University of Nebraska, Lincoln, NE, 68583-0915, 402-472-1503
Increased public awareness of ground water contamination by industrial and agricultural chemicals has created increased attention on the issue of solute movement through soils. This heightened awareness is piqued when models underestimate the transport rate of compounds in question. Most field solute-transport models use a simplified form of the convective-dispersion equation. Attempts to estimate retardation factors and dispersion coefficients used in convective-dispersion equations are complicated by the spatially variable values obtained in structured soils. Additionally, attempts to model field-scale solute transport are usually expensive and exacerbated by climatic conditions not in the control of the researcher. Alternatively, when large cores are used in transport studies, considerable numbers of samples are often generated. In this study we constructed an automated sampling system for large soil columns and compared results to a continuous sampling method (vacuum chamber and fraction collector). Our objectives were to determine if there were statistical differences between the two methods and demonstrate the advantages of the automated sampling system. The automated system consisted of three solenoid values that periodically switched column effluent from a waste receptacle to a sampling vial. Intact soil columns were obtained from the field by slowly pushing a polyvinyl chloride (PVC) tube (0.15 m ID) into the ground and extracting the core. The pipe with soil was secured at both ends using acrylic caps, with the bottom cap sealed with O-rings. A constant 23 kPa suction was applied to the bottom of the column. Columns receive approximately one pore volume 3 mM CaCl2-3H20 at a constant pore water velocity. Results indicated apex concentrations (C/C0) were between 0.64 and 0.68 of initial pulse concentrations of 4000 dpm ml-1; similar breakthrough curves were observed from both sampling methods. These results indicate the automated collection system can be effectively used in large column transport studies and provides additional advantages by reducing sample size (numbers) and time required for sample collection.
ground water quality, solute transport
This paper is from the Proceedings of the 10th Annual Conference on Hazardous Waste Research 1995, published in hard copy and on the Web by the Great Plains/Rocky Mountain Hazardous Substance Research Center.