The ongoing clean-up of a diesel spill after a train derailment has been called “the most challenging recovery operation” in 25 years.
A freight train pulling 25 wagons, each containing up to 100,000 litres of diesel, derailed and spilled into the Loughor Estuary at Llangennech, near Llanelli, in August.
Natural Resources Wales compared it to the Sea Empress disaster in 1996.
The Rail Accident Investigation Branch is investigating the derailment.
In the past two months, around 30,000 tonnes of contaminated soil has been removed from the site.
Incident recovery manager Stuart Thomas, of Natural Resources Wales, said: “This is the most challenging recovery operation we’ve seen since Pembrokeshire’s Sea Empress disaster 25 years ago”.
The stranding of the Sea Empress oil tanker at the mouth of the Cleddau Estuary off Milford Haven in February 1996 resulted in 72,000 tonnes of crude North Sea oil escaping into the sea.
Thousands of birds died and about 120 miles of Welsh coastline were contaminated, with the total cost of the clean-up operation put at £60m.
Mr Thomas said “a phenomenal amount of work” had been carried out at the site of the Carmarthenshire train crash to safely remove the contaminated soil and reinstate the ground, despite being faced with flooding problems during severe weather.
Environmental contractors Adler and Allan aim to complete the remediation works by the end of February.
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Contaminated soil from 164yds (150m) of railway at a depth of 6.5ft (2m) and width of 21yds (20m) has been excavated and taken to a licensed waste management facility near Merthyr Tydfil. The soil has been replaced with clean material from quarries in Carmarthenshire and Pembrokeshire.
Monitoring has shown the safety and quality of shellfish harvested from the area, such as cockles and mussels, is “well within regulatory limits”.
