Under the project, headed by Herefordshire Council, silt will be removed from the pool for the first time in about 200 years and used as a natural fertiliser on sports playing fields at nearby Aylestone Park.
Castle Pool, in Castle Gardens, formed part of the original moat that ran around Hereford Castle when it was built in the 11th century.
The castle has long since disappeared but the moat remains as a scheduled ancient monument of significant importance.
During the summer, when the water level drops, the silt is exposed to the sun often causing unpleasant odours.
English Heritage has already given consent for the works and a series of archaeological surveys have been conducted to establish the historical importance of the site, obtain a full profile of the area and to establish the composition of the silt.
The Royal Society for the Protection of Birds (RSPB) has been consulted over the safety of wild birds and the timing of the works programme in relation to the nesting season.
The Environment Agency (EA) has also been commissioned to net the pool and safely gather fish while the silt is removed.
The works programme, expected to be completed in the early spring, is a partnership between Herefordshire Council, Owen Williams Consultants and Surrey-based specialist contractors Land and Water.
Tony Featherstone, Herefordshire Council’s Parks and Countryside Manager, said: “This is an opportunity to further improve the general appearance of a popular and important open space near the City Centre.
“In addition to the silt being removed, litter and other debris will also be taken away.”
Paul Tucker, Owen Williams engineer, added: “We have consulted a wide range of organisations to ensure that the works are co-ordinated and delivered in a well planned and carefully conducted manner.
“The silt is made of mainly dead leaves and this will prove to be a good natural fertiliser for the sports playing fields.”