Longitudinal stream synoptic (LSS) monitoring to evaluate water quality in restored streams.

Impervious surface cover increases peak flows and degrades stream health, contributing to a variety of hydrologic, water quality, and ecological symptoms, collectively known as the urban stream syndrome. Strategies to combat the urban stream syndrome often employ engineering approaches to enhance stream-floodplain reconnection, dissipate erosive forces from urban runoff, and enhance contaminant retention, but it is not always clear how effective such practices are or how to monitor for their effectiveness. In this study, we explore applications of longitudinal stream synoptic (LSS) monitoring (an approach where multiple samples are collected along stream flowpaths across both space and time) to narrow this knowledge gap. Specifically, we investigate (1) whether LSS monitoring can be used to detect changes in water chemistry along longitudinal flowpaths in response to stream-floodplain reconnection and (2) what is the scale over which restoration efforts improve stream quality. We present results for four different classes of water quality constituents (carbon, nutrients, salt ions, and metals) across five watersheds with varying degrees of stream-floodplain reconnection. Our work suggests that LSS monitoring can be used to evaluate stream restoration strategies when implemented at meter to kilometer scales. As streams flow through restoration features, concentrations of nutrients, salts, and metals significantly decline (p < 0.05) or remain unchanged. This same pattern is not evident in unrestored streams, where salt ion concentrations (e.g., Na+, Ca2+, K+) significantly increase with increasing impervious cover. When used in concert with statistical approaches like principal component analysis, we find that LSS monitoring reveals changes in entire chemical mixtures (e.g., salts, metals, and nutrients), not just individual water quality constituents. These chemical mixtures are locally responsive to restoration projects, but can be obscured at the watershed scale and overwhelmed during storm events.

Shifting stoichiometry: Long-term trends in stream-dissolved organic matter reveal altered C:N ratios due to history of atmospheric acid deposition.

Dissolved organic carbon (DOC) and nitrogen (DON) are important energy and nutrient sources for aquatic ecosystems. In many northern temperate, freshwater systems DOC has increased in the past 50 years. Less is known about how changes in DOC may vary across latitudes, and whether changes in DON track those of DOC. Here, we present long-term DOC and DON data from 74 streams distributed across seven sites in biomes ranging from the tropics to northern boreal forests with varying histories of atmospheric acid deposition. For each stream, we examined the temporal trends of DOC and DON concentrations and DOC:DON molar ratios. While some sites displayed consistent positive or negative trends in stream DOC and DON concentrations, changes in direction or magnitude were inconsistent at regional or local scales. DON trends did not always track those of DOC, though DOC:DON ratios increased over time for ~30% of streams. Our results indicate that the dissolved organic matter (DOM) pool is experiencing fundamental changes due to the recovery from atmospheric acid deposition. Changes in DOC:DON stoichiometry point to a shifting energy-nutrient balance in many aquatic ecosystems. Sustained changes in the character of DOM can have major implications for stream metabolism, biogeochemical processes, food webs, and drinking water quality (including disinfection by-products). Understanding regional and global variation in DOC and DON concentrations is important for developing realistic models and watershed management protocols to effectively target mitigation efforts aimed at bringing DOM flux and nutrient enrichment under control.

Can Common Pool Resource Theory Catalyze Stakeholder-Driven Solutions to the Freshwater Salinization Syndrome?

Freshwater salinity is rising across many regions of the United States as well as globally, a phenomenon called the freshwater salinization syndrome (FSS). The FSS mobilizes organic carbon, nutrients, heavy metals, and other contaminants sequestered in soils and freshwater sediments, alters the structures and functions of soils, streams, and riparian ecosystems, threatens drinking water supplies, and undermines progress toward many of the United Nations Sustainable Development Goals. There is an urgent need to leverage the current understanding of salinization's causes and consequences─in partnership with engineers, social scientists, policymakers, and other stakeholders─into locally tailored approaches for balancing our nation's salt budget. In this feature, we propose that the FSS can be understood as a common pool resource problem and explore Nobel Laureate Elinor Ostrom's social-ecological systems framework as an approach for identifying the conditions under which local actors may work collectively to manage the FSS in the absence of top-down regulatory controls. We adopt as a case study rising sodium concentrations in the Occoquan Reservoir, a critical water supply for up to one million residents in Northern Virginia (USA), to illustrate emerging impacts, underlying causes, possible solutions, and critical research needs.

