Northern Virginia families generally pay nearly twice as much for their water and sewer services as those living in the District of Columbia.

Maryland customers pay more than D.C. residents but usually less than Virginians.

The gaps are narrowing, however.

Rates for Maryland households served by the Washington Suburban Sanitary Commission (WSCC) are scheduled to rise Dec. 1 because customers have been using so little water.

In the District of Columbia, no formal rate has been proposed yet. But officials say privately that customers are sure to pay more when the D.C. pipe and plant network, which dates back to the early 1900s, gets the remodeling it needs.

As water becomes more valuable and the economics of building and financing such service grow more complex, the frustrations for customers are sure to increase.

Consider:

D.C. residents often don't know they have been blessed with low rates because of the way their water is metered and billed. "How much is a cubic foot of water?" asked one customer after scrutinizing his statement. His average daily consumption was shown at the bottom of the bill in gallons, but nowhere did it state the total number of gallons he was being billed for. Nor did it explain how to convert the cubic foot -- which equals 7.481 gallons -- into the more common gallon measure.

Many customers can't even find their meters, which often are located underground in locked pits. The meter reader has a key that opens the metal plate covering the meter, but it generally isn't accessible to the property owner.

A city water official said the plates, frequently located in sidewalks, are locked to prevent accidents. "Someone could trip if the plate isn't fitted into place," he said.

Rates that customers pay sometimes are as confusing to translate as a foreign language. In Maryland there are 100 different fees that a customer may be charged, depending on the amount of water consumed each day.

At present, the price for water and sewer services in the metropolitan area ranges from a high of $243.16 a year in Fairfax City, Va., to a low of $90.04 in the District. That is based on rates for a typical family of four using 80,000 gallons a year. Sewer rates, which are included, are tied to the number of gallons of water consumed.

The differences in the two extremes, water officials say, is roughly akin to the difference between the payments on a house purchased 75 years ago and one purchased 10 years ago. The mortgage on the old house probably has been paid off, and the owner worries only aout maintenance costs. But the mortgage on the new house, because of higher interest rates and inflation, is much more expensive.

Fairfax City rates are designed to cover the cost of a sewer facility built in 1971, said director Richard S. Fruehauf.

The District of Columbia system, with an average age of 75 years, was built when construction and capital costs were much cheaper, said Harold Stearn, a D.C. deputy administrator.

Like the rates, the cubi foot measurement on D.C. water bills is rooted in the system's history. At the turn of the century, when the first meters were installed here, the cubic foot was the standard measure on most meters that manufacturers made, Stearn said.

Since then, the gallon measure has become more popular. But ripping out the old and installing new gallon meters would cost thousands of dollars, an official estimated.

Whether they run by the foot or by the gallon, however, officials insist that meters have a high degree of accuracy.

carl Johnson checks his regularly anyway, just to be sure. With a household wrench, he unfastens the metalplate covering the locked meter outside his home and peers down at the dancing dials. Then he compares the reading he finds to the one on his bill.

Johnson is a cautious man who has never found an error but believes that it is better to be safe than sorry.

He also is the District's top water official.

Johnson relies on the meter and the bill to track family water consumption. "We use about 70 gallons per person per day," he reported.

That's within the normal range of 64 to 76 gallons that area consumers use, according to water department surveys.

Normal water use includes: 29 gallons for flushing, 19 gallons for bathing, 13 gallons for laundry and dishes, 3 gallons for cooking and drinking. That's 64 gallons for indoor use. Theremaining 12 gallons usually goes for lawn watering and car washing although the soggy summer has cut outdoor consumption.

Over a period of one year, a normal family uses 80,000 gallons of water -- enough to fill a tank eight feet high 21 feet wide and 64 feet long.

To document its water consumption, a family can compile an inventory of the waer fixtures in the home and the frequency of use. Each time a conventional toilet is flushed, for instance, it uses 5 to 7 gallons of water.

A clothes washing machine use 42 to 45 gallons per load; a dishwasher, about 25 gallons.

Those who know how much water they traditionally consume are better able to challenge sudden and unexpected increases in water bills. The increase may result from an incorrect reading or an underground leak. But it also can be caused by carelessness.

One Maryland family preparing to leave on vacation waited in the car for a child to make a final trip to the bathroom. The boy flushed the toilet and then joined those in the car outside.

Three weeks later, the family returned home and found the toilet still running.

The incident was duly recorded on their next water bill. It showed they had used about 40,000 gallons more than usual.

That one flush cost about $33.