GC/MS Nontarget Analysis to Examine an Organic Groundwater Contamination. Part II: Graphical and Multivariate Methods for Searching Key Substances

GC/MS Nontarget Analysis to Examine an Organic Groundwater Contamination. Part II: Graphical and Multivariate Methods for Searching Key Substances In the nontarget analysis, a maximum of organic substances was extracted by a sequence which separates high polar, medium polar, and nonpolar compounds. This leads to the detection of 477 different organic substances in the example of the groundwater contamination investigated. To reduce the high expense for an exact identification of 477 compounds as a first step the individual compound is defined as a data set of retention time and the mass spectrum belonging to this retention time. The table of data contains now 477 individual compounds in groundwater samples collected at 10 different locations. The application of mathematical filters helps to reduce the size of the data set. Graphical methods enable large amounts of data to be visualized in a clear manner and enable to detect patterns in a data set. These patterns are the key to select typical compounds as indicator substances for the contamination source as well as the geogenic background. Similarities between the groundwater samples should not be changed by selection of the indicator compounds. Therefore, cluster analysis was applied as a controlling instrument for the final selection of the indicator compounds. The combination of graphical and multivariate data analysis is a useful tool to deduce indicator compounds for monitoring and control of complex environmental pollution states.     

Emissions of Trichloroethylene from Bedroom Furniture Set as a Source of Indoor Air Contamination

Shallow trichloroethene (TCE) groundwater and soil contamination associated with a Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) Superfund Site in Michigan resulted in a vapor intrusion (VI) investigation of overlying condominium units. Units with data suggesting a complete VI pathway received subslab depressurization systems (SSDs). Performance monitoring following the installation of an SSD at one unit indicated that the indoor air TCE concentrations remained elevated, despite pressure field extension tests that showed the system should effectively reduce VI from soil gas. Therefore, a cost-efficient and incremental investigation was launched to identify other potential source(s) of TCE using a field-portable gas chromatograph/mass spectrometer (GS/MS). The combination of room-by-room air sampling, potential VI entry point sampling, and emission tests of potential sources were used, which resulted in successfully identifying a bedroom furniture set as an indoor source of TCE for the unit. Although many common household products are recognized as indoor sources of TCE, emissions from finished furniture products have not been widely discussed in the VI literature. The findings of this study indicate that off gassing from furniture can lead to TCE concentrations in indoor air that exceed regulatory guidelines.     

Emissions of 1,2-Dichloroethane from Holiday Decorations as a Source of Indoor Air Contamination

Groundwater contamination associated with an industrial facility in Utah has led to concerns about potential vapor intrusion into residences outside the facility boundary. Trichloroethylene (TCE) is the main contaminant of concern with 1,2-dichloroethane (1,2-DCA) present in some areas. An air-monitoring program implemented to detect vapor intrusion of these compounds found 1,2-DCA in homes outside areas of groundwater contamination, suggesting indoor sources in these cases. Investigative indoor air and product sampling were conducted to isolate consumer products emitting 1,2-DCA and to quantify the emission rates of identified products. The combination of room-by-room air sampling and emission measurements was successfully used to identify molded plastic holiday ornaments, having measured emission rates as high as 0.3 µg 1,2-DCA/min. Subsequent testing of seven comparable retail items found similar 1,2-DCA emissions. Screening-level calculations show that the measured emission rates of 1,2-DCA from these items can lead to indoor concentrations high enough to be of regulatory concern (0.094 to 9.4 µg/m³ based on 10^–6 to 10^–4 cancer risk levels).     

Use of an Artificial Sweetener to Identify Sources of Groundwater Nitrate Contamination

