Presentations 2016

Evaluating the influence of tile drainage management on shallow groundwater resources

Aquanty

https://www.youtube.com/watch?v=tIVsKm_HMJ8&index=3&list=PLZC-5vj9_JAYJ9uI2abX-KQy56Nl6HTmI

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Tile drainage management is considered a beneficial management practice (BMP) for reducing nutrient loads in surface water. Previous research has shown that controlling tile discharge via control structures with adjustable stop gates can be very effective for reducing tile discharge volume, and that on an annualized basis there is often a reduction in nitrate loads that is proportional to the reduction in discharge. However, the influence of controlled drainage on groundwater resources is rarely considered. In this presentation we will discuss tile drainage experiments that were conducted in Ontario, Canada, that were designed to assess the influence of drainage management on the movement of nutrients and rhodamine WT to surface water and groundwater under controlled (CD) and free drainage (FD) tile management. We will also present results from 2-dimensional dual permeability modeling that was conducted to help develop a better understanding of the flow and transport processes within the soil profile and shallow groundwater system under different drainage management scenarios. Results from the modeling demonstrate that dominant flow and transport characteristics at the field site were successfully replicated, including higher, more continuous tile discharge and lower peak rhodamine concentrations in FD tile effluent; as compared to CD, where discharge was intermittent, peak rhodamine concentrations higher, and mass exchange from macropores into the soil matrix greater. Explicit representation of preferential flow in the modeling framework was essential, as macropores were shown to transmit > 98 % of surface infiltration, tile flow, and tile solute loads for both FD and CD. Incorporating an active 3rd type boundary across the bottom of the model domain in order to facilitate shallow groundwater outflow was imperative for simulating CD, as the higher (relative to FD) water table facilitated greater water and soluble nutrient movement from the soil profile into deeper groundwater. Scenario analysis revealed that in conditions where slight upwards hydraulic gradients exist in tile drained settings, groundwater upwelling can dilute the concentration of surface derived solutes under FD conditions; whereas the higher and flatter water table associated with CD can act to reduce groundwater upwelling. Results show that while CD can reduce tile discharge, and soluble nutrient loads and concentrations in tile effluent and hence surface water receptors, it can promote NO3 loading into groundwater.