Hydrobiogeochemistry of Constructed Wetlands: Intertwined Relationships of Redox Buffer Capacity and Water Treatment Performance

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biological and Environmental Sciences

Date of Award

Summer 8-18-2025

Abstract

Constructed wetlands (CWs) are engineered ecosystems that utilize natural processes to improve water quality. This study examines the hydrogeochemistry and treatment performance of the George W. Shannon Wetland (constructed) in Texas, with a focus on evaluating the resistance and resilience of wetland towards the disturbance of raw Trinity River water injection by calculating redox buffer capacity with spatial and seasonal variations. Water and soil samples were collected from the sedimentary cell, reaction cell, control cell, and outflow cell. Results showed a progressive decrease in electrical conductivity (EC) and total dissolved solids (TDS) along the wetland flow path, from the control cell (EC: 585.8 µS/cm, TDS: 295.4 mg/L) to the outflow cell (EC: 458.1 µS/cm, TDS: 230 mg/L), indicating effective ion removal. Surface soils in the reaction cell exhibited the highest concentrations of nitrate (64.5 mg/kg), sulfate (2740 mg/kg), and potassium (75.1 mg/kg), while depth soils across all cells retained more calcium and magnesium but had lower nitrate and sulfate, indicating prolonged anoxic conditions that favor the retention of divalent cations and reduction of nitrate and sulfate. The reaction cell demonstrated the most dynamic redox behavior, with Eh values ranging from -800 to 500 mV whereas the control cell showed Eh values ranging from 100 to 500 mV and the sedimentary cell ranged from 100 to 400 mV. Throughout the study, calcium (Ca²⁺) and bicarbonate (HCO₃⁻) consistently emerged as the dominant ions in water highlighting the carbonate-buffered nature of the system. PHREEQC modeling identified carbonate dissolution and sulfate reduction as key geochemical processes in the wetland system. Resistance and resilience calculations indicated that the Reaction Cell was more vulnerable to disturbances (resistance: ~4.14; resilience: ~0.07) compared to the Control Cell (resistance: ~0.06; resilience: ~0.92), and Sedimentary Cell (resistance: ~0.27; resilience: ~0.33) during the monitoring period. Across all cells, C/N ratios in surface and depth soils were relatively high (8.91-10.67), especially in the outflow cell, suggesting increased nitrogen immobilization and reduced nitrate removal efficiency. Treatment performance data showed high removal efficiencies for sulfate (78.75%) and phosphorus (45.9%), but lower nitrate removal (20.9%) during the monitoring period. Compared to previous field-scale studies, phosphorus removal efficiencies in this study were similar (phosphorus: 45.9% vs. 45.4% previously), but nitrate removal was substantially lower than the 76.8% removal reported in earlier work, indicating a limitation in the current system’s capacity for denitrification. These findings highlight the importance of cell-specific and seasonal hydrobiogeochemical processes in optimizing constructed wetland design for sustainable and effective pollutant mitigation.

Advisor

Naima Khan

Subject Categories

Environmental Sciences | Physical Sciences and Mathematics

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