The objectives of this study are to:
1) determine the cation exchange capacity and sorption distribution coefficient of hydrophobic organic compounds for Long Island glacial sediments and
2) evaluate the importance of sediment coatings on the cation exchange and sorption capacity of sediments.
Ground water is the sole source of drinking water on Long Island. Thus it is important to understand how cations and near surface contaminants in aqueous solution may interact with sediments in the unsaturated zone as they infiltrate downward to the ground water. The transport of cations (e.g. metal ions, and ammonium ion), and hydrophobic organic compounds in aqueous solution through sediments is affected by sorption to sediments (Holmen and Gscwend, 1997; Coston et al, 1995; Wood et al, 1990; Ryan and Gschwend, 1992). As a result the elements or compounds may be retarded. Also ions in solution may exchange with ions sorbed onto sediments freeing the sorbed ions (Appelo and Postma, 1993, Stumm and Morgan, 1996). The extent of retardation and exchange depends on the nature of the sediments, the chemicals in solution and chemicals already sorbed on the sediments.
The surface sediments of Long Island are dominantly glacial sand consisting of 95% quartz. The sorption capacity of quartz is low but most sand particles are covered with iron stained coatings, which is typical of coastal plain sediments in northeastern USA (Holmen and Gschwend, 1997; Coston et al, 1995; Wood et al, 1990; Ryan and Gschwend, 1992). These iron-stained coatings have been shown to control the sorption capacity of the sediments (Coston et.al, 1995).
Studies of coated sand in environments similar to those of Long Island were conducted in Massachusetts (Coston et al, 1995 and Wood et al, 1990) and in Central New Jersey (Ryan and Gschwend, 1992). The coatings consist of clay minerals (kaolinite, illite), iron hydroxides and organic material. Coston et al (1995) suggested an authigenic origin for coatings in glacial sediments in Cape Cod, that is the minerals formed in place. Ryan and Gschwend (1992) suggested an illuvial origin for coatings on sands in New Jersey, that is the particles were transported to the site and accumulated on the grains.
It is important to know the composition of coatings, because different materials have different cation exchange capacity and surface area. The proportion of the different minerals and organic matter in the coatings determines the cation exchange capacity and the sorption distribution coefficient of the coatings. For example, organic material is the major sorbent for hydrophobic organic compounds (HOC) that also has high cation exchange capacity.
A multi-method approach was used in this study to characterize the coatings. The coated grains were examined with Scanning Electron Microscopy (SEM) to characterize their morphology. X-ray diffraction analysis (XRD) was used to identify the clay mineral assemblage. Transmission Electron Microscopy (TEM) was used to identify the chemical and mineralogical composition of the clays. Total organic carbon concentration (TOC) and surface area (SA) were measured in the coatings and the whole sand.
Hydrophobic organic compounds (HOC) are common contaminants in soils and sediments (Luthy et al, 1997, Holmen and Gschwend, 1997). HOC sorption is a critical process controlling the fate of these chemicals in ground water. It is important to be able to predict the transport properties of HOC for choosing effective remediation procedures. The distribution coefficients (Kd) were measured to estimate capacity of sand to sorb HOC. Holmen and Gschwend (1997) suggested that most of the organic matter in sand is concentrated in the coatings and this organic matter is the main sorbent for HOC.
Knowing the cation exchange capacity of sediments and the distribution of the exchangeable cations in the unsaturated zone will allow modeling the behavior of cations in the soil for evaluating the effects of, for example, acid rain and nitrate contamination of groundwater. Acid rain affects northeastern USA and North Europe (Likens 1996; 1998; Larsen, 1998; Hyman 1998). As the result of acid rain deposition where H+ and Al3+ ions displaced Ca2+ and K+, the soil becomes depleted in these cations and enriched with Al and hydrogen. Plants grown on such soils suffer from lack of nutrients, low pH and high amounts of poisonous Al.
Nitrate contamination of groundwater is a problem in some heavily populated areas of Long Island where septic tanks or cesspools are used. Effluent from septic tanks or cesspools is enriched with ammonium ion (NH4+). Ammonium ion has high sorption affinity and can be preferably sorbed onto sediments surrounding a cesspool (Ceazan, 1989). Under oxidizing conditions ammonium ion (NH4+) is converted to nitrate ion (N03-); this nitrate ion then infiltrates the groundwater. Knowing the cation exchange capacity of the sediments that may be surrounding a cesspool will make it possible to evaluate the amount of nitrogen as ammonium may be absorbed around cesspools.
For this study samples were selected from a range of geologic settings and depths within the unsaturated zone. These settings include: sandy soil samples from an outwash plain, medium-coarse sand from a glacial moraine and a glacial outwash fan, and fine sand from glacial lacustrine sediment. A continuous core consisting of silt, sand and gravel was taken from a glacial stream channel to determine how the distribution of exchangeable cations (Ca, Mg, K, Ca, Na, Mn, Fe, and Al) changes with depth in one locality.
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