The lure of a sudsless Rhine . . . and a 10-foot Lorelei

Take heart, Rhine tourists! Europe's greatest river is no longer sudsy. And as you peer at the gentle crags and hum to yourself ''Ich weiss nicht, was soll es bedeuten,'' you can finally see just where the Lorelei sat, or at least sits. The locale has been fixed forever in concrete.

This month, it seems, the burghers of St. Goarshausen took pity on neck-craning tourists, rectified the omission of centuries, and erected an on-the-spot monument to Heinrich Heine's lyrics and all those boatmen who heeded the temptress rather than tending their helms.

The statue, and its unveiling, can be said to be fully worthy of 19th-century folklore. The sculpture - 10 feet high and several tons large so that no excursion tourist can miss knowing precisely where among the vineyards the Lorelei combed her hair - was dedicated by the mayor of St. Goarshausen himself, along with the 1983 Miss Lorelei winner, the sculptoress (said to be a student of a student of Rodin's), three choirs, and the head of the Lorelei Marketing Firm.

The last named had arranged for souvenir reproductions of the statue to be on sale from the moment of its unveiling. They range in size from ceramic knickknacks for a few marks on up to a ''garden dwarf'' version for 142.50 marks ($60) each.

The realistically sculpted - well, statuesque - nude has not, so far as is known, yet led any barge or excursion-boat skippers astray. But she does presumably divert tourists' eyes from the murky if unsudsy Rhine flowing beneath their hulls.

Few except some brave 12-year-olds swim in the river these days, either at the feet of the Lorelei or elsewhere. Scullers do row on it, but they are careful not to dunk their coxswains in it. Fishermen along its banks put their catch not on the dinner table, but in cat dishes.

The Lorelei's Rhine is, in a word, polluted. It has been cleaned up to a considerable extent, but its salt, chloroform, and 50 other toxic substances (out of some 5,000 chemicals reportedly dumped into it by industry) are still rather more threatening to the locals than sirens ever were.

The Netherlands, which inherits all the gunk spewed into the river upstream, complains repeatedly about the 20,000 tons of salt that French potassium mines discharge into the Rhine daily. This month it won a landmark injunction by a French court in Strasbourg barring the mines from pouring their waste into the river.

(France, which promised in a 1976 treaty to reform and again in an International Rhine Commission compromise in 1981, has never ratified the treaty. And up to the moment of injunction the potassium mines had not started burying the salt in the ground as agreed.)

The Netherlands - which depends on the Rhine for tap water for almost all of its population as well as for irrigation of its hothouse farming industry - also had to turn off its Rhine intake altogether for a time last October because of a high incidence of ortho-chloronitrobenzol in the river.

Another, more arcane, lawsuit is currently in West German courts, with officials suing a waterworks employee (on grounds of defamation, it appears) for charging publicly that unfilterable substances in the Rhine were doing dire things to Bad Godesberg water drinkers. And various angry grass-roots ecological groups, acting on behalf of the Rhine's 28 million drinkers, are staging an International Water Tribunal in Rotterdam in October to protest the disposal of ortho-chloronitrobenzol into the river by a Hoechst subsidiary and other effluents by Bayer and 16 other major European firms.

The Rhine is much better than it used to be, for whatever comfort that gives. The detergent foam of a decade ago has been dispersed by West Germany's general shift to 80 percent biodegradable detergent. More than 70 percent of sewage is by now treated biologically before it enters the Rhine. The oxygen count is reported to be well above critical levels. And recent samplings of the Rhine show a low bacteria count (even if critics scoff that the reason must be that not even bacteria can survive the potent chemical bath).

A welcome lone salmon has even been caught by a startled angler near Karlsruhe, and eels have reappeared upriver from Bad Godesberg.

A witticism by cyclists riding through clouds of Rhine gnats registers the improvement: The river is sufficiently purified to get the bugs back, the saying goes, but not enough to get the fish back to eat the bugs.

Controversy swirls about whether the river should be deemed half dirty or half clean. The tourists hum ''and quietly flows the Rhine.'' The brand new Lorelei smiles enigmatically.