Long-term assessment of floodplain reconnection as a stream restoration approach for managing nitrogen in ground and surface waters.

Stream restoration is a popular approach for managing nitrogen (N) in degraded, flashy urban streams. Here, we investigated the long-term effects of stream restoration involving floodplain reconnection on riparian and in-stream N transport and transformation in an urban stream in the Chesapeake Bay watershed. We examined relationships between hydrology, chemistry, and biology using a Before/After-Control/Impact (BACI) study design to determine how hydrologic flashiness, nitrate (NO3 -) concentrations (mg/L), and N flux, both NO3 - and total N (kg/yr), changed after the restoration and floodplain hydrologic reconnection to its stream channel. We examined two independent surface water and groundwater data sets (EPA and USGS) collected from 2002-2012 at our study sites in the Minebank Run watershed. Restoration was completed during 2004 and 2005. Afterward, the monthly hydrologic flashiness index, based on mean monthly discharge, decreased over time from 2002 and 2008. However, from 2008-2012 hydrologic flashiness returned to pre-restoration levels. Based on the EPA data set, NO3 - concentration in groundwater and surface water was significantly less after restoration while the control site showed no change. DOC and NO3 - were negatively related before and after restoration suggesting C limitation of N transformations. Long-term trends in surface water NO3 - concentrations based on USGS surface water data showed downward trends after restoration at both the restored and control sites, whereas specific conductance showed no trend. Comparisons of NO3 - concentrations with Cl- concentrations and specific conductance in both ground and surface waters suggested that NO3 - reduction after restoration was not due to dilution or load reductions from the watershed. Modeled NO3 - flux decreased post restoration over time but the rate of decrease was reduced likely due to failure of restoration features that facilitated N transformations. Groundwater NO3 - concentrations varied among stream features suggesting that some engineered features may be functionally better at creating optimal conditions for N retention. However, some engineered features eroded and failed post restoration thereby reducing efficacy of the stream restoration to reduce flashiness and NO3 - flux. N management via stream restoration will be most effective where flashiness can be reduced and DOC made available for denitrifiers. Stream restoration may be an important component of holistic watershed management including stormwater management and nutrient source control if stream restoration and floodplain reconnection can be done in a manner to resist the erosive effects of large storm events that can degrade streams to pre-restoration conditions. Long-term evolution of water quality functions in response to degradation of restored stream channels and floodplains from urban stressors and storms over time warrants further study, however.

Tree Trade-Offs in Stream Restoration: Impacts on Riparian Groundwater Quality.

Riparian zones are a vital interface between land and stream and are often the focus of stream restoration efforts to reduce nutrient pollution in waterways. Restoration of degraded stream channels often requires the removal of mature trees during major physical alteration of the riparian zone to reshape streambank topography. We assessed the impact of tree removal on riparian groundwater quality over space and time. Twenty-nine wells were installed across 5 sites in watersheds of the Washington D.C. and Baltimore, Maryland, USA metropolitan areas. Study sites encompassed a chronosequence of restoration ages (5, 10 and 20 years) as well as unrestored comparisons. Groundwater wells were installed as transects of 3 perpendicular to the stream channel to estimate nutrient uptake along groundwater flow paths. Groundwater samples collected over a 2-year period (2018-2019) were analyzed for concentrations of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), total dissolved nitrogen (TDN), and dissolved components of calcium (Ca), potassium (K), magnesium (Mg), sodium (Na), sulfur (S) and other elements. Results showed some interesting patterns such as: (1) elevated concentrations of some nutrients and carbon in riparian groundwater of recently restored (5 year) sites; (2) decreasing linear trends in concentrations of TDN, K and S in groundwater during a 2 year shift from wet to dry conditions; (3) linear relationships between DOC (organic matter) and plant nutrients in groundwater suggesting the importance of plant uptake and biomass as sources and sinks of nutrients; (4) increasing concentrations in groundwater along hydrologic flow paths from uplands to streams in riparian zones where trees were recently cut, and opposite patterns where trees were not cut. Riparian zones appeared to act as sources or sinks of bioreactive elements based on tree removal. Mean TDN, DOC, and S, concentrations decreased by 78.6%, 12.3%, and 19.3% respectively through uncut riparian zones, but increased by 516.9%, 199.7%, and 34.5% respectively through the 5-year cut transects. Ecosystem recovery and an improvement in groundwater quality appeared to be achieved by 10-20 years after restoration. A better understanding of the effects of riparian tree removal on groundwater quality can inform strategies for minimizing unintended effects of stream restoration on groundwater chemistry.

