Ocean County sits on water that USGS calls very corrosive. The Water Quality Association adds that copper can come out of plumbing at low pH. Put the two together and a green streak under the faucet is worth a lab bottle. It is a clue, though, and it does not measure anything by itself.
What should I test for?
Ask a certified lab for three things: pH, copper and lead. A state sale test reports pH and lead. Copper is not among the items we saw in the rule, so it needs asking for. Ask the lab how it wants the copper and lead samples taken, because a sample from water that has stood in the pipes reads differently from a flushed one.
Compare your copper to two numbers from EPA: the secondary level of 1.0 mg/L, about taste and staining, and the action level of 1.3 mg/L. For pH, the range is 6.5-8.5.
Could it be something else?
The stain alone cannot say. EPA ties reddish or orange marks to iron and black ones to manganese, which is a different problem in our iron and manganese brief. If you are on a utility, the reasoning is different again, and your annual water report and utility are the place to start.
If the test confirms low pH, go on to the full story on acid water and copper and then to neutralizer choices. Scrubbing clears the mark and leaves the cause where it was.
Also asked
Is a blue-green stain dangerous?
The stain itself is cosmetic, but it can mean the water carries copper, and lead can travel with low pH. That is why the test is worth doing before anyone decides it is only a cleaning issue.
Why is it only on one fixture?
A drip or a slow faucet leaves more residue in one spot, which may be all that it means. A lab test on the house's water tells you more than the pattern on one basin.