Per Capita Consumption
By Elizabeth N. O'Brien

The Quabbin and Wachusett reservoirs are the main sources of water that the MWRA uses. The Quabbin's maximum capacity is 412 billion gallons and the maximum capacity of the Wachusett is 65 billion gallons. Each day, 155 million gallons are taken from the Quabbin and 118 million gallons are taken from the Wachusett reservoirs. Added together, this means that the MWRA uses approximately 250 million gallons of water each day. According to the MWRA, they have 2.5 million users. Using this data, you can find how much water each person uses daily. 273 million gallons/2.5 million users equals 109.2 g/p/d. This means that each person, from the 2.5 million users, uses 109.2 gallons of water per day.

The Quabbin reservoir can hold a total of 412 billion gallons of water and the Wachusett reservoir can hold a total of 65 billion gallons. The MWRA has access to all of this water, but they, obviously, can not drain the reservoirs dry. To keep the reservoirs from drying-up, the MWRA decided to make a safety yield. To see a graph of this safety yield, click here: http://www.mwra.com/water/html/wsupdate.htm.

The MWRA can safely use 300 million gallons per day. This is a combined total from each reservoir. If you take this number and divide it by the amount of water each person uses each day, you will find the carrying capacity of the MWRA. 300 million gallons of water divided by 109.2 gallons of water per day is approximately 2,747,252 people that the MWRA can serve effectively. [safe yield is defined as follows: The amount of water that a reservoir can provide during both wet and dry periods and still maintain safe delivery levels. Engineers determine safe yield by taking into account the expected rainfall, the size and absorbing capacity of a watershed, the capacity of a reservoir and the amount lost through evaporation. The safe yield of the MDC/MWRA system is generally considered to be 300 mgd on a long-term annual average basis.]

By examining the graphs of Massachusetts and Boston populations, you can readily see that they each are increasing by great numbers. The Massachusetts growth rate from 1900-1990 is .78. I took the growth rate from the Boston graph twice, because of the big population decrease from 1950-1980. The Boston growth rate from 1880-1990 is .63 and the Boston growth rate from 1890-1950 is 1.16.

The Boston population in 1990 was 574,283 people. According to the growth rates stated above, the Boston population should be near 660,000 people by the year 2000. This means that the MWRA is fully qualified to adequately serve the Boston area. If the population increases above 2.7 million, the MWRA will have to rethink their water and sewage processing, for example, the MWRA may have to make additional sewage and water plants to handle all of Boston's waste.

Projections of population growth vary widely among sources. The Metropolitan Area Planning Commission, the State MISER projections, and the U.S. Census all provide different projections. Projections exist for both the Metropolitan Area and just Boston.

MWRA has examined trends for both Boston, the Metropolitan area, and the service area, and has not projected any substantial increase in water demand through 2010. On a daily basis, MWRA supplies about 2 million people with water.

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