Freshwater salinization syndrome on a continental scale.

Salt pollution and human-accelerated weathering are shifting the chemical composition of major ions in fresh water and increasing salinization and alkalinization across North America. We propose a concept, the freshwater salinization syndrome, which links salinization and alkalinization processes. This syndrome manifests as concurrent trends in specific conductance, pH, alkalinity, and base cations. Although individual trends can vary in strength, changes in salinization and alkalinization have affected 37% and 90%, respectively, of the drainage area of the contiguous United States over the past century. Across 232 United States Geological Survey (USGS) monitoring sites, 66% of stream and river sites showed a statistical increase in pH, which often began decades before acid rain regulations. The syndrome is most prominent in the densely populated eastern and midwestern United States, where salinity and alkalinity have increased most rapidly. The syndrome is caused by salt pollution (e.g., road deicers, irrigation runoff, sewage, potash), accelerated weathering and soil cation exchange, mining and resource extraction, and the presence of easily weathered minerals used in agriculture (lime) and urbanization (concrete). Increasing salts with strong bases and carbonates elevate acid neutralizing capacity and pH, and increasing sodium from salt pollution eventually displaces base cations on soil exchange sites, which further increases pH and alkalinization. Symptoms of the syndrome can include: infrastructure corrosion, contaminant mobilization, and variations in coastal ocean acidification caused by increasingly alkaline river inputs. Unless regulated and managed, the freshwater salinization syndrome can have significant impacts on ecosystem services such as safe drinking water, contaminant retention, and biodiversity.

Theoretical Perspectives of the Baltimore Ecosystem Study: Conceptual Evolution in a Social-Ecological Research Project.

The Earth's population will become more than 80% urban during this century. This threshold is often regarded as sufficient justification for pursuing urban ecology. However, pursuit has primarily focused on building empirical richness, and urban ecology theory is rarely discussed. The Baltimore Ecosystem Study (BES) has been grounded in theory since its inception and its two decades of data collection have stimulated progress toward comprehensive urban theory. Emerging urban ecology theory integrates biology, physical sciences, social sciences, and urban design, probes interdisciplinary frontiers while being founded on textbook disciplinary theories, and accommodates surprising empirical results. Theoretical growth in urban ecology has relied on refined frameworks, increased disciplinary scope, and longevity of interdisciplinary interactions. We describe the theories used by BES initially, and trace ongoing theoretical development that increasingly reflects the hybrid biological-physical-social nature of the Baltimore ecosystem. The specific mix of theories used in Baltimore likely will require modification when applied to other urban areas, but the developmental process, and the key results, will continue to benefit other urban social-ecological research projects.

Making 'Chemical Cocktails' - Evolution of Urban Geochemical Processes across the Periodic Table of Elements.

