Radon as a natural tracer of gas transport through trees.

Trees are sources, sinks, and conduits for gas exchange between the atmosphere and soil, and effectively link these terrestrial realms in a soil-plant-atmosphere continuum. We demonstrated that naturally produced radon-222 (222 Rn) gas has the potential to disentangle the biotic and physical processes that regulate gas transfer between soils or plants and the atmosphere in field settings where exogenous tracer applications are challenging. Patterns in stem radon emissions across tree species, seasons, and diurnal periods suggest that plant transport of soil gases is controlled by plant hydraulics, whether by diffusion or mass flow via transpiration. We establish for the first time that trees emit soil gases during the night when transpiration rates are negligible, suggesting that axial diffusion is an important and understudied mechanism of plant and soil gas transmission.

Freshwater algal biofilm assemblages are more effective than invertebrate assemblages at aggregating microplastics.

Microplastics, plastic particles less than 5 mm in length, are a ubiquitous pollutant in the environment, but research on freshwater microplastic contamination is lacking. A possible fate of microplastics in freshwater environments is to become entangled or aggregated in biofilms, which are matrices of algae, bacteria, and micro invertebrates that grow on underwater surfaces, following a progression of settling algae, periphyton, and finally invertebrate colonization. This in-situ study at the Oasis Marina at National Harbor in Oxon Hill, Maryland, examined how the taxonomic assemblages of freshwater biofilms in the Potomac River are associated with the number of microplastics aggregated within them. Aluminum discs, acting as artificial substrate for biofilm growth, were deployed at the water's surface and at 2 m depth to survey biofilm assemblage and were sampled monthly from October 2021-October 2022. Microplastic abundances in the water column were measured every 2 weeks over the same period. Spatial and temporal trends in trapped and suspended microplastics, water quality parameters (temperature, dissolved oxygen, pH, salinity, conductivity, turbidity, ammonia, nitrate, and phosphate), and biofilm assemblages were measured and compared to explore factors affecting the abundance of microplastics and their partitioning between the water column and biofilms. Water quality had no measurable impact on microplastic abundance in the water column at either depth, but temperature was negatively correlated to microplastic abundance in biofilms. As the weather warmed and biofilms progressed to invertebrate settling, they tended to contain fewer microplastics. This may have occurred because less biologically rich biofilms, primarily composed of unicellular algal colonies, provide a favorable surface for microplastic deposition. Understanding seasonal changes in biofilm assemblage and microplastic abundance may help track the fate of microplastics in freshwater systems, particularly in their interactions with lower trophic organisms.

Submarine groundwater discharge data at meter scale (223Ra, 224Ra, 226Ra, 228Ra and 222Rn) in Indian River Bay (Delaware, US).

Submarine groundwater discharge (SGD) was sampled at high-spatial resolution in Indian River Bay, DE, USA, in July 2016 to characterize the spatial variability of the activity of the radium and radon isotopes commonly used to estimate SGD. These data were part of an investigation into the methods and challenges of characterizing SGD rates and variability, especially in the coastal aquifer transition from freshwater to saltwater (Hydrogeological processes and near shore spatial variability of radium and radon isotopes for the characterization of submarine groundwater discharge (Duque et al., 2019)). Samples were collected with seepage meters and minipiezometers to obtain sufficient volumes for analytical characterization. Seepage meter samples (for 223Ra, 224Ra, 226Ra, and 228Ra) were collected at two-hour intervals over a semi-diurnal tidal cycle from 30 seepage meters. Samples for 222Rn characterization were collected with a minipiezometer from 25 cm below the bay bed at each seepage meter location. All samples were analyzed with standard and state of the art procedures.

Caffeine and agricultural pesticide concentrations in surface water and groundwater on the north shore of Kauai (Hawaii, USA).

