1,721,302 research outputs found
The Groundwater Flow Equation
No full textIn this chapter, the differential groundwater flow equation that governs the distribution of the flow directions and rates in an aquifer is derived. The problem is examined at a macroscopic scale, neglecting an analysis of detailed solid–liquid interface distribution, which would entail excessive analytical and computational complexity, without contributing useful information from an operational standpoint. The equation is thus determined as a combination of the equations that express the law of mass conservation (whose terms are described for a representative elementary volume), Darcy’s law, and the storage variation due to changes in hydraulic head. Unsteady state groundwater flow in each aquifer type is described by a different equation, each defining the Laplacian of the hydraulic head as a function of the aquifer’s storage and transport capacity, and of the hydraulic head’s partial derivative with respect to time. In particular, in the case of confined aquifers, flow is a function of specific yield and transmissivity, as is the case even for leaky aquifers, whose hydrodynamic behavior is, however, also affected by the leakage between aquifers (quantified by the leakage factor). A rigorous description of flow in unconfined aquifers would require a nonlinear and nonhomogeneous differential equation due to the inclination of the water table with flow; however, under simplifying conditions an approximate description, analogous to the confined aquifer equation, can be defined as a function of specific yield and transmissivity
Assessment and minimization of potential environmental impacts of ground source heat pump (GSHP) systems
Full text linkGround source heat pumps (GSHPs) gained increasing interest owing to benefits such as low heating and cooling costs, reduction of greenhouse gas emissions, and no pollutant emissions on site. However, GSHPs may have various possible interactions with underground and groundwater, which, despite the extremely rare occurrence of relevant damages, has raised concerns on their sustainability. Possible criticalities for their installation are (hydro)geological features (artesian aquifers, swelling or soluble layers, landslide-prone areas), human activities (mines, quarries, landfills, contaminated sites), and groundwater quality. Thermal alterations due to the operation of GSHPs may have an impact on groundwater chemistry and on the efficiency of neighboring installations. So far, scientific studies excluded appraisable geochemical alterations within typical ranges of GSHPs (±6 K on the initial groundwater temperature); such alterations, however, may occur for aquifer thermal energy storage over 40 °C. Thermal interferences among neighboring installations may be severe in urban areas with a high plant density, thus highlighting the need for their proper management. These issues are presented here and framed from a groundwater quality protection perspective, providing the basis for a discussion on critical aspects to be tackled in the planning, authorization, installation, and operation phase. GSHPs turn out to be safe and sustainable if care is taken in such phases, and the best available techniques are adopted
Water-Energy Nexus in Shallow Geothermal Systems
No full textGround-source heat pumps (GSHPs) reduce CO2 emissions compared to conventional heating and cooling systems. The thermally altered zone (thermal plume) is a key aspect for land management of GSHPs
Performance assessment and monitoring of groundwater remediation by a permeable reactive barrier
No full textHuman health risk assessment for nanoparticle-contaminated aquifer systems
Full text linkNanosized particles (NPs), such as TiO2, Silver, graphene NPs, nanoscale zero-valent iron, carbon nanotubes, etc., are increasingly used in industrial processes, and releases at production plants and from landfills are likely scenarios for the next years. As a consequence, appropriate procedures and tools to quantify the risks for human health associated to these releases are needed. The tiered approach of the standard ASTM procedure (ASTM-E2081-00) is today the most applied for human health risk assessment at sites contaminated by chemical substances, but it cannot be directly applied to nanoparticles: NP transport along migration pathways follows mechanisms significantly different from those of chemicals; moreover, also toxicity indicators (namely, reference dose and slope factor) are NP-specific. In this work a risk assessment approach modified for NPs is proposed, with a specific application at Tier 2 to migration in groundwater. The standard ASTM equations are modified to include NP-specific transport mechanisms. NPs in natural environments are typically characterized by a heterogeneous set of NPs having different size, shape, coating, etc. (all properties having a significant impact on both mobility and toxicity). To take into account this heterogeneity, the proposed approach divides the NP population into classes, each having specific transport and toxicity properties, and simulates them as independent species. The approach is finally applied to a test case simulating the release of heterogeneous Silver NPs from a landfill. The results show that taking into account the size-dependent mobility of the particles provides a more accurate result compared to the direct application of the standard ASTM procedure. In particular, the latter tends to underestimate the overall toxic risk associated to the nP release
Aquifer Characterization
