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In 49 samples of groundwater, sampled in Muzaffargarh District of south-western Punjab, central Pakistan. concentrations of As exceeded the World Health Organisation provisional guideline value. and United States, Environmental Protection Agency (USEPA) Maximum Contaminant Level (MCL), of 10 mug L-1 in 58% of samples and reached up to 906 mug L-1. In this semi-arid region canal irrigation has lead to widespread water-logging. and evaporative concentration of salts has the potential to raise As concentrations in shallow groundwater well above 10 mug L-1. In fact, in rural areas, concentrations stay below 25 mug L-1 because As in the oxic shallow groundwater. and in recharging water. is sorbed to aquifer sediments. In some urban areas, however, shallow groundwater is found to contain elevated levels of As. The spatial distribution of As-rich shallow groundwater indicates either direct contamination with industrial or agricultural chemicals, or some other anthropogenic influence. Geochemical evidence suggests that pollutant organics from unconfined sewage and other sources drives reduction of hydrous ferric oxide (HFO) releasing sorbed As to shallow groundwater. The situation is slightly less clear for seven wells sampled which tap deeper groundwater. all of which were found with >50 pg L-1 As. Here As concentrations seem to increase with depth and differing geochemical signatures are seen, suggesting that As concentrations in older groundwater may be governed by different processes. Other data on parameters of potential concern in drinking water are discussed briefly at the end of the paper. (C) 2004 Elsevier Ltd. All rights reserved.