As many as 5,000 high-tech treasure hunters will descend on Carnation this summer, at a time when the thousand-person city is already thronged with visitors.

Carnation businesses and civic authorities are preparing for GeoWoodstock 2010, set for Saturday, July 3, just before that city’s signature July 4 festival.

GeoWoodstock is an annual event for hobbyists known as “geocachers” — people that use handheld global positioning devices to find hidden caches.

The gathering will be based at Remlinger Farms, which has accommodated like-sized gatherings for corporate picnics.

But geocachers will range beyond the farm, seeking caches in the Carnation vicinity and possibly beyond the Valley.

Jacquie Vaughn, a Sammamish resident and member of Cachers of Washington’s Wet Side, or COWWS, said the caches could be anywhere. Caches can be as small as a finger or as big as a 30-gallon trash can. Geocachers will seek them on foot or in their cars.

“This activity is created to be available to everybody,” Vaughn said. “We have caches that are wheelchair-accessible. There are caches you can drive up to, and they’re 10 feet away.

Vaughn chose Remlinger because of its ease of access from Seattle, and because she knew it could handle the Woodstock-sized crowd.

Cachers will stay at campgrounds and hotels in the Valley and the Eastside. Remlinger will handle meals and parking.

Remlinger Farms Manager Will Hart is a member of Carnation’s recently formed branding committee.

He called the treasure hunt a promising way to expose Carnation to new visitors.

“We’re looking at ways to enhance town visitation,” Hart said.

Carnation Mayor Lee Grumman formed a small group of business owners to explore impacts of the hunt.

“We would love to do something social in Carnation” to draw cachers to the city during the event.

“As a business owner, I do a little more business on the Fourth.

The city already hires extra police to handle its Fourth of July parade and festivities.

“It’s really important to our community — that’s what we’re paying for,” Grumman said.

Carnation’s police agency, Grumman said, are mainly concerned about alcohol drinking and safety during GeoWoodstock.

Vaughn said organizers may set up a beer garden, depending on finances. Children’s entertainment comes first.

“Alcohol is a want, not a need,” Vaughn said. “I have kid things that need to be there, because this is a family activity. Alcohol is not a need to entertain adults.”

She stressed responsibility on the part of the cache seekers.

“We encourage CITO — cache in, trash out — to make this a better place,” Vaughn said. Cachers are asked to pick up litter on the route. Caches generally include a trinket inside. If someone takes a trinket out, they are asked to leave a different one behind.

Participants can sign up for GeoWoodstock before May 15 at www.geowoodstock.com.