CHARLTON – One week after Casella Waste Systems told a crowded Board of Health meeting that 21 residential wells near the Southbridge landfill tested positive for a toxic chemical, residents filled the health board office and demanded answers to their concerns. At the Nov. 3 meeting, Casella identified home wells within 500 feet of the landfill where probable human carcinogen 1,4-dioxane was detected during routine testing.