Geological factors affecting the chemical characteristics of the thermal waters of the carbonate karstified aquifers of Northern Vietnam

In northern Vietnam, exposed carbonate rock formations cover an area of mor e than 50,000 km(2). Their accumulated thickness from the Cambrian to the T riassic is in some places as much as 3000 m. Numerous thermal waters (sprin gs and wells) occur in these strongly karstified carbonate massifs. This is the result of significant ancient and present orogenic activity, as the re gion demonstrates by its strong seismic activity. These karstic formations are water-bearing and strongly recharged by rainfall of between 1600 mm and 2600 mm per year in 90% of the area concerned. In view of the average annu al air temperatures ( 17 degreesC-25 degreesC according to the region), 23 sample springs or wells were chosen with water temperatures of between 29 d egreesC and 68 degreesC. Hydrochemical characteristics of these thermal wat ers emerging in different carbonate-rock units were examined by chemical an alyses of major ions. In this large region, thermal waters are divided into four hydrochemical types: the Na-Cl type resulting from the intrusion of s ea water for distances of up to several kilometres inland and depths of 100 0 m, the Ca-SO4 type, probably resulting from the leaching of deposits of m etallic sulphides that are widely distributed in these carbonate-rock units , and finally the Ca-HCO3 and Mg-HCO3 types which are chemically similar to fresh karstic waters in limestones and dolostones. The occurrence of these thermal groundwaters as well as their chemical characteristics seem to ind icate the existence of large-scale deepseated groundwater flow systems in t he karstic aquifers.