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8. The pre-1993 MSW landfill liner standard (compacted clay) was subject to considerable through-flow, by leachate, at all locations. By adding a rugged plastic membrane on top of the clay layer to produce a single-composite liner, the overall liner cost increased somewhat, but the net through-flow decreased significantly, because the clay layer can only pass liquid in those few locations where there is a flaw in the plastic membrane. Building a double- composite liner would be considerably more expensive than a single-composite liner, and would decrease the normal leakage rate from very small to very very small. In the absence of an effective final cover, it would eventually degrade just as would a single-composite liner. In light of these considerations, the SWRCB's existing single-composite liner standard continues to represent a good minimum design standard. RWQCBs can and should require more stringent design standards in any case where it is warranted, as the regulations indicate [27CCR20080(a)(1)]. 9. There are some emerging technologies that will identify problem areas in the plastic membrane, even in portions hidden from view. Once it matures, RWQCBs should be able to improve the post-construction integrity of single-composite liner(and, perhaps, final cover) systems by requiring the use of such technology. 10. The main problem with assessing the adequacy of the single-composite liner design is that, for any given installation, is that it is not possible to quantify the actual leakage rate. However, it is possible to make this determination, in a general sense, by collecting landfill upper-liner leakage rate data(e.g., gallons per acre per day) at landfills, nationwide, which have double-composite liners. In any case, the SWRCB may consider establishing a more stringent liner design standard should substantial factual evidence show that such a change is warranted.