By Abigail Gilson-Beck and Larry Shilling
The double-lined containment facility featuring a leak detection system (LDS) is the most accurate way of assessing leakage through a primary geomembrane (Bonaparte and Gross 1992). This type of system also represents the gold standard for environmental protection, since leakage through the primary liner is collected and removed so that the hydraulic head over the secondary liner is minimized. Along with a double-lined system comes an action leakage rate (ALR). The definition of ALR is “the maximum design flow rate that the LDS can remove without the fluid head on the bottom liner exceeding 1 foot” (U.S. Federal Regulation 40 CFR 264.222). Inherent to the definition as described in the governing documents, the fluid head on the bottom liner is presumed to come from leakage through the primary geomembrane. In practice, conformance to the regulation is determined simply by the daily quantity of liquid that is pumped out of the LDS. An ALR value is applied to a site as a permit condition, expressed as gallons per acre per day (gpad) or liters per hectare per day (lphd), and if the flow from the LDS is greater than this, a site will not be allowed to operate. The case study detailed here provided a unique opportunity to quantify the flow dynamics of construction water through LDS. In this instance, the site’s leakage exceeded the state-mandated ALR for roughly three months. In light of the free-flowing gravel material used for the LDS, the slow weeping observed, which was unaffected by rainfall, defied all logic. The urgency of the construction process in order to get a site permitted to operate before a site’s airspace is exceeded is shown in Figure 1. Placement of the LDS material, electrical leak location (ELL) testing of the secondary geomembrane, and installation of the primary geomembrane over previously tested areas happened all on the same day.
On liner floors with less than 10% slope, regulations for the case study site require a 1-foot (30.5-cm) thick LDS material. This 1 foot of material provides separation from the primary and secondary liners such that if something were to penetrate the primary liner, it would not likely also penetrate the secondary liner. This layer, commonly referred to as the structural fill layer, has always been required. Prior to the new regulations, there was no minimum requirement for permeability. Generally, this layer was an on-site soil material placed at 95% compaction. The soil layer, once covered with the primary liner, would only weep water into the secondary collection system when weight was added and the water was squeezed out of it. The newer regulations require a minimum permeability of 1 × 10-2 cm/sec. Generally, this requires a large sand or small pea gravel type of material, which is intended to release water more quickly.
The first suspect for the observed leakage was naturally leaks in the primary geomembrane. ELL testing of the primary geomembrane was performed as part of cell construction, per ASTM D8265. The method was performed during active rainfall with extreme sensitivity, and a very small (~1/8 inch [3.2 mm]) knife slice was found. Regardless, the method was performed again when the persistent leakage continued for more than a month after the end of cell construction. No further leaks in the primary were found. In addition, dye testing was performed, confirming no leakage through the primary. This comprehensive testing, along with the lack of leakage response to rainfall, shifted the authors’ focus to other potential culprits.
Construction water has been pointed out as a potential cause for exceeding a site’s ALR before (Gilson-Beck 2019). The flow attributed to construction water can be quantified when the soil properties of the leak detection layer are known, along with the moisture content of the LDS material once encapsulated by the overlying geosynthetics. If the material is placed wet (or experiences rainfall), and the moisture content is greater than the material’s specific retention (Sr), then there will be drainage of construction water from the LDS material. At this site, the ELL testing of the secondary geomembrane was also performed during active rainfall, with additional rainfall in the days following the testing, immediately before the LDS was covered by the overlying geosynthetics. Specific retention quantifies the portion of water that will remain attached to the material particles and will not be released through drainage by gravity. The counterpart of Sr is specific yield (Sy); this is the portion of water that will drain by gravity. If the porosity (ø) of the material is known, either can be calculated if the value of one of them is known using the equation: ø – Sy = Sr.
Construction water flowing to the LDS has been the bane of engineers of double-lined systems since their inception. A set of equations authored by J. P. Giroud was presented 30 years ago to estimate both the quantity to expect from the LDS and the flow rate (Gross et. al 1990). With these equations, one should be able to predict how long to expect the drainage to last if one obtains values for the volumetric moisture content at the time of material placement and the specific retention of the material along with the hydraulic conductivity and the design-specific drainage geometry.
