POROUS CUP SAMPLERS; CLEANING PROCEDURES AND POTENTIAL SAMPLE BIAS FROM TRACE ELEMENT CONTAMINATION

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Abstract
Porous cup samplers are the most commonly used devices for collecting water from the vadose zone. These samplers were originally developed for studies of major cation and nutrient concentrations in agricultural soils. As their use has expanded to investigations of trace metals and other contaminants, concern has arisen about whether originally recommended cleaning procedures are appropriate for the new applications. If the samplers are not prepared adequately before installation, the porous cup material may release contaminants into collected samples and bias study results. To address this concern, two sets of laboratory experiments were performed: one to examine a commonly used cleaning procedure for porous cups in the context of trace element contamination, and a second to evaluate the effectivness and necessity of sampler cleaning under a range of pH conditions. The first experiment entailed leaching ceramic, alundum, and Teflon porous cups with four 250-ml aliquots of 1 N HCl. Analysis of the aliquots demonstrated that the widely used cleaning method proposed by Grover and Lamborn (1979) is applicable for trace element work. In the second experiment, buffered solutions at pH 4, 6, and 8 were evacuated through cleaned porous cups, and pH 6 solutions were drawn through uncleaned cups. Amounts of contaminants leached from the cleaned samplers were comparable or less than those observed in the last aliquot of cleaning solution in the first experiment. Ceramic cups generally released lower contaminant concentrations than alundum or Teflon cups. Concentrations of contaminants released from either cleaned or uncleaned cups were generally less than 10% of concentrations found in typical well-aerated soil water. Thus, cleaned or uncleaned samplers of the types studied here will contribute significant bias only in studies of waters with very low concentrations of contaminants.

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