This data release contains 1-4 dioxane, per- and polyfluoroalkyl substances (PFAS), chlorofluorocarbons (CFCs), methyl tert-butyl ether (MTBE), and trihalomethanes (THMs) concentration data from groundwater samples collected from 78 monitoring wells in Nassau County New York. Samples were collected from May 2019 through December 2022 at wells ranging in depth from twenty-three feet to 875 feet. All wells were sampled with a submersible pump and a subset of five wells were sampled using a dual membrane passive diffusion sampler for method comparison. Samples were analyzed for 28 PFAS compounds and 15 volatile organic compounds (VOCs; 1,4-dioxane, MTBE, and several CFCs and THMs). Field replicates, field blanks, equipment blanks, source solution blanks, and laboratory-control samples, and basic field parameters (water temperature, pH, turbidity, specific conductance, and dissolved oxygen) were collected for quality assurance and quality control purposes and are included. Laboratory-control samples for PFAS included isotope dilution standard recoveries, method blanks, instrument blanks, and blank and matrix spike isotope dilution standard recoveries. Laboratory-control samples for VOCs included surrogate recoveries and a laboratory matrix spike to evaluate extraction efficiency and matrix interference on a sample-specific basis.