No full textAquifer tests are the most appropriate method to determine the hydraulic behavior of an aquifer and the distribution of the hydrodynamic parameters that govern such behavior. This chapter illustrates the different type of aquifer tests (i.e., pumping, recovery and slug tests) and how to plan, execute and interpret them. Pumping tests consist in measuring the drawdown induced by the extraction of water from a well at a constant discharge rate in one or more observation points. They allow to first identify the hydraulic behavior of the aquifer, and thus to classify it as confined, leaky or unconfined, and then to determine, via a type curve matching method, the aquifer’s horizontal hydraulic conductivity, transmissivity and storativity. In the case of leaky aquifers, also the leakage factor can be calculated; and in the case of unconfined aquifers, the effective porosity and the vertical hydraulic conductivity can also be derived. Clearly, this interpretation relies on a number of ideal hypotheses being satisfied; this chapter also illustrates how to interpret pumping tests in the presence of factors that cause a deviation from the ideal behavior (e.g., finite, as opposed to infinitesimal, well radius and volume; partially penetrating well; presence of recharging or impermeable boundaries; inconsistent pumping rate; permeability damage close to the well). During recovery tests, residual drawdown measurements are carried out following the interruption of the pump at the end of a constant discharge pumping test. Theis’ recovery method, based on the superposition principle and normally used for the interpretation of the test, allows to determine the transmissivity of an aquifer. The last type of aquifer test, i.e., the slug test, consists in inducing an instantaneous variation of the static water level in a well or piezometer, and subsequently measuring the recovery over time of the undisturbed level in the same well. This method is used to determine the hydraulic conductivity of the aquifer in proximity of the well. In this chapter, the most common interpretation methods are presented, as well as the strategies to overcome limitations due to the existence of factors that cause a deviation from the ideal behavior. Finally, a suite of correlation-based, laboratory, and field methods available for the determination of an aquifer’s hydrodynamic parameters in alternative to aquifer tests are presented, and the applicability to different aquifer types and situations of each method, as well as their reliability is discussed
Analytical Solutions to the Differential Equation of Mass Transport for Conservative Solutes
No full textIn this chapter, analytical solutions to the differential equation of mass transport for conservative solutes are illustrated. Their derivation relies on a number of simplifying hypotheses, including that: the medium is saturated, homogeneous and isotropic; water has constant density and viscosity, regardless of solute concentration; Darcy’s law is valid; flow directions and rates are uniform; transport parameters are constant within the domain; boundary conditions are constant in time. Solutions for one-, two- and three-dimensional geometries are presented, the former being mainly used for the interpretation of laboratory experiments, the latter two being more relevant for practical applications. Pulse and continuous solute release are considered. Notably, in a three-dimensional geometry a pulse input from a point source and a continuous input from a plane source are illustrated. A solution of the differential equation of mass transfer for the former contamination scenario was derived by Baetslé, while Domenico and Robbins proposed a model for the latter
Basic Concepts
No full textThe largest source of human drinking water is stored and flows in the subsurface. Geological formations saturated in mobile groundwater that can be exploited for human use are called aquifers. This chapter introduces basic notions that set the ground for the understanding and description of subsurface water flow. First, the main properties of water are illustrated, with a particular focus on the forces it establishes with the solid matrix of a porous medium and on how these affect its mobility. Then, broad aquifer classifications are provided, based on their geographical location, their permeability characteristics as a function of the type of porosity (i.e., intergranular, fracture or karst), and their degree of confinement. The latter, which categorizes aquifers as unconfined, leaky or confined, has crucial implications on both their storage capacity and hydrodynamic behavior. The key parameters that characterize an aquifer’s storage capacity are porosity and storativity. While the former is indicative of the total amount of water that can be stored within a porous medium, the latter indicates the fraction that can be released. Both these notions apply to any aquifer type although the mechanism of water release is distinct in unconfined and confined aquifers: in the former, water is released under the effect of gravity alone, and storativity is called specific yield; in the latter, water is released as a result of water expansion that follows a pressure drop. Subsurface water transport, instead, is driven by the existence of a hydraulic gradient (i.e., a drop in hydraulic head, or piezometric level). Under specific hypotheses, groundwater flow can be described by Darcy’s law, which establishes a proportionality relationship between flow rate and hydraulic gradient, and can be used to map an aquifer’s flow field. The relation defined by Darcy’s law is measured by an aquifer-specific parameter called hydraulic conductivity. This parameter is crucial not only in the description of the transport capacity of a porous medium, but also in the calculation of its productivity, which is a function of the hydraulic conductivity and the thickness of an aquifer
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