Detection of heavy metals and VOCs in streambed sediment indicates anthropogenic impact on intermittent streams of the U.S. Virgin Islands.

Global surges in industrialization and human development have resulted in environmental contamination. Streambed sediment contamination threatens ecological and human health due to groundwater leaching and downstream contaminant mobilization. This is especially true in the wider Caribbean region, where streambed sediment contamination is understudied. In the current study, we assessed human impact on intermittent streams by measuring heavy metals and volatile organic compounds (VOCs) in streambed sediment on St. Croix in the United States Virgin Islands (USVI), where intermittent streams receive limited conservation and research attention. In contrast to our hypothesis that streambed sediment pollutant concentrations would be higher in developed, compared to undeveloped areas, contaminant concentrations did not vary significantly according to land cover. Elevated lead, mercury, and zinc concentrations were correlated with commercial building density, suggesting an unnatural origin of these elements in streambed sediment. At some sites, levels of arsenic, cadmium, chromium, nickel, lead, thallium, or zinc exceeded regulatory limits. The most prevalent VOCs at both developed and undeveloped sites were benzene and toluene. Sub-groups of heavy metals identified by principal component analysis indicated potential pollution sources, including fuel combustion (chromium, nickel, arsenic, selenium), vehicle exhaust, oil refining, and gasoline leaks (2-butanone and xylenes), and plastics (acetone and styrene). Our results suggest USVI intermittent streams require further research attention and intervention strategies for pollution reduction.

Greenhouse gas emissions from advanced nitrogen-removal onsite wastewater treatment systems.

Advanced onsite wastewater treatment systems (OWTS) designed to remove nitrogen from residential wastewater play an important role in protecting environmental and public health. Nevertheless, the microbial processes involved in treatment produce greenhouse gases (GHGs) that contribute to global climate change, including CO2, CH4, N2O. We measured GHG emissions from 27 advanced N-removal OWTS in the towns of Jamestown and Charlestown, Rhode Island, USA, and assessed differences in flux based on OWTS technology, home occupancy (year-round vs. seasonal), and zone within the system (oxic vs. anoxic/hypoxic). We also investigated the relationship between flux and wastewater properties. Flux values for CO2, CH4, and N2O ranged from -0.44 to 61.8, -0.0029 to 25.3, and -0.02 to 0.23 µmol GHG m-2 s-1, respectively. CO2 and N2O flux varied among technologies, whereas occupancy pattern did not significantly impact any GHG fluxes. CO2 and CH4 - but not N2O - flux was significantly higher in the anoxic/hypoxic zone than in the oxic zone. Greenhouse gas fluxes in the oxic zone were not related to any wastewater properties. CO2 and CH4 flux from the anoxic/hypoxic zone peaked at ~22-23 °C, and was negatively correlated with dissolved oxygen levels, the latter suggesting that CO2 and CH4 flux result primarily from anaerobic respiration. Ammonium concentration and CH4 flux were positively correlated, likely due to inhibition of CH4 oxidation by NH4+. N2O flux in the anoxic/hypoxic zone was not correlated to any wastewater property. We estimate that advanced N-removal OWTS contribute 262 g CO2 equivalents capita-1 day-1, slightly lower than emissions from conventional OWTS. Our results suggest that technology influences CO2 and N2O flux and zone influences CO2 and CH4 flux, while occupancy pattern does not appear to impact GHG flux. Manipulating wastewater properties, such as temperature and dissolved oxygen, may help mitigate GHG emissions from these systems.

Comparison of N2O Emissions and Gene Abundances between Wastewater Nitrogen Removal Systems.

Biological nitrogen removal (BNR) systems are increasingly used in the United States in both centralized wastewater treatment plants (WWTPs) and decentralized advanced onsite wastewater treatment systems (OWTS) to reduce N discharged in wastewater effluent. However, the potential for BNR systems to be sources of nitrous oxide (NO), a potent greenhouse gas, needs to be evaluated to assess their environmental impact. We quantified and compared NO emissions from BNR systems at a WWTP (Field's Point, Providence, RI) and three types of advanced OWTS (Orenco Advantex AX 20, SeptiTech Series D, and Bio-Microbics MicroFAST) in nine Rhode Island residences ( = 3 per type) using cavity ring-down spectroscopy. We also used quantitative polymerase chain reaction to determine the abundance of genes from nitrifying () and denitrifying () microorganisms that may be producing NO in these systems. Nitrous oxide fluxes ranged from -4 × 10 to 3 × 10 µmol NO m s and in general followed the order: centralized WWTP > Advantex > SeptiTech > FAST. In contrast, when NO emissions were normalized by population served and area of treatment tanks, all systems had overlapping ranges. In general, the emissions of NO accounted for a small fraction (<1%) of N removed. There was no significant relationship between the abundance of or genes and NO emissions. This preliminary analysis highlights the need to evaluate NO emissions from wastewater systems as a wider range of technologies are adopted. A better understanding of the mechanisms of NO emissions will also allow us to better manage systems to minimize emissions.