Water and sewer bills soon may go up again for many Northern Virginians, including residents in Alexandria, Fairfax County and Dale City.

But water officials say the proposed increases are not related to the drought that is draining reservoirs and rivers along the East Coast this winter. Instead, they say, the rate hikes are primarily the result of inflation.

The proposed increases would make water prices for Alexandria and Dale City residents -- supplied by the privately owned Virginia-American Water Co. -- among the most expensive in the Washington region.

Fairfax County Executive J. Hamilton Lambert said this week he may propose a sewer rate increase to meet the costs of inflation and to pay for improvements at sewage treatment plants in the county. Lambert said he plans to meet with county sewer officials next week. Any rate increase, he added, would be included in the fiscal 1982 budget, to be considered by the Board of Supervisors this spring.

Sewer bills for Fairfax residents now average about $112.80 a year, and water bills average $69 a month.

In Alexandria, combined water-sewer bills total about $173 a year. With the proposed water rate increase, that would rise to about $184 a year.

In Dale City, each of the 9,000 families served by Virginia-American now pays an average $166 a year in water bills, the highest in the metropolitan area. Under the proposed rate increases, that water bill would jump to $186 a year.

Despite the high water bills, Dale City residents hav one of the lower sewer rates in the area, paying about $108 a year for service from their community's own sewage treatment plant.

With or without incrases, most Northern Virginians still will be paying the highest water and sewer bills in the area.

District residents, despite sewer and water bill increases last fall, still have the lowest bills -- about $122 a year for the average family. In suburban Maryland, those charges average about $212 a year, although rate increases -- 19 percent for sewer service and 9 percent for water -- have just been proposed for Montgomery and Prince George's counties.

The primary reason for the high prices in Northern Virginia is the unusual network of water and sewer services.

For example, Dale City and Alexandria use water that the Virginia-American Co. purchases from the Fairfax County Water Authority. vUnder a complicated pricing system, Fairfax charges one rate for water usage that is no higher than the amount used in 1974, and a rate three times as high for usage that exceeds the 1974 level.

Sewer bills in Greater Manassas, for instance, average $350 a year, almost double what most area residents pay for water and sewer service combined.

The reason for the high sewer bills in Greater Manassas is the Upper Occoquan River Sewage Treatment Plant, which treats sewage so thoroughly that the effluent is actually drinkable and meets the highest Environmental Protection Agency standards. The plant cost $82 million to build, and because EPA contributed only a small amount toward its construction -- compared to its contribution for most area sewage plants -- the few thousand residents in the area are paying its huge construction costs as well as operating costs.