(FORTUNE Magazine) – BEACH-BOUND LEGISLATORS face an unusually full agenda on their return to Capitol Hill after Labor Day, and many of the unresolved issues, from tax reform to cleaning up toxic dumps, are matters important to business. With the plate so full and so little time to clear it before year's end, business lobbyists fear that Congress might move with undue haste. Norman Ornstein, a political analyst at the American Enterprise Institute, thinks their fears are justified: ''It's going to be rough. Business will ultimately have to reach into its pocketbook to pay for what Congress does.'' The 99th Congress has been kind to business so far. A bill amending the export control law, for instance, contained a long-sought change that makes it more difficult for the President to abrogate business contracts for foreign policy reasons. The new law probably would have prevented President Reagan's 1981 embargo on sales of equipment for the Soviet gas pipeline. Here's how returning legislators are likely to deal with other legislation important to business: TAX REFORM. Tax writers in the House say they'll have a bill by mid-October. It then goes to a more reluctant Senate, which may not act before the new year. The tax plan that President Reagan introduced in May is in for major changes. Under the House bill, Reagan's notion of cutting personal income taxes is likely to survive, but middle-income taxpayers will probably fare better, and the well-off worse, than in the Reagan plan. Business will have to make up for revenue the Treasury loses from the personal tax cuts. Some business tax preferences, such as write-offs of intangible drilling costs, may get the ax. Chances are strong that Congress will do away with the popular 10% investment tax credit, though it may be eliminated in stages rather than all at once. TRADE LEGISLATION. Literally hundreds of trade bills await congressional action. Many seek tariffs on specific goods, such as telecommunications equipment from Japan and garments from Taiwan. At least five bills would impose new tariffs on all imports. For all their frenzy over the trade deficit, legislators are fearful of sparking retaliation abroad and are unlikely to do more than pass a resolution urging the President to get tough on trading partners. Says Ornstein, ''There will be lots of thunder and lightning, but little rain.'' SUPERFUND. Legislation authorizing the fund to detoxify chemical dumps expires at the end of September. A Senate committee has voted to increase the fund from $1.6 billion to a hefty $7.5 billion, and a House committee wants to raise it to $10.1 billion. The prickly question is, who pays? Chemical and oil companies, which now largely finance Superfund through a tax on their raw materials, say they already shoulder too large a burden. A second Senate committee has proposed raising the money mainly through an excise tax that would hit all manufacturers. A coalition of 45 companies and trade groups, led by the Grocery Manufacturers of America, is gearing up to fight the excise tax. Among the alternatives: financing Superfund through general budget revenues -- clearly a tough sell to a deficit-dazed Congress. SOUTH AFRICA SANCTIONS. A bill to impose economic sanctions against South Africa came close to passing in the last days before the recess. When Congress takes it up again this fall, college students will be back on campus and ready to stoke the fire. Chances are excellent that a bill will pass. But Congress has already dropped a tough proposal that would have imposed an immediate ban on new investment in South Africa. The remaining sanctions are largely symbolic, such as the ban on new bank loans to the South African government. Still, the symbolism alone strikes fear among the 350 U.S. companies that have invested $3 billion in South Africa. Chase Manhattan, for one, recently stopped making commercial loans in South Africa, and an exodus of sorts has begun as a dozen small U.S. companies have left the country. IMMIGRATION. Last year business lobbyists opposed a bill to halt the inflow of illegal aliens. The law would have required employers to have documentation of each employee's eligibility to work in the U.S. Employers knowingly hiring illegals would have been subject to stiff fines. A new version of the bill has allayed some business fears and raised others. The bill's likable aspect: keeping eligibility documents would be optional. Its drawback: employers found guilty of hiring illegal aliens would be subject to fines and up to six months in jail. The bill stands a good chance of passing.