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Groundwater quality and contaminant hydrogeology 215

BO X
Groundwater contamination by heavy metals in Nassau County, New York
6.4

The heavy metals of concern in drinking water supplies include Ni, Stratification of the deposits means that the vertical permeability is
Zn, Pb, Cu, Hg, Cd and Cr. In reducing and acidic waters, heavy met- up to five to ten times less than the horizontal permeability.
als remain mobile in groundwater; but in soils and aquifers that As part of the historical investigations in the South Farmingdale-
have a pH buffering capacity, and under oxidizing conditions, heavy Massapequa area, a number of test wells were installed in 1962
metals are readily adsorbed or exchanged by clays, oxides and other driven to depths ranging from 2 to 23 m below ground level. Water
minerals. Sources of heavy metals include, in general, the metal pro- samples were collected at depth intervals of 1.5 m by hand pump
cessing industries, particularly electroplating works with their con- during installation. The results of this investigation are shown in
centrated acidic electrolytes, and other metal surface treatment Fig. 1, and define a pollution plume that is about 1300 m long, up
processes. to 300 m wide, and as much as 21 m thick. The upper surface of the
Infiltration of metal plating wastes through disposal basins in plume is generally less than 3 m below the water table. The plume is
Nassau County on Long Island, New York, since the early 1940s has thickest along its longitudinal axis, the principal path of flow from
formed a plume of contaminated groundwater (Fig. 1). The plume the basins, and is thinnest along its east and west boundaries. The
contains elevated Cr and Cd concentrations. The area is within an plume appears to be entirely within the Upper glacial aquifer.
undulating glacial outwash plain, and there are two major hydro- Differences in chemical quality of water within the plume may
geological units: the Upper glacial aquifer of Late Pleistocene age, reflect the varying types of contamination introduced in the past.
and the Magothy aquifer of Late Cretaceous age, which supplies all In general, groundwater in the southern part of the plume reflects
local municipal water supplies. conditions prior to 1948 when extraction of Cr from the plating
The Upper glacial aquifer is between 24 and 43 m thick, with wastes, before disposal to the basins, commenced. Since the start of
a water table from 0 to 8 m below ground level. The aquifer com- Cr treatment, the maximum observed concentrations in the plume
−1
prises medium to coarse sand and lenses of fine sand and gravel. have decreased from about 40 mg L in 1949 to about 10 mg L −1

Fig. 1 Groundwater contamination by metal plating wastes, Long Island, New York. The location of the investigation area is shown in
(a) and a cross-section of the distribution of the hexavalent chromium plume in the Upper glacial aquifer in 1962, South Farmingdale-
Massapequa area, Nassau County, is illustrated in (b). After Ku (1980).