Formatted Title
What’s after Sequence Stratigraphy? Hydrogeologic Best Practices to Interpret Contaminant Migration Pathways with Case Study Applications
Background/Objectives
In sedimentary aquifers, groundwater flow and mass transport are largely controlled by the geometry and interconnectivity of high and low permeability layers (referred to as lithofacies). Typical data collection for an environmental site characterizes general flow direction, gradients, and hydraulic conductivity. Rarely are these characteristics well understood within the context of the high and low permeability layers identified by sequence stratigraphy, which can lead to inaccuracies in site interpretation. Well networks are typically classified by depth, but do not consider regional hydrogeologic features or depositional environments illustrated by sequence stratigraphy. As a result, wells are mapped together by depth regardless of differences in screened elevation and lithologies. The objective of this work is to discuss best practices of evaluating hydrogeologic information that, when applied together with a sequence stratigraphy foundation, can be used to identify migration pathways and inform remedial investigation and implementation activities. This is demonstrated using case studies at three sites across the United States.
Approach/Activities
Best practices of hydrogeologic evaluation start with a review of regional literature to establish the general context of groundwater flow at the site prior to starting fieldwork. Hydrostratigraphic units should be defined using facies identified through sequence stratigraphy, and well logs should be reviewed to assign screened lithologies. The well network should be classified by hydrostratigraphic units that are not organized by depth but by lithologic characteristics that inform migration pathways. Groundwater data for each hydrostratigraphic unit should be evaluated on maps showing geologic and hydrogeologic features influencing flow in each unit such as topography, surface water with elevations and stream details, top of bedrock and subcrop, geologic unit presence and thickness. Groundwater flow for each hydrostratigraphic unit should be interpreted to reflect conceptual flow through these features. Contamination should be mapped by hydrostratigraphic unit to illustrate the manifestations of those pathways in the subsurface. Real-time data should be evaluated during field investigation to ground truth the conceptual site model understanding if multiple mobilizations are planned.