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An Experimental and Numerical Analysis of Soluble Reactive Phosphorus Removal Mechanisms in Surface-Flow Constructed Stormwater Wetlands Using Soil Amendment Strategies

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Description: Constructed stormwater wetland (CSW) systems were previously reported to be effective for phosphorus (P) removal in the stormwater control community. However, recent studies have found that CSWs are no longer valuable P retention tools past a certain point in their lifetime especially when the dominant form of P is soluble. It is the soluble inorganic form of P (soluble reactive P or SRP) that causes excessive algae growth, which leads to eutrophication, diminished macrophyte, periphyton, benthic, and fish communities, and depleted oxygen levels. The results presented in this research focused on the fate and transport of soluble reactive phosphorus (SRP) in a surface-flow constructed stormwater wetland. A two-phased laboratory mesocosm approach was used to study SRP fate and transport. The phase I objective was to observe and measure SRP pathways through a surface-flow CSW soil profile through a nonvegetated and vegetated mesocosm. Pore water samples were taken at various vertical depths (0, 3, and 8 cm) and horizontal locations throughout the soil profile, analyzed for SRP, and in some instances, chloride. Results from phase I showed there were both horizontal and vertical flow characteristics that ultimately affected SRP movement through the soil profile. Surface water had not fully infiltrated all horizontal and vertical depths prior to the end of the 24 hr experimental replicates. More SRP removal was observed at the inlet region of the mesocosm and within the top 3 cm. There was mass SRP removed within both the nonvegetated and vegetated mesocosms, indicating that sorption processes were the main mechanism of SRP removal as opposed to plant uptake. Phase II consisted of soil amendments mixed into wetland soil taken directly from Villanova University's CSW. Two soil amendments were used in phase II: Iron [Fe (III)] coated sand and Aluminum Water Treatment Residuals (Al WTR). Mesocosms were constructed similarly to phase I. Seven mesocosm studies were employed in phase II: a control, 5% iron coated sand by dry mass WITH plants (FeP), 5% iron coated sand by dry mass WITHOUT plants (Fe), and 2%, 2% duplicate, 5%, and 8% mix by dry mass of Al WTR. A P sorption isotherm analysis determined that the P sorption capacity of the control mesocosm (390 mg kg -1 ) was within the range of both the FeP and Fe mesocosms (380 mg kg -1 ), suggesting SRP removal would be comparable between these three mesocosms. Conversely, the P sorption capacity increases as more Al WTR was added to the original wetland soil (590 mg kg -1 ∼ 900 mg kg -1 ), suggesting mesocosms amended with Al WTR will provide the greatest SRP removal based on its chemical characteristics. Overall, the mesocosms amended with Fe (III) coated sand provided the largest mass of SRP removal amongst all added amendments. The pore water analysis for the mesocosms amended with Al WTR showed that the WTR provided less SRP removal than the control although the initial P sorption capacities were greater in the amended soil than the control. It was thought that the physics are inhibiting the chemical reaction from occurring. A similar conclusion can be made for the soil amended with Fe (III) coated sand: SRP removal using soil amendments is not solely dependent upon the amendment's chemical characteristics, but is also dependent upon its physical and subsequently hydraulic characteristics.
Language: English
Format: Degree Work