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
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Evaluating the Effect of Land Use and Sampling Depth on Ground-Water
Quality, Long Island, New York
by
Paul E. Stackelberg (U.S. Geological Survey, West Trenton, New
Jersey 08628)
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.
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