The Reagan administration wants to drop the national goal of halting even the tiniest water pollution discharges by 1985, the second highest official in the Environmental Protection Agency said yesterday.

Deputy EPA Administrator John Hernandez said the goal as expressed in the preamble to the 1977 Clean Water Act amendments "just isn't realistic" in light of new technology for measuring tiny amounts of pollution. He told a gathering of the National Association of Manufacturers that the administration would submit "modifications" to the act to Congress around March 1.

The target for cleaning up American rivers and lakes will be "something along the lines that people can use them to fish and swim," he said. The law already calls for the protection and propagation of fish, shellfish, wildlife and recreation on inland waters by mid-1983.

In its package of amendments to the law, which is up for reauthorization by the end of September, the administration will seek modifications in several areas, Hernandez said. Among them:

* The so-called "pre-treatment" rules, which require industry to treat its chemical wastes before passing them on to a municipal sewage treatment plant, have "significant problems," Hernandez said. Regulations EPA proposed in 1980 have been held up and "may be unnecessary in terms of monitoring ability and unrealistic in terms of what we can achieve," Hernandez said.

An EPA spokesman said later that the idea is to give local communities more control over industrial wastes. Asked if that meant the administration would seek to drop the requirement in the law for a national program, the spokesman responded that such a move was under consideration. Environmentalists would oppose such a change on the grounds that pressure from local industries would prevent localities from regulating them effectively.

* A court order that EPA require 21 major industries to install the "best available technology" for controlling water pollution by 1984 could be dealt with by modifying the act to extend the deadline, Hernandez said. The order resulted from a Natural Resources Defense Council lawsuit which Hernandez said "continues to plague us" and "misdirects the activities of the agency."

The EPA spokesman said the agency would also seek authority to grant exceptions to industries or plants demonstrating that installation of less advanced technology would not harm water quality.

Hernandez said in response to questions that he favored replacing specific technical requirements with ones mandating a certain level of water purity, leaving it to industry to decide how best to achieve it. Requirements for the use of cost-benefit analysis on installing conventional technology need to be rewritten, he said.

* Planning responsibilities now scattered through the act should be drawn together in one place, Hernandez said.

* Water pollution discharge permits should be for longer than the current five years to ease the paperwork surrounding reissue, the EPA spokesman said.

In all, EPA is aiming for "flexibility" in the act, Hernandez said. Asked what NAM can do to help, he told an ethnic joke.

A German pilot "for the Luftwaffe--I mean Lufthansa; I always get those mixed up," Hernandez said, told his passengers, "ve vill fly at 30,000 feet. You vill have a nice flight. You vill enjoy yourselves--provided you follow directions."

In that sense, Hernandez continued, business can best help by following administration directions. "I told you the last time that you guys got to get your ---- together; not in those words, but that's what I meant," he said. He said industry voices should let him know "the key modifications" they would like to see in the law.

"The act itself is good. I believe we can take out some of the complicated aspects of it with only minor modifications," Hernandez concluded.