In order to compare the results of the equations to what was being collected from the LDS at the case study site, soil testing was performed on the LDS material to augment the typical testing already performed on the material as part of cell construction. The equations predicted that a total volume of 33,986 gallons (128,651 l) would be released from the 3.5-acre (1.4-ha), 1-foot (30.5-cm) thick layer of gravel over a period of 19 hours. This was due to the fact that the material was placed at field capacity (measured to be 0.0378) and the Sr was measured to be 0.008. However, 150 days after the leak detection layer was covered by the primary geomembrane, the drainage was at approximately 10 gpad (94 lphd) and had been slowly decreasing from nearly 80 gpad (748 lphd). The only other trend in the data that could shed light on what might be happening was that the flow rate increased with increasing temperatures with the onset of summer. Data collected during the period of time when ambient temperatures were increasing are shown in Figure 2.
Because of the positive correlation with temperature, the phenomenon of surface tension went under the microscope. The unwillingness of the gravel to give up water has to do with surface tension of the water molecules. As temperatures increase, surface tension decreases, and the water is released more readily under the pull of gravity. The physical property affected by this is Sy. In fact, Sy is so temperature dependent that the testing temperature was standardized to 68°F (20°C) (Johnson 1963). An investigation of the property of Sy resulted in the discovery that it is not only temperature dependent but time dependent as well. In testing for Sy, most of the water will come out early on, but there is virtually no limit to how long the drainage can last. F. H. King, a pioneer in the study of the time dependence of Sy, tested several columns of sand for two and one half years. Results of the coarser sand he evaluated showed that 39.7% of the total drainage occurred during the first 30 minutes, 18.1% occurred in the second 30 minutes, it took nine days to drain an additional 32.5%, and the remaining 9.7% drainage took two and one half years (King 1899). In order to expedite laboratory testing, it was found that if the sample was subjected to 1,000 times the force of gravity, the Sy value had good agreement with the long-term value (Johnson et. al 1963). It is now well established that Sy varies in time. How Sy is measured in the lab will determine where the value lands, but the trend over time will be an exponential curve. The horizontal asymptote of the curve will be the final value for Sy if the sample is left to drain forever. Modern laboratory values of Sy are typically determined by subjecting the sample to –850 bar (Stephens 1996).
In light of this, Giroud’s equations were revisited, but instead of solving for flow and seeing if it matched what was measured, the measured daily flow was used to solve for Sy over time. The result was a logarithmic curve that matched the textbook curve of Sy over time. The variation between the measured curve and theoretical curve can be attributed to site conditions, especially temperature. Each day, as a little more flow is measured, the overall Sy gets slightly higher. Each day the flow is less (on average), raising the Sy a little bit less each time, until it reaches infinitely close to the limit of the curve, in this case the laboratory value of Sy (0.364). This value is shown as the horizontal asymptote in Figure 3. Note that leakage was not measured until two weeks after the LDS layer was sealed. Therefore, the initial leakage volume was changed iteratively in order to fit the measured leakage curve to the laboratory value of Sy.
The curve of Sy versus time will be material specific and will be a function of material shape, texture and mineralogy. Obtaining a laboratory value for Sy will enable estimations of the total flow from the material over an extended period of time. However, the curve must be generated in order to estimate flows in the short term, and leakage must be measured immediately once the layer is sealed up.
Using the equation generated from Figure 3 for Sy, the theoretical daily flow rate was calculated and is shown in Figure 4. This theoretical curve is shown with the actual measured flow rate and the average temperature. Figure 4 shows how the measured flow responded to increasing temperatures by arching away from the theoretical curve until temperatures decreased.
Identifying the culprit and matching the phenomenon to the data was straightforward. The more difficult part is to figure out how to keep this from happening in order to get sites permitted to operate in a timely manner. The type of material used for the LDS will be the biggest factor in whether and for how long it will continue to drain. Clean gravel has a much higher Sy than dirty gravel. In fact, Sy values of 0.21–0.25 are common for clean gravel and 0.05–0.07 for clayey gravel. As with most sites, stockpiling gravel and hauling it to the cell at the case study site caused clayey fines to be placed along with the gravel. Due to precipitation on the day of ELL testing and continuing the few days preceding primary geomembrane placement, the LDS material was encapsulated wet and dirty. Of course, in dry climates the material will be irrelevant. If the moisture content never gets above the Sr, the material will not drain. Note that if ELL testing is performed on the secondary geomembrane, water is added to the LDS material during testing, as shown on Figure 5.
