Interference from HONO in the measurement of ambient air NO2 via photolytic conversion and quantification of NO

The reference method to quantify mixing ratios of the criteria air pollutant nitrogen dioxide (NO₂) is NO-O₃ chemiluminescence (CL), in which mixing ratios of nitric oxide (NO) are measured by sampling ambient air directly, and mixing ratios of NO_x (= sum of NO and NO₂) are measured by converting NO₂ to NO using, for example, heated molybdenum catalyst or, more selectively, photolytic conversion (P-CL). In this work, the nitrous acid (HONO) interference in the measurement of NO₂ by P-CL was investigated. Results with two photolytic NO₂ converters are presented. The first used radiation centered at 395 nm, a wavelength region commonly utilized in P-CL. The second used light at 415 nm, where the overlap with the HONO absorption spectrum and hence its photolysis rate are less. Mixing ratios of NO₂, NO_x and HONO entering and exiting the converters were quantified by Thermal Dissociation Cavity Ring-down Spectroscopy (TD-CRDS). Both converters exhibited high NO₂ conversion efficiency (CF_NO2; > 90%) and partial conversion of HONO. Plots of CF against flow rate were consistent with photolysis frequencies of 4.2 s^-1 and 2.9 s^-1 for NO₂ and 0.25 s^-1 and 0.10 s^?1 for HONO at 395 nm and 415 nm, respectively. CF_HONO was larger than predicted from the overlap of the emission and HONO absorption spectra. The results imply that measurements of NO₂ by P-CL marginally but systematically overestimate true NO₂ concentrations, and that this interference should be considered in environments with high HONO:NO₂ ratios such as the marine boundary layer or in biomass burning plumes.     

Pathogens and indicators in United States Class B biosolids: national and historic distributions

This paper reports on a major study of the incidence of indicator organisms and pathogens found within Class B biosolids within 21 samplings from 18 wastewater treatment plants across the United States. This is the first major study of its kind since the promulgation of the USEPA Part 503 Rule in 1993, and includes samples before and after the Part 503 Rule was promulgated. National distributions collected between 2005 and 2008 show that the incidence of bacterial and viral pathogens in Class B mesophilic, anaerobically digested biosolids were generally low with the exception of adenoviruses, which were more prevalent than enteric viruses. No Ascaris ova were detected in any sample. In contrast, indicator organism numbers were uniformly high, regardless of whether they were bacteria (fecal coliforms) or viruses (phage). Indicators were not correlated with pathogen loads. Historic distributions were collected between 1988 and 2006 at one location in Tucson, AZ. By comparing data collected before and after 1993, the influence of the USEPA Part 503 Rule on indicator and pathogen levels within Class B biosolids can be inferred. In general, the bacterial indicators total and fecal coliforms decreased from the 1980s to present. Enteric virus concentrations after 1993 are much lower than those reported in other studies in the 1980s, although our values from 1988 to 1993 are not significantly different from our values obtained from 1994 to 2006. Presumably this is due to better and more consistent treatment of the wastewater, illustrating that the Part 503 Rule has been effective in reducing public exposure to pathogens relative to 17 yr ago. The percent reduction of both indicators and pathogens during anaerobic mesophilic digestion was between 94 and 99% for all organisms, illustrating that such treatment is effective in reducing pathogen loads.     

Three-dimensional numerical simulation of water quality and sediment-associated processes with application to a Mississippi Delta lake

A three-dimensional water quality model was developed for simulating temporal and spatial variations of phytoplankton, nutrients, and dissolved oxygen in freshwater bodies. Effects of suspended and bed sediment on the water quality processes were simulated. A formula was generated from field measurements to calculate the light attenuation coefficient by considering the effects of suspended sediment and chlorophyll. The processes of adsorption–desorption of nutrients by sediment were described using the Langmuir Equation. The release rates of nutrients from the bed were calculated based on the concentration gradient across the water–sediment interface and other variables including pH, temperature and dissolved oxygen concentration.The model was calibrated and validated by applying it to simulate the concentrations of chlorophyll and nutrients in a natural oxbow lake in Mississippi Delta. The simulated time series of phytoplankton (as chlorophyll) and nutrient concentrations were generally in agreement with field observations. Sensitivity analyses were conducted to demonstrate the impacts of varying suspended sediment concentration on lake chlorophyll levels.     

