Review: Theory-guided machine learning applied to hydrogeology—state of the art,opportunities and future challenges

Hydrogeology Journal - Thanks to recent technological advances, hydrogeologists now have access to large amounts of data acquired in real time. Processing these data using traditional modelling...     

A numerical investigation to illustrate the consequences of hydraulic connections between granular and fractured-rock aquifers

Natural or artificial hydraulic connections between a granular aquifer in contact with a fractured-rock aquifer can have significant physical and chemical impacts at both a local and a regional scale. In this study, numerical simulations are conducted in order to illustrate the hydrogeological consequences of such hydraulic relationships. The numerical investigation, based on a conceptual model, focuses on the effects of the hydraulic connections when conducting a pumping test in a well that is set into a granular confined aquifer overlying a fractured-rock aquifer which presents a few fractures directly connected to the granular aquifer. It is illustrated that when interpreting the pumping test with the conventional methods consisting of plotting the drawdown versus time, a bias is introduced on the estimation of the transmissivity of the granular aquifer due to groundwater flowing from the fractured-rock aquifer via connecting fractures. However, it is underlined that plotting drawdown log-derivative versus time helps to diagnose the existence of these hydraulic relationships and therefore avoids committing a bias on the transmissivity estimation of the granular aquifer. Numerical simulation results also illustrate that hydraulic connections between the two aquifers can have significant impacts on the hydrogeochemical signature of the granular aquifer under investigation.     

A numerical investigation of pumping-test responses from contiguous aquifers

Hydrogeology Journal - Adequate groundwater management requires models capable of representing the heterogeneous nature of aquifers. A key point is the theoretical knowledge of flow behaviour in...     

Spatial distribution of soil shear-wave velocity and the fundamental period of vibration – a case study of the Saguenay region,Canada

The spatial distribution of soil shear-wave velocity and the fundamental period of vibration were selected as input parameters for the determination of potential seismic site effects in the Saguenay region, Canada. The methodology used in this study involved three clear steps. First, a 3D geological model of the surficial deposits was built taking into consideration the type, spatial distribution and thickness of the deposits. Second, representative average V_s values were determined for each of the major soil units. Finally, the average shear-wave velocity from the ground surface to bedrock (V_sav), the shear-wave velocity of the upper 30 m (V_s30) and the fundamental site resonance period (T₀) were calculated over a regular grid for the study area. The results include the spatial distribution of the fundamental site resonance period, the average shear-wave velocity in the first 30 m of the ground and the spatial distribution of National Building Code of Canada seismic soil classes for the Saguenay region.     

Development of a hydrogeological conceptual wetland model in the data-scarce north-eastern region of Kilombero Valley,Tanzania

Understanding groundwater/surface-water interactions in wetlands is crucial because wetlands provide not only a high potential for agricultural production, but also sensitive and valuable ecosystems. This is especially true for the Kilombero floodplain wetland in Tanzania, which represents a data-scarce region in terms of hydrological and hydrogeological data. A comprehensive approach combining hydrogeological with tracer-based assessments was conducted, in order to develop a conceptual hydrogeological wetland model of the area around the city of Ifakara in the north-eastern region of Kilombero catchment. Within the study site, a heterogeneous porous aquifer, with a range of hydraulic conductivities, is underlain by a fractured-rock aquifer. Groundwater chemistry is mainly influenced by silicate weathering and depends on groundwater residence times related to the hydraulic conductivities of the porous aquifer. Groundwater flows from the hillside to the river during most of the year. While floodwater close to the river is mainly derived from overbank flow of the river, floodwater at a greater distance from the river mainly originates from precipitation and groundwater discharge. Evaporation effects in floodwater increase with increasing distance from the river. In general, the contribution of flood and stream water to groundwater recharge is negligible. In terms of an intensification of agricultural activities in the wetland, several conclusions can be drawn from the conceptual model. Results of this study are valuable as a base for further research related to groundwater/surface-water interactions and the conceptual model can be used in the future to set up numerical flow and transport models.     

An analytical solution for ground water transit time through unconfined aquifers

An exact, closed-form analytical solution is developed for calculating ground water transit times within Dupuit-type flow systems. The solution applies to steady-state, saturated flow through an unconfined, horizontal aquifer recharged by surface infiltration and discharging to a downgradient fixed-head boundary. The upgradient boundary can represent, using the same equation, a no-flow boundary or a fixed head. The approach is unique for calculating travel times because it makes no a priori assumptions regarding the limit of the water table rise with respect to the minimum saturated aquifer thickness. The computed travel times are verified against a numerical model, and examples are provided, which show that the predicted travel times can be on the order of nine times longer relative to existing analytical solutions.     

Analytical closed-form solutions for assessing pumping cycles,times, and costs required for NAPL remediation

An analytical solution is given to evaluate the number and duration of pumping cycles required for the remediation by pumping of contaminants, both single component and multi-component non-aqueous phase liquids (NAPLs), when no free product is present in the system. The method can be applied in a homogenous medium if the contamination zones have been delineated and residual total NAPL concentrations assessed. Based on the principle of the NAPL partitioning in unsaturated or saturated porous media, analytical closed-form solutions are provided for both cases of remediation by pumping in saturated and unsaturated conditions: “pump-and-treat” and “soil vapor extraction”. In each case we determine the number of pumping cycles required to reach the residual required concentration of NAPL (for example, according to health-based standards), considering one or more chemicals simultaneously present in an aquifer. The method requires information on the aquifer saturation state and the properties of the chemicals of interest. Calculations are based on the assumption of equilibrium partitioning of chemicals between the pore water, the soil solids, and the soil gas (in the case of unsaturated conditions), and no presence of a NAPL phase.     

Groundwater travel times for unconfined island aquifers bounded by freshwater or seawater

Analytical solutions for calculating groundwater travel times within unconfined island aquifers are given for cases of both a freshwater-bounded island (island located in a fresh water lake) and an oceanic island (freshwater contained above intrusive saltwater). The solutions apply for homogenous aquifers recharged by surface infiltration and discharged by a down-gradient, fixed-head boundary, under steady-state conditions. The solutions are given for two simple island geometries: circular islands and strip islands. A technique is also provided for comparing travel times in inland islands and oceanic islands. Travel times in oceanic islands are found to be shorter than travel times in inland islands, and travel times in circular islands are found to be longer than travel times in strip islands.     

Travel time to a well pumping an unconfined aquifer without recharge

A solution is given for the travel time to a well pumping an ideal, horizontal unconfined aquifer, under steady-state conditions, when recharge from infiltration is negligible. Three forms of the solution are provided: a closed-form solution, an integral to be calculated in a worksheet, and a simple equation. The three forms of the solution give travel times nearly identical to those obtained using a finite-element code for saturated and unsaturated flow and particle tracking.