Appearing before Reading magistrates on 24 May, both pleaded guilty to offences under section 85 of the Water Resources Act 1991.
The court heard that the incident occurred in March 2005 when Wooldridge was demolishing old school buildings at Douai Abbey, Upper Woolhampton.
Bewley had conducted environmental reports on the site and was aware it contained an oil storage system but did not warn Wooldridge.
The Environment Agency was alerted after a farmer spotted oil in a tributary of the river Kennet.
Officers traced the pollution to the Abbey site, although Bewley’s manager initially denied there had been a spill.
However, it was discovered that oil storage tanks had been damaged during preparatory work, allowing oil to seep into a bund. The oil entered the stream after the bund failed to contain it because holes had been drilled into its concrete walls for wiring to pass through.
Both firms initially denied responsibility and the Agency was obliged to begin the clean-up. It estimated that 500 litres of oil were lost.
Wooldridge subsequently paid £5,500 in clean-up costs and Bewley met the soil excavation and fuel removal costs.
The companies were also each ordered to pay £2,367 in prosecution costs.