Thesis (M.Sc.) - Asian Institute of Technology, 2003

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science.

For a country like Cambodia with a significant groundwater arsenic problem, there is an urgent need for the arsenic-contaminated wells to be quickly identified and appropriate action to be taken. This will involve the testing of a large number of wells. A field testing kit made by Hach is currently used national-wide. The field Hach kit is based upon the Gutzeit method, which involves the reduction of arsenic to arsine gas, which reacts with mercuric bromide to form yellow colour. The determination of arsenic concentration is done by visual comparison of the coloured stain with a colour chart. The detection limit of the Hach kit is about 10 og/L. Since Hach kit is widely used it became necessary to evaluate its effectiveness and possible made suggestions for improvement. The Hach kit performance was evaluated in various ways including arsenic standard solution, samples preservation using hydrochloric and nitric acid, quality of the test strip, effect of hydrogen sulphide, colour scale reading and reaction time. Samples from 94 tube wells were tested by kit and by a reliable laboratory technique to ascertain the reliability. Overall the Hach kit performed well with arsenic standard solution as well as with preserved samples. Problems were noted in proper use and storage of test strips as well as interference from hydrogen sulphide. It was also noted that operators usually fail to record arsenic concentration in ranges between the two-gradation values of the colour scale. The 20-minutes reaction time recommended by the kit procedures was not adequate; 30 minutes was necessary to give good results. The comparison of the analytical results of 94 samples showed Hach kit usually gave higher result than either HG AAS or SDDC methods except at arsenic concentration below 10 og/L. The closest agreement between Hach kits and laboratory methods were observed when arsenic concentrations were low. There were 12 (13%) false positives found but no false negatives. All of the finding suggest further development is needed to improve methods of testing arsenic concentrations for less developed countries like Cambodia.