Urbanization contributes to the formation of novel elemental combinations and signatures in terrestrial and aquatic watersheds, also known as 'chemical cocktails.' The composition of chemical cocktails evolves across space and time due to: (1) elevated concentrations from anthropogenic sources, (2) accelerated weathering and corrosion of the built environment, (3) increased drainage density and intensification of urban water conveyance systems, and (4) enhanced rates of geochemical transformations due to changes in temperature, ionic strength, pH, and redox potentials. Characterizing chemical cocktails and underlying geochemical processes is necessary for: (1) tracking pollution sources using complex chemical mixtures instead of individual elements or compounds; (2) developing new strategies for co-managing groups of contaminants; (3) identifying proxies for predicting transport of chemical mixtures using continuous sensor data; and (4) determining whether interactive effects of chemical cocktails produce ecosystem-scale impacts greater than the sum of individual chemical stressors. First, we discuss some unique urban geochemical processes which form chemical cocktails, such as urban soil formation, human-accelerated weathering, urban acidification-alkalinization, and freshwater salinization syndrome. Second, we review and synthesize global patterns in concentrations of major ions, carbon and nutrients, and trace elements in urban streams across different world regions and make comparisons with reference conditions. In addition to our global analysis, we highlight examples from some watersheds in the Baltimore-Washington DC region, which show increased transport of major ions, trace metals, and nutrients across streams draining a well-defined land-use gradient. Urbanization increased the concentrations of multiple major and trace elements in streams draining human-dominated watersheds compared to reference conditions. Chemical cocktails of major and trace elements were formed over diurnal cycles coinciding with changes in streamflow, dissolved oxygen, pH, and other variables measured by high-frequency sensors. Some chemical cocktails of major and trace elements were also significantly related to specific conductance (p<0.05), which can be measured by sensors. Concentrations of major and trace elements increased, peaked, or decreased longitudinally along streams as watershed urbanization increased, which is consistent with distinct shifts in chemical mixtures upstream and downstream of other major cities in the world. Our global analysis of urban streams shows that concentrations of multiple elements along the Periodic Table significantly increase when compared with reference conditions. Furthermore, similar biogeochemical patterns and processes can be grouped among distinct mixtures of elements of major ions, dissolved organic matter, nutrients, and trace elements as chemical cocktails. Chemical cocktails form in urban waters over diurnal cycles, decades, and throughout drainage basins. We conclude our global review and synthesis by proposing strategies for monitoring and managing chemical cocktails using source control, ecosystem restoration, and green infrastructure. We discuss future research directions applying the watershed chemical cocktail approach to diagnose and manage environmental problems. Ultimately, a chemical cocktail approach targeting sources, transport, and transformations of different and distinct elemental combinations is necessary to more holistically monitor and manage the emerging impacts of chemical mixtures in the world's fresh waters.

Seeing the light: urban stream restoration affects stream metabolism and nitrate uptake via changes in canopy cover.

The continually increasing global population residing in urban landscapes impacts numerous ecosystem functions and services provided by urban streams. Urban stream restoration is often employed to offset these impacts and conserve or enhance the various functions and services these streams provide. Despite the assumption that "if you build it, [the function] will come," current understanding of the effects of urban stream restoration on stream ecosystem functions are based on short term studies that may not capture variation in restoration effectiveness over time. We quantified the impact of stream restoration on nutrient and energy dynamics of urban streams by studying 10 urban stream reaches (five restored, five unrestored) in the Baltimore, Maryland, USA, region over a two-year period. We measured gross primary production (GPP) and ecosystem respiration (ER) at the whole-stream scale continuously throughout the study and nitrate (NO3- -N) spiraling rates seasonally (spring, summer, autumn) across all reaches. There was no significant restoration effect on NO3- -N spiraling across reaches. However, there was a significant canopy cover effect on NO3- -N spiraling, and directly comparing paired sets of unrestored-restored reaches showed that restoration does affect NO3- -N spiraling after accounting for other environmental variation. Furthermore, there was a change in GPP : ER seasonality, with restored and open-canopied reaches exhibiting higher GPP : ER during summer. The restoration effect, though, appears contingent upon altered canopy cover, which is likely to be a temporary effect of restoration and is a driver of multiple ecosystem services, e.g., habitat, riparian nutrient processing. Our results suggest that decision-making about stream restoration, including evaluations of nutrient benefits, clearly needs to consider spatial and temporal dynamics of canopy cover and trade-offs among multiple ecosystem services.

Human-accelerated weathering increases salinization, major ions, and alkalinization in fresh water across land use.

Human-dominated land uses can increase transport of major ions in streams due to the combination of human-accelerated weathering and anthropogenic salts. Calcium, magnesium, sodium, alkalinity, and hardness significantly increased in the drinking water supply for Baltimore, Maryland over almost 50 years (p<0.05) coinciding with regional urbanization. Across a nearby land use gradient at the Baltimore Long-Term Ecological Research (LTER) site, there were significant increases in concentrations of dissolved inorganic carbon (DIC), Ca2+, Mg2+, Na+, and Si and pH with increasing impervious surfaces in 9 streams monitored bi-weekly over a 3-4 year period (p<0.05). Base cations in urban streams were up to 60 times greater than forest and agricultural streams, and elemental ratios suggested road salt and carbonate weathering from impervious surfaces as potential sources. Laboratory weathering experiments with concrete also indicated that impervious surfaces increased pH and DIC with potential to alkalinize urban waters. Ratios of Na+ and Cl- suggested that there was enhanced ion exchange in the watersheds from road salts, which could mobilize other base cations from soils to streams. There were significant relationships between Ca2+, Mg2+, Na+, and K+ concentrations and Cl-, SO42-, NO3- and DIC across land use (p<0.05), which suggested tight coupling of geochemical cycles. Finally, concentrations of Na+, Ca2+, Mg2+, and pH significantly increased with distance downstream (p<0.05) along a stream network draining 170 km2 of the Baltimore LTER site contributing to river alkalinization. Our results suggest that urbanization may dramatically increase major ions, ionic strength, and pH over decades from headwaters to coastal zones, which can impact integrity of aquatic life, infrastructure, drinking water, and coastal ocean alkalinization.

