HR: 1340h
AN: H53F-1489    [Abstracts]
TI: A Comparison of Soil-Water Sampling Techniques
AU: Tindall, J A
EM: jtindall@usgs.gov
AF: USGS - National Research Program, MS 413, Box 25046 Federal Center, Denver, CO 80225,
AU: Figueroa-Johnson, M
EM: envsengineer@yahoo.es
AF: Maria Figueroa-Johnson, Los Tilos 1978, Quilpue, Chi 19780, Chile
AU: * Friedel, M J
EM: mfriedel@usgs.gov
AF: USGS - National Research Program, MS 413, Box 25046 Federal Center, Denver, CO 80225,
AB: The representativeness of soil pore water extracted by suction lysimeters in ground-water monitoring studies is a problem that often confounds interpretation of measured data. Current soil water sampling techniques cannot identify the soil volume from which a pore water sample is extracted, neither macroscopic, microscopic, or preferential flowpath. This research was undertaken to compare values of extracted suction lysimeters samples from intact soil cores with samples obtained by the direct extraction methods to determine what portion of soil pore water is sampled by each method. Intact soil cores (30 centimeter (cm) diameter by 40 cm height) were extracted from two different sites - a sandy soil near Altamonte Springs, Florida and a clayey soil near Centralia in Boone County, Missouri. Isotopically labeled water (O18 - analyzed by mass spectrometry) and bromide concentrations (KBr- - measured using ion chromatography) from water samples taken by suction lysimeters was compared with samples obtained by direct extraction methods of centrifugation and azeotropic distillation. Water samples collected by direct extraction were about 0.25 ‰ more negative (depleted) than that collected by suction lysimeter values from a sandy soil and about 2-7 ‰ more negative from a well structured clayey soil. Results indicate that the majority of soil water in well-structured soil is strongly bound to soil grain surfaces and is not easily sampled by suction lysimeters. In cases where a sufficient volume of water has passed through the soil profile and displaced previous pore water, suction lysimeters will collect a representative sample of soil pore water from the sampled depth interval. It is suggested that for stable isotope studies monitoring precipitation and soil water, suction lysimeter should be installed at shallow depths (10 cm). Samples should also be coordinated with precipitation events. The data also indicate that each extraction method be use to sample a different component of soil-pore water. Centrifugation can be used with success, particularly for efficient sampling of large areas. Azeotropic distillation is more appropriate when strict qualitative and quantitative data on sorption/desorption and various types of kinetic studies may be needed.
DE: 1806 Chemistry of fresh water
DE: 1831 Groundwater quality
DE: 1866 Soil moisture
DE: 1875 Vadose zone
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: 2007 Fall Meeting