Environ. Sci. Technol. 2003, 37, 1360-1364
Field Trial of Contaminant Groundwater Monitoring: Comparing Time-Integrating Ceramic Dosimeters and Conventional Water Sampling HOLGER MARTIN,* BRADLEY M. PATTERSON, AND GREG B. DAVIS CSIRO Land and Water, Private Bag No. 5, Wembley, Western Australia, 6913, Australia PETER GRATHWOHL Center for Applied Geosciences (ZAG), University of Tu ¨ bingen, Sigwartstrasse 10, 72076 Tu ¨ bingen, Germany
Passive sampling is a technology that is gaining more and more importance in the field of environmental monitoring. Sampling with ceramic dosimeters (passive samplers) is a new method which is being developed for long-term, timeintegrated monitoring of organic pollutants in groundwater. The concept of time-integrated concentration measurements can theoretically be used for contaminant monitoring in groundwater, rivers, lakes, wastewater sewers, and so on, and can be used to quantify exposures to contaminants for a range of contaminants such as polycyclic aromatic hydrocarbons (PAHs) and volatile aromatic compounds such as benzene, toluene, and xylenes (BTEX), and volatile chlorinated hydrocarbons. This paper presents (1) results from laboratory tests for recovery rates for the extraction of analytes from the adsorbent material Dowex Optipore L-493; (2) tests on the long-term stability of adsorbed BTEX and naphthalenes in ceramic dosimeters; and (3) results from field tests in groundwater wells comparing contaminant (BTEX) concentrations detected with ceramic dosimeters to concentrations determined from conventional pumped groundwater samples over different overlapping time periods. These results demonstrate that ceramic dosimeters are suitable devices for monitoring aqueous contaminant concentrations over long time periods without the artifacts that may arise from pumping, handling, and storing water samples.
Introduction In conventional water sampling, e.g. pumped groundwater or surface water samples, only the momentary concentration of contaminants is detected. To monitor the contaminant concentration over a longer time period, all the variations have to be measured and their average must be found. When taking only a few water samples over a long time period the concentrations detected might represent only short-term extreme values (too high or too low), which would then lead to significant interpretation errors (1). In addition to this, groundwater sampling with pumps leads to a change in the * Corresponding author phone: 61-8-9333 6297; fax: 61-8-9333 6211; e-mail:
[email protected]. 1360
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hydraulic flow field, which could cause an unwanted distortion of the contaminant concentration. For volatile compounds, volatilisation during the sampling procedure may lead to a mass loss and therefore lower contaminant concentrations in the water samples. Sorption of organic contaminants to tubing or accidental sampling of contaminated fine-grained material are other important artifacts. There is a variety of different passive or semi-passive sampling systems for diverse applications (2). Other researchers (3, 4) have proposed passive samplers for the monitoring of surface water and seawater. Polyethylene membranes filled with triolein (semipermeable membrane devices) have been widely used to monitor e.g. PAHs (5, 6), polychlorinated biphenyls (PCBs) (7), and PAHs in groundwater (8). Models describing the contaminant accumulation for this type of passive sampler were developed (5, 9). Polyethylene diffusion samplers filled with water or air have also been used for the monitoring of volatile organic compounds (10). Other passive samplers were proposed for soil air monitoring (11). A system of passive multilevel sampling for ionic compounds in groundwater is commercially available (12). There are also passive sampling systems (i.e., diffusive gradients in thin films, DGT) for inorganic compounds (13-19). Diffusion cell systems that make use of low-flow purging of membrane systems have also been developed for monitoring volatile organic compounds in soil and groundwater (20), and these have been further developed and automated (21). The main advantage of the new passive sampling method presented in this paper compared to conventional pumped groundwater samples is the time-integrated monitoring during a sampling period which may extend over several months. This reduces the number of required analyses to only a few, which leads to a reduction in costs and avoids sources of error that often occur during sampling, transportation, or storage of a conventional water sample. It works qualitatively and quantitatively. Advantages of the ceramic material are that it is inert and that it does not swell in contact