Contaminated groundwater and its associated vapor are a major concern due to the persistence of certain pollutants such as dense non-aqueous phase liquids (DNAPLs). Chlorinated hydrocarbons (CHCs) are particularly important because they constitute the major portion of DNAPLs. Although the government proceeds with remediation at contaminated sites, long-term monitoring (LTM) of these pollutants is needed not only because of their potential hazard, but also due to the reality that complete cleanup of significant DNAPL source zones has not and will not be possible.
Currently, most LTM approaches are relatively conventional, usually involving the installation and maintenance of monitoring wells, labor intensive sampling, and costly laboratory analysis. The emergence of sensitive, robust, and fast-responding ion mobility spectrometry (IMS) technology provides the opportunity to develop an in situ LTM instrument with the ability to detect, identify, and quantify CHCs in groundwater and soil vapors.
The objective of this project was to develop a prototype sensor technology, membrane-extraction (ME)-IMS, that could be used in situ or ex situ to characterize the extent of groundwater plumes, conduct compliance monitoring around waste facilities or at the leading edge of a plume, and monitor remedial actions. The IMS instrument could also be used to monitor CHCs in the vapor phase.
To achieve high reliability and low cost, three main tasks were undertaken in this project: (1) convert contaminants from liquid phase into vapor phase using a novel membrane separation; (2) achieve sensitive identification of contaminants by combining linear and nonlinear IMS; and (3) reduce LTM costs by making the sensor operation unattended and on-site, by adapting miniaturized structures, and by avoiding the use of vacuum equipment. A preliminary field test was conducted at NASA Stennis Space Center (SSC) for the membrane extraction ion-mobility groundwater monitor. Two wells, one with low trichloroethene (TCE) concentration/slow recharge rate and one with high TCE concentration/fast recharge rate, were tested with various water depths and timings.
Laboratory testing showed that the prototype sensor is capable of uniquely identifying 32 volatile organic compounds (VOCs), including TCE and tetrachloroethene (PCE). Limit of detection (LOD) for TCE is 0.37 parts per billion by volume (ppbv), well below regulatory limits. In the field test, the monitor demonstrated a clear identification of CHCs in the wells and reasonably accurate quantification. This qualitative and quantitative capability validates the proof-of-principle prototype for simultaneous sampling and analysis in a single step within a compact in-situ and stand-alone monitor. The field test did also reveal some problems associated with the current prototype stage of the monitor, practical unpredicted environments, or field inexperience.
Based on the current stage of this technology, the following key findings are summarized:
- Identification is reliable. No interference from presence of other contaminants was observed. Only minor revision, such as simultaneous measurements of reactant-ion and sample ion peaks, is needed to achieve identification stability in the field.
- Quantification is excellent in the laboratory, but not highly accurate in the field under current conditions. From laboratory experiments, the LOD can be as low as 0.37 ppbv for TCE, similar order for other chlorinated solvents. The monitor is very reliable for quantification at least in the laboratory. However, quantification was affected by both groundwater temperature and pressure in field testing experiments. The monitor should be modified to address analysis of samples under a variation of environmental conditions.
- ME-gas chromatographic (GC) differential mobility spectrometry (DMS) (ME GC/DMS) monitor may need to work with non-linear quantification curves. Since these curves are stable and reproducible, the non-linear curves should be effective for calibration.
- Dynamic range is limited. Current dynamic range for TCE detection is 0.5-2000 ppbv. In many groundwater wells, such as those at NASA SSC, the TCE concentration is higher than 2000 ppbv. A dilution mechanism will be made so that the monitor can deal with concentrations higher than 2 parts per million (ppm).
- Sensor Dimensions. Sensor configuration and dimensions should be made more flexible to enable placement under different field scenarios (e.g., 2-inch diameter groundwater wells).
The ME-IMS technology may eliminate the need for collecting and shipping samples and expensive offsite laboratory analysis, thereby reducing the cost of monitoring CHCs in groundwater.