Census data outlines future US population growth, immigration

America's population may peak at 309 million in AD 2050, according to the US Census Bureau, based on the first projections of the 1980 census.

As US population growth slows down, demographers ask, will it bring an influx of immigration from other countries?

Statisticians debate the next population chapter of the United States. They find the present US population of 230 million will reach 268 million in 2000, and 309 million in 2050. That number, estimates the Census Bureau, will be an all-time high ''before beginning to decline.''

Signe Wetrogan, a Census Bureau statistician, notes that her agency makes three assumptions, low, middle, and high, based on guesses of mortality, fertility, and immigration. Under the ''high'' projection, she says, population could reach 340 million in AD 2050. It would still be growing at something less than 0.14 percent a year under the high assumption. Under the ''low'' assumption population would peak at around 261 million in AD 2025 and then descend: It would be down to 230 million in AD 2050, or about what it is now.

Demographers grabbed the new census projections. They will be the basis for analysis for the next 10 years, dealing with divisions of age, race, sex, and imponderables like immigration and fertility.

Carl Haub, a demographer of the Population Reference Bureau Inc., a nonprofit research group here, notes one feature of the new study.

''I think it is a bench mark,'' he says, ''that the Census Bureau is now projecting population to eventually enter a period of decline. This results from the fact that women are having less than two children each and that they will continue to do so in the future.''

The population ''replacement'' level requires about a 2.1 percent figure for children whereas the census projection is 1.9 percent in the intermediate range.

Others ask: Is a stationary American population possible when other nations are land hungry? The new study assumes a constant net immigration of 450,000 a year. This is unrealistic, some contend. Mexico, for example, now has a population of 71 million. Projections of growth reach 100 million in 2000, and 140 million in 2020. (The latter equals US population in 1940.) Some estimate a million illegal immigrants from Mexico a year. World population growth rate is projected by UN and other sources at around 1.7 percent; US at 0.7 percent and Mexico at 2.7 percent. Former US Immigration Commissioner Leonard Chapman estimated Mexican illegals in the United States in 1976 as from 6 million to 12 million. Only 1 in 3 illegals is caught, he estimated.