9725194 Davis The effect of geological heterogeneity on fluid flow is one of the most difficult issues when dealing with problems of contaminant transport. Spatial variations in permeability result in spatial variations in velocity which in turn result in dispersion. Significant theoretical advances have provided a quantitative framework for understanding the relationship between geologic heterogeneity and dispersion. However, many questions remain regarding the nature of geologic heterogeneity and how fluids flow through media exhibiting multiple scales of heterogeneity. Two types of field studies have traditionally been used to complement the theoretical models. Large-scale subsurface tracer experiments focus on the spatial variation in hydraulic conductivity and the resulting plume behavior. Outcrop studies have provided insight into the nature of geologic heterogeneity and its relation to stochastic models. However, these two approaches are somewhat isolated in that the large scale tracer experiments have studied the evolution of plumes but are limited in their ability to document geologic heterogeneity and velocity correlation structure. Conversely, outcrop studies provide high-resolution information on the geologic heterogeneity, but historically, have not quantified an explicit flow component. We have found evidence that elements of diagenetic fabrics preserve quantitative information about the paleogroundwater flow in the Sierra Ladrones Formation, New Mexico. The orientations of elongate carbonate concretions ranging in size from centimeters to 10s of meters are interpreted as indicators of paleogroundwater flow direction. The proposed research will obtain important additional data and employ state-of-the-art quantitative methods to further test the hypothesis that concretion orientations record local groundwater flow directions. We will test this hypothesis by 1) obtaining additional data on the hydrogeology and concretion orientations, 2) obtaining high-resolution i mages of concretion chemistry and growth history, and 3) conducting numerical experiments of flow and chemical reactions to test conceptual models of concretion growth. Our mapping will include both meter-scale studies of primary sedimentary structures, concretion orientations, and permeability as well as mapping concretion orientations and facies assemblages at the scale of 100s of meters. We will test alternative models of concretion origin by characterizing geochemical zonation (ICP-MS and Ion Microprobe) and presence of relict microbial populations (SEM). Numerical modeling of flow, transport, and reactions will provide the quantitative means of synthesizing our hydrogeologic, petrographic, and geochemcial results. The research under this collaborative arrangement between Davis (EAR 9725194) and Mozley (EAR 9726416) will focus on hydrogeologic mapping (concretion orientations and hydrogeologic properties) and characterizing concretion growth characteristics (petrography and geochemistry). The reactive transport modeling will be conducted in a cooperative arrangement with Dr. H. Rajaram under EAR-9734404. If these oriented concretions do record paleogroundwater flow directions, they may provide further insight into how fluids flow through porous media over large spatial scale exhibiting multiple scales of heterogeneity.
