Orange staining that appears on a driveway, path or wall in the pattern of a sprinkler arc is almost never rust from a metal object. It is iron dissolved in the irrigation water, oxidizing as it dries on the surface and depositing there. That distinction matters, because the cleaning is similar for both but the prevention is completely different.
Reading the pattern
Look at where the staining is. Fan shapes, arcs and bands that match a sprinkler throw, or a gradient that fades with distance from a head, point at the water. So does staining on a wall or fence at exactly the height a sprinkler reaches. A discrete rusty patch under a garden chair, a railing base or where something metal stood is object rust, and treating the water will do nothing about it. Both types respond to the same cleaning products, which is why the pattern is the only reliable diagnosis.
Why irrigation water stains
Penn State Extension notes that iron and manganese carry secondary, aesthetic standards, with recommended maximums of 0.3 mg/L for iron and 0.05 mg/L for manganese, and that oxidized iron is what causes visible staining. Groundwater often carries iron in dissolved form, clear as it leaves the well, and it oxidizes on contact with air. Sprayed onto a porous surface and left to evaporate in sunlight, it oxidizes and deposits in the surface pores. The same water that stains laundry indoors stains concrete outdoors, for the same reason.
Removing what is already there
Iron stains need a product formulated to dissolve or chelate iron rather than a general cleaner. Ordinary pressure washing spreads it and can drive it deeper into porous concrete. Chlorine bleach is the wrong choice, since it oxidizes the iron further and sets the stain darker, which is the same mistake people make with laundry. Apply the treatment to a wet surface, give it contact time rather than scrubbing, and rinse thoroughly away from planting. Expect to repeat on old, heavily saturated concrete.
Stopping it coming back
Three options in order of cost. Change the irrigation pattern so heads do not throw onto concrete, walls or fences, which is free and often enough. Fit a phosphate or polyphosphate injector on the irrigation line, which sequesters the iron so it stays in solution and does not deposit as the water dries. Or treat the water with an iron removal system, which is the complete answer and the one to choose if the same iron is causing problems inside the house. Our quote page covers treatment sized to a measured iron level.
Questions people ask about what causes rust stains on concrete
How do I know if the stain is from the water?
By the pattern. Arcs, fans and bands matching a sprinkler throw are iron in the irrigation water. A discrete patch under a metal object is object rust.
Will bleach remove rust stains from concrete?
No, it makes them worse by oxidizing the iron further. Use a product formulated to dissolve or chelate iron instead.
Can pressure washing fix it?
It often spreads the stain and can drive it deeper into porous concrete. Chemical treatment with contact time works better than pressure alone.
How do I stop the staining recurring?
Redirect the sprinklers off hard surfaces, inject a sequestering agent on the irrigation line, or treat the iron at the source with a removal system.