The clean up operation is underway after a freight train carrying a large amount of fuel derailed and spilt into a river near Llanelli on Wednesday 26.
Natural Resources Wales confirmed that diesel from the spill was "no longer confined to the upper reaches of the estuary" and had been found miles downstream.
Over the bank holiday weekend, wagons on the track continued to be drained of their contents and a crane was brought in to help clear the track.
Two workers escaped unhurt after the train derailed in Llangennech and several carriages caught fire.
Nearby homes were evacuated and a major incident was declared.
On Saturday, Natural Resources Wales said they had found diesel as far away as Crofty as the oil moved downstream.
They are continuing to monitor the situation, sampling areas where oil has been found to assess the impact on local wildlife.
The Maritime Coastguard Agency has also been using aerial images to identify areas to which the oil has spread. This work is likely to continue for the next few weeks.
Concerns were raised in the immediate aftermath of the incident about how the local environment would be affected. The Loughour River is a protected marine area and home to a large number of birds that feed in the salt marshes.
Fishermen local to the area sent ITV News pictures and videos of oil on the water's surface and coating rockpools. Some said the effect on the estuary would be "devastating".
One cockler who uses the area said the spill will have "a massive impact on the river", killing a lot of the wildlife and affecting the livelihoods of cocklers and fisherman alike.
The cockle beds and shellfisheries have been closed following advice from the Food Standards Agency.
Anyone who finds animals who have been contaminated by the spill are being advised to contact the RSPCA and not to try and clean the creatures themselves.
Network Rail said that they have already pumped thousands of litres of fuel from the tanks and surrounding area. A crane was brought in on Monday with the aim to remove two wagons each day, starting on Tuesday.
The train consisted of 25 wagons, each with the capacity to hold 100,000 litres of diesel or gas oil. 14 of those have already been removed from the site.
A large number of homes were evacuated following the derailment and the fire was finally extinguished on Friday morning.
An investigation is being carried out to discover what caused the incident but for now the railway line remains shut, with bus replacement services operating for passengers wanting to use the route.