Nassau County advised against swimming at 19 beaches Saturday because runoff from heavy rainfall Friday may have contaminated the water.

Heavy rainfall can cause substantial stormwater runoff that can raise bacteria levels; those levels mean the waters fail to meet state standards for swimming, health officials said.

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The advisory covers the following 14 North Shore beaches: Centre Island Sound, Bayville; Creek Club, Lattingtown; Lattingtown Beach, Lattingtown; Laurel Hollow Beach, Laurel Hollow; Morgan Sound, Glen Cove; North Hempstead Beach Park, Port Washington; Piping Rock Beach Club, Locust Valley; Pryibil Beach, Glen Cove; Ransom Beach, Bayville; Theodore Roosevelt Beach, Oyster Bay; Sea Cliff Village Beach, Sea Cliff; Soundside Beach, Bayville; Stehli Beach, Bayville; and Tappen Beach, Glenwood Landing.

The advisory also covers the following four South Shore beaches: Biltmore Beach Club, Massapequa; Hewlett Point Beach, East Rockaway; Island Park Beach, Island Park; Merrick Estates Civic Association, Merrick; and Philip Healey Beach, Massapequa.

For recorded information on beach closings and openings in Nassau, call 516-227-9700. Health department representatives are available weekdays from 9 a.m. to 4:45 p.m., at 516-227-9717.

With Gary Dymski