USING INVERSE METHODS AND MULTIPLE AQUIFER TESTS TO CALIBRATE A MODEL OF GROUNDWATER FLOW THROUGH CONTAMINATED FRACTURED SEDIMENTARY ROCKS

Multiple aquifer tests were conducted using a network of pumping wells at the former Naval Air Warfare Center (NAWC), West Trenton, New Jersey, which is underlain by dipping fractured sedimentary rocks highly contaminated with trichloroethene and its biodegradation daughter products. The aquifer tests were monitored at multiple observation wells, yielding a rich data set of water level responses that were used to calibrate a site-scale model of groundwater flow. The flow model is a step toward development of reactive transport models of contaminant fate and transport that are used to compare different remediation alternatives at the site. The U.S. Geological Survey (USGS) groundwater modeling software MODFLOW (Harbaugh and others, 2000), and the USGS universal inverse modeling code UCODE_2005 (Poeter and others, 2005) were used to develop and calibrate the flow model.

A high-resolution three-dimensional geologic framework depicting the stratigraphy of the dipping sedimentary beds was critical to delineating the distribution of rock hydraulic properties in the model (Lacombe and others, in press; Tiedeman and others, 2009). Using the aquifer test data in combination with the geologic framework yielded a hydrogeologic framework showing that the dominant groundwater flow paths occur in the near-surface saprolite and in a few thin fissile or laminated dipping mudstone beds. Cross-bed fractures also play a role. On the basis of this hydrogeologic framework, a relatively small number of model parameters were used to represent the hydraulic conductivity and specific storage distribution in the model.

UCODE_2005 was used to estimate the model parameters by weighted least-squares regression solved with a modified Gauss-Newton method. Use of inverse modeling enabled quantitatively examining the effect of including different subsets of the multiple aquifer tests. Inclusion of observation data from multiple tests provides additional information about the true flow system and can more tightly constrain its representation in the model, yet it can be difficult to develop conceptual and numerical models that consistently explain the full suite of observation data. For the NAWC flow model, results showed that calibrations using data from multiple aquifer tests produced models with more reasonable parameter estimates and reduced parameter uncertainty, compared to those using data from only one test.

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 2000, MODFLOW-2000, the U.S. Geological Survey modular ground-water model -- User guide to modularization concepts and the Ground-Water Flow Process: U.S. Geological Survey Open-File Report 00-92, 121 p.

Lacombe, P.J., and Burton, W.C., in press, Hydrogeologic framework of USGS research site for contaminated fractured rock, Newark Basin, New Jersey: Groundwater Monitoring and Remediation.

Poeter, E.P., Hill, M.C., Banta, E.R., Mehl, S., and Christensen, S, 2005, UCODE_2005 and Six Other Computer Codes for Universal Sensitivity Analysis, Calibration, and Uncertainty Evaluation: U.S. Geological Survey Techniques and Methods 6-A11, 283p.

Tiedeman, C.R., Lacombe, P.J., and Goode, D.J., 2009, Multiple well-shutdown tests and site-scale flow simulation in fractured rocks, Ground Water, online early.

List of acronyms:

USGS: U.S. Geological Survey

NAWC: Naval Air Warfare Center