Field water balance of landfill final cover

Protection of the environment from contaminants in solid waste landfills is a matter of growing interest. To date there has been no geographically dispersed quantitative evaluation of cover performance at landfill sites. A systematic study was conducted of 24 cover test sections at 11 sites across the US to evaluate the field-scale performance of both conventional and alternative cover designs. Water balance variables of the covers were monitored with large instrumented drainage lysimeters. Average drainage rates for alternative covers ranged between 33 and 160 mm/yr (6 and 18% of precipitation) in humid climates and generally less than 2.2 mm/yr (0.4% of precipitation) in arid/semi-arid/sub-humid climates. One-half (five) of the alternative covers in arid/semi-arid/sub-humid climates transmitted less than 0.1 mm of percolation, but two transmitted much more percolation than expected. Conventional covers with composite barrier systems performed well and limited drainage to <1.5% of precipitation (0--12 mm/yr) at all sites when proper geomembrane installation procedures were followed during construction. Drainage through the composite covers was typically coincident with heavy precipitation, field capacity water contents of the relatively thin surface soils and lateral flow across the geomembrane. The initial performance of the three conventional compacted clay covers confirmed that current construction guidance is sufficient for successful placement of the low-conductivity layers on which these designs are based. The performance of all compacted clay covers deteriorated, however, during the monitoring period and drainage rates increased at all sites by a factor of 3--15 to 8--30% of precipitation (22--300 mm/yr). In situ measurements and laboratory analyses of large intact samples from one site after four years of field service showed increases in saturated hydraulic conductivity of about three orders of magnitude. Results presented here indicate that (i) properly designed alternative covers at arid and semi-arid sites and some sub-humid sites can be expected to limit drainage to <1% of precipitation, (ii) alternative covers in humid locations can limit drainage to 5--20% of precipitation and are unlikely to meet regulatory equivalency requirements at humid sites when the standard of comparison is a composite design, (iii) composite cover designs perform well at all sites, (iv) compacted clay covers that are not protected from the effects of the surface environment including desiccation, root penetration and freeze/thaw conditions are likely to fail within the design life time of the facility. (Abstract shortened by UMI.).