Question 4 of 16: Manure in Tile Outlets

Question 4: Manure in Tile Outlets (5 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The absence of visible overland runoff does not rule out a tile discharge, because liquid manure can bypass the soil matrix entirely through preferential (macropore) flow — earthworm burrows, old root channels, and shrinkage cracks that connect the surface directly to subsurface tile drains. Even a "normal rate" surface application can travel this pathway to a tile outlet within hours, especially on a clay-loam soil that was dry (cracked) before application, on a field with shallow tile, or where an old, undocumented "blind" surface inlet or a break in a tile line sits near the application area.

What to tell the farmer: (1) this is a recognized and reportable risk distinct from surface runoff — a discharge to a tile drain that outlets to a watercourse can still be a reportable spill under the Ontario Water Resources Act / Environmental Protection Act even though no surface runoff occurred, so document the event and, if the tile discharges to a watercourse, contact the Ontario Spills Action Centre and the local conservation authority as a precaution; (2) stop applying to that field immediately and inspect it (and its tile map) for cracks, old surface inlets, or shallow tile runs near where manure was spread; (3) going forward, avoid liquid application on that field when the soil is cracked/dry or immediately before a rain event, consider incorporating/injecting rather than surface-applying to reduce macropore contact, and consider a controlled tile outlet or blind-inlet shutoff that can be closed during and immediately after application on high-risk fields.