In 1992, LEA began to use PC ARC/ INFO and ArcView GIS to collate, analyze, and plot watershed data. Since that time, LEA has developed a "Hotspots" watershed model based exclusively in PC ARC/INFO and ArcView GIS to quantify nutrient export to lakes based on known watershed parameters. Through a series of queries, the model identifies areas of maximum nutrient export within the watershed.
Phosphorus is the main threat to lake quality in this region of Maine, as well as throughout most of North America. Phosphorus is an element found naturally in the air, soil, rock, and organic materials within a watershed. It is transported to the lake from throughout the watershed by a variety of routes.
Phosphorus is also used within the watershed by humans in many form including fertilizers, some detergents, and petroleum products. Human activities that increase soil erosion and the use of materials high in phosphorus within the watershed increase phosphorus export to the lake. Phosphorus availability leads to a series of events that may eventually cause extreme degradation of water within a lake.
Phosphorus export is quantified by land use type and can be measured in units of kg/hectare/year. These values are referred to as phosphorus export coefficients. They are typically reported as a range, so that local conditions can be better incorporated by the user. Phosphorus export coefficients do not consider factors such as proximity to the water body, soil type, and slope, which all significantly affect phosphorus export. The "Hotspots" model that LEA has developed modifies phosphorus export coefficients based on the three factors. The model is user friendly, based on simple algebraic principles, and is run by a series of macros. In designing the phosphorus export modification process, LEA worked with the Portland Water District and the State of Maine Department of Environmental Protection.
To run the model, LEA first develops land use, hydrologic soil, slope, and proximity coverages for the subject watershed. These four coverages are combined to create a single coverage. Through the use of a macro, the phosphorus export coefficients for each land use are modified based on the relative importance of the modification factor and the value of the factor. A series of queries is the run in Arc/View GIS to select and plot ranges of phosphorus export. In this way, geographic areas of high phosphorus export can be identified.
LEA uses this "Hotspots" model in conjunction with field verification to determine where within the watershed to focus mitigative measures. In the spring of 1997, LEA will begin to implement mitigative measures based on "Hotspots" analysis in a threatened watershed, with the support and assistance of local town officials, land owners, and students, as part of a USEPA-sponsored Section 319 Non-Point Source Pollution Grant.
