Application of 15N isotopes to identify sources of nitrate in groundwater in the Khon Kaen area, N.E. Thailand

Nitrate pollution of village bore water supply is a major health concern in the Northeast Thailand province of Khon Kaen. Locally high concentrations of NO3 in excess of World Health Organization (WHO) guidelines occur in the two main aquifers used for village water supply west and northwest of Khon Kaen City. These aquifers are the shallow sand and gravel aquifer and the fractured shale, sandstone and siltstone aquifer. Evaporites and rocksalt of the Mahasarakham. Formation underly these aquifers in the eastern half of the area studied. Minerals associated with the rocksalt, such as sylvite (KCl) and carnallite (KCl MgCl2 6H2O) show traces of nitrogen and high concentrations of ammonia N have been reported elsewhere in clays and of sal ammoniac (NH4Cl) in sylvite and carnallite. Brines derived from the rocksalt have been considered as a potential source of nitrates where deep groundwater discharge occurs. Other potential sources of nitrates considered are nitrogenous fertilisers and animal manure leachate, and wastewater and septic tank effluents Identification of these various sources based on hydrogeology, local land use and proximity to likely contaminants can become more precise if nitrogen isotope analyses of NO3 - N are included. Samples of suspected contaminants including rocksalt clay, nitrogenous fertiliser, pig and water-buffalo manure were analysed for their 15N content to establish an isotopic signature range for each contaminant. These signatures were compared with 15N values obtained from samples of nitrate-rich groundwater. The groundwater values of 16.3 to 39.7 per mil are the closest to the 15N determinations for wastewater effluent of 21.8 and 27.6 per mil. The range of the remaining potential sources was 3 5 to 10.3 per mil with the nitrogen content of the rocksalt being too low to obtain a valid sample for analysis. The contamination pathway appears to be through the corroded bore casing and cracked concrete platform at the borehead. Thus, in many cases the implementation of better bore maintenance and borehead protection measures should control the problem at individual sites