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Dr Emma Paris

Hydro-geochemical modelling of a partially lined landfill

Understanding the generation, character, long-term evolution and control of leachate discharges from unlined landfills still provides an important scientific and engineering challenge, requiring integration of hydrogeological and geochemical modelling.

Supervisors: Professor Charles Harris and Dr Simon Wakefield (Cardiff), Funding: Blaenau Gwent County Borough Council

This project is based at Silent Valley landfill site, Blaenau Gwent. The research aims to develop a hydro-geochemical model of the leachate system, to allow prediction of volumes and quality. Flow and conductivity monitoring are installed where the leachate exits the site and rainfall is monitored automatically. Leachate samples are collected and analysed using Ion Chromatography (IC). Resistivity surveys have been performed across the site to help gain an understanding of the internal structure.

All the data collected are analysed statistically and relationships between rainfall, discharge and leachate chemistry are explored through regression analysis. Research outcomes will include empirical modelling of the hydro-geochemical system at the landfill as an aid to landfill management.

Figure 1 Georeferenced photograph of Silent Valley taken in 2000. From Getmapping.com.

Aim

Dr Emma Paris
Figure 1 Georeferenced photograph
of Silent Valley taken in 2000.
From Getmapping.com. 

The research aims to develop a hydro-geochemical model of the leachate system, to allow prediction of volumes and quality.

The Site

This project is based at Silent Valley landfill site (Fig. 1), Blaenau Gwent, which has been in operation as a landfill site since 1981. Before accepting municipal waste it received large quantities of waste from the iron and steel works at Ebbw Vale. The site is currently licensed to accept household waste.

The site is located in the Cwm Merddog, an eastern tributary valley that joins the Ebbw Fawr at the village of Cwm and is situated at a level between 310 and 440 m AOD.

Dr Emma Paris
Figure 2 Instrumentation in the
Settlement Tank at Silent Valley
landfill site. 

Silent Valley is located within the Carboniferous Upper Coal Measures, Westphalian C and D series and rests on the Rhondda Beds. The solid geology is overlain by a mixture of Boulder clay, Head Deposits and made ground. The site is unlined and works on a dilute and attenuate principal with the clay rich deposits covering the valley floor acting as a relatively impermeable layer. The clay permeability met the NRA guideline of for an acceptable lining material. Any extension to the site will be fully engineered.

Monitoring

Leachate exits the site via a number of pipes at the base of the landfill. The flows are collected and pumped into the Settlement Tank, which is divided into two compartments. The first acts as a settling tank to remove suspended solids. Leachate flows from this tank over a weir to the second larger compartment, which provides a degree of retention. Water is discharged from this tank to the foul sewer.

Flow and conductivity monitoring are installed (Fig. 2) where the leachate exits the site. Weather data are collected on site and recorded. A long-term annual rainfall of 1257.8 mm for the region has been obtained from the Meteorological Office. Site specific MORECS data have been bought from the Meteorological Office. Leachate samples are collected and analysed using Ion Chromatography (IC).

Dr Emma Paris

Figure 3 A typical resistivity profile obtained from the
monthly resistivity survey at Silent Valley. 

Resistivity surveys (Fig. 3) have been performed across the site to help gain an understanding of the internal structure. A permanent set of electrodes was installed on the southern slope at Silent Valley in December 2001. This enabled resistivity surveys to be repeated across the same area of landfill throughout a year. The results show how resistivities within the landfill vary across the year, which give an indication of how the saturation within the landfill is varying.

In June 2002 five parallel resistivity surveys were performed. The sequence of two-dimensional vertical resistivity data points were integrated using the Surfer contouring computer programme to provide a three-dimensional model of resistivity in the fill material beneath the survey area. This 3D model was then sliced at different depths below the surface and parallel to it, to show spatial distribution of resistivity at each depth (Fig. 4).

Dr Emma Paris

Figure 4 Resistivity 11.87m below the surface. Low resistivity (blue)
values indicate areas of leachate saturated waste. It is likely that the high
resistivity values (red/purple) to the west of the survey area represent
the presence of slag within the fill material. 

Research Outcomes

All the data collected are analysed statistically and relationships between rainfall, discharge and leachate chemistry are explored through regression analysis. Research outcomes will include empirical modelling of the hydro-geochemical system at the landfill as an aid to landfill management.