Formatted Title

Efficient Numerical Groundwater Models from Environmental Sequence Stratigraphy

Background/Objectives

Recent advances in borehole logging and visualization software combined with more rigorous evaluation of depositional environments have made sophisticated subsurface geologic interpretation and models accessible to the environmental consulting industry. However, in practice, conceptual site models are still very often based on generalized and often arbitrarily subdivided layered stratigraphy. Important hydrostratigraphic effects related to the discontinuity of strata are often overlooked early in site characterization because of this tendency to force observations to fit into a layered framework. These effects may go unappreciated until late in the lifecycle of a project, when stalled remedial efforts lead to a re-appraisal of the underlying site conceptual model.  Environmental sequence stratigraphy (ESS) provides the conceptual tools to identify and appreciate such hydrostratigraphic effects earlier in the life cycle of contaminated site remediation, but standard numerical groundwater modeling workflows remain biased towards strictly layered systems.

Approach/Activities

Stratigraphic interpretations and related groundwater models for contaminated groundwater sites are reviewed in the context of informing remedial design and scope. ESS products tend to be arranged around a suite of intersecting subsurface cross sections. While immensely valuable for analyzing hydrostratigraphy, these products do not lend themselves to the efficient prototyping or construction of groundwater models the same way as a layered interpretation would. Here, we focus on the process of building functional groundwater models from actual ESS interpretations from several Department of Defense sites. We evaluate performance of different numerical discretization schemes for representing complex stratigraphy and discuss efficient workflows for converting to scale-appropriate numerical models.

Results/Lessons Learned

Application of detailed stratigraphic analysis offers many benefits to contaminated site management. Of these, more accurate site conceptualization is arguably the most valuable. The ability to develop and communicate a geologically consistent interpretation at the correct resolution adds context for heterogeneous data and uncertainty inherent in standard environmental projects. Numerical groundwater models are often seen as an end product of site conceptualization. However, numerical groundwater models are equally useful as a method of testing the validity of a given site conceptual model, but only if it is feasible to efficiently translate the conceptual model into its numerical equivalent. To that end, we advocate the development of intermediate bridging products such as structural surfaces as part of the initial ESS evaluation.