Use of a Three-Dimensional Reactive Solute Transport Model for Evaluation of Bioreactor Placement in Stream Restoration.

A three-dimensional groundwater flow and multispecies reactive transport model was used to strategically design placement of bioreactors in the subsurface to achieve maximum removal of nitrate along restored stream reaches. Two hypothetical stream restoration scenarios were evaluated over stream reaches of 40 and 94 m: a step-pool scenario and a channel re-meandering scenario. For the step-pool scenario, bioreactors were placed at locations where mass fluxes of groundwater and nitrate were highest. Bioreactors installed over 50% of the total channel length of a step-pool scenario (located to intercept maximum groundwater and nitrate mass flux) removed nitrate-N entering the channel at a rate of 36.5 kg N yr (100 g N d), achieving about 65% of the removal of a whole-length bioreactor. Bioreactor placement for the re-meandering scenario was designed using a criterion of either highest nitrate mass flux or highest groundwater flux, but not both, because they did not occur together. Bioreactors installed at maximum nitrate flux locations (53% of the total channel length) on the western bank removed nitrate-N entering the channel at 62.0 kg N yr (170 g N d), achieving 85% of nitrate-N removal of whole-length bioreactors for the re-meandering scenario. Bioreactors installed at maximum groundwater flux locations on the western bank along approximately 40% of the re-meandering channel achieved about 65% of nitrate removal of whole-length bioreactors. Placing bioreactors at maximum nitrate flux locations improved denitrification efficiency. Due to low groundwater velocities, bioreactor nitrate-N removal was found to be nitrate limited for all scenarios.

Evolution of sediment plumes in the Chesapeake bay and implications of climate variability.

Fluvial sediment transport impacts fisheries, marine ecosystems, and human health. In the upper Chesapeake Bay, river-induced sediment plumes are generally known as either a monotonic spatial shape or a turbidity maximum. Little is known about plume evolution in response to variation in streamflow and extreme discharge of sediment. Here we propose a typology of sediment plumes in the upper Chesapeake Bay using a 17 year time series of satellite-derived suspended sediment concentration. On the basis of estimated fluvial and wind contributions, we define an intermittent/wind-dominated type and a continuous type, the latter of which is further divided into four subtypes based on spatial features of plumes, which we refer to as Injection, Transport, Temporary Turbidity-Maximum, and Persistent Turbidity-Maximum. The four continuous types exhibit a consistent sequence of evolution within 1 week to 1 month following flood events. We also identify a "shift" in typology with increased frequency of Turbidity-Maximum types before and after Hurricane Ivan (2004), which implies that extreme events have longer-lasting effects upon estuarine suspended sediment than previously considered. These results can serve as a diagnostic tool to better predict distribution and impacts of estuarine suspended sediment in response to changes in climate and land use.

Shallow groundwater denitrification in riparian zones of a headwater agricultural landscape.

Riparian zones adjacent to cropped lands are effective at reducing nitrate (NO) loads to receiving water bodies primarily through plant assimilation and denitrification. Denitrification represents a permanent removal pathway and a greenhouse gas source, converting NO to inert N gas or nitrous oxide (NO), and has been the subject of many studies in agricultural landscapes. Despite the prevailing notion that riparian zones can be areas of enhanced denitrification, there is a lack of in situ denitrification measurements from these areas that buffer streams and rivers from NO originating in upland cropped soils, especially over time scales that capture seasonal dynamics. We measured in situ groundwater denitrification rates in two riparian zones of an intensive dairy farm located in the headwaters of the Susquehanna River. Denitrification rates determined monthly over a 1-yr period with the N-NO push-pull method ranged from 0 to 4177 µg N kg soil d (mean, 830 ± 193 µg N kg soil d). Denitrification showed a distinct seasonal pattern, with highest rates observed in the spring and summer, concomitant with warmer temperatures and decreasing dissolved oxygen. We estimate an annual N loss of 470 ± 116 kg yr ha of riparian zone via denitrification in the shallow saturated zone, with the potential for >20% of this amount occurring as NO. Total denitrification from shallow groundwater in the riparian zone was equivalent to 32% of manure N spread on the adjacent upland field, confirming the importance of riparian zones in agricultural landscapes at controlling N loads entering downstream waters.

