Geo-engineering problems at Llwyneinion hazardous waste site near Rhosllanerchrugog, North Wales

At Llwyneinion, a clay pit excavated in Carboniferous Bettisfield Formation was utilised for hazardous waste disposal prior to 1972. Approximately 94,000 t of acidic asphaltic sludges and pitches was poured directly into the pit together with 7500 t of spent bentonite containing absorbed oil. Numerous chemical drums were also dumped in the sludge. The unlined acidic asphaltic sludge and pitch lagoon occupies 1.3 ha, ranges up to 10 m deep and is underlain by fissured strata and extensive mineworkings including three shafts. The heterogeneous asphalt waste has developed a semi-solid asphalt layer at the base but mostly remains in a viscous and mobile condition. A surface layer of rainwater less than 0.4 m deep covers the waste. Surprisingly, detailed hydrogeological studies of the area surrounding the acidic asphaltic sludge and pitch lagoon failed to disclose any significant contamination of aquifers, however regular monitoring of water conditions is performed as a safeguarding measure. Various remediation strategies have been assessed and rejected. They fall into two main categories; those involving on site treatment of the acidic asphaltic sludges and pitches and encapsulation and those aimed at removal of the material to facilities elsewhere in the country. Investigations and deliberations continue in order to determine the optimum way forward.