By Glenn T. Darilek and Daren L. Laine
The logical criteria for specifying an action leakage rate (ALR) is not whether a certain rate is achievable with good construction practices, but whether there is a practical solution if the ALR is not achieved.
Specifying a low ALR can be a disaster if the source of the leakage cannot be located or logically explained. In that case, there is no practical solution other than to reline the facility and hope that the leak rate of the new geomembrane does not also exceed the low ALR.
When specifying an action leakage rate, it should not necessitate legal actions if the specification cannot be achieved with the existing technology. The practical approach is to specify an ALR to be compatible with the leak rate that would be caused by a leak (or leaks) that can be reliably located using geoelectric leak-location methods.
This article illustrates the use of a mathematical model of the geoelectric leak signal vs. size of the leak, and a mathematical model of the leak rate vs. size of the leak to give a new perspective of what is achievable and what is unrealistic for the specification of action leakage rates.
The geoelectric leak signal model takes into consideration the geomembrane thickness, leak diameter, and the survey parameters, and assumes that the resistivity of the material in the leak and the resistivity of
Action leakage rates should be specified to correspond to the leak rate that would be observed from the smallest reliably-detectable leak. Actually, even with such a specification, one assumes that the leakage is from one leak and not from many smaller undetectable leaks. The unsolvable problem continues to exist if the leakage is from a multitude of smaller undetectable leaks. So a safety factor must be applied to cover that contingency.
For geoelectric leak-location surveys, with only water covering the geomembrane and with the usually-specified ALRs, there is no problem currently because a properly-applied geoelectric survey can locate the very small leaks that would contribute to the ALRs.
Figure 1 shows a graph of the free flow leakage rate vs. the leak signal for some typical leak-location survey parameters for surveys with water on the geomembrane. This is only an illustration. Actual survey parameters will be different.
Because the geoelectric leak signal is related to the leak diameter, Figure 1 also has scales that correspond to the leak diameters that would cause the leak signals. Note that the leak diameter scales are not linear with the leak signal. ASTM D7007 currently specifies a 1.3mm (0.05in.) test leak.
Assuming free flow, which is the case with a properly designed leak-detection system, the graph shows that the leak rate from such a leak is about 80 and 180 gallons per day for water depths of 1m (3.3ft) and 5m (16ft), respectively. Such a leak is very practical to detect, and those leak rates are typically a fraction of the usual ALR for water-filled impoundments.
This illustration is for only one leak. For large impoundments, it is reasonable to assume there would be multiple leaks. If one would expect one leak per acre (0.4 hectare), the illustration can be extrapolated to multi-acre impoundments by using the dimensions of gallons per acre per day (gpa/d) instead of gallons per day (gpd). However, one cannot logically interpolate for impoundments less than one acre, because that would result in a leak rate that may not be located using leak-location methods.
In general, this analysis suggests that typical ALRs could be made more stringent for ponds larger than one acre.
Impoundments and landfills with earth materials on the geomembrane
The problem gets much more interesting with earth materials covering the geomembrane.
Specifying an ALR that is too low for a geomembrane covered with earth materials can result in a problem with no practical solution. This is because the geoelectric leak-detection sensitivity is lower with earth materials on the geomembrane.
The thickness of the earth materials prevents detection of the smallest leaks for two reasons. One is that the leak-detection measurements are made from a greater distance from the leaks. This distance is at least the depth of the earth materials. Secondly, there is much more measurement noise when making the measurements on the earth materials.
Figure 2 shows a graph of the leak rate vs. leak signal for some typical leak-location survey parameters with earth materials on the geomembrane. Again, this is only an illustration. The corresponding leak diameters are again shown on the abscissa. ASTM D7007 currently specifies a 6mm (0.236in.) test leak.
Assuming free flow, which is often the case with granular drainage materials or some geosynthetic materials above and below the geomembrane, the graph shows the leak rate from such a leak is about 950 and 1,700 gallons per day for typical hydrostatic heads of 0.3m (1ft) and 1m (3.3ft), respectively.
Such a leak is usually practical to detect, but these leakage rates are typically more than an order of magnitude higher than some present ALRs for landfills. This illustration shows that the existing typical ALRs can be unrealistic and can result in disastrous consequences that will not allow the permitting of the landfill cell.
A common geoelectric leak-location specification for landfills calls for detecting all leaks that could contribute to an ALR of 20 gallons per acre per day. The analysis below shows the paradox that meeting that geoelectric leak-location specification depends almost entirely on having intimate contact between the geomembrane and an underlying geosynthetic clay liner (GCL), or other leak-sealing layer, and not on the geoelectric leak-location survey.
It is an incorrect to assume a maximum of 1ft of water head on a landfill cell because it is common to have a few feet of water standing on the lowest part of a cell after rainfall or snowmelt. But assuming 1ft (0.3m) of head, Figure 2 shows that 20 gallons per day will flow through a 0.034-in.-diameter leak assuming free flow. For all practical purposes, free flow conditions will exist with such a small hole with drainage material above the geomembrane and a geonet or drainage material under the geomembrane.
Figure 2 shows the signal from such a leak is in the low-mill volt range. Measurement noise is typically much greater than that, so reliable detection of a leak of that size is problematic to say the least. Even without free flow conditions, the size of the leak will likely be small and probably undetectable.
In some cases, a saving grace for the specification of 20 gallons per acre per day is that a low permeability earth material or GCL often underlies the primary geomembrane. Therefore, the flow is greatly reduced, and larger leaks can be located or tolerated. However, if the geomembrane is not in intimate contact with the low-permeability layer, the unsolvable problem re-emerges.
Another approach is to test the bare geomembrane for leaks before the earth materials are placed to detect the smallest installation leaks and then assume no more small leaks will be caused as the earth materials are being placed on the geomembrane. Of course, another leak-location survey is needed to test for larger construction damage after the earth material is put on the geomembrane.
There has been at least one case where the ALR was set too low and the leaks could not be detected under 2 feet of sand. So the sand had to be removed from the lowest area of the landfill cell and that area had to be surveyed with water on the bare geomembrane. Needless to say, many more leaks were caused while removing the sand.
When the U.S. Environmental Protection Agency (EPA) promulgated the Final Rule for ALRs, it may not have considered the practical consequences of failing to meet the ALR, particularly for geomembranes covered with earth materials. In hindsight, the ALR for solid-waste facilities should be greater than the ALR for liquid-waste impoundments, which is the opposite of the guidance.
The misconception of zero leakage
Specifying too low of an ALR, as untenable as that may be, is still a giant step more reasonable than specifying zero leakage.
Some engineers and owners are still specifying zero leakage or no leaks. A reasonable person could interpret zero leakage as never a drop. Ever.
Although one strives to obtain the best attainable results with a specification, it is naive to specify something that cannot be remedied if the specification is not met. That is the case when specifying zero leakage in a geomembrane of any practical size. Sometimes, one is fortunate and the leakage is zero or ignorable. But specifying zero leakage or specifying that a liner has no leaks almost always results in disputes and sometimes unsolvable problems.
A workable specification
An important part of engineering and specification writing is to balance the desire for perfection with what is suitable for the purpose at a reasonable cost.
Although perfection may be the goal, specifying perfection without regard to consequences or cost is not good engineering. It does not make sense to provide a specification that if it is not met, there is no practical solution. So instead of specifying an ALR that may be legally actionable, proper engineering practice is to specify an ALR for which there is an action leakage plan that is workable.