Stroud Water Research Center

ABSTRACT Controls of hydrology, landscape and soil characteristics on redox potentials, particularly at fine spatial and temporal scales, are poorly quantified and understood. Using high‐frequency sensors (30 min), the variability of redox potentials (Eh) in riparian soils was investigated for multiple depths and across diel, event and seasonal time scales over a period of 2 years. Sampling was performed for two riparian sites upstream of dams with contrasting soil textures. Incremental changes in soil moisture triggered abrupt and disproportionately large shifts in soil Eh values, a threshold‐like behaviour that has not been reported before. Contrary to expectations, Eh values initially decreased as the soil dried and then abruptly increased with further drying and oxidation of soil. Diel Eh patterns differed between sites, with the sand‐rich site oxidising by day and reducing at night, while the clay‐ and silt‐rich site showed the opposite pattern. These differences were attributed to differential oxygen diffusion and consumption by processes such as photosynthesis, evapotranspiration and microbial respiration. Sharp Eh shifts also occurred at soil depths with textural discontinuities. These redox hot spots and hot moments were attributed to differences in site topography, hydrology and soil characteristics such as texture, moisture, organic matter content and microbial biomass. Identifying and understanding these fine‐scale spatial and temporal patterns of Eh is critical for improved characterisation and quantification of biogeochemical processes in redox‐sensitive environments. This study underscores the value of high‐frequency redox sensors in making such assessments.

Abstract Freshwater urban wetlands are important ecosystems that can naturally filter and remove excess nitrogen (N) through the process of denitrification (DNF). However, anthropogenic inputs such as road salt application may affect the N removal capacity of urban wetlands by affecting the relative rates of DNF and another competing reductive process that retains N – dissimilatory nitrate reduction to ammonium (DNRA). Here, we assessed 13 roadside wetlands in urban/suburban areas of Delaware, USA to determine the effects of road salt sodium (Na+) on soil physical, chemical, and biological properties and the rates of DNF and DNRA. Based on soil Na+ concentrations, wetlands were grouped into three categories: low (Na+ < 70 mg kg− 1), medium (70 mg kg− 1 < Na+ < 150 mg kg− 1) and high (Na+ >150 mg kg− 1). Rates of DNF and DNRA ranged from 0.8–83 and 0.2–24 µg N L− 1 slurry h− 1, respectively. DNF was significantly lower in high Na+ category wetlands whereas DNRA did not show any significant differences. Similarly, macroaggregates and bioavailable Fe were lowest in the high Na+ category, whereas concentrations of soil NH4+, NO3−, TOC, TN, and microbial metrics (biomass and nosZ and nrfA functional genes) did not reveal any consistent patterns. These findings imply that road salt Na+ input exhibited mixed effects on soil properties in these wetlands. Overall, elevated Na+ from road salt could undermine the N removal capacity of the roadside urban wetlands. Therefore, strategies should be implemented to reduce the application of road salt or identify effective alternatives.

Abstract The seasonal controls of hydrology, temperature, hypoxia, and biogeochemical conditions for groundwater ammonium–N (NH4+) concentrations are not well understood. Here we investigated these controls for riparian groundwaters located upstream of two milldams over a period of 4 years. Groundwater chemistry was sampled monthly while groundwater elevations, hydraulic gradients, and temperatures were recorded sub‐hourly. Distinct seasonal patterns for NH4+ were observed which differed among the wells. For wells that displayed a strong seasonal pattern, NH4+ concentrations increased through the summer and peaked in October–November. These elevated concentrations were attributed to ammonification, suppression of nitrification, and/or dissimilatory nitrate reduction to ammonium (DNRA). These processes were driven by high groundwater temperatures, low hydraulic gradients (or long residence times), hypoxic/anoxic groundwater conditions, and increased availability of dissolved organic carbon as an electron donor. In contrast, NH4+ concentrations decreased in the riparian groundwater from January to April during cool and wet conditions. A groundwater well with elevated total dissolved iron (TdFe) concentrations had elevated NH4+ concentrations but displayed a muted seasonal response. In addition to hydrologic controls, we attributed this response to additional NH4+ contribution from Fe‐driven autotrophic DNRA and/or ammonification linked to dissimilatory Fe reduction. Understanding the temporal patterns and factors controlling NH4+ in riparian groundwaters is important for making appropriate watershed management decisions and implementing appropriate best management practices.

