Work is continuing at Roughton Woods near Woodhall Spa following on from last week’s recovery of canisters containing mustard gas.
Agencies are still at the site conducting tests and removing the contaminated items.
Chief Insp Dan Whyment said: “We appreciate this is a frustrating time for people who want to enjoy the woods, but we ask you to just bear with us for a little while longer.
“Work is likely to continue into next week and possibly beyond. We’re very much aware that people want to see the woodland reopened and we’re doing our best to make sure that this happens as soon as possible.
“We hope that people aren’t unduly worried as we’ve been working at the location for some time now. I’d like to reiterate that this is simply to ensure that we are being thorough and public safety remains our priority.
“We really appreciate your patience and would ask people to avoid the area whilst work continues.”