Persimmon Homes has been fined more than £430,000 after pleading guilty in relation to several river pollution incidents in Monmouthshire, South Wales, in 2019.
A total of seven pollution offences occurred at the Willow Court development site in Abergavenny between 11 February and 11 November 2019, impacting the River Gavenny.
An additional offence also occurred on 2 February 2021.
The house-building company admitted the charges at Newport Magistrates' Court today (5 April) and expressed its "deep regret", saying improvements have since been made.
Inspections carried out by Natural Resources Wales (NRW) revealed that silt run-off from the building site affected nearby water courses over a period of several months.
Samples revealed that fish and other invertebrates would have been negatively impacted by the "significant" levels of sediment in the River Gavenny.
Although there was no evidence that fish had died, experts say it would have clogged their gills and reduced light penetration in the water.
Officers from Natural Resources Wales say they met with the management of Persimmon Homes in March 2019 to discuss ways to minimise the risk of pollution.
This included jet-washing the silt back onto the site from the road and digging a trench across the entrance, in which clean stone and straw bales could be placed to absorb any silt water run-off.
Natural Resources Wales said the trench initially stopped the immediate flow of contaminated water.
But additional measures - including digging more ditches, closing the site entrance to traffic, and effectively recording inspections of pollution measures - failed to be carried out, it added.
A silt netting placed at a discharge point into the River Gavenny, designed to prevent silt from entering the river, was also not maintained.
Anthony Bruten, environment officer for Natural Resources Wales said: "Environmental regulations are in place to help protect people, wildlife, our rivers, and land.
"The construction industry has a duty of care to the communities in which they operate, to ensure the correct controls and safeguards are in place in order to prevent incidents such as these occurring.
"In this case, Persimmon Homes’ failure to put in place suitable mitigation methods meant that silt run-off from the site continued to negatively impact the nearby water courses and the River Gavenny over a period of 10 months.
"I hope this fine will send out a clear message that environmental legislation is to be taken seriously. We will not hesitate to take appropriate action against those who disregard regulations and jeopardise the natural environment we all know and love."
A spokesperson for Persimmon Homes East Wales said: "We would like to sincerely apologise for these incidents which occurred some years ago in Abergavenny.
"While we are relieved that there has been no material harm caused to the local environment, and it has been acknowledged that they were not deliberate acts, we fully understand their potential significance.
"These incidents should not have happened and we have made a number of changes to local personnel and procedures in South East Wales to improve the on-site implementation of appropriate preventative measures.
"As a company we take our environmental responsibilities very seriously and express our deep regret that these incidents took place."
Persimmon Homes was fined £53,000 per offence, as well as costs, bringing the total fines to £433,331.