Brown Gritty Coating on Waterfall Rocks: Iron Deposits from Softened City Water

A Houston pond (no fish/plants) has brown, gritty deposits on upper rock surfaces only. Lime-Away dissolves it; chlorine doesn't. Water source: city water run through a water softener. Rocks are large, flat, reddish.

Diagnosis: Iron Staining

Water softeners exchange calcium/magnesium for sodium — they do NOT remove iron. If your city water has iron (common in well water, but possible in municipal), the softener passes it through. When water cascades over rocks, iron oxidizes (rusts) and deposits as brown grit.

The reddish rocks likely contain iron minerals too, accelerating the process.

Solutions

1. Pre-filter the fill water: Install a whole-house iron filter (greensand, Birm, or air-injection) before the softener.
2. Seal the rocks: Clear marine spar varnish or two-part epoxy resin (e.g., Pond Armor) creates a barrier. Test on one rock first.
3. Switch to non-softened water for pond top-offs if possible (bypass softener).
4. Accept it: In a fishless, plantless feature, iron stains are cosmetic. Scrub annually with Lime-Away or vinegar.

Why Only Upper Surfaces?

Splash zone = maximum evaporation = maximum mineral concentration. Submerged rocks stay wet, so iron stays dissolved.

For more on iron in water: water-research.net/iron.htm

Bottom line: Your softener isn't the problem—it's the iron in your source water. Filter it out or seal the rocks.