Groundwater is an important freshwater resource in Thailand and is increasingly vulnerable to impacts from rapid urbanization. This study investigated groundwater quality and microplastic in Nakhon Pathom Province, an urbanizing area in central Thailand, between 2023 and 2025. Groundwater samples were collected from 12 monitoring wells representing shallow (26–62 m) and deep (122–184 m) aquifers during 4 sampling campaigns. Water quality parameters, major ions, and heavy metals were analyzed along with microplastic characterization using Laser Direct Infrared (LDIR) spectroscopy. Shallow groundwater exhibited higher salinity (1.02–8.93 ppt) and total dissolved solids (710–3150 ppm) than deep groundwater. Microplastics were consistently detected in both aquifers, with concentrations ranging from 484 to 2,007 particles/L in shallow wells and from 943 to 1,210 particles/L in deep wells. On average, shallow wells contained higher microplastic abundance (1,114 particles/L) than deep wells (1,050 particles/L). Polypropylene (PP), polytetrafluoroethylene (PTFE), and polyamide (PA) were the most abundant polymers, reflecting inputs from commonly used consumer plastics. Mass calculation indicated that shallow wells also contained average comparison of 35% higher in total microplastic mass than deep wells, however, rubber particles contributed substantially to the overall concentration and frequently exceeded the combined mass of all other detected microplastics. Rubber accounted for approximately 40–45% of detected particles and is likely associated with tire wear particles transported through surface runoff and preferential flow paths. No significant correlations were observed between microplastic abundance and hydrochemical parameters, suggesting that microplastic occurrence is primarily governed by physical transport processes rather than groundwater chemistry. These findings demonstrate that groundwater quality in urbanizing environments is influenced by both conventional plastic and rubber-derived particles and highlight the importance of incorporating mass-based assessments alongside particle abundance when evaluating microplastic in groundwater.