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

Evaluating the Effect of Land Use and Sampling Depth on Ground-Water Quality, Long Island, New York

Abstract

The effect of nonpoint-source contamination was statistically evaluated through a comparison of water-quality data collected from two depth intervals at 153 wells in five areas of differing land use on Long Island. Each area was delineated to represent a predominant land use; the areas were categorized as (1) suburban with long-term sewering, (2) suburban with recent sewering, (3) suburban without sewers, (4) agricultural, and (5) undeveloped. The depth zones were delineated on the basis of estimated traveltime of ground water along vertical flow paths from the water table to each well screen's midpoint. Wells were classified as shallow (estimated traveltimes of less than 10 years) or intermediate (estimated traveltimes between 10 and 100 years).

Concentrations of several inorganic constituents and values of field properties were found to differ significantly among land-use areas and depth zones. Median constituent concentrations tend to be (1) highest, and the concentration ranges the widest, in samples from the agricultural area, (2) lowest, and the concentration ranges the smallest, in samples from the undeveloped area, and (3) intermediate to high in samples from the suburban areas. Volatile organic compounds (VOC's) were detected only in the suburban areas. A common source of nonpoint-source contamination in agricultural and residential areas is fertilizers used on commercial crops, lawns, gardens, and golf courses. Other sources of inorganic contaminants and VOC's in residential areas include (1) effluent from cesspools, septic tanks, and leaking sewers, (2) road-deicing salts, and (3) runoff contaminated by road residues and by chemicals commonly used at industrial and commercial facilities. A decrease in concentrations of most inorganic constituents with depth is attributed to (1) physical and chemical reactions that remove constituents from solution, and (2) dilution by advection and hydrodynamic dispersion along flow paths.