Monitoring for Holding Pond Seepage
To determine potential seepage of liquid from the manure holding ponds, an interceptor trench 200 ft. in length was constructed approximately 150 feet downslope of the holding ponds. Any leakage from the ponds would initially move vertically, but horizontal flow will be induced at any permeability contrast in the subsurface. The trench was designed to intercept horizontal flow that would be induced over a low-permeability restricting layer existing at a soil depth of 4 feet that was identified during site evaluation. The trench was excavated to about 10 feet below the base of the holding ponds. The trench was designed to drain to outlets at both ends, with a minimum design gradient of 0.25 inch/ft. maintained from the high midpoint of the trench to each of the two outlets to ensure unimpeded flow.
The base of the interceptor trench was lined with impermeable plastic sheeting to capture all flow moving through trench walls into the trench. A 4-inch perforated pipe was laid on top of the plastic sheet and the trench filled with washed gravel to a depth of 2-3 feet below the soil surface. The remaining trench was then backfilled with compacted soil and seed and hay placed on the surface to encourage surface vegetation regrowth. The design will capture any flow moving horizontally into the trench, and direct flow for collection and transmittal to the outlets where flow rate can be measured and samples is collected for water-quality analyses. Water exiting the interceptor trench pipe outlets can be sampled by hand. We will instrument both pipes with flow monitoring, sonde and ISCO sampler to collect continuous flow and chemistry information.