A groundwater hazard assessment scheme for solid waste disposal. (Report No. WC/95/007)
This study reports on work carried out under the Overseas Development Administration (ODA) and British Geological Survey (BGS) Technology, Development and Research (TDR) Programme (Project 93/5, R5565) as a contribution to the British Governement's provision of aid through technical assistance to developing countries. It focuses on the contamination threat to groundwater quality posed by landfill leachate. The main objective is to provide an approach for a robust means of ranking an existing or prospective waste disposal site within a hazard framework.
The controls on contaminant migration from landfills are examined and the parameters which are important in establishing the hazard posed by a waste facility to groundwater quality identified.
The report considers: the factors which contribute to landfill hazard; the mathematical description of flow and contaminant transport in aquifers
numerical modelling and parameter estimation for landfill in a number of different hydrogeological environments; case studies from Indonesia and Mexico to demonstrate the proposed scheme and to compare the methodology with existing empirical assessment schemes.
The factors, which need to be evaluated in assessing landfill or landraise impact on groundwater quality, have been identified by Klinck, (1995), and relate to site and geology and hydrogeology, and also to variables relating to the fate of the pollutants (proximity of local population, distance to nearest abstraction borehole/spring, volume of groundwater abstracted, distance to the nearest surface water)
A number of empirical methods have been employed in evaluating groundwater vulnerability which take account of the above listed risk factors. Three of them have been considered in this study. The schemes used were DRASTIC, Aller et al., (1987); GOD, Foster, (1987) and Foster and Hirata, (1991), and the Waste Aquifer Separation Principle, WASP, of Parsons and Jolly, (1994a and 1994b). The latter scheme is specifically designed for landfill hazard ranking and makes an assessment of the impact on the aquifer directly below the waste disposal site.
The hazard assessment scheme proposed here is based on the travel time of a conservative contaminant, chloride, from a waste facility. It can be used to establish a hazard zone around a waste site for different hydrogeological and climatic settings. For the purposes of the present study a generic, deterministic model has been designed in order to examine a number of hydrogeological scenarios. The use of mathematical models offers the advantage of being able to test the sensitivity of the prediction to parameter variations where there is uncertainty in their value. The model has both 1-D and 2-D numerical, contaminant transport simulators. The impact of waste disposal is assessed not only directly below the waste site, but also in the environs of the site and in this respect the approach is novel compared with existing schemes.
A comparison of the above mentioned ranking schemes and the numerical simulators was conducted using data from the case studies. The following observations can be made: DRASTIC is insensitive to high recharge, i.e. greater than 250mma-1 and considers a narrow range of hydraulic conductivity from 4.7 x 10-5 to 9.4 x10-4 ms-1. GOD is the quickest scheme to use; the computerised version of WASP was easy to use. Because the scheme is intended for hazard assessment of prospective waste disposal sites, groundwater usage is considered; the numerical simulator requires considerable expertise to run and obtain results. Using the 1-D simulator, developed for this study, vertical travel times from the waste to the water table arc obtained. These times give a quantified groundwater pollution potential which are directly comparable with the results of the less mathematically rigorous scoring schemes.
All of the schemes tested can be used either to evaluate prospective sites or sites already in existence.
The following procedure is suggested to evaluate the impact of a single site on the groundwater system or to compare a number of sites for their suitability for waste disposal.
Carry out a preliminary desk study of the prospective areas. This will involve obtaining topographical, geological and hydrogeological maps; climate data; site investigation reports, and any other information in the public domain.
In many cases the basic information will not be available and a site visit will be essential. During this visit a record of the geology, and geographic setting of the area should be made.
Apply GOD to rank the sites. GOD is the simplest scheme to implement and does not require very detailed data to make an assessment. It gave comparative rankings to WASP and DRASTIC.
The sites with the lowest GOD score should then be modelled using the WASP scheme or the numerical, l-D transport simulator. The sites with the lowest barrier factor scores in WASP and the longest travel times using the simulator should then be considered for waste disposal. The effect of the waste disposal operation on the fate group can be assessed using the 2-D contaminant transport simulator.
Using the model and the guidelines provided, it should be possible to derive an approximate time at which the landfill starts to pose a hazard at the distance of interest.
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