for researchers, environmental managers, and anyone interested in water quality, hydrological modeling, and stream solute dynamics.   These experiments were collected from various sources, including published studies, unpublished data, and technical reports from different authors. The original data were in diverse formats and units; all data were curated and standardized to a consistent format and to the Imperial (U.S. customary) units.   Visit TIERRAS at https://www.tierras.org/ Cite: Rodríguez, L., Tunby, P., Abusang, A., Tartakovsky, A., Carroll, K., Ginn, T., & González-Pinzón, R. (2025). TIERRAS Tracer Injection Experiments in RiveRs And Streams (2.0) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.15794259 

Despite advances in wastewater treatment plant (WWTP) efficiencies, multiple contaminants of concern, such as microplastics, pharmaceuticals, and per- and poly-fluoroalkyl substances (PFAS) remain largely untreated near discharge points and can be highly concentrated before they are fully mixed within the receiving river. Environmental agencies enforce mixing zone permits for the temporary exceedance of water quality parameters beyond targeted control levels under the assumption that contaminants are well-mixed and diluted downstream of mixing lengths, which are typically quantified using empirical equations derived from one-dimensional transport models. Most of these equations were developed in the 1970s and have been assumed to be standard practice since then. However, their development and validation lacked the technological advances required to test them in the field and under changing flow conditions. While new monitoring techniques such as remote sensing and infrared imaging have been employed to visualize mixing lengths and test the validity of empirical equations, those methods cannot be easily repeated due to high costs or flight restrictions. We investigated the application of Lagrangian and Eulerian monitoring approaches to experimentally quantify mixing lengths downstream of a WWTP discharging into the Rio Grande near Albuquerque, New Mexico (USA). Our data spans river to WWTP discharges ranging between 2-22x, thus providing a unique dataset to test long-standing empirical equations in the field. Our results consistently show empirical equations could not describe our experimental mixing lengths. Specifically, while our experimental data revealed “bell-shaped” mixing lengths as a function of increasing river discharges, all empirical equations predicted monotonically increasing mixing lengths. Those mismatches between experimental and empirical mixing lengths are likely due to the existence of threshold processes defining mixing at different flow regimes, i.e., jet diffusion at low flows, the Coanda effect at intermediate flows, and turbulent mixing at higher flows, which are unaccounted for by the one-dimensional empirical formulas. Our results call for a review of the use of empirical mixing lengths in streams and rivers to avoid widespread exposures to emerging contaminants. 

Land cover changes alter hydrologic (e.g., infiltration-runoff), biochemical (e.g., nutrient loads), and ecological processes (e.g., stream metabolism). We quantified differences in aquatic ecosystem respiration in two contrasting stream reaches from a forested watershed in Colorado (1st-order reach) and an agricultural watershed in Iowa (3rd-order reach). We conducted two rounds of experiments in each of these reaches, featuring four sets of continuous injections of Cl as a conservative tracer, resazurin as a proxy for aerobic respiration, and one of the following nutrient treatments: (a) N, (b) N + C, (c) N + P, and (d) C + N + P. With those methods providing consistent information about solute transport, stream respiration, and nutrient processing at the same spatiotemporal scales, we sought to address: (1) Are respiration rates correlated with conservative transport metrics in forested or agricultural streams? and (2) Can short-term modifications of stoichiometric conditions (C:N:P ratios) override respiration patterns, or do long-term physicochemical conditions control those patterns? We found greater respiration in the reach located in the forested watershed but no correlations between respiration, discharge, and advective or transient storage timescales. All the experiments conducted in the agricultural stream featured a reaction-limited transformation of resazurin, suggesting the existence of nutrient or carbon limitations on respiration that our short-term nutrient treatments did not remove. In contrast, the forested stream was characterized by nearly balanced transformation and transient storage timescales. We also found that our short-lived nutrient treatments had minimal influence on the significantly different respiration patterns observed between reaches, which are most likely driven by the longer-term and highly contrasting ambient nutrient concentrations at each site. Our experimental results agree with large-scale analyses suggesting greater microbial respiration in headwater streams in the U.S. Western Mountains region than in second-to-third-order streams in the U.S. Temperate Plains region. 

The benthic biolayer is a shallow zone of reactive streambed sediments, widely believed to contribute disproportionately to whole‐stream reactions such as aerobic respiration and contaminant transformation. Quantifying the relative contribution of the biolayer to whole‐stream reactions remains challenging because it requires that hyporheic zone solute transport and reaction heterogeneity are explicitly captured within a single modeling framework. Here, we use field experiments and modeling to quantify the biolayer's aerobic reactivity relative to other stream compartments. We co‐injected and monitored several fluorescent tracers, including the reactive tracer resazurin, into a controlled experimental stream. We characterized reactive transport in the water column and at multiple depths in the hyporheic zone by fitting all data to a new mobile‐immobile model, using resazurin‐to‐resorufin conversion as an indicator of aerobic bioreactivity. Results show that the biolayer converted 8 times more resazurin to resorufin than all other stream compartments, and 80% of this conversion occurred within 2 reach advection times. This hotspot and hot moment behavior is attributed to the biolayer's ability to rapidly acquire, transiently retain, and rapidly degrade stream‐borne solutes. The model analysis shows that the majority of raz‐to‐rru conversion occurs in the biolayer across streams with a wide range of biolayer structural properties, including streams with a biolayer that is less reactive than deeper regions of the hyporheic zone. Together, our results show that the biolayer is a common feature of streams and rivers that should be considered in network‐scale models of aerobic reactivity.