For mobile, landscape view is recommended.
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.
Two-dimensional display (right) of chlorinated hydrocarbons (left) predicted with micro ion mobility sensor (center)
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:
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.