Specifying allowable leakage rates

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Introduction
Allowable Leakage Rates or Action Leakage Rates (ALRs) are specified for installed geomembranes to quantify the maximum allowable liquid leakage.

If the ALR is exceeded, corrective action must be taken to reduce the leakage rate below the set limit. To lower the leakage, the holes in the geomembrane are located and repaired. In some cases, the ALR is set so low that, if exceeded, there is no technology to locate the small leak or leaks for repair.

In other cases, surface impoundments are allowed to leak an order of magnitude more than what could be easily attained by locating and repairing leaks using currently available leak-detection technologies. Therefore, the capabilities of currently available technologies should be considered when specifying ALRs.

The logical criteria for specifying an ALR is not whether a certain low leakage rate is achievable with good construction practices, but whether there is a practical solution if the low leakage rate is not achieved. Specifying a leakage rate that is too low can be a disaster if the source of the leakage cannot be located by current technology. If the source of the leakage cannot be located then the only alternative is to reline the facility and hope that the new geomembrane does not also exceed the specified ALR.

Specifying an ALR that is too high, and easily improved with the application of a readily available technology, allows for lower construction standards and possible future environmental damage. The practical approach is to specify ALRs that are achievable in all reasonable cases when currently available leak-detection technologies are applied.

The best current technologies for locating leaks in geomembranes are geoelectric leak-location methods. They are described in ASTM Standard D6747.

Various implementations are used to locate leaks in bare geomembranes, geomembranes covered with water, and geomembranes covered with earth materials. Other methods include visual inspection, vacuum box testing, and pressure testing of seams. These methods are usually limited to pre-service testing and are not applicable after earth materials are installed on the geomembrane.

No other viable technologies are more effective than geoelectric methods in locating leaks in geomembranes. Therefore, ALRs should be established based on the technical capabilities of these leak-location methods. It is illogical and untenable to specify an ALR that may not be met in some situations and, if not met, there is no technology or practical way to meet the specified ALR. This is not a limitation of the geoelectric survey methods but a limitation of all currently available leak-detection technologies.

This article examines what is achievable—and what is unrealistic—for specifying ALRs. It considers the existing ASTM D7007 for geomembrane leak location with water or earth materials on the geomembrane and uses a mathematical equation for leakage rate vs. size of the leak. The equation is derived from Bernoulli’s equation for free flow through an orifice (Giroud and Bonaparte, 1989).

Paradoxically, the usual general technical guidance that geomembranes covered with water are allowed to leak some 10 times more than geomembranes covered with earth materials is opposite from what is attainable with technology available today.

The result is that there are landfills that may never meet their allowable leakage specification by a factor of about 10, and there are surface impoundments that are allowed to leak 10 times more than necessary. For the second case, some evaporation ponds are allowed to leak as much as they evaporate, which clearly is not a prudent environmental goal!

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