Avoiding Relying Too Much on the HELP Model for Forecasting Leachate Generation Rates from Landfill Sites

Uncontrolled Leachate Ponding

There is a method for estimating leachate generation rates. It has been produced by the American authorities to assist in the estimation of leachate production rates for a wide variety of MSW Landfills, and Hazardous Waste Landfills. In itself it represents a good exercise in rationalising the highly complex set of climatic and physical interactions which take place when rainfall incident on a real life landfill becomes leachate.

The problem is that just like any computer model, it produces results by mathematical processes which the average user will never be able to understand fully. To anyone other than academics who have studied the programming of the Model, and then furthermore have verified the calculations which they have produced by using it, against real-life data for existing closely landfills in similar climates, it should be thought of as a “black box”.

Even if the data these users put into the model is good, and that is by necessity rare as landfills are difficult to quantify in the terms that the model requires inputs, and the site operator actually then complies with the “plan” in detail. Also, the average HELP Model user does not get the chance to validate the results for the site in question. So, the results should be considered of doubtful accuracy, and yet leachate management plans are commonly based upon HELP models alone.

The result has been a tendency for leachate management systems designers underestimate landfill leachate generation that starts sooner than expected and is of higher volumes than predicted.

They would do better if landfill engineers would ensure that only HELP Model experienced hydrologists were engaged to do the modelling work, as well.

Unfortunately, there is no easy fix to this problem, and although the experienced hydrologist will know more clearly where his skills end in using the programme, and uncertainties remain, no magic wand can be waived without good verification data available when the HELP Model is run, and a close match can be shown.

The science of leachate volume prediction is not simple, and the methods to predict it need further research. However, landfill designers would do well to appreciate more fully the limits to the tools available for leachate flow prediction, and pass this knowledge on to landfill operators. By making the uncertainty inherent within the predictions clearer, such that more flexibility is provided within leachate management plans, and in forward planning, everyone would be better served.

After all, the lack of availability of an adequate method for controlling leachate can in some cases, over a period of only weeks allow leachate spillages into watercourses and aquifers which may take years to remedy. This may threaten the health and livelihoods of many people and the negative publicity from such incidences will only hasten the demise of the landfill in the long term. If the industry cannot avoid such instances of pollution, it probably should.

This article is supported by a more detailed discussion of this subject, which quotes papers and authors who have written on this subject in the past. Read the full article and if you wish, follow up on this with its sources, at: HELP Model at http://leachate.co.uk/main/