Time-integrated monitoring of polycyclic aromatic hydrocarbons (PAHs) in groundwater using the Ceramic Dosimeter passive sampling device

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Abstract

Passive sampling relies on the uptake of contaminants into appropriate sampling devices along a diffusion gradient without using pumps or bailers. Thus, for example, in groundwater sampling, changes to flow due to pumping can be avoided. If the diffusion gradient can be maintained for extended periods, contaminants can be sampled continuously over time without any action, allowing to determine time-weighted average contaminant concentrations. We here show that the Ceramic Dosimeter, a solid receiving phase passive sampler using a ceramic membrane as sorbent container and diffusion barrier, can be used without calibration for the long-term monitoring of polycyclic aromatic hydrocarbons (PAHs) in groundwater.

Introduction

One of the most important steps in analyzing water samples for contaminants is the sampling of water itself [1]. Disturbances of water composition during sampling as well as alterations during transport and storage, all can irreversibly affect the outcome and subsequent interpretation of water analysis results. The conventional, and most common, sampling technique is the active removal or so-called snap-shot sampling, of a defined volume of water at a given time using bailers or pumps. The caveats of such an active sampling technique are well known. Contaminants can sorb to tubes and sampling containers. As well, they may be degraded and/or may decay during transport. For groundwater, in particular, disturbed flow regimes due to pumping may lead to the pulling in of clean or contaminated water from zones not intended for sampling. Pumping can also cause losses of volatile compounds [2]. For these reasons, alternatives to the common way of water sampling are being sought.

One alternative to the conventional snap-shot sampling approach is the passive water sampling. In passive sampling, contaminants are being taken up into appropriate sampling devices along a diffusion gradient without the use of pumps or bailers [3]. The diffusion gradient is established and maintained by means of a receiving sorbent with a high affinity to the analytes to be explored. If the diffusion gradient can be maintained over extended periods by providing sufficient sorption capacity, contaminants can be accumulated continuously over time and time-weighted average contaminant concentrations determined. Thus, in addition to providing undisturbed water sampling, passive approaches can be used to integrate fluctuations in contaminant concentrations over time without the need for excessive snap-shot sampling.

A variety of passive sampling devices are available for water sampling [3], [4]. However, only a few samplers have so far been applied to groundwater (e.g. [5], [6], [7], [8], [9], [10]). Among them, the Ceramic Dosimeter [11] is a time-integrative passive sampling device which is based on a ceramic tube as diffusion-limiting barrier enclosing a receiving phase that consists of solid sorbent beads. Recently, the utility of the Ceramic Dosimeter as a robust groundwater sampling device was shown for benzene, toluene, ethylbenzenes, xylenes and naphthalenes, using Dowex Optipore L-493 as the receiving phase [10]. Over up to 90 days of sampling in a contaminated aquifer, the Ceramic Dosimeters showed an excellent performance, which was judged by comparing time-weighted average contaminant concentrations derived from the Dosimeters with average aqueous concentrations determined by frequent conventional snap-shot sampling. Based on the same principle, the same group postulated the use of Amberlite IRA-743 as a solid receiving phase for the sampling of polycyclic aromatic hydrocarbons (PAHs). This is an ion exchange resin on polystyrene basis which was chosen due to its capacity for binding hydrophobic contaminants and its wettability, which is of importance for use in a water-saturated sampling device [12]. Indeed, initial laboratory and field experiments focusing largely on phenanthrene showed promise for the long-term sampling of PAHs by means of Amberlite IRA-743 in the field [12], [13], [14]. Among the important features determined in these studies were the high affinity of Amberlite IRA-743 to the model PAHs and its ability to preserve the adsorption of PAHs even when the Ceramic Dosimeters were placed in PAH-free, de-ionized water for up to 100 days [12].

The aim of the current study was to explore the ability of the Ceramic Dosimeter passive sampling device, filled with Amberlite IRA-743, to perform well under field conditions with regard to the sampling of PAHs. Over the course of 1 year, a total of 51 Ceramic Dosimeters were deployed in three groundwater boreholes at a former gas works site known to be contaminated with PAHs.

Section snippets

Passive sampler material and chemicals

The ceramic tube, caps as well as the stainless steel holder of the Ceramic Dosimeter passive sampling device were provided by IMW (Tübingen, Germany). More detailed properties of the ceramic tube can be found in Table 1. The cap material was PTFE. Caps had an inner diameter of 1 cm, thus closing the ceramic tube by a tight fit. Filled ceramic tubes were fixed in stainless steel holders of 6 cm length. Amberlite IRA-743, which was used as the receiving phase, was from Sigma–Aldrich (Steinheim,

Appearance of the Ceramic Dosimeters after field exposure

Sampling devices removed from boreholes 1 and 3 occasionally displayed a slight greyish discoloration whereas Ceramic Dosimeters obtained from borehole 2 showed compelling discolorations ranging from yellow-reddish to dark black (Fig. 1). These alterations in colour were visible from the first sampling point (1 month) on. They were attributable to a tar oil phase occurring unexpectedly in this borehole upon an operational change at the gas works site. The tar oil phase filled the borehole from

Discussion

The Ceramic Dosimeter was originally developed by Grathwohl [11] for the passive, time-integrative sampling of water or air. Two key features to this simple, small device are its robust ceramic membrane and the solid receiving phase, which has to be chosen according to the analytes under investigation. Previously, Martin et al. [10] had filled the ceramic tube with Dowex Optipore L-493 as the solid receiving phase in order to derive time-weighted average concentrations of BTEX and napthalenes

Conclusions

The Amberlite IRA-743-filled Ceramic Dosimeter proved to be a reliable device for long-term groundwater monitoring of PAHs at contaminated sites. It greatly reduces the sampling and analysis efforts compared to active snap-shot sampling and due to its robust design, does not require laborious calibration steps. Thus, the Ceramic Dosimeter can be recommended as a simple and straightforward alternative to conventional snap-shot sampling if contaminated groundwater needs to be monitored for

Acknowledgements

We would like to thank Prof. Peter Grathwohl, Dr. Holger Martin as well as Matthias Piepenbrink for helpful discussions in the course of this work. Dr. Thomas Wendel and Annegret Walz are thanked for their fantastic help in chemical analysis. This work was supported by the German Federal Ministry of Education and Research (Project RETZINA, 02WT0041).

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