Caffeine has been associated with wastewater pollution in temperate and subtropical locations, but environmental caffeine concentrations in tropical locations have not been reported. The objectives of this study were to measure caffeine and agricultural pesticide (carbaryl, metalaxyl, and metribuzin) concentrations in environmental waters on the tropical north shore of Kauai (Hawaii, USA) and assess whether patterns in caffeine concentration were consistent with a wastewater caffeine source. Groundwater, river, stream and coastal ocean samples were collected in August 2006 and February 2007. Caffeine was detected in all August 2006 samples and in 33% of February 2007 samples at concentrations up to 88ngL(-1). Metribuzin was detected in five samples collected in February 2007. Carbaryl and metalaxyl were not detected in any sample. Caffeine was not detected in offshore ocean samples or river samples upstream of human development. A positive correlation between caffeine and enterococci suggested a possible wastewater caffeine source.

Submarine groundwater discharge of total mercury and monomethylmercury to central California coastal waters.

Fluxes of total mercury (Hg(T)) and monomethylmercury (MMHg) associated with submarine groundwater discharge (SGD) at two sites onthe central California coast were estimated by combining measurements of Hg(T) and MMHg in groundwater with the use of short-lived, naturally occurring radium isotopes as tracers of groundwater inputs. Concentrations of Hg(T) were relatively low, ranging from 1.2 to 28.3 pM in filtered groundwater, 0.8 to 11.6 pM in filtered surface waters, and 2.5 to 12.9 pM in unfiltered surface waters. Concentrations of MMHg ranged from < 0.04 to 3.1 pM in filtered groundwater, < 0.04 to 0.53 pM in filtered surface waters, and 0.07 to 1.2 pM in unfiltered surface waters. Multiple linear regression analysis identified significant (p < 0.05) positive correlations between dissolved groundwater concentrations of Hg(T) and those of NH4+ and SiO2, and between dissolved groundwater concentrations of MMHg and those of Hg(T) and NH4+. However, such relationships did not account for the majority of the variability in concentration data for either mercury species in groundwater. Fluxes of Hg(T) via SGD were estimated to be 250 +/- 160 nmol day m(-1) of shoreline at Stinson Beach and 3.0 +/- 2.0 nmol m(-2) day(-1) at Elkhorn Slough. These Hg(T) fluxes are substantially greater than net atmospheric inputs of Hg(T) reported for waters in nearby San Francisco Bay. Calculated fluxes of MMHg to coastal waters via SGD were 10 +/- 12 nmol day(-1) m(-1) of shoreline at Stinson Beach and 0.24 +/- 0.21 nmol m(-2) day at Elkhorn Slough. These MMHg fluxes are similar to benthic fluxes of MMHg out of surface sediments commonly reported for estuarine and coastal environments. Consequently, this work demonstrates that SGD is an important source of both Hg(T) and MMHg to coastal waters along the central California coast.

Identifying sources of nitrogen to Hanalei Bay, Kauai, utilizing the nitrogen isotope signature of macroalgae.

Sewage effluent, storm runoff, discharge from polluted rivers, and inputs of groundwater have all been suggested as potential sources of land derived nutrients into Hanalei Bay, Kauai. We determined the nitrogen isotopic signatures (delta(15)N) of different nitrate sources to Hanalei Bay along with the isotopic signature recorded by 11 species of macroalgal collected in the Bay. The macroalgae integrate the isotopic signatures of the nitrate sources over time, thus these data along with the nitrate to dissolved inorganic phosphate molar ratios (N:P) of the macroalgae were used to determine the major nitrate source to the bay ecosystem and which of the macro-nutrients is limiting algae growth, respectively. Relatively low delta(15)N values (average -0.5% per hundred) were observed in all algae collected throughout the Bay; implicating fertilizer, rather than domestic sewage, as an important external source of nitrogen to the coastal water around Hanalei. The N:P ratio in the algae compared to the ratio in the Bay waters imply that the Hanalei Bay coastal ecosystem is nitrogen limited and thus, increased nitrogen input may potentially impact this coastal ecosystem and specifically the coral reefs in the Bay. Identifying the major source of nutrient loading to the Bay is important for risk assessment and potential remediation plans.