Environ. Sci. Technol. 1988,22,1365-1367
Application of Passive Dosimetry to the Detection of Trace Organic Contaminants in Water Francis A. DiGiano," Daniel Elliot, and David Leith Department of Environmental Sciences and Engineering, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7400
Passive dosimetry, used to monitor air pollutants, was applied to monitoring organic contaminants in water. A 5-cm-diameter dosimeter was designed and constructed. Contaminants diffused across l-cm-long, parallel diffusion channels and were adsorbed onto activated carbon. After the dosimeter was exposed for a fixed time, days to weeks, the carbon was extracted and analyzed by gas chromatography for the contaminants of interest. The concentration in the water was calculated from the contaminant mass adsorbed, exposure time, contaminant diffusion coefficient, and the physical dimensions of the dosimeter. The dosimeter was tested in tanks containing known concentrations of p-xylene or atrazine. The concentrations obtained from dosimetry agreed fairly well with those in the tank. The range of concentrations tried was 0.054-10 ppm. A modified dosimeter design was proposed to lower the detection limit or decrease the required exposure time.
Introduction Monitoring of organic contaminants in surface waters and water and waste water treatment plants is mandated by various legislation, the most recent example being the Safe Drinking Water Act Amendments of 1986. Passive dosimetry could provide a simple and easy method to monitor. It has long been used in the industrial hygiene field for monitoring exposure of workers to chemicals in the air, and it has recently been applied to measurement of contaminants in soil gas from the unsaturated zone (1). We believe passive dosimetry offers advantages over conventional grab-sampling or compositing methods used in monitoring surface waters. I t allows convenient measurement of an average concentration over a long time period, on the order of several weeks; this gives a better picture of the state of the environment than would a simple grab sample. Further, it requires no mechanical device to collect the sample or a series of samples; this makes the method inexpensive, convenient to use at remote sites, and perhaps less prone to vandalism. We report here our initial work, which includes construction of a first-generation dosimeter and measurement of xylene and atrazine at ppm and ppb levels in a laboratory experiment. The results are promising enough to warrant further development of this concept. Basis of Operation In a passive monitor, contaminants diffuse from the ambient air or water along parallel tubes that connect to an adsorbent such as activated carbon or Tenax. The contaminant concentration is assumed to be zero at the end of the diffusion tubes next to the adsorbent. After sampling, the adsorbent is extracted with a solvent or desorbed thermally and then analyzed to give the concentration of each organic contaminant of concern. The contaminant concentration, C (g/cmS), is determined by exposing the monitor for a given time, t ( s ) , extracting the adsorbent to measure the mass, M (g), of the contaminant collected, and using Fick's law C = ML/(DAt) (1) 0013-936X/88/0922-1365$01.50/0
where L (cm) is the length, A (cm2) is the total crosssectional area of the diffusion pathways, and D (cm2/s) is the molecular diffusivity of the contaminant in water calculated for each contaminant used here by standard techniques (2). Inherent in the calculation of M is knowledge of the extraction efficiency, E , of the contaminant from the sorbent, Le., M = mass extracted/E. Changes in contaminant concentration will affect the accuracy of the passive dosimeter if the changes are so rapid that the dosimeter cannot respond quickly enough. Hear1 and Manning (3) have shown that a passive dosimeter will be accurate as long as t