Sediment Source Assessment in a Lowland Watershed Using Nitrogen Stable Isotopes1

Fox, James F., Charles M. Davis, and Darren K. Martin, 2010. Sediment Source Assessment in a Lowland Watershed Using Nitrogen Stable Isotopes. Journal of the American Water Resources Association (JAWRA) 46(6):1192–1204. DOI: 10.1111/j.1752-1688.2010.00485.x Abstract: Sediment sources and transported sediments were sampled in a lowland watershed with pronounced fine sediment storage in the streambed. Sediments were analyzed for carbon and nitrogen content and stable nitrogen isotopic composition. Analysis of the data shows that temporarily stored streambed sediments dominate the sediment load during moderate- and low-flow hydrologic events. Modeling of sediment transport and nitrogen elemental and isotopic mass balance was performed for the watershed for a 12-month time period using a continuous, conceptual-based model. The model results show that during moderate- and low-flow hydrologic events, the streambed is slowly downcutting. During very high-flow hydrologic events, deposition is pronounced in the streambed and sediment is replenished to the bed. Nitrogen model results show that elemental and isotopic nitrogen of streambed sediments vary substantially over the simulation period. In this manner, the streambed in a lowland watershed functions as a temporary storage zone that, in turn, can impact the nitrogen elemental and isotopic signature of sediments. The variation could significantly impact estimates of sediment provenance using nitrogen tracer-based methods. Future work should consider both hydrologic and biogeochemical control on the nitrogen isotopic signature of sediments in small lowland watersheds and streams where a significant portion of deposited fines are temporarily stored.     

红树林呼吸根上生物固氮作用的初步观测研究

在热带海洋环境中,红树林是具有较高生产力的生态系[1,2].通常认为,红树林湿地区域可为毗连的沿岸水域和相关的生物群落生境提供有机物和无机物[3～5].尽管在红树林区域中具有很高的生产率,但红树林的树木一般是生长在营养比较贫瘠的热带地区.     

Household willingness to pay for stream restoration on private and public land: Evidence from the Baltimore metropolitan region

Stream restoration is one of the most widely used interventions to mitigate urban stormwater impacts and improve water quality. Government agencies have typically focused urban stream restoration efforts on public lands that they already own, even though a substantial portion of stream miles in highly urbanized areas occur on privately owned land. Yet, limited research exists to distinguish household willingness to pay (WTP) for stream restoration occurring on private versus public land. In this study, we use a choice experiment to analyze how household WTP for stream restoration attributes vary by land ownership and distance to the restoration project. Our empirical results indicate that streambank stabilization approaches have positive WTP estimates that are substantially larger in magnitude than those related to riparian vegetation management for clearing or planting trees. In general, estimated total household WTP for each of the four restoration design scenarios on public land is higher than when the same restoration design is located on private land. Nonetheless, estimated household WTP for each restoration design scenario on private land is substantial, retaining the majority of the value found on public land in all cases.     

Featured Collection Introduction: Connectivity of Streams and Wetlands to Downstream Waters

Connectivity is a fundamental but highly dynamic property of watersheds. Variability in the types and degrees of aquatic ecosystem connectivity presents challenges for researchers and managers seeking to accurately quantify its effects on critical hydrologic, biogeochemical, and biological processes. However, protecting natural gradients of connectivity is key to protecting the range of ecosystem services that aquatic ecosystems provide. In this featured collection, we review the available evidence on connections and functions by which streams and wetlands affect the integrity of downstream waters such as large rivers, lakes, reservoirs, and estuaries. The reviews in this collection focus on the types of waters whose protections under the U.S. Clean Water Act have been called into question by U.S. Supreme Court cases. We synthesize 40+ years of research on longitudinal, lateral, and vertical fluxes of energy, material, and biota between aquatic ecosystems included within the Act's frame of reference. Many questions about the roles of streams and wetlands in sustaining downstream water integrity can be answered from currently available literature, and emerging research is rapidly closing data gaps with exciting new insights into aquatic connectivity and function at local, watershed, and regional scales. Synthesis of foundational and emerging research is needed to support science‐based efforts to provide safe, reliable sources of fresh water for present and future generations.