Exploring Manganese, Zinc and Iron behavior in a Constructed Wetland

The constructed wetland on Western Washington University’s (WWU) south campus has been a constant presence in mitigating the campus’s stormwater runoff. This area is sampled regularly as part of the Environmental Science’s Water Quality course, taught every quarter throughout the year for the past decade. However, only recently have metals been sampled, where in Fall of 2023 metal concentrations (ppb) decreased from the inlet to the outlet (manganese concentrations fell from 264.0 ppb to 76.1 ppb; zinc concentrations fell from 13.5 ppb to 5.47 ppb). In Spring of 2023, manganese concentrations on average increased from the inlet to the outlet (7.57 ppb to 461.0 ppb). The Spring 2023 data seems incongruent with current literature which suggests that metal concentrations decrease after passing through a constructed wetland as they are subject to uptake from sedimentation and vegetation, which are each additionally influenced by redox conditions. As a result of this knowledge gap, from late January 2025 through mid-May 2025, the redox conditions of WWU’s south campus constructed wetland were investigated to determine the speciation, bioavailability, transport, and retention of three metals: zinc, manganese, and iron. Samples were collected from the incoming parking lot stormwater runoff, the inlet of the wetland, the wetland’s outlet, and connecting surface water of Connelly Creek. Aqueous samples were collected and water quality parameters assessed (temperature, pH, conductivity, and ORP). Metal concentrations were analyzed using inductively coupled plasma-mass spectrometry (ICP-MS). It was determined that temperature, conductivity, pH, and ORP did not provide enough information to fully explain the data collected, showing that future work involving more sampling sites, parameters, and year-long sampling would benefit the future understanding of this constructed wetland.