Acknowledging that his "timing at this moment in the Sheriff's Department's history is not good," Arlington Sheriff James A. Gondles Jr. said yesterday he will resign, as courthouse speculation on a replacement focused on his chief deputy and the county corrections director.

Gondles noted that his announcement comes as the county is building a new jail and considering a jail release center that has created an uproar among South Arlington residents. But he said that as executive director of the Laurel-based American Correctional Association, he "will be able to do more than just administer a jail."

Gondles, a Democrat, said he plans to leave office in October after 11 years as sheriff. Arlington's four Circuit Court judges will appoint an interim sheriff to serve the remainder of Gondles' term, which expires next year. If the judges haven't made a selection by the time Gondles leaves, the chief deputy, Tom Faust, would become acting sheriff.

Faust, 36, a 14-year member of the Sheriff's Department with strong ties to the county's Democratic Party, is considered by many courthouse observers to be the favorite to replace Gondles. Faust said yesterday he would be "very interested" in the sheriff's job.

Also mentioned as a possible replacement is David Bogard, 35, Arlington's director of corrections. Bogard, who came to Arlington 19 months ago after working as a lawyer in Philadelphia, said he would be "flattered" to be considered.

A third person who might be interested in the appointment is Ronald Hager, 36, who was Gondles' chief deputy before resigning and jumping to the Republican Party to run against the sheriff in a muddy campaign in 1987. Hager has said he is interested in running for sheriff again in 1991.

Observers say that because Gondles' departure comes at a critical time for the county's correctional system, his successor is likely to come from within the department, a theory that favors Faust and Bogard.

Arlington's current jail houses about 400 prisoners in space designed for 164, and the county is about to begin construction of a $42 million jail. Gondles led the successful campaign for a $35 million bond issue to pay for the jail.

He has supported a proposed facility for jail inmates, drug and alcohol addicts and the homeless near Barcroft Park. South Arlington residents have angrily protested.

Sources say Gondles will make more than the $77,000 he was paid last year as sheriff but less than $100,000 in his new job at the corrections association, which promotes work, housing and safety standards for jails, prisons and other correctional institutions.