Browsing by Author "Lin, Lixiang"

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    Evaluation of groundwater flow theories and aquifer parameters estimation
    (University of the Western Cape, 2014) Xiao, Liang; Xu, Yongxin; Lin, Lixiang
    This thesis deals with some fundamental aspects of groundwater models. Deterministic mathematical models of groundwater are usually used to simulate flow and transport processes in aquifer systems by means of partial differential equations. Analytical solutions for the deterministic mathematical models of the Theis problem and the transient confined-unconfined flow in a confined aquifer are investigated in the thesis. The Theis equation is a most commonly applied solution for the deterministic mathematical model of the Theis problem. In the thesis, a most simplified similarity transformation method for derivation of the Theis equation is proposed by using the Boltzmann transform. To investigate the transient confined-unconfined flow towards a fully penetrating well in a confined aquifer, a new analytical solution for the deterministic mathematical models of interest is proposed in the thesis. The proposed analytical solution considers a change of hydraulic properties (transmissivity and storativity) during the confined-unconfined conversion. Based on the proposed analytical solution, a practical method to determine distance of the conversion interface from pumping well and diffusivity of the unconfined region is developed by using a constant rate test. Applicability of the proposed analytical solution is demonstrated by a comparison with previous solutions, namely the MP and the Chen models. The results show that the proposed analytical solution can be used to assess the effect of the change of diffusivity on the transient confined-unconfined flow. The MP model is only accepted if the transmissivity during the confined-unconfined conversion is constant. The Chen model, given as a special case of the proposed analytical solution, is limited to the analysis of the transient confined-unconfined flow with a fixed diffusivity. An important application of groundwater models is to estimate parameters, such as hydraulic properties and flow dynamics, of groundwater systems by assessing and analysing field data. For instance, the pumping and the hydrochemistry and environmental tracer tests are two effective ways to obtain such data. To evaluate hydraulic properties of aquifer systems by derivative interpretation of drawdown data from pumping tests, a new diagnostic analysis method is proposed based on a lg-lg drawdown derivative, dlgs/dlgt, and the differentiation algorithm namely Lagrange Interpolation Regression (LIR) in the thesis. Use of a combined plot of dlgs/dlgt and a semi-lg drawdown derivative (ds/dlgt) is made to identify various flow segments during variable discharge tests with infinite conditions, constant rate tests in bounded aquifers and tests involving double-porosity behaviours. These can be applied to further characterize pumped aquifers. Compared to traditional diagnostic analysis method using plot of ds/dlgt alone, the combined drawdown derivative plot possesses certain advantages identified as: (1) the plot of dlgs/dlgt is strikingly sensitive for use in unveiling differences between pumping and its following recovery periods in intermittent variable discharge tests; (2) storativity (S) of pumped aquifers can be evaluated by using the combined plot; and (3) quantitative assessments of double-porosity behaviours can also be achieved. Based on two case studies, advantages and disadvantages of uses of the LIR and other existing differentiation methods in calculations of numerical drawdown derivative are demonstrated in practice. The results suggest that the LIR is a preferred method for numerical differentiation of drawdown data as it can be used to effectively minimise noisy effects. The proposed derivative approach provides hydrologists with an additional tool for characterizing pumped aquifers. Use of hydrochemistry and environmental tracer tests to assess flow dynamics of groundwater systems is demonstrated via a case study in the dolomite aquifer of South Africa. An emphasis is on determining mean residence times (MRTs) of the dolomite aquifer by means of an appropriate box model with time series of 14C values of dissolved inorganic carbon (14C-DIC) and initial 14C activities of spring samples during 1970s and 2010s. To obtain the calibrated 14C MRTs, 13C values of dissolved inorganic carbon (δ13C-DIC) of the spring samples are applied to estimate mineral dissolution in the dolomite aquifer and calculate the initial 14C activities. The results indicate that the spring samples have about 50%-80% initial 14C activities. By using the appropriate box model, the calibrated 14C MRTs of the spring system are given within a range from ≤ 10 to 50 years. Additionally, the flow dynamics, including the recharge source and area, the effect of climate change on the temporal trend of the groundwater MRTs and the groundwater flow circulation, of the dolomitic spring system are also discussed for further possible management interventions in the dolomite aquifer.