with organic compounds in contrast to some polymer membranes used in other passive sampling devices. The new passive sampling system described here is operated in a manner analagous to that of dosimeters and relies on a ceramic membrane to control diffusive fluxes of contaminants from water into the device, which allows the calibration of the system and thereby enables its application for quantitative contaminant monitoring. Ceramic dosimeters (22) monitor the contaminant concentrations (representative mean values) over the whole sampling period, enabling the determination of the integrated dose and exposure to a receptor of contaminants over a specified time interval, e.g. for water discharging into a river, rather than an indicative once-off contaminant concentration measured by conventional means. The time-integrated monitoring of organic contaminants, e.g. in groundwater (as shown in this study), by ceramic dosimeters that are installed in sampling wells and continuously accumulate contaminants from the contact water onto a suitable adsorbent material, allows the determination of the average concentrations over the sampling period. The aim of this paper is to evaluate the potential of ceramic dosimeters as suitable devices for monitoring organic contaminants in groundwater by testing their performance in the laboratory as well as by comparing field results determined using ceramic dosimeters with conventional monitoring. 10.1021/es026067z CCC: $25.00
2003 American Chemical Society Published on Web 02/11/2003
De ) DW‚m
FIGURE 1. Dosimeter design and cross section through a ceramic dosimeter (10 mm o.d., 1.5 mm wall thickness) filled with adsorbent material with second cap lying on ceramic dosimeter.
FIGURE 2. Concentration gradients across the ceramic membrane into the adsorbent bed.
Materials and Methods Setup of the Ceramic Dosimeters. Figure 1 shows the setup of the ceramic dosimeters developed in this study which consist of a ceramic tube (1 cm o.d., 5 cm length, with 5 nm pore size, USF Schumacher, Crailsheim, Germany) that is filled with water-saturated adsorbent material and closed with a Teflon cap at each end. The contaminants accumulate by diffusing from the contact water through the ceramic membrane into the adsorbent bed. There the contaminants accumulate linearly with time, depending on the concentration gradient and the mass transfer coefficient across the ceramic membrane (effective diffusion coefficient/membrane thickness ∆x). During the monitoring period the contaminant diffusion rate through the membrane occurs at a quasi-steady state, and the concentration inside the dosimeter is close to zero because of the adsorption of the solutes by the adsorbent material (Figure 2). The accumulated mass (M) which has diffused through the membrane into the adsorbent bed can be calculated from Fick’s first law:
M ) F‚A‚t ) De
∆C ‚A‚t ∆x
(1)
where F is the mass flux through the ceramic membrane [M t-1 L-2], A is the membrane surface area [L2], t is the sampling time [t], and ∆C/∆x is the concentration gradient across the membrane. Because of variations of concentrations in the water, quasi-steady-state conditions exist and therefore timeintegrated average concentration gradients were used. De is the effective diffusion coefficient, which is given by the following:
(2)
where DW is the diffusion coefficient in water [L2 t-1], is the porosity of the ceramic material, and m is Archie’s law exponent, respectively. The average contaminant concentration in the water which has been in contact with the ceramic dosimeter can be calculated from the accumulated mass of contaminants during the sampling period and is equal to ∆C as long as the concentration in the water inside the ceramic dosimeter is being kept close to zero. The mass uptake is limited by the diffusion distance which is specified by the thickness of the water-saturated ceramic membrane. This leads to a low mass flux and therefore enables an extended monitoring period. For the monitoring of very low water concentrations, the surface area can be increased by using longer ceramic tubes leading to higher uptake and allowing for shorter monitoring periods if desired. Potentially the most suitable, long-term, flux-controlling membranes are porous ceramic materials that are commercially available as disks or tubes of different (wall) thickness, pore size, and porosity. These ceramic membranes allow high diffusive fluxes of the solute due to their high porosity. A further advantage is that they are practically inert and do not adsorb organic solutes (which would lead to a long time-lag of diffusion before reaching a steady-state flux across the membrane) and they do not swell, in contrast to some organic polymers. Also, the