Appearing before Grantham magistrates on 12 July, WRG pleaded guilty to causing polluting matter to enter controlled waters, contrary to sections 85(1) and 85 (6) of the Water Resources Act 1991.
In April last year, the company - then trading as Shanks - began spreading food and vegetable waste on fields near Grantham.
An Environment Agency officer visited the site and expressed concern that the waste could get into the nearby Cringle brook. WRG said land drains were too deep underground for that to happen.
However, on 14 May a member of the public phoned the Agency to report that the brook had turned grey. Agency officers visited the scene and found that the waste had poured off the field into a nearby wood and on into the brook. Minnows were gasping for air at the brook's surface, and nearly two kilometres of it was polluted.
Investigations later revealed that the pollution had been caused the previous day. During work to inject the waste into the soil, a pipe had split open causing the waste to run down the fields.
The company's operations manager had seen the spill occur, but had had to run half a mile uphill to tell the tanker driver to turn the pump off. By the time he got there, the operation was complete. He said that he had been too busy cleaning up the spill to phone the Agency.
In addition to the £6,000 fine, the company was ordered to pay costs of £3,000. The fine was WRG's second of the year. In June it was fined £2,500 after pouring sludge onto roads in the Swansea area (ENDS Report 365, p 55 ).