More than 50 inches of snow blanketed Rhode Island this winter — nearly double the annual average of 27 inches.

Unfortunately, all of the snowmelt carried salt, sand, gas, deicers and other harmful gunk from streets and sidewalks straight into Narragansett Bay, its tributaries and our drinking water supplies.

Through April 1, 104,066 tons of salt were loaded on state snowplows and sand trucks to melt snow and ice. That’s a 32 percent jump from last winter, but just a slight increase from the 2008-09 snow season. Each city and town in the state also adds to the total with their own salt and sand road treatments.

Determining the impact of all the salt and sand on waterways each year is a challenge, said John Torgan of Save The Bay. “We don’t really know how serious it is or how to measure it.” But, he added, “It’s among the greatest pollution challenges we face in Narragansett Bay.”

This winter, the state published guidelines for the dumping of excess snow into the bay and tributaries such as the Seekonk River. According to the state Department of Environmental Management (DEM), only a boatyard in Warwick and another in Barrington reported depositing untreated snow into basins that fed into the bay.

However, much of the damage from the salt and sand is inflicted on freshwater tributaries and groundwater. Saltwater regions can withstand the salinity from road salt, Torgan said, but wetlands, rivers and streams are less resilient. Sand blankets pebble-covered habitats, smothering spawning grounds for the already fragile brook trout population. Only a fraction of the local trout are still able to lay their eggs in the wild. Sand-covered river and streambeds have already contributed to the demise of natural-spawning salmon in the region.

Drinking water sources such as groundwater and reservoirs also are disrupted by increased sodium levels.

“The biggest threat tends to be drinking water and the water table,” said Chris Deacutis of the Narragansett Bay Estuary Program.

In the meantime, one oddity occurring in recent months has been the proliferation of brown and yellow algae blooms across the bay. This “brown tide” is made of decomposed plankton, pollen and other organic debris. Local scientists aren’t sure about the cause, but so far there’s no concern about health effects. It’s just one of the new changes that have coincided with the 4-degree increase in water temperature during the past 20 years.

“We don’t know why it decided to bloom like crazy,” Deacutis said.