The reintroduction of the water voles took three years to plan.
Councillor Kirsti Wenn, Mayor of Harpenden, said: “It has been a real pleasure to watch this conservation project, from its infancy to the release of our new residents, and I can’t wait to see how the water voles will help the town council continue to improve the environment and local ecosystem.
“Batford Springs Local Nature Reserve is not only a beautiful green space, but it is the home of hugely important habitats, and we are immensely proud of all we have achieved here.”