[en] Full text: Water supplying for domestic and product activities in Hochiminh City is being taken from two both sources: surface water and groundwater. Due to the amount of surface water did not satisfy the water demand, groundwater is extracted ever more. In this area there are more than 1000 large production wells and about 83000 tubewells now tapping groundwater mainly in two aquifers: Q_I-III , Pleistocene in age and N₂, Pliocene in age. The over extraction causes the water level depression continuously. The observation data of DHES from 1991 to 1998 shown that water level depression of 7m in Q_I-III aquifer and 12m in N₂ aquifer were found in comparison with those in 1991. In the past (1960s) saline groundwater (TDS ≥1g/L) was observed in Saigon area when the amount of groundwater exploited was 170,000 cubic meters per day. The ever more groundwater exploitation caused aquifers salinisation. The enlargement of saline water is not able to shirk and becomes a hard problem for the exploiting groundwater manager. Therefore to study and assess the groundwater salinisation using suitable techniques is very necessary for water resources managers in establishing better exploitation strategy. In this situation, environmental isotopes technique is emphasized to determine the salinisation mechanism of groundwater. The objectives studied are groundwater of two aquifers mainly being exploited (Q_I-III and N₂) in Hochiminh City area. Based on the national water monitoring wells existing in the studied area and the hydro-geological setting a network of 70 sampling points for both two aquifers was set up. Water samples were collected two times (in rainy season of 2001 and in the end of dry season of 2002). All collected samples were analyzed for hydrochemical and stable isotopes. 30 of them were analyzed for tritium and 15 of them were done for ¹⁴C. Analyzing hydrochemical results of collected samples show that the quality of groundwater varies from fresh to saline, soft to very hard and high iron contents in some regions (TDS: 17.0-33000mg/L; Salinity: 0-25%; Cl^-: 4.6-16600mg/L; Na⁺: 0.75-5230mg/L; K⁺: 0.10-176mg/L; HCO₃^-: ND-319mg/L; Ca²⁺: 0.2-410mg/L; Mg²⁺: 0.18-732mg/L; SO₄^2-: 1.15-1613mg/L; NO₃^-: ND-40.4mg/L). Groundwater quality problem in a half of studied area is due to high chloride, total dissolved solids, iron contents and sulfate. The analyses of cations and anions by Piper Trilinear Diagram show that in aquifers where saline groundwater distributed the cations are mainly sodium, calcium, and magnesium type while the anions are mainly chloride and sulfate type but in the part where fresh groundwater the cations are mainly sodium, calcium, and type while the anions are mainly bicarbonate, carbonate and nitrate type. According to TDS values the distribution of fresh and saline groundwater in studied aquifers is mapped and fresh-saline groundwater boundaries in 1990 and 2000 is also demonstrated. The stable isotopes data show that the isotopic compositions scatter widely (values of δD range from -7.72 per mille to -50.55 per mille and values of δ¹⁸O range from -2.18 per mille to -12.3 per mille) along the GMWL and a part of them fall on the mixing with sea water line. The distribution of groundwater samples collected along the GMWL show that groundwater in this area is recharged directly by rainfall and surface water. High tritium contents and ¹⁴C relative radioactivity of groundwater in the area also support this process. Delta values of ¹⁸O and Chloride contents plot show that it exists two main salinisation mechanism. The first one is the leaching process and the second one is mixing with seawater process and both these mechanisms are always accompanied by evaporation process. Each process dominates in different region: In the southwest part of area groundwater salinisation caused mainly by leaching process and in the southeast is dominated by mixing with seawater. There is a small brackish region in the northeast of studied area in which the salinisation was caused by mixing with sea water too but the relation between ratios of Na⁺/Cl^- and Cl^- contents show that the refreshing is taking place. It could be explained that groundwater salinisation in this region was happened in the past by infiltration of seawater coming inland through Dongnai river and its distributaries (due to the tidal effect, especially in dry season) but now when Trian reservoir constructed on the upstream, fresh river water push seawater far towards downstream and the infiltration of fresh river water makes groundwater fresher and fresher. This is one of evens proving that groundwater in studied area is recharged by surface water. The study results are new and useful for water resources managers to make better groundwater exploitation plan for limiting the enlargement of saline groundwater in aquifers as well as make faster the refreshing of brackish in aquifer in the northeast part of Hochiminh City. (author)