Groundwater (GW) is a vital freshwater resource extensively exploited in the Vietnamese Mekong Delta, especially during the dry seasons. This study applies the Cumulative Rainfall Departure (CRD) method to estimate GW recharge in deep aquifers of Soc Trang Province, located in the southernmost region of Vietnam under a tropical climate. Monthly rainfall records and daily GW level data of the aquifers from 2010 to 2020 were used. The Pearson correlation between observed GW levels and CDR model GW levels exceeds 0.995, indicating high model accuracy. The analysis reveals that the CRD fractions for the Upper Pleistocene (qp₃), Middle Pleistocene (qp_2-3), Lower Pleistocene (qp₁), and Middle Pliocene (n₂²) aquifers are 0.085%, 0.104%, 0.130%, and 0.180%, respectively, totaling approximately 0.5% of the annual rainfall. This corresponds to an annual GW recharge of 25.86 million m³, or 70,850 m³/day, equivalent to 70% of the current GW abstraction rate of 101,000 m³/day. Given the critical role of GW as a freshwater source, implementing an enhanced GW recharge program using surface water and rainwater is strongly recommended. Additionally, the analysis suggests that the decline in GW levels due to abstraction corresponds to 0.85 times the mean annual precipitation, a finding that warrants further investigation.

The groundwater tracer injection and withdrawal tests are often carried out for the determination of aquifer solute transport parameters. However, the parameter analyses encounter a great difficulty due to the radial flow nature and the variability of the temporal boundary conditions. An adaptive methodology for the determination of groundwater solute transport parameters using tracer injection and withdrawal test data had been developed and illustrated through an actual case. The methodology includes the treatment of the tracer boundary condition at the tracer injection well, the normalization of tracer concentration, the groundwater solute transport finite element modelling and the method of least squares to optimize the parameters. An application of this methodology was carried out in a field test in the South of Hanoi city. The tested aquifer is Pleistocene aquifer, which is a main aquifer and has been providing domestic water supply to the city since the French time. Effective porosity of 0.31, longitudinal dispersivity of 2.2 m, and hydrodynamic dispersion coefficients from D = 220 m²/d right outside the pumping well screen to D =15.8 m²/d right outside the tracer injection well screen have been obtained for the aquifer at the test site. The minimal sum of squares of the differences between the observed and model normalized tracer concentration is 0.00119, which is corresponding to the average absolute difference between observed and model normalized concentrations of 0.035 5 (while 1 is the worst and 0 is the best fit).