HAMDEN — For two decades, the reason behind the mass die-offs of coho salmon in Washington state remained a mystery. Then, in 2020, a group of researchers published a study naming 6PPD-Quinone, a tire-derived chemical, as the likely culprit.   In Connecticut, where illegal tire dumping has been a longtime problem for municipalities, environmentalists say the practice also could threaten the state’s natural resources and have pointed to research like the coho salmon study as cause for concern. “(Tires) can leach some toxic materials into both the soil and the water,” said Rhea Drozdenko, a river steward with the Connecticut River Conservancy. Generally, pollution from the wear-and-tear of tires on…