Landfills hydrology

Depression method. At locations where natural or artificial depressions exist, it is often possible to use them effectively for land-filling operations. Canyons, ravines, dty borrow pits, and quarries have oeen used for this purpose. The techniques to place and compact solid wastes in depression landfills vary with the geometiy of the site, the characteristics of the cover material, the hydrology and geology of the site, and access of the site. 

Solid-wa.ste-filling plan. The specific method of filling will depend on the characteristics of the site, such as the amount of available cover material, the topography, and local hydrology and geology. To assess future development plans, it will be necessary to prepare a detailed plan for the layout of the individual solid-waste cells. On the basis of the characteristics of the site or the method of operation (e.g., gas recovery), it may be necessaiy to incorporate special features for the control of the movement of gases and leachate from the landfill. 

Numerical models are used to predict the performance and assist in the design of final cover systems. The availability of models used to conduct water balance analyses of ET cover systems is currently limited, and the results can be inconsistent. For example, models such as Hydrologic Evaluation of Landfill Performance (HELP) and Unsaturated Soil Water and Heat Flow (UNSAT-H) do not address all of the factors related to ET cover system performance. These models, for instance, do not consider percolation through preferential pathways may underestimate or overestimate percolation and have different levels of detail regarding weather, soil, and vegetation. In addition, HELP does not account for physical processes, such as matric potential, that generally govern unsaturated flow in ET covers.39 42 47... 

The ET cover cannot be tested at every landfill site so it is necessary to extrapolate the results from sites of known performance to specific landfill sites. The factors that affect the hydrologic design of ET covers encompass several scientific disciplines and there are numerous interactions between factors. As a consequence, a comprehensive computer model is needed to evaluate the ET cover for a site.48 The model should effectively incorporate soil, plant, and climate variables, and include their interactions and the resultant effect on hydrology and water balance. An important function of the model is to simulate the variability of performance in response to climate variability and to evaluate cover response to extreme events. Because the expected life of the cover is decades, possibly centuries, the model should be capable of estimating long-term performance. In addition to a complete water balance, the model should be capable of estimating long-term plant biomass production, need for fertilizer, wind and water erosion, and possible loss of primary plant nutrients from the ecosystem. 

A major requirement of a landfill cover is to control the amount of precipitation that enters the waste. The amount of water that percolates through the cover and may enter the waste is called PRK. PRK is a part of a much bigger hydrologic system and must be assessed in parallel with the other parts. Therefore, it is necessary to estimate the entire hydrologic water balance for the cover in order to assess its behavior.49... 

Because the quantity of water on or near the earth is believed to be constant, the hydrologic water balance for a landfill cover may be expressed by the statement14 ... 

Lateral flow (L) within the soil layer containing plant roots is small for most landfill cover situations and is zero for lysimeters with sidewalls. During the course of a hydrologic year, ASW is usually small in comparison to the other terms, but it may be large on a daily basis. A primary focus for the design is PRK below the ET landfill cover as represented by the rearranged equation ... 

Surface runoff (Q) is the second-largest part of the hydrologic water balance for ET landfill covers at many sites in humid regions. Even at dry sites where surface runoff is small, errors in estimates of Q are important, and especially so if the model estimates significant Q on days with no runoff. Estimates of Q are therefore important to the design process at all sites. 

This section describes currently available models and presents important features of each that are pertinent to ET landfill cover design. These models have diverse origins however, each was intended for use in evaluating the hydrologic cycle and included features that are pertinent to landfill covers. The model developer and/or other reviewers have tested each of these models. The purpose of this evaluation is to determine the level of accuracy and usefulness of a model as it might be applied to ET landfill cover design and evaluation. 

Some models require calibration to optimize input parameters they are best used in a research setting where it is possible to make measurements with which to calibrate the model for a particular site. Appropriate measured hydrologic data are seldom available to calibrate a model for a particular landfill site. Therefore, engineering models used for ET cover design should not require calibration. 

Development of the Environmental Policy Integrated Climate (EPIC) model and its predecessor, the Erosion Productivity Impact Calculator, began in the early 1980s.69 70 The first version of EPIC was intended to evaluate the effects of wind and water erosion on plant growth and food production. More recent versions also evaluate factors important to other environmental issues. EPIC is a onedimensional model however, it can estimate lateral flow in soil layers at depth. All versions of EPIC estimate surface runoff, PET, AET, soil-water storage, and PRK below the root zone—these complete the hydrologic water balance for an ET landfill cover. 

Source Hauser, V.L. and Gimon, D.M., Evaluating Evapotranspiration (ET) Landfill Cover Performance Using Hydrologic Models, Air Force Center for Environmental Excellence (AFCEE), Brooks City-Base, TX, January 2004. 

Warren, R.W., Hakonson, T.E., and Bostik, K.V., The hydrologic evaluation of four cover designs for hazardous waste landfills, in Landfill Capping in the Semi-Arid West Problems, Perspectives, and Solutions, Reynolds, T.D. and Morris, R.C., Eds, Environmental Science and Research Foundation, Idaho Falls, ID, 1997. 

Schroeder, P.R., Dozier, T.S., Zappi, P.A., McEnroe, B.M., Sjostrom J.W., and Peyton, R.L., The Hydrologic Evaluation of Landfill Performance (HELP) Model Engineering Documentation for Version 3, EPA/600/R-94/168b, U.S. Environmental Protection Agency, Cincinnati, OH, 1994. 

Hakonson, T.E., et al., Hydrologic Evaluation of Four Landfill Cover Designs at Hill Air Force Base, Utah, LAUR-93-4469, Los Alamos National Laboratory, Los Alamos, NM, 1994. 

HELP Hydrologic Evaluation Landfill Performance Model... 

Lagas, P., J.P.G. Loch, and K. Harmsen. 1982. The behaviour of cyanide in a landfill and the soil beneath it. Pages 169-178 in R. Perry (ed.). Effects of Waste Disposal on Groundwater and Surface Water Intern. Assoc. Hydrolog. Sci., Publ. No. 139. 

No water wells were located down gradient from the landfill and significant leaching past the landfill liner was not found hence no further hydrological assessments were undertaken. Airborne emissions from the landfill were characterized by air samples taken at the landfill and airborne concentrations were also measured at the nearest community. 

Johnson, C. A., Kaeppeh, M., Brandenberger, S., Ulrich, A. Baumann, W. 1999. Hydrological and geochemical factors affecting leachate composition in municipal solid waste incinerator bottom ash. Part II. The geochemistry of leachate from Landfill Lostorf, Switzerland. Journal of Contaminant Hydrology, 40, 239- 259. 

Hydrological and geochemical factors controlling leachate composition in incinerator ash landfills... 

A landfill can be seen as a spatially heterogeneous porous and, by design, unsaturated system. Flow paths and physical mechanisms of runoff generation play a crucial role for the hydrology and ultimately leaching reactions within a landfill. Figure 1 illustrates the flow... 

This approach has been used in the well-known model HELP (Hydrological Evaluation of Landfill Performance, Schroeder et al. 1994) and a number of complementary models (Nixon et al. 1997). These models mostly assume the landfilled material to be idealized layers with homogenous properties. One such model, HYDRUS, has been used to model flow through Landfill Lostorf, but it was found that it could not fully catch the dynamics of flow, particularly after rain events (Johnson et al. 2001). Water passes through the... 

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