Abstract
Constructed wetlands (CWs) are used to remediate runoff from a variety of agricultural, industrial, and urban sources. CW remediation performance is often evaluated at the laboratory scale over durations less than one year. The purpose of this study was to characterize the effect of CW design (cell depth) and residence time on nitrogen (N) speciation and fate across season and years in two free water surface wetlands receiving runoff from irrigated plant production areas at an ornamental plant nursery. Water quality (mg·L−1 of nitrate, nitrite, and ammonium, dissolved oxygen and oxidation reduction potential) was monitored at five sites within each of two CWs each month over four years. Nitrate-N was the dominant form of ionic N present in both CWs. Within CW1, a deep cell to shallow cell design, nitrate comprised 86% of ionic N in effluent. Within CW2, designed with three sequential deep cells, nitrate comprised only 66% of total N and ammonium comprised 27% of total N in CW2 effluent. Differences in ionic N removal efficacies and shifts in N speciation in CW1 and CW2 were controlled by constructed wetland design (depth and hydraulic retention time), the concentration of nutrients entering the CW, and plant species richness.
Highlights
Constructed wetlands (CWs) are engineered biological treatment systems used by agricultural producers [1,2,3], stormwater managers in urban and suburban communities [4,5,6], wastewater treatment plants [7,8,9], industry [10,11], and landfills to remove contaminants from leachate, stormwater runoff and wastewater using physical, chemical and biological treatment processes
Ammonium and nitrate were the dominant forms of nitrogen in waters flowing from Sites 1.1, 1.2, and 1.3 into Constructed Wetland 2 (CW2) Site 2 (Figure 2, Table 2)
Ammonium concentrations are greater in the inflow ditch than in the retention pond or inflow to Constructed Wetland 1 (CW1) from April (p = 0.031) to July (p = 0.019)
Summary
Constructed wetlands (CWs) are engineered biological treatment systems used by agricultural producers [1,2,3], stormwater managers in urban and suburban communities [4,5,6], wastewater treatment plants [7,8,9], industry [10,11], and landfills to remove contaminants from leachate, stormwater runoff and wastewater using physical (settling), chemical (sorption and oxidation reduction potential) and biological (microbial and plant uptake) treatment processes. Many studies evaluating contaminant remediation efficacy within CWs are laboratory and mesocosm-scale experiments [1,10,15]. Laboratory and mesocosm scale studies are highly useful to parameterize factors that influence remediation efficacy for specific contaminants and design parameters because experimental replication at a smaller scale is less expensive. Extrapolation of laboratory scale CW functionality to field-scale applications typically result in over-estimation of contaminant remediation potential and may inadequately simulate changes in plant growth and physicochemical parameters within the CW [16,17]. Laboratory scale experiments are a critical step toward determining how design factors influence remediation efficacy; only field-scale applications permit accurate validation of predicted remediation efficacy as influenced by specific design factors
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