You'll be fine. Flush it out as best you can with distilled (demineralized) water. Then make sure you drain at least half the water out (I have to open the block drain on the 3.0L to get at least half out.) Then fill with pure BMW antifreeze until you've added half the listed capacity. Finish the fill/bleed process with your distilled water. You'll be good. The tap water isn't going to cause significant damage in the short run.

AM.

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E70 2010 3.0 M57 AT Titanium Silver with Black Leather
E53 2004 3.0 M54 AT Toledo Blue with Gray Leather
E83 2008 3.0 N52 AT Silver Gray Metallic with Black Vinyl
E46 1999 2.8 M52TU 5MT Black with Tan Leather 250K+ miles (Sold, but not forgotten)