GROUNDWATER MAPPING AND ANALYSIS IN A COASTAL ENVIRONMENT USING GIS

By combining resources from many diverse fields the ability to accurately reconstruct the three dimensional shape of the saltwater/freshwater interface in the coastal environment of Long Island, Maine can be achieved with reproducible and accurate results. GIS software creates a workspace that allows input of data in different formats to be analyzed, structured, converted, and drawn into a visual environment that can be manipulated into a user friendly graphical interface. To build the maps in this project, data from well boring logs had to be geocoded to existing homes using aerial photography and tax parcel maps. Water table and geologic features data had to be taken from existing PDF files and georeferenced into the projected coordinate system to provide accurate spatial representations of interpreted physical features. Data such as well depth, well yield, surficial geology, bedrock geology, potentiometric elevation and surficial topography are combined to create a multilayered three dimensional digital map.

To get the most accurate data for surface elevations, raw LiDAR files had to be broken down and clipped into usable formats and then loaded into ArcMap software. The surface topography was then rebuilt in the GIS environment to create a 3D model from which all subsurface features could be measured from. Features such as well location, depth, yield, surface elevation, potentiometric elevation, bedrock elevation, water body elevation, and freshwater cell interface could all be mapped in a projected coordinate system with accepted tolerances applicable to the data being entered. GPS data logging was also employed to improve the accuracy of individual well locations, but the data proved to be only as accurate as the homeowner's recollection of well location and well specifics.