Surface Water Quality Modelling Considering Riparian Wetlands

water quality;hydrology;Civil Engineering

Jenkinson, Robert Wayne

University of Waterloo

关键词: water quality;    hydrology;    Civil Engineering;   

Others  :  https://uwspace.uwaterloo.ca/bitstream/10012/4279/1/Jenkinson_PhDThesis_Final_1.2.pdf

瑞士|英语

来源: UWSPACE Waterloo Institutional Repository

【 摘 要 】

Riparian wetlands are believed to play an important role in mitigating non-point source pol-lution, acting as physical and biochemical buffers between diffuse pollution sources and receivingwaters. Many studies examined riparian wetlands at the field scale, but there is a dearth of re-search at the watershed scale, particularly in the region of Southern Ontario, where agriculturalland use predominates.This study examined the impacts of riparian wetlands on surface water quality at the water-shed scale. A field study was conducted on two sub-watersheds at the northern headwaters of theCanagagigue Creek within the Grand River Watershed in Southern Ontario. The two watershedswere similar in area and land use but with differing riparian wetland extent adjacent to the sub-watershed main channels. A two-year study was conducted examining the hydrology, hydraulics,water quality and nutrient fluxes from the two sub-basins. Water quality data were obtained atthe outlet of each sub-basin during base-flow conditions and during 16 rainfall and snow meltrunoff events. The hydrology was simulated using the WatFlood model and the water quality(nitrate and total suspended solids) was simulated using an enhancedWatFlood/AGNPS modelthat was modified to account for continuous simulation, in-stream contaminant fate/transportand riparian wetland influences.The hydraulics and hydrological characteristics of the two basins were distinct. The basinwithout riparian wetland protection (;;West Basin”) exhibited ephemeral tendencies, going dryfor several months in the summer, whereas the basin with extensive riparian wetland protection(;;East Basin”) showed a persistent base-flow throughout the year with a consistently more rapidhydrological response. This study showed higher nutrient concentrations including nitrate, totalnitrogen (TN), and total phosphorus (TP) in the West basin than the East basin, attributedto the lack of riparian wetland protection in the West sub-basin. Total Suspended Solids (TSS)concentration were higher in the east sub-basin than the west sub-basin attributed to differencesin sediment grain size distributions and differences in local stream bed slope. Constituent loadingestimates from the two sub-basins were conducted on an event-basis and on an average monthlyload basis. This study showed that during events most constituents (Nitrate, TP, and TSS) weredischarged in greater quantities from the East sub-basin than the West sub-basin for both rainfalland snowmelt events. Event-based TN loading was also higher for the East sub-basin but thedifference was not statistically significant. Monthly average loading was significantly higher inthe East sub-basin than the West sub-basin for Nitrate, TN and TSS. Monthly average loading was higher in the East basin than the West basin for TP as well, but the difference was notstatistically significant. In spite of the generally higher nutrient concentrations in the West sub-basin, the east sub-basin exhibits higher loads due to the differing hydrological conditions in thatbasin. The persistent stream flow in the East basin continuously transports nutrients of a lowerconcentration than the West, but the consistent flow dominates the loading calculations resultingin a greater constituent mass transported.The modelling of sediment and nitrogen loading was conducted over the study period. Sedi-ment modelling results showed that the dominant process in the model was in-channel transportwith the calibrated model showing very little sensitivity to overland transport parameters andriparian wetland retention. The ability to hydrologically model the basin accurately dictated theperformance of the sediment transport model. Nitrogen modelling results demonstrated an abilityto generally simulate the nitrogen profiles trends during storm events. However, the WatFloodgroundwater storage model provided limitations in terms matching the nutrient concentrationvariability observed in the measured data. The processes that dominated model performancewere fertilizer loading and nitrogen mineralization coefficients, with the riparian wetlands playinga small role in nitrogen removal in the calibrated model.