When Dr. Helen Hackman made up her mind last month to resign her $45,000-a-year post as director of Arlington County's Department of Human Resources, she didn't tell anyone, not even her husband, Marvin Wechsler. "But then we've been married nearly 25 years," Hackman said last week," so nothing I do surprises him anymore."

While Hackman's husband might not have been surprised, some county officials were.

County board chairman Joseph S. Wholey, who has clashed with Hackman publicly in recent weeks, said he did not expect her resignation, which was submitted to county manager Vernon Ford in a letter dated Nov. 25. Hackman, who has worked for the human resources department since 1965, resigned less than a week after an angry exchange with Wholey at a county board meeting. Her resignation is effective March 31.

At that meeting Hackman called the Department of Health Education and Welfare "venal" and branded the Health Systems Agency of Northern Virginia, which has developed a far-reaching and controversial health care plan, " a menace to public health."

Physicians and medical personnel, including Hackman, have mounted a vocal campaign against the health care plan which allow consolidation of some facilities and services in Northern Virginia in an effort to improve the health care services and stem rising costs. Opponents claim that the plan would create the opposite effect, leaving the residents with inferior medical care.

Hackman said her recent clashes with Wholey was a factor in her decision to resign, but were not the primary reasons.

"We have had our differences but I wouldn't say that was the major reason. I've been thinking about leaving for some time now . . . the focus of power is definitely at the federal level and that's where I want to be. I'd like to work as an assistant to a congressman."

Hackman would not say where she would prefer to work since she still is a county employee and is not permitted to express partisan political views. So far, Hackman said, she has neither sought nor received job offers on the Hill.

According to Hackman, the countywide system of zero-based budgeting instituted several years ago by Wholey, has made her job of administering a 400-person department increasingly difficult and frustrating. In Arlington, a department must justify its expenses from the previous year and give the county board various options for budget increases or decreases.

In an interview several months ago, as she was preparing next year's budget, Hackman said: "The first year under zero-based budgeting we cut out the frills. The second year we were down to the bare bones. I'm agonizing now. Do we stop a VD clinic? A TB clinic?"

Last week, she reiterated those views.

'When Arlington's financial situation was better it was very exciting in the department," she said. "Now there's no longer any opportunity to do new and different things. I'm not blaming the (county) board because no one wants higher taxes, but it's very grinding working 70 to 80 hours per week in this environment."

As DHR director Hackman has helped create several innovative programs, most notably an award-winning day care program for the eldery established last year.

A native of Great Britain, Hackman attended medical school in Ireland and has a master's degree in public health from Johns Hopkins. She spent 1976 in England as a World Health Organization fellow.

County manager Ford said that a nationwide recruitment drive is being launched to find her successor.