Why do sprinklers spot my windows?

USGS defines hard water in plain terms: "Hard water is high in dissolved minerals, largely calcium and magnesium." Those minerals do not evaporate with the water. USGS points to a household result homeowners already know from the dishwasher: "One of the most common causes of cloudy dishes and glassware is hard water," and the spots left behind are what USGS calls "hard-water residue."

USGS writes about dishes, not windows, and says nothing about sprinklers. The mechanism carries over all the same: a drop of sprinkler water that lands on a pane and sits there dries the same way, the water leaving as it evaporates and whatever it was carrying staying on the surface.

Some Florida landscapes are irrigated with reclaimed water rather than well or utility water, and UF/IFAS is direct about what that water still carries: "All reclaimed waters contain dissolved mineral salts, for example, calcium carbonate, sodium chloride, calcium sulfate, among others." The same publication traces those salts back to their source: "These soluble salts come from the salts in the original potable water and the salts added during its use as potable water."

No source read for this page measures a color or names a specific stain chemistry for reclaimed water on glass. What is sourced is the same drying mechanism as any other hard water: mineral-bearing water that sits on a surface and evaporates leaves what it carried, whether that water came from a well, a utility main or a reclaimed line.

NGA's paper records a pure-water technique that some professional window cleaning contractors use, set out on pure water and the water-fed pole, and its reason is this same drying problem: "Rinse water is generally allowed to evaporate from freshly cleaned surfaces. Therefore, the pure water used in both the wash and rinse must have a total dissolved solids content (TDS) of 20 parts per million (PPM) or less to prevent spotting and streaking of cleansed surfaces." The limit is a ceiling, not zero, and why that water still dries clear is answered on why pure water dries without leaving spots.

A sprinkler runs on the opposite end of that same logic. NGA's own quick-reference list gives the general rule cleaning is built around: "DO NOT allow water or cleaning residue to remain on the glass or adjacent materials." That rule is written for cleaning, and no source read addresses sprinkler water reaching windows, so the connection is reasoning: irrigation water is still water left to sit and dry, which makes hard water reaching a window a plausible source of spotting.

No read source measures the hardness of a particular Naples property's irrigation water. USGS routes that question to the people who actually know it: "information that is directly relevant to water hardness and other chemical properties at a home or immediate locale should be provided by the local health agency or local water utility, or by the vendor of a local water-softening system." Whether a given reclaimed water carries more or less salt than a given well or utility line is the provider's own data to state, not something this page estimates.

Whether and how to redirect a sprinkler head away from a window is a call for the homeowner or an irrigation contractor, and no source read here ranks that fix by cost or ease. Whether reclaimed water stains exterior surfaces more broadly, and whether a mineral deposit can be polished off glass once it has dried, are answered on their own pages.