very small pore sizes of the ceramic material (5 nm) protect the interior of the sampling device from contamination by microorganisms, thereby preventing biodegradation of the analyte(s). Adsorbent Material. To keep the concentration gradient at a maximum during the whole monitoring period (thus guaranteeing linear uptake during the long-term operation of ceramic dosimeters) it is important to use an adsorbent material with a high sorption capacity. On the other hand, for analysis the extraction of the adsorbent with a simple method and high recovery rates is desirable. The adsorbent material Dowex Optipore L-493 (Supelco, Bellefonte, PA) has been identified as a suitable adsorbent for BTEX (benzene, toluene, ethylbenzene, and the xylene isomers) and naphthalene and alkylnaphthalenes. It is easily wetted by water without swelling and shows high accumulation rates as well as high recovery rates during solvent extraction of these hydrophobic organic compounds. It is pre-cleaned with acetone and water and then packed into the ceramic tubes. Therefore, the ceramic dosimeters contain a water-saturated bed of adsorbent material. Conventional Water Analysis. Water samples were collected using glass syringes and nylon tubing. The nylon tubes were kept short in order to prevent losses due to sorption and to keep the purge volume low. The volume of the tubing was purged several times before taking the samples. A 10mL sample was acidified with 10 drops of 1 N HCl and spiked with 10 µL of surrogate standard solution (10.9 µg of 1,2dibromoethane, 4.3 µg of p-xylene-d10, and 5 µg of naphthalene-d8 in 10 µL of methanol; C/D/N Isotopes, Quebec). The samples were then extracted with 3 mL of diethyl ether by shaking for 30 min. Extraction of the Adsorbent Material. After sampling, the adsorbent material was removed from the ceramic dosimeter and extracted with 2 × 10 mL of acetone (Rowe Scientific Pty Ltd, Perth, Australia). The adsorbent material was initially washed out of the dosimeter into a glass vial with 10 mL of acetone and left for 24 h before filtering over a glass column containing a sintered glass filter and washing with a second 10-mL portion of acetone. Prior to analysis, acetone solutions were spiked with an internal standard (1,2dibromoethane, p-xylene-d10 and naphthalene-d8 in methanol) and dried with 5 g of anhydrous sodium sulfate. VOL. 37, NO. 7, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Extraction Efficiency Tests. To test the efficiency of this extraction method, a 0.4-g sample of Dowex Optipore L-493 was added to 10 mL of water and then the mixture was spiked with a standard containing approximately 100 µg each of benzene, toluene, ethylbenzene, m-, p-, and o-xylene, 1,3,5trimethylbenzene, naphthalene, 2-methylnaphthalene, 1methylnaphthalene, trichloroethene (TCE), and tetrachloroethene (PCE) in methanol. These experiments were conducted in triplicate and one additional sample was treated as a blank. After two weeks the water was removed with a pipet and extracted with diethyl ether using the conventional water sample extraction method described above. The adsorbent material was extracted with acetone as described above. To assess the extraction efficiency, triplicate control samples were prepared consisting of the same amount of standard injected directly into 20 mL of acetone. Analysis. BTEX analyses were performed using a Varian Star 3400 CX gas chromatograph equipped with a Varian Saturn 3D mass selective detector. Splitless injections of 1 µL were made with a Varian 8200 CX autosampler. A 30-m BPX5 crossbonded capillary column (SGE, Australia) of 0.25 mm i.d. and 1 µm film thickness was used. Helium (ultrahigh purity) was used as the carrier gas at 7.5 psi. The oven temperature program was 40 °C for 1 min, 5 °C/min to 60 °C, 15 °C/min to 280 °C, and 280 °C for 5.34 min. The data were acquired by scanning with scan times of 0.75 s and a mass range of 50-320 m/z and quantified using selective ion monitoring (SIM). Background Contamination. To assess background contamination, ceramic dosimeters not exposed to the contaminated water (blank) were extracted and analyzed. The results showed no significant background contamination for the naphthalenes but some background contamination was found for the BTEX, especially benzene. Consistent levels of BTEX contamination were also observed for blank water samples. This background seems mainly due to contamination in the solvents. Therefore, during the experiments blank ceramic dosimeters and duplicate blank water samples were analyzed and background concentrations were subtracted from the respective sample concentrations. Test of the Teflon Caps. It is important that the caps used to seal the ceramic tubes have a