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    Hydraulic properties of the Table Mountain group (TMG) aquifers
    (University of the Western Cape, 2006) Lin, Lixiang; Xu, Yongxin
    The Table Mountain Group (TMG), located at the southmost cape of African Continent, is one out of three major regional aquifer systems in South Africa, which has a potential of bulk water supply to meet the requirements of irrigation and local municipalities in the Western and Eastern Cape provinces. The TMG aquifers comprising a thick sequence of hard sedimentary rocks dominated by fractured sandstones have the outcrop area of 37000 km2, the deposit area of 248000 km2 and the thickness ranging from 900m to 4000m. Large-scale distribution of the TMG over various geological structures leads to a big diversity in its hydrogeological properties, especially the hydraulic properties which are critical in determining the aquifers’ abstraction potential and sustainable yield. A proper estimation of hydraulic properties, with focus on the investigation of aquifer porosity, permeability and storativity, is important for the sound evaluation and sustainable utilization of the groundwater resource in the TMG area. Data from previous studies and current research have been collated and analyzed to help establish conceptual models of the TMG aquifers and to quantify the intrinsic aquifer properties − hydraulic properties. Based on the study of hydrogeological settings and aquifer types, combined with the interpretation of aquifer hydraulic tests, it is realized that the hydraulic properties of the TMG rocks are strongly controlled by fractures regarding the groundwater flow path within the TMG rocks. Media study on the nature of the fractures or fracture networks therefore is conducted in detail. Subsequently, the establishment of the fractured-media conceptual models on the basis of stochastic analyses is helpful for the better understanding of groundwater behaviors in the TMG aquifers. With the data derived from field measurements and interpretation of remote sensing data, the fractured rock hydraulic conductivities are estimated by using a hydraulic conductivity tensor approach. Considering the influential factors such aperture, roughness, stress condition, and most importantly the connectivity of fractures, the tensor model is accordingly modified to meet different boundary conditions for the estimation of the hydraulic conductivities on the surface and at depth. As a result, the estimated hydraulic conductivities at most sites fall in the range of 10-2~10-3 m/d that is roughly consistent with site pumping test results. However, it decreases with depth following a negative power law, which implies that the majority of fractures tend to be closed at depth. Site hydraulic tests also show the similar tendency of vertically spatial variation of the hydraulic conductivities. The study of fracture connectivity shows another hydrogeological significance. Fracture networks on the measurement scale present the feature of various fracture blocks in the system rather than they are well connected. The 3-D model demonstrates that very few fractures in the TMG sandstones are competent for groundwater flow. With this regard the computation of hydraulic conductivities is hence calibrated Multiple approaches are employed to estimate porosity and associated aquifer storativity. Results show that porosity of the TMG sandstones is strongly scale-dependent, of which the value of core sample laboratory tests yield a upper limit of 1.0%~3.6%. The porosity of pumping tests and in-situ fracture measurements fall in the middle rang of 0.05%~0.6%, whilst the application of lineament interpretation from remote sensing data produce its lower limit of 1.2×10-8. Assuming the TMG rocks are homogeneous media, the storativity value should have the same trend at various scales. These results indicate that the TMG groundwater resource at a larger scale may be overestimated if use the aquifer parameters derived from a smaller-scale study. Research findings in current study provide a new insight into the fractured rock aquifers in the TMG area. Some of the results will have wide implications on the groundwater management and forms a solid basis the further study of the TMG aquifers.
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    Hydraulic properties of the Table Mountain Group (TMG) aquifers
    (University of Western Cape, 2008) Lin, Lixiang; Xu, Yongxin; Titus, Rian
    Research findings in current study provide a new insight into the fractured rock aquifers in the TMG area. Some of the results will have wide implications on the groundwater management and forms a solid basis the further study of the TMG aquifers.
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    A tensor approach to the estimation of hydraulic conductivities in Table Mountain Group aquifers of South Africa
    (Water Research Commission, 2006) Lin, Lixiang; Xu, Yongxin
    Based on the field measurements of the physical properties of fractured rocks, the anisotropic properties of hydraulic conductivity (HC) of the fractured rock aquifer can be assessed and presented using a tensor approach called hydraulic conductivity tensor. Three types of HC values, namely point value, axial value and flow direction one, are derived for their possible applications. The HC values computed from the data measured on the weathered or disturbed zones of rock outcrops tend to give the upper limit values. To simulate realistic variations of the hydraulic property in a fractured rock aquifer, two correction coefficients, i.e. the fracture roughness and combined stress conditions, are adapted to calibrate the tensor model application. The application results in the Table Mountain Group (TMG) aquifers show that the relationship between the HC value and fracture burial depths follows an exponential form with the power hyperbola.