The objectives of this study were to quantitatively estimate the distribution of arsenic with its speciation and to identify potential pathways for transformation of arsenic species from samples of water, sediments, and plants in the ecosystem affected by the Cheongog Spring, where As(V) concentration reached levels up to 0.270 mg L−1. After flowing about 100 m downstream, the arsenic level showed a marked reduction to 0.044 mg L−1 (about 84% removal) without noticeable changes in major water chemistry. The field study and laboratory hydroponic experiments with the dominant emergent plants along the creek (water dropwort and thunbergian smartweed) indicated that arsenic distribution, reduction, and speciation appear to be controlled by, (i) sorption onto stream sediments in exchangeable fractions, (ii) bioaccumulation by and possible release from emergent plants, and (iii) transformation of As(V) to As(III) and organic species through biological activities. Biogeochemical reactions with emergent plants and sediments control the fate of arsenic along creeks originating from a high-As Spring.

As wastewater treatment systems that strengthen natural processes, constructed wetlands provide both ecosystem services and disservices. Manipulating both the physical and ecological structures of constructed wetlands has been the key to improve ecosystem services while reducing disservices. Herein, an experiment using simulated constructed wetlands was conducted to explore the effect of two different substrate sizes (fine sand or coarse sand), plant richness (1, 3, or 4 species), and plant species identity on ecosystem services. Results indicated that (1) only in microcosms with coarse sand, species richness enhanced nitrogen removal efficiency while reduced nitrous oxide emissions and that (2) the presence of Phalaris arundinacea increased nitrogen removal rate, and the presence of Rumex japonicus or Oenanthe javanica decreased nitrous oxide emissions; (3) however, the net ecosystem services (nitrogen removal, greenhouse gas emissions, biofuel production) of microcosms with fine sand were higher than those of microcosms with coarse sand, and (4) interestingly, there was no difference in net ecosystem services between microcosms with coarse sand (1033 yuan ha⁻¹ day⁻¹; 1 yuan ≈ 0.14 USD) and those with fine sand (1071 yuan ha⁻¹ day⁻¹) for the four-species mixtures. Hence, in practice, ensuring plant species richness with appropriate species in microcosms with coarse sand can improve ecosystem services to a level equal to that of microcosms with fine sand and help to prevent constructed wetlands from clogging.

Constructed wetlands (CWs) have been widely used for treating wastewater. CWs also are the sources of greenhouse gas (GHG) due to high pollutant load. It has been reported that plant species diversity can enhance nitrogen (N) removal efficiency in CWs for treating wastewater. However, the influence of plant species diversity on GHG emissions from CWs in habitats with high N levels still lack research. This study established four species richness levels (1, 2, 3, 4) and 15 species compositions by using 75 simulated vertical flow CWs microcosms to investigate the effects of plant species diversity on the GHG emissions and N removal efficiency of CWs with a high N level. Results showed plant species richness reduced nitrous oxide (N₂O) emission and N (NO₃ ⁻-N, NH₄ ⁺-N, and TIN) concentrations in wastewater, but had no effect on methane (CH₄) emission. Especially, among the 15 compositions of plant species, the four-species mixture emitted the lowest N₂O and had under-depletion of N (DₘᵢₙTIN < 0). The presence of Oenanthe javanica had a significantly negative effect on the N₂O emission but had no effect on N removal efficiency. The presence of Rumex japonicus significantly reduced the N (NO₃ ⁻-N and TIN) concentrations in wastewater but had no effect on the N₂O and CH₄ emissions. The N concentrations and GHG emissions in the community of R. japonicus × Phalaris arundinacea were as low as those in the four-species mixture. Assembling plant communities with relatively high species richness (four-species mixture) or particular composition (R. japonicus × P. arundinacea) could enhance the N removal efficiency and reduce the GHG emissions from CWs for treating wastewater with a high N level.