ANALISIS PENGARUH NILAI KONDUKTIVITAS HIDRAULIK DAN DISPERSIVITAS DINAMIK TERHADAP REMEDIASI AIR TANAH MENGGUNAKAN SIMULASI NUMERIK

Groundwater remediation is one of the solutions to restore the contaminated groundwater. This study was conducted to determine the effect of hydraulic conductivity and dynamic dispersivity on the groundwater remediation effectiveness. As a case study, in 2020, in an area located in Balikpapan, groundwater remediation will be carried out by injecting water containing NaOH through five wells and pumping it back through five wells to form a cycle. The method used is a numerical simulation consisting of groundwater flow simulation, mass transport, and sensitivity analysis. The results show that it takes 124 to 300 days for the injected NaOH to arrive at the pumping wells. The sensitivity analysis results show that when the hydraulic conductivity value is ten times greater, the time required is reduced to 84 to 172 days. Meanwhile, when the dynamic dispersivity is twice larger, the time required is reduced to 75 to 189 days. These results indicate that the groundwater remediation method will be effective for aquifers with high hydraulic conductivity and dynamic dispersivity values. For the study area, the groundwater remediation is suggested to be carried out by increasing the number of injection and pumping wells with a relatively close distance, i.e., around 10 meters, so that NaOH arrives at the pumping wells more quickly.Keywords: groundwater, remediation, hydraulic conductivity, dynamic dispersivity, numerical simulation

Delineation of groundwater recharge zones in the Mitidja plain, north Algeria, using multi-criteria analysis

This work aims to identify the potential groundwater recharge zones in the Mitidja plain (north Algeria) using the multi-criteria approach. The analysis was based on the use of a geographical information system (GIS) and remote sensing to establish eight thematic maps, weighted, categorized and inserted, that allowed us to establish the potential zones’ map for groundwater recharge. Three potential groundwater recharge classes were defined corresponding, respectively, to low (26%), moderate (47%) and high (27%). The best groundwater potential zones are situated in the piedmont of the Blidean Atlas (the south of the study area), precisely, upstream near to wadis (wadi El Harrach, wadi Djemaa, wadi Mazafran) and the western aquifer limit, where the hydrogeological units are formed by the alluvium formation which is characterized by high hydraulic conductivity, high water flow, low slope, low drainage, low quantity transported sediments and good water quality. The obtained results, in this work, describe the groundwater recharge potential areas and supply information for a suitable mapping and the management of aquifer resources in the study area.

Self-Potential signals produced by pumping test of heterogeneous aquifer

In this paper, we presented results of interpretation of Self-Potential (SP) signals produced by pumping test experiment of heterogeneous aquifer. Heterogeneity is represented by zone with low and high hydraulic conductivity in two types of configuration: planned boundary and strip-layer. We studied five models for each type of heterogeneity with two variants of hydraulic conductivity. We carried out three-dimensional modelling with the use of Modflow (for hydraulic heads and electrical potentials) and ElSources (for current source term). We obtained SP distributions Self-Potential produced by pumping test which indicate heterogeneity. We calculated the values of the anomalous field for a detailed study of the heterogeneties. We found a correlation between SP and drawdown for the case of heterogeneity with low hydraulic conductivity. We defined the both boundaries of the strip clearly for zone with low hydraulic conductivity. The outer boundary of the strip-layer was not fixed due to the high hydraulic conductivity of the heterogeneity and low hydraulic gradient at the boundary. We estimated dependence of horizontal thickness of heterogeneity on SP distribution. The results can serve as a base for defining the heterogeneties in the aquifer.

Characteristics of groundwater seepage with cut-off wall in gravel aquifer. I: Field observations

This paper presents a case history of the leakage behavior during dewatering tests in the gravel strata of an excavation pit of a metro station in Hangzhou, China. The groundwater system at the test site is composed of a phreatic aquifer underlain by an aquitard and a confined aquifer with coarse sand and gravel. The sandy gravel stratum has very high hydraulic conductivity. The maximum depth of the excavation is 24 m below the ground surface, which reaches the middle of the aquitard strata, where the thickness of the clayey soil is insufficient to maintain the safety of the base of the excavation. To understand the hydrological characteristics of gravel strata, single- and double-well pumping tests were conducted, where a cut-off wall was installed 43 m deep with its base penetrating 2 to 3 m into the aquifer. Test results show that this partial cut-off of the aquifer cannot effectively protect the base of the excavation from the upward seepage force of the groundwater during excavation. Therefore, a new cut-off wall (second phase) was constructed to a depth of 54 m to cut off the confined aquifer. A second pumping test was conducted after the construction of the second phase cut-off wall, and test results show that this full cut-off combined with dewatering can control groundwater effectively during excavation. This finding indicates that when a deep excavation is conducted in a confined aquifer with high hydraulic conductivity, determination of the depth of the retaining wall should be based on three factors: the stability of the base, the upward seepage stability, and settlement control.

Coupled Mechanical and Hydraulic Analysis of the Effect of a Front High Hydraulic Conductivity Zone on Shield Tunnelling

A localized high hydraulic conductivity zone with high water pressure is denoted as a hazardous zone. This study focuses on the 3D coupled mechanical and hydraulic analysis of the effect of a front hazardous zone on shield tunnelling. The tunnelling process is modelled by a step-by-step excavation. And a series of three-dimensional coupled mechanical and hydraulic analysis are performed to estimate the distribution of the TSR of the ground ahead of the tunnel face. The result show that the tunnelling directly influences on the distribution of the pore water pressure and the TSR. The maximum influence range of the hazardous zone on the face stability is within 4 times the R value. And the high water pressure can induce a failure of the ground at the tunnel face by itself in conjunction with a poor ground condition.