QUATERNARY MAPPING FOR RESOURCE MANAGEMENT IN AN URBANIZED WATERSHED

We continue to upgrade existing surficial (Quaternary) maps for the urbanized watersheds of northeastern Rhode Island. Three maps, East Greenwich (Smith, 1955a), Bristol (Smith, 1955b) and Crompton (Smith, 1956) are 1:31,680 scale on pre-World War II topographic bases. We remapped portions of the 7½ minute quads utilizing 1987 digital 1:24,000 scale bases with all data input into MapInfo™. Lack of field exposures was compensated for by examining lithofacies data obtained from bore holes and by completing seismic reflection profiles in Greenwich Bay. We also used 1997 1:5,000 scale orthophotographs, significant aquifer coverage and other cultural data from the RI Geographic Information System (RIGIS). Greenwich Bay (GBay) and watershed (26.16 mi2, 6,774 hectares) is presented as a single 1:24,000 scale map. This map will assist in the implementation of a Special Area Management Plan for Greenwich Bay and watershed by the RI CRMC, the state coastal-zone management agency.

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 lake floor, most delta slopes, and parts of delta plains to create present day Narragansett Bay. The Warwick Plains delta extends from the Pawtuxet River on the north, south to Greenwich Bay, continues under present-day Gbay as a submerged delta plain to the Potowomut delta system on the south. The watershed boundary bisects T.F. Green airport (PVD), which sits on the delta-plain surface. Three till uplands, one on the east and two on the west, function as ground-water recharge areas for the stratified sediment in valleys and lowlands. Significant meltwater flow during deglaciation deposited coarse-gravel alluvial fans in smaller sub-watersheds that open onto the large delta plains.

Subsequent industrialization and high-density home development occurred on the highly permeable stratified deposits and along the post-glacial watercourses. The high hydraulic conductivity of the alluvial fan and delta plain (braided river) deposits and their flow paths in the GBay watershed have resulted in both groundwater and surface water transport of industrial contaminants, septic waste and nitrogen inputs toward, and into, Greenwich Bay.