An 18-year-old Falls Church man, on a final fling with school friends before beginning college this fall, died Sunday night after being shot in the abdomen during an incident at a Culpeper County lake resort.

The death of Joseph Thomas Maybury, a six-foot, athletic graduate of George Mason High School, upset friends who witnessed the late afternoon shooting and generated flurries of sympathy for his family in the community where he grew up.

Culpeper County Sheriff R.E. Peters said yesterday that Eugene K. Wells of the county's Reva section had been charged with murder in the death of Maybury and is being held in lieu of $50,000 bond. He was arrested at the hospital where Maybury was taken after the shooting.

Peters declined to give details of the killing and witnesses could not be reached. But Maybury's mother, Theresa Maybury, said yesterday that friends of her son who were present told her the tragedy began Sunday morning, when Joseph and two friends who were staying at a vacation cottage in Culpeper, about 60 miles from the Maybury home, went hiking.

In a field, they came across a locked car that appeared to be abandoned, she said. The youths broke one of its windows. Shortly afterward, Theresa Maybury said, an elderly man shouted at them to get away from the vehicle.

The three left the area and returned to the cottage. That afternoon, the group of friends was swimming in a pond near the cottage when the man arrived at the scene with a shotgun and fired a blast into the air.

According to the witnesses, Theresa Maybury said, Joseph, one of the older members of the group, attempted to take control of the situation. Walking toward the man, he told him: "Don't shoot. I'll pay for any damage done to your car."

The man placed the barrel of the shotgun against his stomach, according to the account given Theresa Maybury. As Joseph brushed it away, she said, the gun discharged into his abdomen.

The man told the youths to call a doctor. Joseph was lifted onto a vehicle and taken to Culpeper Memorial Hospital, where he was admitted about 4:30 p.m. He died about six hours later.

Wells had been arrested at the hospital earlier in the evening and charged with felonious assault, Sheriff Peters said. After Maybury died, the charge was changed to murder, according to Peters.

Yesterday, Maybury's parents took calls of sympathy in their home in Falls Church and tried to comprehend what had happened. "It was just some kind of freak thing," said his mother. " . . . He was never one to take chances."

An athletic student who swam and played tennis and racquetball, her son was set to enroll at Virginia Polytechnic Institute this fall to study engineering.

He left the house Saturday morning, telling her he would return Monday night. "I said, 'How late?' " she recalled. "He said, 'I'll be here for the Redskins game.' "

Howard Hurley, a neighbor of the Mayburys, said he and other acquaintances are looking for a way to ease the family's grief. He said Joseph "was the last of the nice kids. No smoking, no drinking, no drugs. He always had odd jobs in the neighborhood before he got to be a teen-ager. Every time he drove by, it was always a wave or a honk of the horn."