Salty chemical cocktails as water quality signatures: Longitudinal trends and breakpoints along different U.S. streams.

Along urban streams and rivers, various processes, including road salt application, sewage leaks, and weathering of the built environment, contribute to novel chemical cocktails made up of metals, salts, nutrients, and organic matter. In order to track the impacts of urbanization and management strategies on water quality, we conducted longitudinal stream synoptic (LSS) monitoring in nine watersheds in five major metropolitan areas of the U.S. During each LSS monitoring survey, 10-53 sites were sampled along the flowpath of streams as they flowed along rural to urban gradients. Results demonstrated that major ions derived from salts (Ca2+, Mg2+, Na+, and K+) and correlated elements (e.g. Sr2+, N, Cu) formed 'salty chemical cocktails' that increased along rural to urban flowpaths. Salty chemical cocktails explained 46.1% of the overall variability in geochemistry among streams and showed distinct typologies, trends, and transitions along flowpaths through metropolitan regions. Multiple linear regression predicted 62.9% of the variance in the salty chemical cocktails using the six following significant drivers (p < 0.05): percent urban land, wastewater treatment plant discharge, mean annual precipitation, percent silicic residual material, percent volcanic material, and percent carbonate residual material. Mean annual precipitation and percent urban area were the most important in the regression, explaining 29.6% and 13.0% of the variance. Different pollution sources (wastewater, road salt, urban runoff) in streams were tracked downstream based on salty chemical cocktails. Streams flowing through stream-floodplain restoration projects and conservation areas with extensive riparian forest buffers did not show longitudinal increases in salty chemical cocktails, suggesting that there could be attenuation via conservation and restoration. Salinization represents a common urban water quality signature and longitudinal patterns of distinct chemical cocktails and ionic mixtures have the potential to track the sources, fate, and transport of different point and nonpoint pollution sources along streams across different regions.

Where the rubber meets the road: Emerging environmental impacts of tire wear particles and their chemical cocktails.

About 3 billion new tires are produced each year and about 800 million tires become waste annually. Global dependence upon tires produced from natural rubber and petroleum-based compounds represents a persistent and complex environmental problem with only partial and often-times, ineffective solutions. Tire emissions may be in the form of whole tires, tire particles, and chemical compounds, each of which is transported through various atmospheric, terrestrial, and aquatic routes in the natural and built environments. Production and use of tires generates multiple heavy metals, plastics, PAH's, and other compounds that can be toxic alone or as chemical cocktails. Used tires require storage space, are energy intensive to recycle, and generally have few post-wear uses that are not also potential sources of pollutants (e.g., crumb rubber, pavements, burning). Tire particles emitted during use are a major component of microplastics in urban runoff and a source of unique and highly potent toxic substances. Thus, tires represent a ubiquitous and complex pollutant that requires a comprehensive examination to develop effective management and remediation. We approach the issue of tire pollution holistically by examining the life cycle of tires across production, emissions, recycling, and disposal. In this paper, we synthesize recent research and data about the environmental and human health risks associated with the production, use, and disposal of tires and discuss gaps in our knowledge about fate and transport, as well as the toxicology of tire particles and chemical leachates. We examine potential management and remediation approaches for addressing exposure risks across the life cycle of tires. We consider tires as pollutants across three levels: tires in their whole state, as particulates, and as a mixture of chemical cocktails. Finally, we discuss information gaps in our understanding of tires as a pollutant and outline key questions to improve our knowledge and ability to manage and remediate tire pollution.

Increased river alkalinization in the Eastern U.S.

