Spatiotemporal Changes in Trace Metal Bioavailability in the Sediment Pore water of a Constructed Wetland Using Passive Pore water Samplers

Abstract

Sediments in aquatic systems often act as a major sink for contaminants. Diffusive gradient in thin films (DGTs) and in situ equilibrium dialysis samplers (peepers) are two major in situ porewater sampling devices that overcome the problems associated with conventional porewater sampling methods. In this study, DGTs and peepers were used to study the spatial and seasonal effects (cool months: October‐February, warm months: May‐September) on metal bioavailability in the H‐02 constructed wetland, and the sink vs. source role of the sediments by calculating the metal resupply capacity (R). Data showed similar seasonal trends in metal concentrations using both passive samplers, peepers and DGTs. Pooled Cu and Zn concentrations measured using DGTs were lower in warm months (1.67 ± 1.50 and 2.62 ± 0.68 μg.L‐1, respectively, p < 0.001) vs. (2.12 ± 0.65 and 5.58 ± 1.33 μg.L‐1, respectively, p < 0.001) in cool months (mean ± 95% Confidence Intervals). Sulfate (SO42‐) concentrations were significantly (p = 0.0139) lower in warm months (averaged at 0.22 ± 0.05 mg.L‐1) compared to (0.16 ± 0.05 mg.L‐1) in cool months. The increase in SO42‐ concentration is an indicator of the lower activity of sulfate reducing bacteria (SRB) which need SO42‐ during the anaerobic respiration, in which SO42‐ is reduced to sulfide (S2‐) which forms insoluble salts with Cu and Zn, which could partially explain higher bioavailability of these metals in the cool season. Metal resupply capacity of the sediments was mostly < 0.2 for Cu and Zn. Taken together, the H0‐2 wetland sediments mostly acted as a sink to both Cu and Zn over the course of this study.

Microbial Community Changes across Time and Space in a Constructed Wetland

“…Compared to the cool months, which exhibited a population of 5.37 × 10 06 ± 25 g.cell –1 , the Desulfuromonas population was significantly ( p <0.0001) lower during the warm months (1.70 × 10 04 ± 2 g.cell –1 ) because sulfur oxidation is less likely as O 2 solubility decreases with increasing temperatures, implying less sulfur oxidation and more sulfur staying in reduced forms (Table S2). Qin et al, 2022, and Elhaj Baddar et al, 2023, recently reported that sulfate concentrations in the pore water of the H-02 constructed wetlands were significantly lower during the warm months. So, it is highly likely that the higher sulfate concentrations in the cool months were not due to increased SO 4 2– production, but rather to the lower rates of SO 4 2– removal by the smaller SRB population.…”

“…This study constituted a comprehensive 10-month investigation spanning both the cooler period (from October 2021 to February 2022) and the warmer season (from May 2022 to September 2022), as extensively detailed in ref . In brief, passive water samplers were deployed once a month throughout the specified time frame mentioned above, with each deployment lasting 8 days.…”