Optimal locations of groundwater extractions in coastal aquifers

A regional water supply management model for coastal aquifers was developed. One of its outcomes is the definition of the optimized locations for groundwater withdrawal. Such a tool permits the analysis of alternative plans for groundwater extraction and the sustainable use of water resources in a coastal aquifer subject to saltwater intrusion. The principal components are the evolutionary optimization and the analytical/numerical simulation models. The optimization technique looks for the best well locations taking into consideration the economic results and the satisfaction of the societal water demand. However these two concerns are conditioned by trying to control the saltwater intrusion, i.e., preserving the environmental equilibrium. The simulation model uses the governing mathematical equations for groundwater movement to find the interface between freshwater and saltwater. Because of the non-linearity in the system and the possibility of a jumping interface, a security distance was defined. This is a controlling variable which can be set by the decision makers. The model was applied to a typical case with interesting results. For example, diagrams showing the relationship between the location of the wells and the security distance(s) are of importance to the managers. It was also crucial to have an understanding of the tradeoffs between groundwater withdrawals, positions of the wells from the coast line, and the security distance. The model was also applied to a real case in order to relate the extractions, distances and artificial recharge (not presented in this paper).Civil Engineering Research Centre of the University of Minho.Science and Technology Foundation -POCTI/ECM/2512/9

Radial Dupuit interface flow to assess the aquifer storage and recovery potential of saltwater aquifers

A new accurate numerical solution is presented for aquifer storage and recovery (ASR) systems in coastal aquifers; flow is approximated as radial Dupuit interface flow. The radial velocities of points on the interface are a function of time, the vertical coordinate, and the dimensionless parameter D (the discharge of the well divided by the product of the hydraulic conductivity, the square of the aquifer thickness, and the dimensionless density difference). The recovery efficiency of an ASR system (the ratio of the recovered volume of water divided by the injected volume of water) is determined by D and by the relative lengths of the injection, storage and recovery periods. Graphs are produced for the recovery efficiency as a function of parameter D for ASR operations with and without storage periods and for multiple cycles. The presented solutions and graphs are to be used as screening tools to assess the feasibility of specific injection, storage and recovery scenarios of planned ASR systems in saltwater aquifers without having to run complicated flow and transport models. When the screening tool indicates that recovery efficiencies are acceptable, the consideration of other features such as mixing and chemistry is warranted.WatermanagementCivil Engineering and Geoscience

An analytic solution for groundwater uptake by phreatophytes spanning spatial scales from plant to field to regional

Phreatophytes are important to the overall hydrologic water budget, providing pathways from the uptake of groundwater with its nutrients and chemicals to subsequent discharge to the root zone through hydraulic lift and to the atmosphere through evapotranspiration. An analytic mathematical model is developed to model groundwater uptake by individual plants and fields of plant communities and the regional hydrology of communities of fields. This model incorporates new plant functions developed through aid of Wirtinger calculus. Existing methodology for area-sinks is extended to fields of phreatophytes, and Bell polynomials are employed to extend existing numerical methods to calculate regional coefficients for area-sinks. This model is used to develop capture zones for individual phreatophytes and it is shown that the functional form of groundwater uptake impacts capture zone topology, with groundwater being extracted from greater depths when root water uptake is focused about a taproot. While individual plants siphon groundwater from near the phreatic surface, it is shown that communities of phreatophytes may tap groundwater from greater depths and lateral extent as capture zones pass beneath those of upgradient phreatophytes. Thus, biogeochemical pathways moving chemical inputs from aquifer to ecosystems are influenced by both the distribution of groundwater root uptake and the proximity of neighboring phreatophytes. This provides a computational platform to guide hypothesis testing and field instrumentation and interpretation of their data and to understand the function of phreatophytes in water and nutrient uptake across plant to regional scales