Bhutan in the eastern Himalaya contains some of the last pristine watersheds in the world, yet there has been limited monitoring of streams and rivers. Eighteen streams in three watersheds were surveyed for chemistry, bacteria, and macroinvertebrates in post-monsoon (2015) and monsoon (2016) seasons. Many water quality variables, including temperature, pH, specific conductivity, nitrite, nitrate, E. coli, and total coliform bacteria differed between seasons and between areas upstream and downstream of anthropogenic disturbance. In both seasons, total coliform bacteria and E. coli were significantly higher downstream of anthropogenic disturbance, with many urban sites having high coliform levels (>2000 cfu/100 ml) indicative of sewage inflow. A total of 50 insect families and six non-insect taxa were identified. During the post-monsoon, eight of 13 metrics (e.g., total richness, Ephemeroptera, Plecoptera, and Trichoptera (EPT) richness, % EPT, % non-insects, HKHbios, BMWP1983, ASPT1983, and ASPT2021) based on kick samples (qualitative) indicated impairment, while in the monsoon season composite Surbers (quantitative) had two metrics (e.g., total richness and Shannon) that differed between sites up and downstream of disturbance. DNA barcoding for cytochrome c oxidase subunit I (COI) in 63 morphological species of mayfly, stonefly, and caddisfly indicated 18 additional species, 17 mayflies and one stonefly. Forty-two barcode species were new additions to the Barcode of Life Data database. Results suggest macroinvertebrates are a viable method for evaluating human impacts on Bhutan streams. Bhutan faces future challenges of sanitation management, climate change, and shared river systems, and monitoring will need to be expanded. The monsoon season may be an ideal time to measure water chemistry and bacteria due to increased runoff, but macroinvertebrate sampling should occur in the post-monsoon season to obtain the best sampling conditions and larger individuals. Increasing the knowledge of species in the region, potentially with the help of DNA barcodes, will document the diversity of the region and help amplify the capacity for macroinvertebrates with future biomonitoring.

Abstract Scientific experts from different disciplines often struggle to mesh their specialized perspectives into the shared mindset that is needed to address difficult and persistent environmental, ecological, and societal problems. Many traditional graduate programs provide excellent research and technical skill training. However, these programs often do not teach a systematic way to learn team skills, nor do they offer a protocol for identifying and tackling increasingly integrated interdisciplinary (among disciplines) and transdisciplinary (among researchers and stakeholders) questions. As a result, professionals trained in traditional graduate programs (e.g., current graduate students and employed practitioners) may not have all of the collaborative skills needed to advance solutions to difficult scientific problems. In the present article, we illustrate a tractable, widely implementable structured process called RISE that accelerates the development of these missing skills. The RISE process (Route to Identifying, learning, and practicing interdisciplinary and transdisciplinary team Skills to address difficult Environmental problems) can be used by diverse teams as a tool for research, professional interactions, or training. RISE helps professionals with different expertise learn from each other by repeatedly asking team-developed questions that are tested using an interactive quantitative tool (e.g., agent-based models, machine learning, case studies) applied to a shared problem framework and data set. Outputs from the quantitative tool are then discussed and interpreted as a team, considering all team members’ perspectives, disciplines, and expertise. After this synthesis, RISE is repeated with new questions that the team jointly identified in earlier data interpretation discussions. As a result, individual perspectives, originally informed by disciplinary training, are complemented by a shared understanding of team function and elevated interdisciplinary knowledge.

Abstract Vertical motion is an important driver of sunlight exposure in aquatic environments, shaping the growth and fate of materials and organisms. We derive a simple model accounting for turbulent depth fluctuations of particles to predict the depth that contributes the most sunlight exposure (effective depth) as well as the single depth that, if measured at one place over time, produces the same total sunlight exposure as a moving particle (functional depth). Field measurements of light and depth in rivers using neutrally buoyant drifters and buoys validate our model. Effective depth varied from 0.1 to 1.5 m below the water surface and was ~ 30% of the overall water depth on average. Functional depth varied from 0.67 to 2.3 m and was ~ 50% of the overall water depth on average. Functional and effective depth are physically based concepts incorporating turbulent motion, spatial variability, and water clarity offering new approaches to characterize light exposure in aquatic environments.

