Where is the proof that these elevated caffeine levels in the water are from humans? Couldn't there be some naturally occurring source of ocean caffeine? Caffeine producing sea weed, for example.
Also the "high" level they measured was 45 nanograms per liter of water, which is 0.000001 parts per million. The molecular weight of caffeine is 10 times that of water which means there is one caffeine molecule for ever 10 trillion water molecules. Seems pretty insignificant to me.
It repeatedly says things like "effects are unknown" and "we're not yet sure about its environmental effects," he said. "But it's a very nice tracer, even if it doesn't have a large effect, because in most parts of the world, you know that this is coming from a human waste source."
Just because something seems insignificant, doesn't mean that it is.
As to the caffeine sources, this summary linked to the paper's abstract. The highlights section, with bullet points, says: "► Caffeine concentrations corresponded with storm event occurrence. ► Caffeine concentrations in rivers and estuaries draining to the coast measured up to 152.2 ng/L."
This means it's not a ocean-based source of caffeine. You would need to spend US$40 to see the references for the statement that "Caffeine, a biologically active drug, is recognized as a contaminant of freshwater and marine systems." Or make an interlibrary loan request and wait a few weeks.
One of the processes that strips the berry from the coffee bean can use thousands of gallons of water a day. If not treated, it is toxic. There are better ways to process coffee, but most of the poor 3rd world farmers can't afford the upgrades to the processing plants.
Still no proof, but with storms flushing these toxic pools out, it's definitely possible.
Also the "high" level they measured was 45 nanograms per liter of water, which is 0.000001 parts per million. The molecular weight of caffeine is 10 times that of water which means there is one caffeine molecule for ever 10 trillion water molecules. Seems pretty insignificant to me.