Cornwall, UK is a region with high concentrations of environmental arsenic (As). Previous studies found elevated concentrations in urine and toenails of some local residents. Private water supply (PWS) usage is relatively common in Cornwall with an estimated 60,000 people served. A recent British Geological survey and Public Health England survey of 512 households across Cornwall found 5.5 % of drinking water samples with As exceeding 10 µg/L, the UK Prescribed Concentration Value.. We report here a subsequent urinary biomonitoring survey in a sample of these households to (1) quantify human exposure to inorganic As, and (2) assess the importance of PWS as an exposure route. Volunteers (n=207) in 127 households provided a first morning void or spot urine sample for As speciation (by HPLC-ICP-MS), along with hair and toenail samples to reflect longer term exposure (Middleton et al, submitted and presented at the 6th International Conference on Medical Geology, Aveiro, Portugal, July 2015). Environmental sampling was conducted to assess the relevance of various exposure routes. A tap water sample from the private supply was taken for total As determination (by ICP-MS), to capture the exposure level relevant via this pathway at the time of bio monitoring. Where possible, a garden topsoil sample, an indoor dust wipe sample and a small amount of rice (a foodstuff known to contribute to population arsenic intake) were collected from each household. Eight urine samples (4%) exceeded the guidance value of 35 μg/l of inorganic arsenic plus methylated metabolites set by the American Conference of Governmental Industrial Hygienists (ACGIH). The significant correlation between water and urine concentrations reflects drinking water as the main route of exposure. Individual results have been fed back to participants along with tailored advice on exposure reduction. Ongoing work is assessing the relative contributions of food, contaminated soil and household dust.