Methodology of determining effective porosity and longitudinal dispersivity of aquifer and the application to field tracer injection test in Southern Hanoi, Vietnam

Abstract

Groundwater field pumping out and tracer injection test had been carried out at Nghiem Xuyen commune, Thuong Tin district, Hanoi where salinized and fresh groundwater boundary exist in the Pleistocene aquifer. The test was executed with pumping out rate of 9l/sec and tracer injection rate of 0.7l/sec of water with the salt concentration of 5g/l. The interpretation and analysis of the groundwater solute transport parameters by the field pumping out and tracer injection test is a rather complicated and delicate task due to the variability of the temporal boundary conditions. The test results have shown that although the tracer injection time is rather long (up to 60 hours), the tracer breakthrough curve of the tracer concentration of the pumped out water has its very specific characteristic shape, however with some variation due to the test invisible variability of conditions. The results of the parameter identification based on the method of least squares have given effective porosity of 0.32 and longitudinal dispersivity of 2.5m (which give the hydrodynamic dispersion of from D=250m 2 /day right outside the pumping well screen to D=18m 2 /day right outside the injection well screen). 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 0.0355 (while 1 is the worse and 0 is the best). The results have also shown that the maximal tracer concentration right outside the pumping out well screen is 6.1 times greater than the tracer concentration of the pumped out water. The distortion flow coefficient αW (the ratio between the flow rate through the injection well section without its presence) and the groundwater flow into the tracer injection well is from 18.66 (at the early testing time) to 10.76 (at the later testing time).

Classification of hydrogeological structure along the Red River in the Hanoi area

“…The parameters showing the hydrogeological structure in each area are the aquifers and aquitards' thickness (minimum, maximum, average), the depth of the river bottom cutting into layers (minimum, maximum, intermediate), the width and the area cutting into the middle layer, and the proportion (%) of cutting into layers regarding the cutting area and the cutting depth [11].…”

Inverse analysis for transmissivity and the Red river bed's leakage factor for Pleistocene aquifer in Sen Chieu, Hanoi by pumping test under the river water level fluctuation

“…The estimated values of the parameters may be of very high errors if the boundary conditions and boundary conditions' values and one or some parameters' values are far from the real values. Tong Ngoc Thanh et al (2017) and Nguyen The Chuyen (2017) have presented some arguments of wrong utilization of of a single Pleistocene confined aquifer without leakage from underlying Neogene aquifer in Thuong Tin district and Mo Lao-Ha Dong areas in determination of the Pleistocene aquifer transmissivity. Besides, the study of true hydrogeological aquifer structure is very important including the determination of the Groundwater level change during pumping test relatively to the groundwater level at the pumping started (at t=0): A 0 =0.503 days; T=1511.5m 2 /day; S*=0.00113 nature of the over-lying and lower-lying forformations in regards to the leakage to the main aquifer in the setting up the conceptual aquifer scheme, for which geophysical prospecting would be very helpful and effective (Nguyen Van Giang et al, 2014).…”

“…The main productive groundwater aquifer in Hanoi area is the Pleistocene aquifer. General hydrogeological conditions of the area may be referred to many publications, for example, Nguyen Minh Lan, 2014;Tong Ngoc Thanh et al, 2017;Nguyen The Chuyen et al, 2017. This work is dealing with a particular site in Sen Chieu commune, Phuc Tho district, Hanoi city where a group-well pumping test was carried.…”