A citizens task force looking into ongoing financial problems in Anne Arundel County's utilities department has concluded that the agency, which operates all county sewer and water systems, is headed toward a $12 million deficit by 1990, and that rates may need to be raised as much as 23 percent over the next five years.

Despite a healthy surplus reported in the department's operating and capital budgets at the end of the last fiscal year, the eight-member task force predicted that the agency could face financial difficulties within the next decade if "inadequate" rates are not increased to meet costs.

"If utility rates and charges are not significantly increased, two things are clear: Annual deficits in the debt service fund used to pay off construction loans will occur very soon . . . and the size of the accumulated deficit will quickly grow to substantial amounts," the task force said.

If the administration follows the task force's recommendations, the average water and sewer bill would increase from $308 this year to $379 by 1990, according to projections attached to the report. Officials say the recommendations could ease the county into correcting funding problems without placing a large burden on users.

To avoid future deficits, the task force recommended:

* System users should be charged for construction and maintenance costs as part of their quarterly bills. Currently, county utility users pay one-time capital and connection fees when they receive sewer and water service and pay nothing toward additional capital costs required to keep the system operational.

That is unfair to future users, who may be charged higher fees to allow the county to meet debt payments, the task force said.

* Water and sewer service rates should reflect actual costs and be determined by a formula. Rates are now set on an ad hoc basis by the County Council at the request of the county executive and do not accurately reflect operational costs, the report said.

Use of a formula would alleviate political pressure on county officials to keep the rates artificially low, task force members suggested.

* A 10 percent environmental surcharge should be reimposed on quarterly water bills to reduce the deficits and stabilize rates so that dramatic increases are not needed by 1990. Such a fee is currently levied on wastewater customers, but not on water users.

* The cost of constructing and operating water and sewer systems should continue to be borne by users. In this recommendation, the task force contradicted an increasingly popular argument that, because everyone benefits from a clean environment, general county tax revenues should be used to subsidize operational costs.

However, the task force said the county should subsidize such costs as pollution controls on sewage treatment plants, rate breaks to nonprofit corporations, fire hydrants and planning and development costs.

County Executive James Lighthizer, utilities Director Thomas H. Neel and Director of Administration Adrian Teel are expected to meet with the task force soon to discuss which recommendations the administration may adopt.

Currently, the utilities department is in its best financial shape in 10 years, Teel said. It ended the 1984 fiscal year in June with a $3.5 million surplus in its operating budget and $10.5 million in its debt service, or capital, account.

But officials agree with the task force's general finding that changes must be made or those surpluses will disappear.

The largest problem facing the utility is its capital debt, the task force said. Capital charges have not covered actual construction and financing costs and fewer customers than anticipated have hooked up to the systems, leaving a real deficit.

The department faced repeated financial crises during the administration of Robert A. Pascal, Lighthizer's predecessor, because rates and capital connection charges were considered too low.