Forensic analysis of excessive leakage from lagoons lined with a composite GCL

A forensic investigation of excessive leakage from two lagoons is described. The lagoons were lined with a composite geosynthetic clay liner (GCL) consisting of a thin geomembrane laminated to one side of a conventional GCL (i.e. granular bentonite encased between two geotextiles). The GCL was installed with the geomembrane in contact with the subgrade. Layers of sand (150 mm) and gravel (300 mm) were placed on top of the GCL to protect it from wave action. Field-scale leakage rates were estimated by monitoring water levels with a graduated rod and by measuring the pressure at the bottom of each lagoon using pressure transducers. In addition, leakage through the GCL panels was measured in situ with sealed double-ring infiltrometers and in the laboratory using flexible-wall permeameters. Leakage rates for the overlaps were measured in the laboratory with flow box tests on GCL specimens from the lagoons. Tests were also conducted to assess the swelling capacity and ionic composition of the exchange complex of the bentonite component of the GCL, the ionic composition of the lagoon water, and the carbonate content of the soils adjacent to the GCL. Inspection of the GCL revealed minimal damage and showed that the installation was in general accordance with the manufacturer's recommendations. However, one of the lagoons had no supplemental bentonite in the panel overlaps, and the other lagoon had supplemental bentonite in less than one-half of the overlaps. Comparison of leakage rates estimated with the pressure transducers and those computed using the field and laboratory test data indicates that leakage through the overlaps without supplemental bentonite was the most likely cause of the excessive leakage. Leakage rates from overlaps without bentonite were as much as eight times higher than leakage rates from overlaps containing bentonite. Extensive ion exchange also occurred in the bentonite, but this factor appears not to have affected the leakage rate because of the hydraulic impedance provided by the geomembrane. Water contained in the lagoons is believed to be the primary source of the divalent and polyvalent ions involved in cation exchange.