Surface Water Quality Modelling Considering Riparian Wetlands

Riparian wetlands are believed to play an important role in mitigating non-point source pol-
\nlution, acting as physical and biochemical buffers between diffuse pollution sources and receiving
\nwaters. Many studies examined riparian wetlands at the field scale, but there is a dearth of re-
\nsearch at the watershed scale, particularly in the region of Southern Ontario, where agricultural
\nland use predominates.
\nThis study examined the impacts of riparian wetlands on surface water quality at the water-
\nshed scale. A field study was conducted on two sub-watersheds at the northern headwaters of the
\nCanagagigue Creek within the Grand River Watershed in Southern Ontario. The two watersheds
\nwere similar in area and land use but with differing riparian wetland extent adjacent to the sub-
\nwatershed main channels. A two-year study was conducted examining the hydrology, hydraulics,
\nwater quality and nutrient fluxes from the two sub-basins. Water quality data were obtained at
\nthe outlet of each sub-basin during base-flow conditions and during 16 rainfall and snow melt
\nrunoff events. The hydrology was simulated using the WatFlood model and the water quality
\n(nitrate and total suspended solids) was simulated using an enhancedWatFlood/AGNPS model
\nthat was modified to account for continuous simulation, in-stream contaminant fate/transport
\nand riparian wetland influences.
\nThe hydraulics and hydrological characteristics of the two basins were distinct. The basin
\nwithout riparian wetland protection (“West Basin”) exhibited ephemeral tendencies, going dry
\nfor several months in the summer, whereas the basin with extensive riparian wetland protection
\n(“East Basin”) showed a persistent base-flow throughout the year with a consistently more rapid
\nhydrological response. This study showed higher nutrient concentrations including nitrate, total
\nnitrogen (TN), and total phosphorus (TP) in the West basin than the East basin, attributed
\nto the lack of riparian wetland protection in the West sub-basin. Total Suspended Solids (TSS)
\nconcentration were higher in the east sub-basin than the west sub-basin attributed to differences
\nin sediment grain size distributions and differences in local stream bed slope. Constituent loading
\nestimates from the two sub-basins were conducted on an event-basis and on an average monthly
\nload basis. This study showed that during events most constituents (Nitrate, TP, and TSS) were
\ndischarged in greater quantities from the East sub-basin than the West sub-basin for both rainfall
\nand snowmelt events. Event-based TN loading was also higher for the East sub-basin but the
\ndifference was not statistically significant. Monthly average loading was significantly higher in
\nthe 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 not
\nstatistically significant. In spite of the generally higher nutrient concentrations in the West sub-
\nbasin, the east sub-basin exhibits higher loads due to the differing hydrological conditions in that
\nbasin. The persistent stream flow in the East basin continuously transports nutrients of a lower
\nconcentration than the West, but the consistent flow dominates the loading calculations resulting
\nin a greater constituent mass transported.
\nThe modelling of sediment and nitrogen loading was conducted over the study period. Sedi-
\nment modelling results showed that the dominant process in the model was in-channel transport
\nwith the calibrated model showing very little sensitivity to overland transport parameters and
\nriparian wetland retention. The ability to hydrologically model the basin accurately dictated the
\nperformance of the sediment transport model. Nitrogen modelling results demonstrated an ability
\nto generally simulate the nitrogen profiles trends during storm events. However, the WatFlood
\ngroundwater storage model provided limitations in terms matching the nutrient concentration
\nvariability observed in the measured data. The processes that dominated model performance
\nwere fertilizer loading and nitrogen mineralization coefficients, with the riparian wetlands playing
\na small role in nitrogen removal in the calibrated model.

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