Potential solutions to this problem include washing the LDS material to remove fines, applying some kind of surfactant to break the surface tension before the LDS layer is covered, using larger stone for the LDS material, keeping rainwater out of the LDS layer using temporary tarps, waiting a potentially extended period of time for cell permitting and regulators allowing a higher leakage rate if it can be demonstrated that the primary geomembrane is not the source of leakage. Some of these solutions are less practical (and more expensive) than others. The least disruptive to construction would be the allowance of leakage above a site’s ALR if it can be attributed to construction water.
It could certainly be dangerous for a regulatory board to allow sites to exceed a mandated leakage rate by claiming the issue is caused by construction water, since every site could point the blame on this phenomenon. However, true leaks in the primary geomembrane will cause a spike in the leakage after a rain event. A site could have both construction water flow and leaks in the primary geomembrane. Any spike in the flow corresponding with rain events should be treated as a probable leak and ELL methods should be applied. However, if the flow is unresponsive to rainfall and exhibits a flow rate that corresponds with this phenomenon, it is likely construction water and should not be considered leakage through the primary geomembrane if evidence can be generated to support this. Drainage caused by the slow release of construction water from the LDS material can be accurately quantified with the following approach during cell construction.
Immediately before the LDS material is covered by the overlying geosynthetics, a sample should be taken of in situ material. The sample should be taken all the way down to the surface of the secondary geomembrane for a representative cross section. This location can be backfilled with material from the stockpile. A partially filled five-gallon bucket of material will suffice. The following material properties must be evaluated: moisture content, dry bulk density, porosity, hydraulic conductivity and specific yield. Flow monitoring of the LDS layer should start on the day the sample is taken. Daily measurements of flow from the LDS, rainfall and ambient temperature should be recorded. The general trend indicating that flow is from construction water drainage will be less flow each day, no response to rainfall, and likely an increase in flow with increasing temperatures. Also, the volumetric moisture content of the sample must be larger than the value of Sr. Calculate the difference between the volumetric moisture content and the Sr and multiply that by the volume of the LDS material. This will be the total quantity of flow that should come out of the LDS, starting on the day of sampling. Create a “best fit” curve for the daily leakage rate data set in order to project future flows. Solve for the “x” value where the “y” value is equal to the site’s ALR. This can estimate the number of days remaining until the flows get down to an acceptable level.
In tandem with ELL testing for leaks in the primary geomembrane, LDS material testing and matching the measured flows to the theoretical curve should provide sufficient evidence that the leakage is attributed to residual construction water and not to leakage of the primary geomembrane.
References
Bonaparte, R., and Gross, B. A. (1992). “LDCRS flow from double-lined landfills and surface impoundments.” U.S. Environmental Protection Agency, Cincinnati, Ohio, Contract No. 68-C0-0068.
Gilson-Beck, A. (2019). “Controlling leakage through installed geomembranes using electrical leak location.” Geotextiles and Geomembranes 47, 697–710.
Gross, B. A., Bonaparte, R., and Giroud, J. P. (1990). “Evaluation of flow from landfill leakage detection layers.” Proc., 4th Int. Conf. on Geotextiles, Vol. 2, The Hague, Netherlands, 481–486.
Johnson, A. I., Prill, R. C., and Morris, D. A. (1963). “Specific yield: Column drainage and centrifuge moisture content.” U.S. Geological Survey Water-Supply Paper.
Johnson, A. I. (1963). “Specific yield: Compilation of specific yields for various materials.” U.S. Geological Survey Water-Supply Paper 1662-D.
King, F. H. (1899). “Principles and conditions of the movements of ground water.” U.S. Geological Survey 19th Annual Report, pt. 2, 86–91.
Meinzner, O. E. (1923). “The occurrence of ground water in the United States, with a discussion of principles.” U.S. Geological Survey Water-Supply Paper 489.
Stephens, D. B. (1996). Vadose zone hydrology, CRC Press, Boca Raton, Fla., 11–12.
U.S. EPA (1992). “Action leakage rates for leak detection systems.” U.S. Environmental Protection Agency.
Abigail Gilson-Beck, M.S., P.E., is director of electrical leak location services with TRI/Environmental Inc. and is based in Lansing, N.Y.
Larry Shilling is market area landfill manager at Casella Waste Systems Inc.
in Angelica, N.Y.