U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Modeling Statistical Relations Among Shallow Ground-Water Quality, Human Activities, Land Use, and Thickness of the Unsaturated Zone on Long Island, New York

Abstract

Water-quality data from 90 shallow observation wells in five areas of differing land use in Nassau and Suffolk Counties were used to develop statistical models that relate shallow ground-water quality to variables representing human activities and hydrogeologic conditions. The five study areas represent the full range of land uses in the two-county area and lie along the regional ground-water divide, where ground-water has a vertically downward component and thus recharges deep zones of the aquifer system.

Maximum-likelihood logistic-regression analysis of explanatory variables that describe the degree and type of human activities and hydrogeologic conditions at the 90 well sites was used to develop statistical models that can predict the probability of the presence of contaminants in shallow ground water within the two-county area. Variables found useful for representing human activities and hydrogeologic conditions include (1) population density within a 1/2-mile radius of each well, (2) percentages of land-use categories within the same area, and (3) thickness of the unsaturated zone at each well site. One model that relates the probability of the presence of volatile organic compounds (VOC's) at concentrations of at least 1 microgram per liter to commercial and high- and medium-density residential land was selected to illustrate model application and testing procedures. Resulting predictions were compared with an independent set of water-quality data from 378 wells in Nassau and Suffolk Counties. The comparison indicates that VOC's are more prevalent in areas with relatively little commercial and dense residential land than predicted by data from the five study areas.