Leachate Balances

Evaluation of the inputs to a landfill, basal leakage or groundwater ingress, and the volumes of drainable leachate held in the waste through use of a leachate balance has the potential to provide a sound basis for managing:

• New leachate generation through cap infiltration and any groundwater ingress;
• Risk to groundwater from basal leakage;
• Volumes to extract to achieve compliance.

Landfill leachate balances often fail however due to uncertainties in a range of input values at the landfill cell scale.  Whilst at Entec UK Ltd, Nick developed an approach which looked at the scale local to a retrofit leachate extraction well.  This produced good results in terms of comparing estimated inputs (rainfall, groundwater and liquid waste) with outputs (depth dependent absorptive capacity below and above the measured leachate level, and leachate level dependent basal leakage) and the volume of leachate held within the depth-dependent drainable porosity of the waste.

Nick applied this approach to two large WRG landfill sites (Brogborough and Calvert) and in addition to preparing comprehensive reports, presented the results to the local Environment Agency officers.  The work brought confidence to the volumes of leachate to extract to achieve compliance and the implications for leachate management from surcharging existing waste with new waste (see Impacts of Overtipping / Surcharging).  The work also highlighted that leachate levels in relatively recently landfilled waste appeared unrealistic from a leachate balance viewpoint and this was consistent with evidence of significant foam in leachate wells and gas in the waste (see Gas in Waste Issues).

Whilst at Rukhydro, Nick has undertaken detailed leachate balances on sites for FCC, Caird Bardon (Aggregate Industries), SITA and Natural Resources Wales. For one of these sites, leachate extraction targets were developed in 2011 and progress against these targets was reviewed in early 2015 and again in late 2016. There is good agreement between the 2011 estimated volumes to extract to gain compliance and the volumes removed to date. This brings confidence to Nick's leachate balance estimates.