State and local county health departments have detected per- and polyfluoroalkyl substances (PFAS) in groundwater downgradient of airports and military and firefighting training areas in Long Island, New York. However, the occurrence and spatial distribution of PFAS throughout the surficial aquifer is not well established. Shallow groundwater samples were collected in 2018 from two observation well networks; the Sentinel network and Wastewater Treatment Plant groundwater (WTPGW) network. The Sentinel network is an island-wide group of wells screened within the shallow upper glacial (surficial, <100 feet deep) aquifer, which were sampled to assess the occurrence of PFAS in different land-use settings. The WTPGW network consisted of shallow (<60 feet deep) wells located downgradient of decentralized wastewater treatment systems discharging to groundwater, which were sampled to determine if these systems are a source of PFAS to groundwater. Each sample was analyzed for 18 PFAS compounds. This sample collection documents the first Long Island-wide assessment for ambient PFAS contamination within the shallow aquifer system. An extensive quality-assurance and quality-control program developed specifically for the expected low level (<200 nanograms per liter [ng/L]) PFAS environmental concentrations within the groundwater networks revealed no detections of the 18 PFAS compounds analyzed. This indicates that selected equipment, cleaning, sampling, and handling procedures are sufficient to provide data that reflects environmental conditions. Twenty-six of the 37 wells sampled in the Sentinel network had one or more PFAS compounds detected, and up to 8 compounds were detected in a single sample. Ten PFAS compounds have been detected in these samples, with individual compound concentrations ranging from 3.4 to 93 ng/L. Land-use settings for these sample sites varied between medium- and high-density residential and mixed-use commercial properties. Seven monitoring wells surrounding leach fields from three different decentralized wastewater treatment systems (WTPGW network) found four or more PFAS in each groundwater sample. Twelve PFAS compounds have been detected in these samples, with individual compound concentrations ranging from 5 to 620 ng/L. The greatest variety of PFAS was detected at the site that served an assisted living and rehabilitation facility and a hotel while the highest PFAS concentrations were detected at the site that served a strip mall (commercial property).