QUATERNARY MAPPING FOR WATER QUALITY AND TRANSPORTATION MANAGEMENT IN AN URBAN ENVIRONMENT

We are upgrading existing surficial (Quaternary) maps and completing new maps in the highly urbanized northeastern section of Rhode Island. These maps will assist in the implementation of a management plan for Greenwich Bay and watershed, help determine the best use for dredged material from the Providence River, and aid in design of road fill and bridge piers for the relocation of I-195. Three existing maps, Providence (Smith, 1956), East Greenwich (Smith, 1955a) and Bristol (Smith, 1955b), are 1:31,680 scale on pre-World War II topographic bases. We are completing upgrades of all three 7½ minute quads utilizing 1987 digital 1:24,000 scale bases with all data input into a GIS mapping system (MapInfo™). New mapping was completed for the East Providence 7½ minute quad for the USGS StateMap program. Lack of field exposures in this urbanized area was partially compensated for by examining lithofacies and other data obtained from bore holes. Bore-hole data, the result of past road and bridge construction, ongoing road realignments, new sewer system construction and the Providence River dredging project, is being organized into a comprehensive database by other geoscientists in a cooperative project with the RI Department of Transportation. We also used 1997 1:5,000 scale orthophotographs, significant aquifer coverage and other cultural data from the Rhode Island Geographic Information System (RIGIS). The Quaternary geology is dominated by a series of very large deltas deposited into Glacial Lake Narragansett during the recession of Laurentide ice through Rhode Island during Late Wisconsinan time. Subsequent lake drainage and post-glacial sea-level rise covered the lakefloor, most delta slopes and parts of delta plains to create the present day Narragansett Bay (NB). Greenwich Bay (GB), an arm of NB, is one such submerged delta plain. The high hydraulic conductivity of delta plain (braided river) deposits in the GB watershed has resulted in significant septic waste and nitrogen inputs into GB. Alternatively, the discovery of a large glacial lacustrine fan beneath the area to be dredged for shipping in the Providence River has allowed some of the coarse-grained gravel to be removed for fill for hurricane elevation for buildings and the I-195 project. Contaminated dredged material will be placed in the disposal cells created and capped with glacial lakefloor deposits.