A comprehensive plan to double sewer capacity and sharply increase the water supply for the rapidly growing north end of Charles County is being submitted to the county commissioners next week for final approval.

The plan, which is required by state law and updated every two years, calls for at least doubling the capacity of the Mattawoman Waste Treatment Plant to keep pace with expected growth in the north central part of the county.

Without such an expansion, that facility would exceed its present capacity by 6.3 million gallons per day in 1990, according to the plan.

The plan recommends that the county drill wells for the area served by the Waldorf water system into a different and deeper underground river known as the Upper POatapsco Acquifer in order not to maintain reserves in the heavily used Magothey Acquifer. Most of Charles County depends on this system of underground rivers for its individual wells and community water supplies.

In the late '70s and early '80s ''there was such rapid, centralized growth; it was almost uncontrolled, it happened so quickly,'' said Nancy Ehrlich, head of planning and engineering in Maryland's water resources administration.

In the next 20 years, planners say, Charles will witness a population explosion of 38 percent, from 81,799 persons to 113,024. Most of this growth is expected to take place by 1990.

Much of the recent and predicted growth centers along the Rte. 301 corridor in the Waldorf, St. Charles and La Plata areas and in the Indian Head area along Rte. 201 to the west, said Andrew Paszkowski, planning engineer with Nassaux-Hemsley consultant group in Chambersburg. Pa., which was paid $40,000 to produce the water and sewer plan.

According to the plan, St. Charles, a sprawling 12-square-mile housing development outside of Waldorf that opened its first units in 1968, will account for 57,189 people or 52.3 percent of the county's total population by 1990.

Roy E. Hancock, director of public works, said the plan calls for one or two more deep water wells a year for the Waldorf area system, in addition to the one already drilled in the Upper Patapsco Acquifer, each at a cost of about $300,000.

Also, the plan recommends tapping into the Washington Suburban Sanitary Commission's water supply within the next three years as the county's quickest and most cost-efficient way of increasing that resource.

Discussions among officials from the WSSC and Charles and Prince Georges' counties about such a hookup have been going on since last February, Hancock said.

Like much of Southern Maryland, Charles County has soil conditions unsuitable for septic systems in 50 percent of the county, although for many residents this method of waste disposal had been the traditional route, Hancock said.

The long-range plan says areas such as Cobb Island -- where a high rate of failed septic systems has plagued residents for years -- must be addressed, as must be communities with contaminated individual wells.

Hancock said mound systems and on-site spraying are among disposal options under consideration for areas such as Cobb Island.