Self-Sealing of Earthen Liquid Manure Storage Ponds: I. A Case Study

A monitoring system was established on an unlined, earthen storage pond near Kitchener, ON prior to the addition of liquid manure from a 4500-head beef (Bos taurus) feeding operation. The bottom of the pond, which had a surface area when full of 2 ha, was a coarse textured sand. This material, with some gravel layers, extended below the water table, which was initially at 13.7 m below the ground surface. A platform, at which soil moisture measurement and groundwater sampling tubes were installed, was constructed within the pond. Additional groundwater sampling tubes were installed at several points surrounding the pond. Moisture content of the soil immediately below the pond reached saturation when liquid manure was first added but began to decrease within 2 weeks and reached a steady state at a water potential of about −0.03 MPa within 90 d. The infiltration rate at this time was estimated to be less than 10⁻⁸ m s⁻¹, a value considered to indicate that the bottom was effectively sealed. There was a rapid increase in Cl content of the groundwater within 2 weeks of manure addition but the concentration declined to initial values within 12 weeks. There was no evidence of elevated Cl concentrations in groundwater outside the boundaries of the pond. The NO₃-N content of groundwater below the pond decreased to non-detectable values very shortly after addition of manure but returned to background values within 12 weeks except in the upper portions of the groundwater. A similar depression of NO₃-N was observed at one sampling position within a few meters of the pond but not at more distant points. It was concluded that the NO₃-N depression was due primarily to denitrification in the groundwater as it passed below the pond. It is concluded that, with some limitations, unlined earthen manure ponds are environmentally acceptable, even in sandy material.