Road salt applications mobilize trace elements from roadside soil to shallow groundwater.

In regions where deicers are applied to roadways, micronutrients and toxic trace elements may be mobilized from soil material into soil porewater. These elements may subsequently migrate with soil porewater to surface waters and groundwaters, potentially leaching the soil of micronutrients or introducing toxins to water resources. Our study thus aims to quantify the timing and extent of trace element releases from soil material to soil porewater and groundwater in response to deicing events. We sampled soil porewater near a road at a rural site for trace elements and compared the results to salt applications and soil porewater Na and Cl levels. We also assessed trace element, Na, and Cl concentrations in a karst spring at the rural site and a karst spring at an urban site to evaluate the role of land use in conveying these contaminants to groundwater. We found that certain trace elements (e.g., As, Ba, Fe, Sr) peaked concomitantly with Na and Cl in soil porewater at the rural site after road deicing events, suggesting their release due to excess salt inputs to the soil. We did not observe increases in trace element concentrations at the rural karst spring following individual road salt applications, likely due to low deicer inputs and trace element levels across its recharge basin. However, at the urban site, we observed that other assemblages of trace elements (e.g., As, Cu, Li) in the karst spring peaked with deicing-related Na and Cl pulses. We also found positive and significant correlations between salt applications to the recharge basin and exports of some trace elements (e.g., As, Cu, Li, Se) at the urban karst spring, indicating deicing events triggered trace element releases to groundwater. Overall, we detected road salt-driven trace element release from soil material to soil porewater and groundwater that was exacerbated by urbanization.

Cave sediment sequesters anthropogenic microparticles (including microplastics and modified cellulose) in subsurface environments.

Anthropogenic microparticles (of synthetic, semisynthetic, or modified natural compositions) are globally pervasive, yet little is known about their distribution and storage in the subsurface despite their potential threats to belowground environments. We therefore assessed their amounts and characteristics in water and sediment from a cave in the United States. During a flood, water and sediment samples were collected at 8 sites every ~25 m along the cave passageways. Both sample types were evaluated for anthropogenic microparticles, while water was assessed for geochemistry (e.g., inorganic species) and sediment was evaluated for particle sizes. Additional water samples were collected during low flow at the same sites for further geochemical analysis to determine water provenance. We found anthropogenic microparticles in all samples that were mainly fibers (91 %) and clear (59 %). Both suspected (identified visually) and confirmed (identified with Fourier transform infrared spectroscopy; FTIR) anthropogenic microparticle concentrations were positively correlated between the compartments (r ≥ 0.83, p ≤ 0.01), but quantities in sediment were ~100 times those in water. These findings indicate that sediment sequesters anthropogenic microparticle pollution in the cave. Microplastic concentrations were similar among all sediment samples, but only one water sample at the main entrance contained microplastics. Treated cellulosic microparticle abundances in both compartments generally increased along the cave stream's flowpath, which we suspect is due to both their flood and airborne deposition. Water geochemical and sediment particle size data collected at a branch indicated at least two distinct water sources to the cave. However, anthropogenic microparticle assemblages did not differ between these sites, implying minimal variation in sourcing across the recharge area. Our results reveal that anthropogenic microparticles intrude karst systems and are stored in sediment. Karstic sediment consequently represents a potential source of "legacy" pollution to the water resources and fragile habitats found in these globally distributed landscapes.

Floods enhance the abundance and diversity of anthropogenic microparticles (including microplastics and treated cellulose) transported through karst systems.

Microplastics (plastics <5 mm) are emerging contaminants that have been detected in virtually all environments. While microplastic research in terrestrial surface waters has been proliferating, microplastic contamination in subsurface environments remains understudied. Karst terrains may be particularly susceptible to microplastic pollution because the presence of large dissolution openings allows fast transport of water through these systems, facilitating the introduction of surface contaminants into subsurface habitats. Furthermore, few studies address the prevalence and movement of microparticles composed of semisynthetic and modified natural materials, despite their known ecotoxicity. Our study therefore aims to identify anthropogenic (i.e., synthetic, semisynthetic, and treated natural) microparticle extent, sourcing, and transport in subsurface karst environments. To do so, we examined a cave spring under variable flow conditions, finding that anthropogenic microparticles were present in all samples and were most frequently fibrous and clear. The mean anthropogenic microparticle concentration during baseflow was 9.2 counts/L but increased up to 81.3 counts/L during floods, indicating their enhanced mobilization when relatively dilute, acidic, and sediment-rich event water entered the cave. These results suggest that anthropogenic microparticles may originate from surface recharge or sediment resuspension within the cave. When we analyzed a subset of microparticles with Fourier transform infrared spectroscopy (FTIR), we found that cellulose of known (i.e., dyed) and suspected (i.e., clear) anthropogenic origin was the most abundant material type. We nevertheless confirmed the presence of microplastics in the cave stream under all flow conditions, with the most common polymer being polyethylene. Both the concentrations and relative fractions of microplastics were higher during floods compared to baseflow, indicating their increased transport during high flow events. We also observed that microplastic polymer types diversified as discharge increased. Our study gives new insight into how anthropogenic microparticle contamination is transported through karst landscapes that can help inform debris mitigation strategies to protect ecosystems and water resources.

Integrating land cover, point source pollution, and watershed hydrologic processes data to understand the distribution of microplastics in riverbed sediments.

Microplastics are emerging contaminants ubiquitously distributed in the environment, with rivers acting as their main mode of transport in surface freshwater systems. However, the relative importance of hydrologic processes and source-related variables for benthic microplastic distribution in river sediments is not well understood. We therefore sampled and characterized microplastics in river sediments across the Meramec River watershed (eastern Missouri, United States) and applied a hydrologic modeling approach to estimate the relative importance of river discharge, river sediment load, land cover, and point source pollution sites to understand how these environmental factors affect microplastic distribution in benthic sediments. We found that the best model for the Meramec River watershed includes both source-related variables (land cover and point sources) but excludes both hydrologic transport-related variables (discharge and sediment load). Prior work has drawn similar and dissimilar conclusions regarding the importance of anthropogenic versus hydrologic variables in microplastic distribution, though we acknowledge that comparisons are limited by methodological differences. Nevertheless, our findings highlight the complexity of microplastic pollution in freshwater systems. While generating a universal predictive model might be challenging to achieve, our study demonstrates the potential of using a modeling approach to determine the controlling factors for benthic microplastic distribution in fluvial systems.

Road salt retention and transport through vadose zone soils to shallow groundwater.

Increasing background salinity in watersheds has largely been attributed to road salt retention in groundwaters due to their long residence times. However, laboratory studies demonstrate that soils temporarily store salts, either in porewater or adsorbed onto particles. Field studies of road salt retention in soils are nevertheless rare, and mechanisms of salt transport across multiple hydrological reservoirs (e.g., from soil to groundwater) are unknown. Thus, we collected roadside soil porewater and karst spring water weekly for ~1.5 yr to determine salt transport through the vadose zone into the phreatic zone. We observed dual retention mechanisms of sodium (Na+) and chloride (Cl-) in soils due to slow porewater movement, causing ion movement through the soil as slow as 1.3 cm/day, and cation exchange processes, leading to initial Na+ retention followed by later release months after application. Cation exchange processes also caused base cation loss from exchange sites into mobile porewater. Rapid Na+ and Cl- delivery to groundwater occurred through karst conduits during the winter. However, elevated background levels of salt ions in groundwater during the non-salting months indicated accumulation in the catchment due to slower porewater flow in the soil and rock matrix and delayed Na+ release from soil exchange sites.