The interaction between human activities and watershed geology is accelerating long-term changes in the carbon cycle of rivers. We evaluated changes in bicarbonate alkalinity, a product of chemical weathering, and tested for long-term trends at 97 sites in the eastern United States draining over 260,000 km(2). We observed statistically significant increasing trends in alkalinity at 62 of the 97 sites, while remaining sites exhibited no significant decreasing trends. Over 50% of study sites also had statistically significant increasing trends in concentrations of calcium (another product of chemical weathering) where data were available. River alkalinization rates were significantly related to watershed carbonate lithology, acid deposition, and topography. These three variables explained ~40% of variation in river alkalinization rates. The strongest predictor of river alkalinization rates was carbonate lithology. The most rapid rates of river alkalinization occurred at sites with highest inputs of acid deposition and highest elevation. The rise of alkalinity in many rivers throughout the Eastern U.S. suggests human-accelerated chemical weathering, in addition to previously documented impacts of mining and land use. Increased river alkalinization has major environmental implications including impacts on water hardness and salinization of drinking water, alterations of air-water exchange of CO2, coastal ocean acidification, and the influence of bicarbonate availability on primary production.

Ecological engineering practices for the reduction of excess nitrogen in human-influenced landscapes: a guide for watershed managers.

Excess nitrogen (N) in freshwater systems, estuaries, and coastal areas has well-documented deleterious effects on ecosystems. Ecological engineering practices (EEPs) may be effective at decreasing nonpoint source N leaching to surface and groundwater. However, few studies have synthesized current knowledge about the functioning principles, performance, and cost of common EEPs used to mitigate N pollution at the watershed scale. Our review describes seven EEPs known to decrease N to help watershed managers select the most effective techniques from among the following approaches: advanced-treatment septic systems, low-impact development (LID) structures, permeable reactive barriers, treatment wetlands, riparian buffers, artificial lakes and reservoirs, and stream restoration. Our results show a broad range of N-removal effectiveness but suggest that all techniques could be optimized for N removal by promoting and sustaining conditions conducive to biological transformations (e.g., denitrification). Generally, N-removal efficiency is particularly affected by hydraulic residence time, organic carbon availability, and establishment of anaerobic conditions. There remains a critical need for systematic empirical studies documenting N-removal efficiency among EEPs and potential environmental and economic tradeoffs associated with the widespread use of these techniques. Under current trajectories of N inputs, land use, and climate change, ecological engineering alone may be insufficient to manage N in many watersheds, suggesting that N-pollution source prevention remains a critical need. Improved understanding of N-removal effectiveness and modeling efforts will be critical in building decision support tools to help guide the selection and application of best EEPs for N management.

Stormwater Best Management Practices: Experimental Evaluation of Chemical Cocktails Mobilized by Freshwater Salinization Syndrome.

Freshwater Salinization Syndrome (FSS) refers to the suite of physical, biological, and chemical impacts of salt ions on the degradation of natural, engineered, and social systems. Impacts of FSS on mobilization of chemical cocktails has been documented in streams and groundwater, but little research has focused on the effects of FSS on stormwater best management practices (BMPs) such as: constructed wetlands, bioswales, ponds, and bioretention. However emerging research suggests that stormwater BMPs may be both sources and sinks of contaminants, shifting seasonally with road salt applications. We conducted lab experiments to investigate this premise; replicate water and soil samples were collected from four distinct stormwater feature types (bioretention, bioswale, constructed wetlands and retention ponds) and were used in salt incubation experiments conducted under six different salinities with three different salts (NaCl, CaCl2, and MgCl2). Increased salt concentrations had profound effects on major and trace element mobilization, with all three salts showing significant positive relationships across nearly all elements analyzed. Across all sites, mean salt retention was 34%, 28%, and 26% for Na+, Mg2+ and Ca2+ respectively, and there were significant differences among stormwater BMPs. Salt type showed preferential mobilization of certain elements. NaCl mobilized Cu, a potent toxicant to aquatic biota, at rates over an order of magnitude greater than both CaCl2 and MgCl2. Stormwater BMP type also had a significant effect on elemental mobilization, with ponds mobilizing significantly more Mn than other sites. However, salt concentration and salt type consistently had significant effects on mean concentrations of elements mobilized across all stormwater BMPs (p<0.05), suggesting that processes such as ion exchange mobilize metals mobilize metals and salt ions regardless of BMP type. Our results suggest that decisions regarding the amounts and types of salts used as deicers can have significant effects on reducing contaminant mobilization to freshwater ecosystems.

