Running water can wash it directly into the streams, lakes and rivers that supply drinking water in many cities. Or, it can seep down into aquifers that supply wells. Don't flush it down sink drains or toilets. Most sewage treatment plants can not handle HHW because these facilities work by using bacteria to break down biodegradable wastes. Since toxic wastes can't be broken down, they pass into the sewage sludge or waste water.
Much waste water is discharged into the waterways that supply our drinking water.
Don't throw them in the regular trash. Most trash is landfilled or incinerated. Ordinary landfills aren't designed to handle HHW because rain water runs through the wastes and picks up the toxic materials. HHW shouldn't be incinerated either because, despite technology advances, incineration can spew dangerous materials into the air, producing toxic fumes and residual toxic ash. Don't store partially used containers for long periods of time.