The artificial sweetener acesulfame (ACE) is a potentially useful tracer of waste water contamination in groundwater. In this study, ACE concentrations were measured in waste water and impacted groundwater at 12 septic system sites in Ontario, Canada. All samples of septic tank effluent (n = 37) had ACE >6 µg/L, all samples of groundwater from the proximal plume zones (n = 93) had ACE >1 µg/L and, almost all samples from the distal plume zones had ACE >2 µg/L. Mean mass ratios of total inorganic nitrogen/ACE at the 12 sites ranged from 680 to 3500 for the tank and proximal plume samples. At five sites, decreasing ratio values in the distal zones indicated nitrogen attenuation. These ratios were applied to three aquifers in Canada that are nitrate‐stressed and an urban stream where septic systems are present nearby to estimate the amount of waste water nitrate contamination. At the three aquifer locations that are agricultural, low ACE values (<0.02‐0.15 µg/L) indicated that waste water contributed <15% of the nitrate in most samples. In groundwater discharging to the urban stream, much higher ACE values (0.2‐11 µg/L) indicated that waste water was the likely source of >50% of the nitrate in most samples. This study confirms that ACE is a powerful tracer and demonstrates its use as a diagnostic tool for establishing whether waste water is a significant contributor to groundwater contamination or not.     

Identification of Multiple Sources of Groundwater Contamination by Dual Isotopes

Chlorinated solvents are one of the most commonly detected groundwater contaminants in industrial areas. Identification of polluters and allocation of contaminant sources are important concerns in the evaluation of complex subsurface contamination with multiple sources. In recent years, compound‐specific isotope analyses (CSIA) have been employed to discriminate among different contaminant sources and to better understand the fate of contaminants in field‐site studies. In this study, the usefulness of dual isotopes (carbon and chlorine) was shown in assessments of groundwater contamination at an industrial complex in Wonju, Korea, where groundwater contamination with chlorinated solvents such as trichloroethene (TCE) and carbon tetrachloride (CT) was observed. In November 2009, the detected TCE concentrations at the study site ranged between nondetected and 10,066 µg/L, and the CT concentrations ranged between nondetected and 985 µg/L. In the upgradient area, TCE and CT metabolites were detected, whereas only TCE metabolites were detected in the downgradient area. The study revealed the presence of separate small but concentrated TCE pockets in the downgradient area, suggesting the possibility of multiple contaminant sources that created multiple comingling plumes. Furthermore, the variation of the isotopic (δ¹³C and δ³⁷Cl) TCE values between the upgradient and downgradient areas lends support to the idea of multiple contamination sources even in the presence of detectable biodegradation. This case study found it useful to apply a spatial distribution of contaminants coupled with their dual isotopic values for evaluation of the contaminated sites and identification of the presence of multiple sources in the study area.     

Modeling Volatile Chemical Transport, Biodecay, and Emission to Indoor Air

A model is presented for estimating vapor concentrations in buildings because of volatilization from soil contaminated by non- aqueous phase liquids (NAPL) or from dissolved contaminants in ground water. The model considers source depletion, diffusive- dispersive transport of the contaminant of concern (COC) and of oxygen and oxygen-limited COC biodecay. Diffusive-advective transport through foundations and vapor losses caused by foundation cross-flow are considered. Competitive oxygen use by various species is assumed to be proportional to the product of the average dissolved-phase species concentration and a biopreference factor. Laboratory and field data indicate the biopreference factor to be proportional to the organic carbon partition coefficient for the fuel hydrocarbons studied. Predicted indoor air concentrations were sensitive to soil type and subbase permeability. Lower concentrations were predicted for buildings with shallow foundations caused by flushing of contaminants by cross-flow. NAPL source depletion had a large impact on average exposure concentration. Barometric pumping had a minor effect on indoor air emissions for the conditions studied. Risk-based soil cleanup levels were much lower when biodecay was considered because of the existence of a threshold source concentration below which no emissions occur. Computed cleanup levels at NAPL-contaminated sites were strongly dependent on total petroleum hydrocarbon (TPH) content and COC soil concentration. The model was applied to two field sites with gasoline-contaminated ground water. Confidence limits of predicted indoor air concentrations spanned approximately two orders of magnitude considering uncertainty in model parameters. Measured contaminant concentrations in indoor air were within model-predicted confidence limits.     

Containment of Sources of Groundwater Contamination: Analysis of Mass Fluxes

Cutoff walls and liners are used frequently as barriers to isolate contaminants at both controlled and uncontrolled hazardous waste sites. Neville and Andrews (2006) presented a containment criterion for contaminant isolation by a barrier. The analysis of Neville and Andrews (2006) yields the inward Darcy flux that balances the diffusive mass flux from the source so that the net mass flux is zero. A requirement of zero net mass flux may not be achievable in all situations. The analysis developed by Neville and Andrews (2006) is extended to develop straightforward expressions for the long-term mass fluxes across a barrier for any conditions. In cases where it may not be possible to satisfy a criterion of zero net mass flux, the results from an exact solution for transient solute transport are used to show how the mass fluxes evolve to their long-term values.     

