Natural Resources Wales are digging trenches to intercept the diesel spilt after a freight train caught fire at a scientifically important site.
They said that booms and absorbent pads were being used and regularly replaced to contain as much of the diesel as possible in the River Loughor.
The site is one of special scientific interest, which is home to wildfowl and wading birds.
Natural Resources Wales said that diesel is no longer confined to the upper reaches of the estuary (around the Loughor Bridge and upstream) and had been observed five miles away, as far as Crofty.
The estuary’s cockle beds and shellfisheries have been closed as a precaution, following advice from the Food Standards Agency.
The Maritime and Coastguard Agency has undertaken aerial surveys to aid the monitoring of the pollution.
At 11.15pm on Wednesday, August 26, a freight train carrying 25 tank wagons with around 75.5 tonnes of diesel in each, derailed at Llangennech near Carmarthenshire.
10 of the 25 wagons were derailed, causing a large diesel spill which also seeped into the nearby Loughor Estuary, and a major fire.
Residents were told to leave their homes and the two employees who were aboard the train were uninjured and able to limit the number of wagons to catch fire.
It took fire crews until the morning of Friday, August 28 to extinguish the fire, which limited the initial efforts to contain the diesel due to safety and priority of extinguishing the fire.
Network Rail is arranging for a crane to be delivered to the site today to start removing the wagons from tomorrow.
“These techniques are proved to be working well and are removing a considerable quantity of diesel from the watercourses,” they said.
Ten wagons, each containing 75 tonnes of diesel, derailed and spilled oil into the Loughor Estuary near Llanelli in Carmarthenshire on Wednesday night.
British Transport Police’s initial probe has ruled out criminal intent.
“Initial findings are that the derailment is not believed to have been caused by criminal activity,” said Det Ch Insp Paul Langley.
“We are therefore handing primacy of the scene to the Office of Rail and Road, so that it can carry out its own specialist investigation.”
Please report sightings of oil pollution around the Loughor Estuary to NRW by calling 03000 65 3000. If you come across any contaminated birds or animals, call the RSPCA on 0300 1234 999.