Comparison of nonlinear solvers within continuation method for steady-state variably saturated groundwater flow modelling

Nonlinearity continuation method, applied to boundary value problems for steady-state Richards equation, gradually approaches the solution through a series of intermediate problems. Originally, the Newton method with simple line search algorithm was used to solve the intermediate problems. In the present paper, other solvers such as Picard and mixed Picard–Newton methods are considered, combined with slightly modified line search approach. Numerical experiments are performed with advanced finite volume discretizations for model and real-life problems.

The effect of river regulation on the hydrological conditions of the aapa mire in a mining development site in Northern Finland

<p>Central Lapland Greenstone Belt is highly prospective for gold and Ni-Cu-PGE deposits. The study area in Sodankylä, in northern Finland, has been glaciated during last ice ages forming complex sedimentary succession with low conductivity till and highly variable sorted sediments, which hydraulic conductivity can be orders of magnitudes higher. The complex Quaternary sediments usually cover weathered/fractured bedrock, which is preserved due to weak glacial erosion and can host bedrock aquifers, as well. Rivers, lakes, streams and mires are common features in northern boreal and subarctic regions and their hydraulic interactions are usually poorly understood.</p><p> </p><p>Planning of mining operations in such environments needs a detailed understanding of water balance and groundwater discharge and recharge patterns, which are linked to subsurface sediments. In baseline studies, present hydrogeology, hydrology and ecology of the development site has usually been studied intensively. However, main rivers in northern Finland have been regulated since the 1970s and surrounding environments are not in their natural stage. The understanding, how much the environments could have been changed due to the regulation, is needed.</p><p> </p><p>The study area locates in the western part of Natura 2000 protected Viiankiaapa mire, which lies about 300 meters above high-graded Ni-Cu-PGE deposit. The regulated River Kitinen is running close to the western edge of the Viiankiaapa mire. The construction of the hydroelectric power plants and the regulation of the River Kitinen has changed the hydrology of the study area from the 1970s onwards. The Matarakoski power plant built in 1995 affected the study area most directly by ending the regular spring floods and rising the river stage.</p><p> </p><p>The changes in the groundwater flow and recharge/discharge patterns were studied with 3D groundwater flow modelling with MODFLOW-NWT and flood modelling with HEC-RAS. Pre-regulation situation was compared to the present stage with two different groundwater flow models in order to understand how regulation of river has affected the groundwater recharge/discharge patterns and flow patterns of the mire. Flood modelling was used to simulate the pre-regulation flood distribution.</p><p> </p><p>The regulation of the River Kitinen has affected the western part of Viiankiaapa mire by raising the water table and smoothing the hydraulic gradient towards the river leading to partial wetting of the mire. Annual water table variations decreased due to ending of the flooding and the regulation created a more stable hydrological environment in mire area.  The stabilization of the hydrological environment, as well as the rising of the water table, might have affected the distribution of habitats of endangered moss species <em>Hamatocaulis vernicosus</em>. The mire might have become more favourable for <em>Hamatocaulis vernicosus</em>, which is resistant to flooding and high water table. This study emphasizes the importance of understanding the interactions of surface water and groundwater and the present and pre-regulated stage of the river in order to assess the difference between the present and natural stage of the mire.</p>

Monitoring data analysis and groundwater flow modelling at a former uranium mine in France

<p>Mining companies are responsible for after-mining and environmental monitoring to ensure that mining waters released in the environment meet environmental quality standards. Water treatment plants can be used to mitigate surface waters when impacts related to past mining activities is evidenced. Indeed, meteoric recharge through waste rocks and tailings stored on sites often creates Acid Mining Drainage (AMD) or metallic signature that can be transferred to rivers through groundwater flow.</p><p>We studied a former uranium mine in Bertholène, Aveyron (France), where tailings, covered by waste rock, are stored in a valley behind a waste rock dyke. Mining waters, coming from both tailing drainage and mine facilities (galleries and open pit mine), are collected to a water treatment plant before release in the environment, meeting the environmental standards. Groundwater flow modelling is required to understand and quantify the different sources of AMD and their fate. The objective of this study is to give new insights on flows to guide the potential additional remediation of the site by testing different management solutions. For this purpose, we have developed a 3D hydrogeological model (MODFLOW) for the entire watershed.</p><p>This work is divided into two parts. We first analysed all available climatic and hydrogeological data (precipitation, water level, surface water flow, electric conductivity) using auto-correlations, cross-correlations and water balance calculations. These data come from long-term monitoring (14 years) on 13 piezometers and 4 discharge points. Recent two-year daily monitoring of groundwater levels completes the data set and provides a better understanding of the dynamic of the hydrosystem after precipitation. There is a 5 to 10-day time lag between rainfall and increases in water level and flow rate. The analysis also concludes that flows in the tailings occur under unsaturated conditions and that the water level in the gneiss aquifer never reaches the tailings.</p><p>Steady-state modelling developed at the watershed scale confirms that water level does not reach the tailings and allows simulating the impact of different management scenarios. Particle tracking has also been used to identify hydrogeological sub-watersheds of interest, such as those of the former open pit mine or the tailings to compute their water balance.</p>

Estimation of parameters in groundwater modelling by modified Clonalg

The purpose of this study was to improve the optimization model for predicting more parameters in more difficult conditions (more grid cell numbers and high time interval numbers) than other studies in groundwater flow modelling. Also, the model needs fewer observation numbers for estimating parameters than other studies. In the present study, an optimization model based on model calibration was developed to estimate simultaneously four groundwater flow parameters – hydraulic conductivity, transmissivity, storage coefficient and leakance. The modified clonal selection algorithm, a class of artificial immune systems, was used as a heuristic optimization method. In order to simulate the groundwater flow, MODFLOW was used in conjunction with the model in MATLAB. The input files for MODFLOW were obtained by GMS groundwater simulator. The model was applied to two different hypothetical groundwater systems (two- and three-dimensional) under transient conditions to evaluate its performance. The results showed that the model was feasible for groundwater flow modelling and it could determine the groundwater flow parameters successfully with less observations and more grid cell numbers than the other studies.