The Walkerton, Ontario investigation as noted by BillThor determined that people drinking 4+ liters of water per day experienced proteinuria (i.e. kidney damage):
"Canadian doctors are warning that drinking too much water may cause loss of kidney function... Researchers... identified 100 otherwise healthy adults who had a condition called proteinuria, or abnormal amounts of protein in their urine. ... Proteinuria can cause kidney failure and is a sign of microvascular disease, where the heart's tiny arteries are damaged, causing cardiac disease and cardiac death. ... They were drinking, on average, at least four litres of fluid per day. ... Some people were drinking six litres. One woman, a health care worker, was drinking eight."
And here's a scholarly paper about it: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2175005/
In other words, humans cannot safely drink 15 liters per day.
This information does not appear to be widely known, since google search results are dominated by (1) the old weakly supported 8x8 advice and (2) that the only possible danger from water is water intoxication by rapid ingestion of large quantities (http://en.wikipedia.org/wiki/Water_intoxication).