sufficiently low diffusion coefficient to the contaminants (are virtually impermeable) so uptake is controlled solely by the ceramic membrane. To check the performance of the Teflon caps, 3 ceramic dosimeters and 3 metal tubes were filled with water-saturated Dowex Optipore L-493 and sealed with Teflon caps at each end, then exposed to an aqueous solution (3 mg/L) of the compounds used in the previous extraction test. After five weeks of exposure the ceramic dosimeters and metal tubes were removed from the aqueous solution and the adsorbed contaminant was extracted with acetone and quantified by GC-MS analysis as described above. Long-Term Stability of the Ceramic Dosimeters. An advantage of the new method is that, once collected, contaminants do not significantly degrade or desorb and do not significantly diffuse out of the ceramic dosimeter back into the water when the concentration in the water declines. To validate this, loaded ceramic dosimeters containing a specified mass of the contaminants were immersed into uncontaminated water. The masses found within the ceramic dosimeters after prescribed time periods were compared with the initial concentration. In detail, twelve ceramic dosimeters were exposed for five weeks to a highly concentrated aqueous solution of the contaminants mentioned above, after which the contaminant masses sorbed in three ceramic dosimeters were measured using the procedure described above. The remaining nine ceramic dosimeters were immersed in clean water for different periods of time, up to four weeks. 1362
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TABLE 1. Recovery Rates (Percent) for the Extraction of Compounds from Triplicate Samples of the Adsorbent Material Dowex Optipore L-493
compound benzene toluene ethylbenzene m- and p-xylene o-xylene 1,3,5-trimethylbenzene naphthalene 2-methylnaphthalene 1-methylnaphthalene TCE PCE
recovery rates for replicate samples (%) 1 2 3 98 100 94 97 101 90 100 106 100 96 94
89 91 92 93 91 82 85 106 99 90 94
91 90 82 90 82 90 81 90 86 116 83
average standard (%) deviation 93 93 89 93 91 87 89 100 95 101 90
4.7 5.5 6.2 3.3 9.2 4.4 9.9 9.2 7.9 13 6.4
Field Tests. Several dosimeters were tested in two groundwater wells at a field site situated on the foreshore of the Canning River, approximately 6 km south-southwest of the city center of Perth, Western Australia. Groundwater at the site is contaminated with dissolved-phase petroleum hydrocarbons, including the BTEX compounds and polycyclic aromatic hydrocarbons (naphthalene and methylnaphthalenes were investigated). These contaminants originate from light nonaqueous-phase liquid (LNAPL) petroleum hydrocarbons released from underground storage tanks approximately 70 m from the river’s edge. The hydrogeology at the site is dominated by highly permeable alluvial sands of Quaternary age. The sediments comprise well-sorted, mediumgrained (effective grain size of 1 mm), well-rounded quartz and shell grains. BTEX-contaminated groundwater discharges into the tidally and seasonally forced Swan-Canning estuarine system on the Swan Coastal Plain (23). The field site was selected because of significant, previously monitored, variations in concentrations with time, partly due to tidal movements. Ceramic dosimeters were placed in two wells (wells A and B, screened 10 cm above the bottom) at 1.6 m below the average water level at the river’s edge for 1 to 3 months. For comparison, groundwater samples (10 mL) were collected weekly in triplicate during the field test period. Six ceramic dosimeters were exposed over three months in well A. In well B, triplicate ceramic dosimeters were changed every month, another three remained for the first two months, and yet another three remained for the whole period of three months. The aim was to check both the reproducibility and the effects that different sampling time periods had on the results. The ceramic dosimeters were extracted using the procedure described above. In the field test the ceramic dosimeters should take into account all the variations during the test period and therefore detect the average concentrations.
Results and Discussion Extraction Efficiency Tests. The extraction test showed (Table 1) high recovery rates from Dowex Optipore L-493 for all the compounds, suggesting the simple acetone extraction method would be suitable for further application of the ceramic dosimeters. No significant concentrations were found in the supernatant water due to the high sorption capacity of the adsorbent material. Test of the Teflon Caps. The tests of the Teflon caps used as seals of impermeable metal tubes showed concentrations similar to background concentrations with masses found on the adsorbent material in the metal tubes of
