Here's an irony: Walden Pond, immortalized by writer Henry David Thoreau for its "crystalline, pellucid" waters, was last week banned to swimmers by conservation officials in Concord, Mass., because of dangerous bacteria. Thoreau lived there for two years from 1845, idealizing the spot's tranquillity, and espousing what we nowadays would call an environmentalist agenda.

Excerpts from Thoreau's "Walden" provide the text for Minneapolis-based composer Dominick Argento's choral work "Walden Pond," centerpiece of Sunday afternoon's concert at the Minnesota Beethoven Festival in Winona. Argento wrote the work 20 years ago, and the Dale Warland Singers, who commissioned it, subsequently made a Grammy-nominated recording.

Warland himself marshaled the Festival Chorale in Sunday's performance. Now in his 80s, he led the singers with undiminished authority. Thoreau's quietist philosophy infuses the prose passages set by Argento, meaning there are few moments of explicit drama and many prolonged passages of musing and meditation. That means extra work for singers and conductor to ensure the music's placidity of temperament doesn't seem bland or induce somnolence.

The performance was, in the event, a triumph. The choir of 43 singers, hand-picked from across the United States, had rehearsed all week, and seemed thoroughly imbued with conductor Warland's very detailed view of how Argento's music should be presented. He asked much of them — sudden shifts of perspective and dynamic, micro-alterations of tempo to reflect the ever-shifting cadences of Thoreau's writing and outstanding breath control to sustain phrases liquidly beyond the points where punctuation might normally have been inserted.

It was a virtuoso performance, but also one of great nuance and sensitivity. Argento's unconventional accompaniment for a harp and three cellos was deftly balanced with the choir and yielded further significant strands of insight. High cello harmonics set the air atingle in "Angling," and a plunging cello subtly darkened the atmosphere in "Walden Revisited," where woodchoppers have partly wrecked the verdant shoreline of Thoreau's idyll. Warland calls "Walden Pond" an unmitigated masterpiece; on this showing it was difficult to disagree with him.

Water, as a theme, also pervaded the first half of the program. It was topped and tailed by Eric Whitacre's "Water Night" and "Cloudburst," the former fresh and luminous in impact, the latter spangled by Whitacre with a technical chicanery and tricksiness that occasionally suggested a preoccupation with surface effect rather then deeper motivations. Both works were limpidly executed by the singers, handbells and finger-clicking ramping up the atmospherics at "Cloudburst's" conclusion.

A group of spirituals explored river imagery in a religious context, and in Lionel Daunais' "Le Pont Mirabeau" the choir showed idiomatic French and an ability to float the caressing melodies with a beguiling elegance.

Terry Blain writes about classical music.