“The person who walks with his/her eyes on the stars, is susceptible to the puddles in the road.” We all would like to follow our stars, but at times we need to do some “puddle watching” too! Sometimes we need to know what not to do, what things and actions to avoid, and why…
There’s a correct way and a wrong way to use the Hydrologic Evaluation of Landfill Performance (HELP) computer program Version 3: issued September 1994. This informative article deals with the negative, problem avoiding side of things, pointing out 5 things you really should avoid if your goal would be to use the HELP computer program to predict the design daily volume of leachate which a landfill will produce.
Here then, will be the 5 criticisms to be aware of to avoid over-reliance on the HELP Model Version 3:
1. Take care not to rely on the HELP Model alone to generate predicted leachate volume forecasts, even when the landfill is in a high rainfall region for which researchers have validated the model and provided guidance on the applicaton of the model in that region. The reason you shouldn’t do this is that the originators of the HELP model say themselves for the current Version 3 that;
“The primary purpose of the model is to assist in the comparison of design alternatives as judged by their water balances”.
Please note, the use of the model is primarily for comparative analysis to enable designers to select landfill designs which produce low leachate volume when compared with other options.
The originators do not suggest that the model should be used for sizing leachate treatment facilities specifically at all, merely for evaluation of design alternatives, and yet it is common that the HELP model alone is relied upon by site onwers and designers to size leachate treatment facilities. Instead, you ought to be aware that researchers have repeatedly warned about this.
2. For example, Klaus Berger, has published his “Validation of the hydrologic evaluation of landfill performance (HELP) model for simulating the water balance of cover systems”, in Environmental Geology Volume 39, Number 11, 1261-1274, and says that HELP version 3 was examined in a German climate in an extensive validation study, and a German adaptation was developed. However, limitations, weak points, and errors of the HELP model were shown to exist during the validation work.
The HELP model is a suitable tool for experts in hydrology; but good knowledge of the model and its behaviour and critical review of the simulation results are essential. The reasoning behind this is that unless the software is used by an expert in hydrology, who has validated the model using existing data for other landfills in similar climates, and if possible also data from the site itself, it should not be considered reliable. Alternatively, you could potentially simplify things and just say that using software models which act to the user like a “black box”, and produce an output which may appear authoritative, can be very deceptive, and the output may be worthless.
3. R Parsons in his paper; “Water Balance Method To Predict Leachate Generation-Geohydrological Experiences”, in the Proceedings Sardinia 95, (Fifth International Landfill Symposium ‘. Margherila di Pula, Cagliari, Italy; 2-6 October 1995 ) 1995 by CISA, Environmental Sanitary Engineering Centre, Cagliari, Italy) states that;
“The application of the Waste Site Water Balance (WSWB) method to predict leachate generation is not only widespread in South Africa, but also throughout the world. The results obtained using the WSWB method are, however, not compatible with knowledge concerning groundwater recharge.”
This is particularly true under arid and semi-arid conditions. Clearly this is a major limitation of the HELP model which uses the WSWB method. And, what you need to do instead is treat the HELP results with particular caution under arid and semi-arid conditions.
4. The HELP Model makes insufficient allowance for dry periods when the surface will be dry for long periods and no evapotranspiration will be occurring. The reason is clearly described by Parsons, where he says;
“Evapotranspiration is recognised in the literature as the single biggest component of water loss from a waste pile. A small change in the evapotranspiration value used can have a significant impact on the final estimate of leachate generated. This high sensitivity, coupled with the inability to obtain an accurate estimate of the volume of water lost through evapotranspiration, is in itself a major limitation”.
And, so you should, as we have previously suggested, once again take great care using the HELP model.
5. In the paper titled the; “Use Of Water Balances For Landfill Site Monitoring”, By B. Nolting, P. Gossele, H. Wefer And M. Bender, in the Proceedings Sardinia 95, (Fifth International Landfill Symposium ‘. Margherila di Pula, Cagliari, Italy; 2-6 October 1995 ) 1995 by CISA, Environmental Sanitary Engineering Centre, Cagliari, Italy) the authors support Parsons, but go further. They state that:
“even in humid climates longer periods without rainfall occur, thus reducing the exhaustible water store of the waste surface. In this case, the potential evaporation values exceed the available soil water resources and do not reflect the real evaporation from the landfill surface. On the other hand, it is obvious that the steepness of slopes and high rainfall intensities must bring about considerable runoff quantities, especially when sections of the landfill area are intermediately covered with little or non-permeable layers. But this factor is omitted for open landfills and seldom considered for landfills with final cover”.
It is sometimes hard to avert this but one has to be inclined to ask what the HELP Model is good at, but I would not say that. Instead, do not forget that the designers of leachate treatment facilities do, in the end, have to agree with their landfil owner/ operator clients upon a flow rate for design purposes. However, no party should delude themselves that, even when experienced landfill leachate modelling hydrologists, such as may be numbered within these these authors, and not generalists, undertake this work, they admit to a science of leachate flow prediction which is not well developed. All those involved in leachate management and leachate treatment must reconise this, and that is my point in writing this article
Avoid these 5 frequent mistakes and you will greatly boost your results. Stick to the suggested alternatives if you would like, but by all means steer clear of the mistakes…