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A geochemical survey was conducted on thermal water and cold water around non-volcanic geothermal fields at Mahaoya and Marangala in Sri Lanka. One hundred forty-two samples were analyzed for
fifteen selected irons to investigate geochemical relationships resulting from water-rock interactions
and mixing. Based on measurements using a Na-K-Mg geothermometer, the maximum temperatures of
thermal reservoirs were estimated to be 148 °C in Mahaoya and 191 °C in Marangala, which were higher
values than those obtained using Na-Li and Li-Mg geothermometers. This suggests that the reservoirs
extend from intermediate to deep levels. Hydrogeochemistry of thermal waters is distinct from that of
cold water; higher contents of Na, K, Cl, F, S04 and TDS in thermal water are most likely due to the dissolution of feldspar, mica, and sulfide minerals in the granitic rocks. Conversely, lower values of Fe, Mg,
Cu, Zn, and Pb imply less ferromagnesian minerals in the basement. Classification based on major ions
reveals a Na-K-S04 type of thermal water for Mahaoya and Marangala. Cold water is dominated by the
Na-K-HC03 type, which indicates deep groundwater influence by iron exchange. Non-mixing cold water
indicates a Ca-HCCh type. In general, chemistry of cold water wells (<400 m) close to the thermal water
changes significantly due to direct mixing of thermal water and cold water. In comparison, a contrasting
action occurs with increasing distance from the geothermal field. Gradual decline of S04 with increasing
distance from thermal water may indicate a trend of dear oxidation. However, the chemistry of more
distant wells demarcates deep circulations through fractures and faults in the basement
