UPDATE / ILLEGAL IMMIGRATION : Uncountable Problem at the Border : Up to 7 million aliens may be in the U.S. improperly, experts estimate as employer sanctions are debated.

Illegal immigration is, by definition, impossible to document precisely because most illegal immigrants sneak into the country undetected.

But the U.S. Border Patrol keeps track of the number of people arrested when attempting to cross the U.S.-Mexico border, where an overwhelming majority of illegal entries occur. The accompanying map shows where the crossings are the heaviest.

The numbers represent apprehensions during the fiscal year that ended Sept. 30, 1989, in each of the Border Patrol’s districts along the U.S.-Mexico border. The cities shown are the sites of the patrol’s district offices.

The Border Patrol does not estimate the number of illegal aliens in the United States based on its illegal crossing arrest statistics because it believes such extrapolations would be unreliable. Many of those captured are repeat offenders who are sent back routinely.

Some experts, including those at the Washington-based Center for Immigration Studies, estimate that 6 million to 7 million illegal immigrants currently reside in the United States. But Dave Simcox, the center’s director, acknowledged that his figures are imprecise.

“Nobody’s going to have a hard and fast mathematical formula for tracking illegal immigrants,” Simcox said. “You’re counting a population that doesn’t want to be counted.”

Concern about the influx of illegal immigrants crossing the U.S.-Mexico border in search of jobs led Congress to pass the Immigration Reform and Control Act of 1986. It requires employers to check worker documentation and mandates penalties against those who knowingly hire illegal aliens.

The General Accounting Office recently estimated that 19% of the nation’s employers engaged in some form of discrimination against job applicants who appear or sound foreign, rather than risk violating the sanctions law. Immigrant rights organizations and civil liberties groups have called for repeal of the employer sanctions, but the Bush Administration has urged that they be retained.

A task force composed of officials representing the Justice Department, the Civil Rights Commission and the Equal Employment Opportunity Commission is reviewing the GAO report. Its recommendations regarding employer sanctions are due in about four months. Meanwhile, Congress is unlikely to repeal the sanctions.