Most readily available landuse/landcover (LULC) data are developed using growing season remote sensing images often at annual time steps. We used the Dynamic World near real-time global LULC dataset to compare how geospatial environmental models of water quality and hydrology respond to growing vs. non-growing season LULC for temperate watersheds of the eastern United States. Non-growing season LULC had more built area and less tree cover than growing season data due to seasonal impacts on classifications rather than actual LULC changes (e.g., quick construction or succession). In mixed-LULC watersheds, seasonal LULC classification inconsistencies could lead to differences in model outputs depending on the LULC season used, such as an increase in watershed nitrogen yields simulated by the Soil and Water Assessment Tool. Within reason, using separate calibration for each season may compensate for these inconsistencies, but lead to different model parameter optimizations. Our findings provide guidelines on the use of near real-time and high temporal resolution LULC in geospatial models. Environmental models incorporating landuse/landcover (LULC) data are common in many fields including hydrology, biogeochemistry, ecology, and climate science, often with decision-making implications (Hu et al., 2021;

IntroductionDamming has substantially fragmented and altered riverine ecosystems worldwide. Dams slow down streamflows, raise stream and groundwater levels, create anoxic or hypoxic hyporheic and riparian environments and result in deposition of fine sediments above dams. These sediments represent a good opportunity to study human legacies altering soil environments, for which we lack knowledge on microbial structure, depth distribution, and ecological function.MethodsHere, we compared high throughput sequencing of bacterial/ archaeal and fungal community structure (diversity and composition) and functional genes (i.e., nitrification and denitrification) at different depths (ranging from 0 to 4 m) in riparian sediments above breached and existing milldams in the Mid-Atlantic United States.ResultsWe found significant location- and depth-dependent changes in microbial community structure. Proteobacteria, Bacteroidetes, Firmicutes, Actinobacteria, Chloroflexi, Acidobacteria, Planctomycetes, Thaumarchaeota, and Verrucomicrobia were the major prokaryotic components while Ascomycota, Basidiomycota, Chytridiomycota, Mortierellomycota, Mucoromycota, and Rozellomycota dominated fungal sequences retrieved from sediment samples. Ammonia oxidizing genes (amoA for AOA) were higher at the sediment surface but decreased sharply with depth. Besides top layers, denitrifying genes (nosZ) were also present at depth, indicating a higher denitrification potential in the deeper layers. However, these results contrasted with in situ denitrification enzyme assay (DEA) measurements, suggesting the presence of dormant microbes and/or other nitrogen processes in deep sediments that compete with denitrification. In addition to enhanced depth stratification, our results also highlighted that dam removal increased species richness, microbial diversity, and nitrification.DiscussionLateral and vertical spatial distributions of soil microbiomes (both prokaryotes and fungi) suggest that not only sediment stratification but also concurrent watershed conditions are important in explaining the depth profiles of microbial communities and functional genes in dammed rivers. The results also provide valuable information and guidance to stakeholders and restoration projects.

Abstract The compounding effects of anthropogenic legacies for environmental pollution are significant, but not well understood. Here, we show that centennial‐scale legacies of milldams and decadal‐scale legacies of road salt salinization interact in unexpected ways to produce hot spots of nitrogen (N) in riparian zones. Riparian groundwater and stream water concentrations upstream of two mid‐Atlantic (Pennsylvania and Delaware) milldams, 2.4 and 4 m tall, were sampled over a 2 year period. Clay and silt‐rich legacy sediments with low hydraulic conductivity, stagnant and poorly mixed hydrologic conditions, and persistent hypoxia in riparian sediments upstream of milldams produced a unique biogeochemical gradient with nitrate removal via denitrification at the upland riparian edge and ammonium‐N accumulation in near‐stream sediments and groundwaters. Riparian groundwater ammonium‐N concentrations upstream of the milldams ranged from 0.006 to 30.6 mgN L−1 while soil‐bound values were 0.11–456 mg kg−1. We attribute the elevated ammonium concentrations to ammonification with suppression of nitrification and/or dissimilatory nitrate reduction to ammonium (DNRA). Sodium inputs to riparian groundwater (25–1,504 mg L−1) from road salts may further enhance DNRA and ammonium production and displace sorbed soil ammonium‐N into groundwaters. This study suggests that legacies of milldams and road salts may undercut the N buffering capacity of riparian zones and need to be considered in riparian buffer assessments, watershed management plans, and dam removal decisions. Given the widespread existence of dams and other barriers and the ubiquitous use of road salt, the potential for this synergistic N pollution is significant.