We describe drinking water sources and water quality for a large agricultural cohort. We used questionnaire data from the Agricultural Health Study (N = 89,655), a cohort of licensed pesticide applicators and their spouses in Iowa (IA) and North Carolina (NC), to ascertain drinking water source at enrollment (1993–1997). For users of public water supplies (PWS), we linked participants’ geocoded addresses to contaminant monitoring data [five haloacetic acids (HAA5), total trihalomethanes (TTHM), and nitrate-nitrogen (NO₃-N)]. We estimated private well nitrate levels using random forest models accounting for well depth, soil characteristics, nitrogen inputs, and other predictors. We assigned drinking water source for 84% (N = 74,919) of participants. Among these, 69% of IA and 75% of NC participants used private wells; 27% in IA and 21% in NC used PWS. Median PWS nitrate concentrations (NO₃-N) were higher in IA [0.9 mg/L, interquartile range (IQR): 0.4–3.1 mg/L] than NC (0.1 mg/L, IQR: 0.1–0.2 mg/L), while median HAA5 and TTHM concentrations were higher in NC (HAA5: 11.9 µg/L, IQR: 5.5–33.4 µg/L; TTHM: 37.7 µg/L, IQR: 10.7–54.7 µg/L) than IA (HAA5: 5.0 µg/L, IQR: 3.7–10.7 µg/L; TTHM: 13.0 µg/L, IQR: 4.2–32.4 µg/L). Private well nitrate concentrations in IA (1.5 mg/L, IQR: 0.8–4.9 mg/L) and NC (1.9 mg/L, IQR: 1.4–2.5 mg/L) were higher than PWS. More private wells in IA (12%) exceeded 10 mg/L NO₃-N (regulatory limit for PWS) than NC (<1%). Due to the proximity of their drinking water sources to farms, agricultural communities may be exposed to elevated nitrate levels.