Charles County officials last week said the county's incidence of drug abuse and addictions--the fourth highest among the state's 24 jurisdictions, according to a recent study--accentuates Charles' problems as it changes from a rural area to part of metropolitan Washington.

The growth of northern Charles County, which is becoming an increasingly popular suburb with Washington's work force, accompanied the rise in reported cases of drug abuse, officials noted.

The county's population shot up from 47,678 in 1970 to 72,751 in 1980 as St. Charles, a federally financed "new town."

Local newspapers blamed "newcomers" for bringing a "flood" of drugs and for adding sophistication to local drug abuse habits. But county health officials said the expanding population has highlighted existing problems that foster drug use.

Unemployment and the pressures of growing up continue to be factors that bring young people to experiment with drugs, said Larry McAvoy, director of the county Department of Mental Health.

"Kids do it out of rebellion, kids do it out of boredom, some kids do it out of curiosity," McAvoy said. "You can't just say it's growth, and you can't just say it's because we are becoming part of Washington. It's a very sticky combination of things."

Drug arrests have climbed from 193 in 1981 to 247 last year, "but our population has increased, too," said Capt. Ross Pitrelli of the County Sheriff's Department--the county's principal police force. The department has stepped up drug investigations and school and community awareness programs, he said.

Ironically, Charles County's attention to drug problems has heightened its standing in statistics for drug abuse cases. The study that state Department of Drug Abuse Administration officials released last week projected the total number of drug abuse and addiction cases based on the use of Maryland's 65 drug teatment centers.

Lewis Kelley, a crisis worker at Bumpy Oak Counseling Center, Charles' drug-counseling and treatment center, said local judges are cracking down on even minor and first-time offenders and commonly are making treatment at Bumpy Oak part of their sentences.

Based on treatment center reports between 1979 and 1982, the report found Charles County fourth in the incidence of drug abuse, behind Baltimore City and Prince George's and Baltimore counties, as drug abuse cases rose 40 percent throughout Maryland. Montgomery County was fifth in the report. He said most of the center's clients have been county residents more than five years.

Alcohol remains the most commonly abused drug, according to Bumpy Oak director Mike Smith. A recent check showed a slight decline recently in marijuana use by young county residents, he said.