Ghislain de Marsily1

Hydrostratigraphy is a term that has increasingly gained traction by practioners of in situ remediation methods, primarily because of the need to develop quantitative means of describing behavior of contaminants or reagents in aquifer systems. The genesis of hydrostratigraphy stems from “hydro” meaning water and “stratigraphy” meaning study of sedimentary sequences or even more simply, sedimentary layers. As introduced in Chapter 2, the sedimentary sequences comprising aquifer systems constitute the megascopic building blocks that control three-dimensional aquifer architecture. While sedimentary deposits are typically laid down in more or less horizontal layers, the individual facies comprising sequences and depositional elements demonstrate a self-organizing three-dimensional structure that translates into a complex, but characteristic relationship that is determined by the sequence of depositional and erosional processes from the macro-to the micro-scale. The result is that the “layer-cake” approach to stratigraphy is oversimplifi ed, lacking the detail in mapping permeability structures that control reagent distribution and contaminant transport.