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FIELD-SCALE STUDIES: HOW DOES SOIL SAMPLE PRETREATMENT AFFECT REPRESENTATIVENESS ? (ABSTRACT)
Citation:
Shan, Y., M. J. Kupferle, A. Balz, S. Qi, AND M. Hardebeck. FIELD-SCALE STUDIES: HOW DOES SOIL SAMPLE PRETREATMENT AFFECT REPRESENTATIVENESS ? (ABSTRACT). The Seventh International Symposium, Orlando, FL, 06/2-5/2003.
Description:
Samples from field-scale studies are very heterogeneous and can contain large soil and rock particles. Oversize materials are often removed before chemical analysis of the soil samples because it is not practical to include these materials. Is the extracted sample representative of the experimental unit despite excluding oversize materials? This question was studied using samples from land treatment studies for a field site in Bedford, IN. This field site was contaminated by polynuclear aromatic hydrocarbon (PAH) compounds from a former manufactured gas plant.
At each sample event, twenty-four soil cores were received from the field. The cores were weighed and then sieved through a 1/4-inch screen. The oversize materials removed from samples were visually inspected and separated into two categories: "inorganic" (e.g. bricks, plastic, mortar or stone) and "organic" (tarry or asphalt-like chunks). The three categories (<1/4-inch, inorganic, and organic) were weighed. The average mass fractions for the three categories were: 74.2% <1/4-inch; 24.3% inorganic oversize, and 0.09% organic oversize. The <1/4-inch screened soil was mixed, extracted and measured using GC/MS. The total PAH concentrations over the site ranged from 330 to 3200 mg/kg ash soil. The concentration if oversize had been included was estimated using the mass fraction of the three categories, assumed concentrations of the organic and inorganic samples, and measured concentrations of the <1/4-inch soil. The concentration was lower but not significantly different from the concentration based on the <1/4-inch soil.
This estimate was confirmed by analyzing 34 samples randomly selected from the seven sampling events for the three reactors for Bedford Land Treatment Studies Year 3. Based on preliminary studies, subsampling oversize samples introduces significant errors so the whole oversize sample was extracted with methylene chloride using an orbital shaker (35 oC, 150 rpm). The PAHs in the extracts were measured using GC/FID. The total PAH concentration ranges for "inorganic" oversize are 100 - 400 mg/kg ash solids, and for "organic" oversize are 1000-5000 mg/kg ash solids. PAH concentrations per ash basis including oversize material was calculated by weighting the concentration of each category by the mass fraction. Based on this formula, PAH concentrations per ash basis including oversize material would be about 15 to 23% lower than the concentrations based on the <1/4-inch material. Since the RSD of the <1/4-inch data is often around 20%, the extracted sample is representative of the experimental unit despite excluding oversize materials.