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Title: WRRCTR No.178 Hydrogeologic characteristics of subsoil and saprolite and their relation to contaminant transport central Oahu, Hawaii 
Author: Miller, Mark E.; Green, Richard E.; Peterson, Frank L.; Jones, Rollin C.; Loague, Keith
Date: 1988-06
Publisher: Water Resources Research Center, University of Hawaii at Manoa
Citation: Miller ME, Green RE, Peterson FL, Jones RC, Loague K. 1988. Hydrogeologic characteristics of subsoil and saprolite and their relation to contaminant transport central Oahu, Hawaii. Honolulu (HI): Water Resources Research Center, University of Hawaii at Manoa. WRRC technical report, 178.
Abstract: A detailed characterization was made of soil and saprolite core samples taken from seven locations in central Oahu pineapple fields. Groundwater in the area is contaminated with trace amounts of the soil fumigants EDB, DBCP, and TCP. Samples were collected to a depth of 30m in the unsaturated zone and tested for hydraulic and mineralogic properties to evaluate the geologic controls on contaminant transport from the surface soil to the groundwater. The geologic profile in the area consists of Oxisol surface soils grading into saprolite (highly weathered basalt). Soil water retention measurements on ducted on tension table and pressure plate apparatus provided data for the determination of porosity, bulk density, water-retention curves, and pore-size distributions. Hydraulic conductivity, as a function of water content and water tension, was estimated using the Marshall method. X-ray diffraction was used to determine the bulk mineralogy, and scanning electron microscopy (SEM) was used for detailed study of mineral morphology and composition and the degree of pore interconnections. The hydrogeologic conditions of the unsaturated zone were found to be extremely heterogeneous relative to hydrologic and mineralogic properties. Porosities were high (0.455-0.723), but generally over half of the pore space consisted of micropores. While not contributing much to water flow, the significant quantity of micropore water provides a temporary reservoir for solutes and could be expected to contribute to the retardation of pesticide movement in percolating water. The calculated saturated hydraulic conductivities ranged over five orders of magnitude (10-3-10-8 m/s). The SEM results suggested that preferential flow was occurring in channels between macropores and along joints. The mineralogy results showed the most intense weathering at shallower depths, with primarily Fe and Ti oxides occurring to a depth of 6 to 12m, below which were mainly clay minerals. The highly weathered upperzone had lost the parent basalt structure and showed lower conductivities. The presence of kaolinite and halloysite closely correlated with pesticide residues in the subsurface; these clay minerals might have been entrapping the pesticides within their tubular structure. These residues are suspected to be weakly bound and might easily leach downward to the groundwater. The future impact of the residual pesticides on groundwater quality will depend on the rate of release of the entrapped pesticides and the degree of dilution by infiltrating water.
Series/Report No.: WRRC Technical Report
Description: Director's Office
Pages/Duration: viii + 76 pages
Keywords: geohydrology, mineralogy, soil physical properties, X-ray diffraction, groundwater contamination, fumigants, EDB, BDCP, TCP, Pearl Harbor aquifer, saprolite, halloysite, scanning electron microscopy, Oahu, Hawaii
LC Subject Headings: Groundwater -- Pollution -- Hawaii -- Oahu.
Hydrogeology -- Hawaii -- Oahu.
Saprolites -- Hawaii -- Oahu.
Soils -- Hawaii -- Oahu.

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