Freshwater Salinization Syndrome Alters Nitrogen Transport in Urban Watersheds.

Anthropogenic salt inputs have impacted many streams in the U.S. for over a century. Urban stream salinity is often chronically elevated and punctuated by episodic salinization events, which can last hours to days after snowstorms and the application of road salt. Here, we investigated the impacts of freshwater salinization on total dissolved nitrogen (TDN) and NO3-/NO2- concentrations and fluxes across time in urban watersheds in the Baltimore-Washington D.C. metropolitan area of the Chesapeake Bay region. Episodic salinization from road salt applications and snowmelt quickly mobilized TDN in streams likely through soil ion exchange, hydrologic flushing, and other biogeochemical processes. Previous experimental work from other studies has shown that salinization can mobilize nitrogen from sediments, but less work has investigated this phenomenon with high-frequency sensors and targeted monitoring during road salt events. We found that urban streams exhibited elevated concentrations and fluxes of TDN, NO3-/NO2-, and specific conductance that rapidly peaked during and after winter road salt events, and then rapidly declined afterwards. We observed plateaus in TDN concentrations in the ranges of the highest specific conductance values (between 1000 and 2000 µS/cm) caused by road salt events. Plateaus in TDN concentrations beyond a certain threshold of specific conductance values suggested source limitation of TDN in watersheds (at the highest ranges in chloride concentrations and ranges); salts were likely extracting nitrogen from soils and streams through ion exchange in soils and sediments, ion pairing in soils and waters, and sodium dispersion of soils to a certain threshold level. When watershed transport was compared across land use, including a forested reference watershed, there was a positive relationship between Cl- loads and NO3-/NO2- loads. This relationship occurred across all sites regardless of land use, which suggests that the mass transport of Cl- and NO3-/NO2- are likely influenced by similar factors such as soil ion exchange, ion pairing, sodium dispersion of soils, hydrologic flushing, and biogeochemical processes. Freshwater salinization has the potential to alter the magnitude and timing of total dissolved nitrogen delivery to receiving waters during winter months following road salt applications, and further work should investigate the seasonal relationships of N transport with salinization in urban watersheds.

Freshwater salinization syndrome limits management efforts to improve water quality.

Freshwater Salinization Syndrome (FSS) refers to groups of biological, physical, and chemical impacts which commonly occur together in response to salinization. FSS can be assessed by the mobilization of chemical mixtures, termed "chemical cocktails", in watersheds. Currently, we do not know if salinization and mobilization of chemical cocktails along streams can be mitigated or reversed using restoration and conservation strategies. We investigated 1) the formation of chemical cocktails temporally and spatially along streams experiencing different levels of restoration and riparian forest conservation and 2) the potential for attenuation of chemical cocktails and salt ions along flowpaths through conservation and restoration areas. We monitored high-frequency temporal and longitudinal changes in streamwater chemistry in response to different pollution events (i.e., road salt, stormwater runoff, wastewater effluent, and baseflow conditions) and several types of watershed management or conservation efforts in six urban watersheds in the Chesapeake Bay watershed. Principal component analysis (PCA) indicates that chemical cocktails which formed along flowpaths (i.e., permanent reaches of a stream) varied due to pollution events. In response to winter road salt applications, the chemical cocktails were enriched in salts and metals (e.g., Na+, Mn, and Cu). During most baseflow and stormflow conditions, chemical cocktails were less enriched in salt ions and trace metals. Downstream attenuation of salt ions occurred during baseflow and stormflow conditions along flowpaths through regional parks, stream-floodplain restorations, and a national park. Conversely, chemical mixtures of salt ions and metals, which formed in response to multiple road salt applications or prolonged road salt exposure, did not show patterns of rapid attenuation downstream. Multiple linear regression was used to investigate variables that influence changes in chemical cocktails along flowpaths. Attenuation and dilution of salt ions and chemical cocktails along stream flowpaths was significantly related to riparian forest buffer width, types of salt pollution, and distance downstream. Although salt ions and chemical cocktails can be attenuated and diluted in response to conservation and restoration efforts at lower concentration ranges, there can be limitations in attenuation during road salt events, particularly if storm drains bypass riparian buffers.