Formatted Title
Pressure Transient Analysis of Drawdown and its Derivative Provide Insight on Complex Flow Regimes Affecting Groundwater and Contaminant Transport in a Bedrock Aquifer in the North Carolina Piedmont
Background/Objectives
Over 140 wells have been drilled to various depths to characterize the subsurface controls on groundwater flow and contaminant plume transport at a site in the North Carolina Piedmont. Many wells are cased to the top of competent bedrock, which limits evaluation of the weathered and partially weathered bedrock unit that overlies the competent bedrock. Furthermore, the lithologic descriptions provided on soil boring logs lack sufficient detail to enable an accurate depiction of the lithologic units as either saprolite, transition zone, or competent bedrock. The inability to differentiate these lithologic units has impeded the ability to correlate the various geologic units, to construct accurate geologic cross-sections, or to map the spatial distribution of these units in the subsurface. Development of a robust conceptual site model (CSM) is also hindered by the lack of key hydraulic data for these units across the site.
Approach/Activities
Diagnostic plots (drawdown and derivative plots) were used to analyze aquifer test data from six open boreholes and over 120 observation wells equipped with transducers, supplemented with fracture analysis from borehole geophysical logs, and non-invasive geophysical surveys (seismic, magnetics, electrical resistivity, and MASW) to better understand the geologic and hydrogeologic conditions that control the behavior of a VOC plume in a fractured bedrock aquifer.
Analytical solutions for the evaluation of pumping tests using derivative plots have been developed and can be applied to diagnose groundwater flow regimes and characteristics (including well-bore storage, skin, linear flow, radial flow, double porosity, etc.) that would otherwise be difficult to detect on linear time-drawdown plots. Derivative analysis was also used to obtain detailed information about the spatial distribution of hydraulic conductivity, transmissivity, storage coefficient, and diffusivity within the aquifer. Knowledge of the hydraulic characteristics of the aquifer and its geometry is important in developing a robust CSM. Plots of drawdown (s) versus time (t) divided by the square of radial distance from the pumping well (t/r2) were used to assess well connectivity.
Results/Lessons Learned
Based on drawdown and derivative responses, the proposed CSM indicates the plume trajectory is not controlled by regional eastward groundwater flow but occurs within a fracture corridor that extends to the north and to the south of the site for about 1,500 feet. The corridor consists of a series of parallel to subparallel, hydraulically separate blocks, each with a different static head. High angle vertical faults within the corridor act as conduits parallel to the fault plane but as leaky barriers perpendicular to the fault plane. To the north of the Site, groundwater flow and the plume trajectory is to the north within the fracture corridor. Along the southern half of the site, groundwater flow and the plume trajectory is southerly and occurs along a similarly narrow, restricted flow path. Bifurcation of groundwater flow is likely related to the presence of an east-west trending fault along the middle of the site. Leakage along a permeable segment of the fault may have provided the mechanism to account for this offset. The fault mimics a topographic divide exerting structural control on groundwater flow and plume trajectory. The almost instantaneous hydraulic communication between these faults, suggest a system of faults that results in an anisotropic transmissivity structure.
Employing the derivative of the drawdown as a complementary approach for pump test data interpretation improved the quality of the analysis and the interpretation by considering all measured data with an improved sensitivity. In addition to providing information on the hydraulic properties of the aquifer, this method of interpreting pumping tests allows for the identification of important flow regimes in the aquifer, the detection of aquifer boundaries, and selection of appropriate aquifer model. These benefits are not available using typical type-curve analysis of drawdown data only.