Aquifer simulation models are important tools for managing groundwater resources. Wagner [this issue] describes numerous studies in which aquifer simulation models have been combined with optimization methods for the purpose of designing aquifer remediation plans, managing pumping and injection systems, evaluating groundwater policies, controlling aquifer hydraulics, preventing saltwater intrusion and for the optimal allocation of surface and groundwater resources. Wagner [this issue] laments, as did Rogers and Fiering [1986] for surface water optimization problems a decade ago, that given the plethora of applications of systems methodologies in the literature, there are conspicuously few studies which describe the implementation of groundwater management strategies derived from the combined use of simulation and optimization models. Interestingly, a decade after the Rogers and Fierings [1986] criticisms, the combined use of simulation and optimization is now almost routinely applied in industry, for example, to hydropower scheduling problems. Perhaps it takes a few decades of research applications until such procedures find their way into actual practice.
Much of the research in the area of groundwater modeling focuses on the important problem of designing aquifer remediation systems [ Wagner, this issue; Rathfelder et al., this issue]. For example, soil vapor extraction (SVE) is now a fully established and widely exploited groundwater remediation technology [ Rathfelder et al., this issue]. Further advancements in SVE, bioventing and other subsurface remediation technologies will result from future advances relating to the description of heterogeneous soil properties and fluid distributions and from advances relating to the accurate mathematical description of non-equilibrium mass exchanges between phases and microbial degradation kinetics.
The development of accurate models for the prediction of multiphase multicontaminant transport in groundwater will require advances in a variety of different areas. Russell [this issue] provides an overview of recent practical models of multiphase multicontaminant transport in groundwater along with a discussion of the experimental and theoretical issues relating to the formulation and application of mathematical models. Celia et al. [this issue] describe recent advances in the use of pore-scale models for describing multi-phase porous media systems.
Again, as argued earlier by Hornberger and Boyer [this issue] significant advances in our understanding of subsurface heterogeneity are likely to result from data-driven studies such as isotope and environmental tracer studies [ Phillips, this issue]. It is encouraging to observe an increasing number of studies which employ isotopic and environmental tracers in addition to other investigative techniques for exploring subsurface flow regimes [ Phillips, this issue]. Field studies which reflect the full spatial and temporal heterogeniety of the subsurface are increasingly exploiting scale invariance to reduce the complexity of large databases. Sposito [this issue] provides an encouraging review of vadose-zone hydrology, when he notes that both empirical and theoretical studies are exploiting scale invariance leading to a connection between field and theoretical studies which has provided deeper insight into the mechanisms of soil water processes at field scales.
On another scale, Person and Baumgartner [this issue] summarize our current state of knowledge relating to regional and long-distance fluid migration within the earth's crust. Their review demonstrates that shallow groundwater flow systems are expected to receive considerable and significant fluid input if they are above active metamorphic and tectonic terrains.