In article 3p1Ae.153066$nG6.66010@attbi_s22,
says...
I've been using a sprinkler to keep my plantings watered, and the rock
around the plantings and the nearby concrete is turning rust colored.

Is there anything I can buy and put in-line to prevent this from
happening? Ie. some kind of filter or whatever to remove the iron/rust
before it puddles on the rock and concrete?

A water softener will take out the minerals, but may add traces
of salt which is bad for most plants.

As other posters have suggested, various mild acids will clean
the stains from rock and concrete, but may turn the soil acid,
so you may need to add lime.

Some folks collect rain water in barrels to use for plants.