Have you tried spiking your sample with Sr and Ba? If you can't see a spike by ICP-OES either, then interference is likely. The other thing would be to dilute by a factor of 10 or 100. Sr should still be detectable by ICP-OES at 20 ppb. Any ground water sample that contains 2 ppm Sr will have ~200 ppm Ca and lots of Mg as well.
Besides flame AA or emission, you could try drying some sample down and analyze the residue by XRF, if that's available.
I'm betting on some sort of interference, though. Well water is usually very hard and so should contain traces of Sr and Ba along with all the Ca and Mg.
Kingsport, TN 37663