Hydraulic analysis of a leachate collection system at a closed landfill was performed to evaluate the effectiveness of the collection system and the relationship between shallow ground water, surface water, and leachate. The leachate collection system was installed along the downgradient edge of the unlined landfill, where leachate seeps formerly discharged to surface water, including a tidally influenced river. The system includes a geomembrane cover on the downgradient sideslopes of the landfill, 4000 feet of lined leachate collection trench, pump stations, and a sanitary sewer force main conveying collected liquids to a wastewater treatment plant.

Operational pumping data suggested inflow of water into the system from outside the landfill. The hydraulic analysis included ground water and surface water level monitoring during periods of pumping and recovery of the collection system, at 24 shallow piezometers, drainage ditches, river, and leachate wet wells/pumping stations. The study evaluated the performance of the collection system, contribution of leachate vs. clean water, degree of hydraulic gradient control, and surface water (including tidal) interactions.

Analysis of the data collected indicated the leachate collection system was capturing leachate and shallow ground water from the landfill, preventing migration of leachate or ground water off the landfill and into adjacent surface water, as well as capturing some ground water from the other (non-landfill) side of the trench.