A Field-Scale Study of Controlled Tile Drainage and a Pond-Wetland to Attenuate Nutrients from Agricultural Overland Runoff and Subsurface Drainage on a Farmer-Operated Seed Farm in Saint-Isidore, ON

Abstract

Agricultural runoff and drainage are significant contributors to the deterioration of water quality in receiving water bodies. Ponds and constructed wetlands are proven treatment systems that can remove nutrients and reduce peak flow. A combined controlled drainage structure (CDS), free drainage structure (FDS) and pond-wetland system was installed on a 64-acre seed farm in St. Isidore, Ontario as a flood mitigation and nutrient attenuation measure. The effectiveness of the combined use of controlling drainage and a pond-wetland system to attenuate flow, solids and nutrients was studied during the growing season (GS) and non-growing season (NGS) of 2017. The GS was subdivided into wet (GS-W) and dry (GS-D) periods.

The mean daily precipitation was 3.95±8.86 mm d⁻¹ and the mean daily flow at the drainage structures was greater (p>0.05) at the CDS (1.05±1.59 mm d⁻¹) than the FDS (0.58±0.93 mm d⁻¹) during the GS-W and significantly lower between the CDS (0.13±0.21 mm d⁻¹) and the FDS (0.21±0.13 mm d⁻¹) during the GS-D (p≤0.05). In the NGS, all DS were free flowing. In comparison to the GS, however, flow was reduced at the CDS (0.41±0.64 mm d⁻¹) in comparison to the FDS (0.56±0.81 mm d⁻¹). Reductions in turbidity (81%), and concentrations of TSS (40%), NH₄⁺-N (19%), NO₃⁻-N (43%), soluble reactive phosphorus or SRP (78%) were observed during the GS-D. Mean daily mass flux reductions of TSS (53%), NH₄⁺-N (54%), NO₃⁻-N (46%) and SRP (82%) were observed in the dry period of the growing season between the CDS and the FDS.

Composite flow data over the study period indicate that on average, the pond-wetland inflow was reduced by 17% compared to the pond-wetland outflow. Reductions in turbidity (52%), and concentrations of TSS (55%), TP (21%) and NH₄⁺-N (27%) were observed between the pond inlet and outlet for the study period. The combined flow and nutrient attenuation contributed to effective mass removal of nutrients daily and seasonally. On average, the pond-wetland system reduced mean mass flux of TSS (33%), TP (43%), TN (41%), NH₄⁺-N (61%). The pond-wetland attenuated NO₃-N (48%) and SRP (27%) mass in the GS-W and released both nutrients in the GS-D and NGS suggesting an effect of seasonality for nitrate and SRP attenuation in small pond-wetland systems. These results show that a combined controlled drainage and pond-wetland system can be a beneficial tool to reduce flow as well as reduce the impact of nutrient migration from farmlands with a low economic footprint, especially in the context of particulate-bound nutrients.