A decision support framework for fit for purpose assessments in brackish groundwater units



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Brackish groundwater resources in deeper aquifers are finite and require that the water allocation decisions be optimal. Therefore, it is important to evaluate whether the groundwater available in deeper aquifers are - 1) fit for what purpose(s) (i.e., agriculture, energy and municipal requirements)? and 2) What physical constraints are likely to constrain production from these geological units? Decision support frameworks that help address these questions are therefore of practical interest and their development is the primary focus of this dissertation. The framework is illustrated by applying it to evaluate the feasibility of developing Dockum-Hydrostratigraphic Group (HSG) as an alternative resource to augment supplies from dwindling Ogallala Aquifer in the Southern High Plains (SHP), Texas. A GIS-based Fit-for-Purpose (FFP) assessment framework that is free from eclipsing and ambiguity problems was developed using distance-based multi-criteria decision making (MCDM) technique to identify the suitability of water based on prescribed water quality standards. The results from FFP assessment indicate that Dockum HSG has high potential for use especially for hydraulic fracturing applications. Some level of treatment is warranted for agriculture applications and the compatibility is winter wheat > corn > sorghum > cotton considering locations within SHP where these crops are grown. Total dissolved solids (TDS) and Sodium Absorption Ratio (SAR) are limiting factors. Groundwater in Dockum HSG does not meet drinking water standards warranting treatment prior to its municipal use. A copula-based framework was developed to evaluate water quality violation risks and how these risks differ in fresher and brackish portions of a geological unit. A new metric (Kth Analyte Recovered Information Metric – KARIM) was developed to identify whether the knowledge of site-specific TDS can improve upon the copula-based regional-scale risk quantification. Considerable differences in water quality violation risks were noted between brackish and fresher portions of Dockum-HSG which are attributable to the geochemical evolution of the groundwater in this unit. Water quality violation risks for major crops are Peanuts > corn > sorghum ~ cotton > winter wheat. The site-specific knowledge of TDS reduces uncertainty in estimated water quality violation risks, except for boron. The reduction in uncertainty is however heterogeneous and is greater in the fresher portions of the aquifer. Density stratification is commonly noted in brackish units which can lead to unstable rise of the freshwater-saltwater interface under pumping (this phenomenon is called upconing). The groundwater production limitation under upconing was evaluated using Monte-Carlo simulations. An entropy-based global sensitivity analysis (GSA) was used to identify the influences of aquifer hydrogeology, well design and the overlying aquitard. The likelihood of upconing being a limiting factor (as compared to well yields) is high (p-value ~ 0.55). Comparison of stochastic supply and irrigation demands indicate that water availability for sorghum > winter wheat > peanuts ~ cotton > corn. The developed framework is generic and can be applied to evaluate the suitability of other data-poor brackish units to develop insights on future monitoring and water allocation decisions.



Ogallala Aquifer, Brackish groundwater, Intended use, Fit for purpose, Sensitivity analysis, Copulas, Upconing, Entropy, GIS