View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by DSpace at VU Integrating Geophysical, Hydrochemical, and Hydrologic Data to Understand the Freshwater Resources on Nantucket Island, Massachusetts Andee Jean Marksamer Submitted to the faculty of the University Graduate School in partial fulfillment of the requirements for the degree Master of Science in the Department of Geological Sciences, Indiana University May 2007 Acknowledgements I would like to thank my committee members, Dr. Mark Person, Dr. Gary Pavlis and Dr. Arndt Schimmelmann for their thorough review and helpful suggestions for this manuscript. I would like to mention a special appreciation to Dr. Mark Person, my faculty advisor, for his continued encouragement and support during my time at Indiana University. The knowledge I’ve gained in both hydrogeology and in basic scientific principles from Dr. Person will be forever remembered and appreciated as I advance through my professional career. In addition to the individuals above, I would also like to thank Dr. John Lane, Dr. Frederick Day-Lewis, Dr. Denis Cohen, Dr. Brandon Dugan, and Dr. Henk Kooi for their contributions to help get my thesis published. I must also give a big thanks to my fellow labmates in the GeoFluids Computational Laboratory for always keeping me smiling even during the long days and late nights in the lab. It is also necessary to recognize Ken Dehart for his help at fixing all the computer problems that I incurred during my time at Indiana University. I would also like to thank the Wannacomet Water Company and Sarah Oktay and Tony Molis of the University of Massachusetts Boston Field Station for their logistical field support. Finally, I would like to thank my family and friends, but especially my husband, Ken Downey, for his continued love and support in all aspects of my life. This research was supported by a grant from the National Science Foundation (EAR-0337634) to Mark Person. USGS contributions to this work were funded in part by the USGS Groundwater Resources Program. iii Preface This thesis is a draft of a manuscript that was recently submitted to the American Geophysical Union (AGU) for publication in an AGU Geophysical Monograph Series entitled, Data Integration in Subsurface Hydrology, edited by, David W. Hyndman, Frederick D. Day-Lewis, and Kamini Singha. The manuscript was submitted under the same title as this thesis. This monograph will be in publication Spring 2007. Co-authors for the published manuscript include Mark Person of Indiana University, Bloomington, IN, Frederick D. Day-Lewis and John Lane Jr. of the United States Geological Survey in Storrs, CT, Denis Cohen from Iowa State University, Ames, Iowa, Brandon Dugan from Rice University, Houston, TX, Henk Kooi from Vrije Universiteit, Amsterdam, The Netherlands, and Mark Willett from the Wannacomet Water Company on Nantucket, MA. The geophysical time-domain electromagnetism (TDEM) work presented in this thesis was completed by Dr. John Lane and Dr. Frederick Day-Lewis. iv Abstract In this study we integrate geophysical, hydrologic, and salinity data to understand the present-day and paleo-hydrology of the Continental Shelf near Nantucket Island, Massachusetts. Time-domain electromagnetic (TDEM) soundings collected across Nantucket and observed salinity profiles from wells indicate that the saltwater/freshwater interface is at least 120 m below sea level in the northern and central portions of the island, far deeper than predicted (80 m) by modern sea level conditions. TDEM soundings also indicate that higher salinity conditions exist on the southern end of the island. These findings suggest a relatively high-permeability environment. Paradoxically, a deep, scientific borehole (USGS 6001) on Nantucket Island, sampling Tertiary and Cretaceous aquifers, is over-pressured by about 0.08 MPa (8 m excess head), which is suggestive of a relatively low-permeability environment. We constructed a series of two-dimensional, cross-sectional models of the paleohydrology of the Atlantic Continental Shelf near Nantucket to understand the flushing history and source of overpressure within this marine environment. We considered two mechanisms for the emplacement of freshwater: (1) meteoric recharge during sea level low stands; and (2) sub-ice-sheet and glacial-lake recharge during the last glacial maximum. Results indicate the sub-ice-sheet recharge from the Laurentide Ice Sheet was needed to account for the observed salinity/resistivity conditions and overpressures. Both TDEM soundings and model results indicate that a lateral transition from fresh to saltwater occurs near the southern terminus of the island due to ice sheet recharge. We also conclude that the overpressure beneath Nantucket represents, in part, “fossil pressure” associated with the last glacial maximum. v Table of Contents Acknowledgements............................................................................................................iii Preface................................................................................................................................ iv Abstract............................................................................................................................... v Table of Contents...............................................................................................................vi List of Figures..................................................................................................................viii List of Tables ..................................................................................................................... xi Introduction......................................................................................................................... 1 Modeling Approach and Hypothesis .................................................................................. 9 Purpose of Investigation ................................................................................................... 10 Study Area ........................................................................................................................ 10 Geophysical Data.............................................................................................................. 12 Time Domain Electromagnetic Survey Results............................................................ 13 Mathematical Modeling.................................................................................................... 16 Mathematical Modeling.................................................................................................... 17 Numerical Methods....................................................................................................... 17 Numerical Mesh............................................................................................................ 22 Hydrologic Stresses and Model Input........................................................................... 27 Variable Sea Level Simulation ................................................................................. 27 Ice Sheet Simulation ................................................................................................. 30 Constant Sea Level Simulation................................................................................. 33 Results............................................................................................................................... 34 Discussion......................................................................................................................... 38 vi Conclusion ........................................................................................................................ 42 References......................................................................................................................... 46 Vitae vii List of Figures Figure 1. Location of wells on Nantucket Island, Martha’s Vineyard Island and the Atlantic Continental Shelf. Numbered squares on the Nantucket map indicate the location of time-domain electromagnetism (TDEM) soundings. Cross section A- A’ is associated with the cross section on Figure 6. Salinity profiles are shown of wells on the North Atlantic Continental Shelf. Bathymetry contours are in meters [Kohout et al., 1977; Folger et al., 1978; Hathaway et al., 1979; Hall et al., 1980]. Note the change in scale for wells PT-12 and PT-13……………….…………….2 Figure 2. Conceptual models for freshwater plumes: (a) lateral incursion of freshwater during the Pleistocene sea level low stands [Meisler et al., 1984] and vertical infiltration of meteoric water induced by local flow cells on the Continental Shelf [Groen, 2002]; (b) sub-ice-sheet recharge from the Laurentide ice sheet [Person et al., 2003]; (c) infiltration beneath pro-glacial lakes……………………………....4 Figure 3. Distribution of proglacial lakes and position of maximum ice sheet extent during the Late Quaternary on the Atlantic Continental Shelf [after Uchupi et al., 2001]………………………………………………………………………………6 Figure 4. Head data comparing the local water table elevation in well 228 and Cretaceous aquifers in well 6001 beneath Nantucket Island between 1976-1993. The data indicated that the deep aquifers are anomalously pressured and disconnected from the water table flow system. The locations of the wells are shown on Figure 1.……………………………………………………………………….......8 viii Figure 5. TDEM sounding data and 3-layer inversion results for (a)