Sewer Gas: An Indoor Air Source of PCE to Consider During Vapor Intrusion Investigations

The United States Environmental Protection Agency (USEPA) is finalizing its vapor intrusion guidelines. One of the important issues related to vapor intrusion is background concentrations of volatile organic compounds (VOCs) in indoor air, typically attributed to consumer products and building materials. Background concentrations can exist even in the absence of vapor intrusion and are an important consideration when conducting site assessments. In addition, the development of accurate conceptual models that depict pathways for vapor entry into buildings is important during vapor intrusion site assessments. Sewer gas, either as a contributor to background concentrations or as part of the site conceptual model, is not routinely evaluated during vapor intrusion site assessments. The research described herein identifies an instance where vapors emanating directly from a sanitary sewer pipe within a residence were determined to be a source of tetrachloroethylene (PCE) detected in indoor air. Concentrations of PCE in the bathroom range from 2.1 to 190 µg/m³ and exceed typical indoor air concentrations by orders of magnitude resulting in human health risk classified as an “Imminent Hazard” condition. The results suggest that infiltration of sewer gas resulted in PCE concentrations in indoor air that were nearly two orders of magnitude higher as compared to when infiltration of sewer gas was not known to be occurring. This previously understudied pathway whereby sewers serve as sources of PCE (and potentially other VOC) vapors is highlighted. Implications for vapor intrusion investigations are also discussed.     

A Semi‐Structured MODFLOW‐USG Model to Evaluate Local Water Sources to Wells for Decision Support

In order to better represent the configuration of the stream network and simulate local groundwater‐surface water interactions, a version of MODFLOW with refined spacing in the topmost layer was applied to a Lake Michigan Basin (LMB) regional groundwater‐flow model developed by the U.S. Geological. Regional MODFLOW models commonly use coarse grids over large areas; this coarse spacing precludes model application to local management issues (e.g., surface‐water depletion by wells) without recourse to labor‐intensive inset models. Implementation of an unstructured formulation within the MODFLOW framework (MODFLOW‐USG) allows application of regional models to address local problems. A “semi‐structured” approach (uniform lateral spacing within layers, different lateral spacing among layers) was tested using the LMB regional model. The parent 20‐layer model with uniform 5000‐foot (1524‐m) lateral spacing was converted to 4 layers with 500‐foot (152‐m) spacing in the top glacial (Quaternary) layer, where surface water features are located, overlying coarser resolution layers representing deeper deposits. This semi‐structured version of the LMB model reproduces regional flow conditions, whereas the finer resolution in the top layer improves the accuracy of the simulated response of surface water to shallow wells. One application of the semi‐structured LMB model is to provide statistical measures of the correlation between modeled inputs and the simulated amount of water that wells derive from local surface water. The relations identified in this paper serve as the basis for metamodels to predict (with uncertainty) surface‐water depletion in response to shallow pumping within and potentially beyond the modeled area, see Fienen et al. (2015a).     

Prediction of Indoor Air Quality from Soil-Gas Data at Industrial Buildings

Subslab or shallow soil-gas data are often compared with indoor air concentration data in vapor intrusion (VI) evaluations. If no indoor air data are available or confounding sources are present, or if future scenarios are considered, the soil-gas data may be used to estimate the indoor air concentrations due to VI. The typical approach in risk assessments is to use the 95th percentile values from a set of concentration data. For VI studies, however, this rarely is an option because the data sets tend to be quite small. Therefore, various guidance documents urge the use of maximum soil-gas values. This may be reasonable for small residential buildings, but can lead to very conservatively biased estimates if applied to large industrial buildings with localized areas of contamination, especially given that the sampling locations may not be randomly selected and instead are biased toward worst-case locations. By this approach, VI guidance implicitly tolerates a large percentage of false positive decision errors to minimize the number of false negative decision errors. In this paper, implications of using maximum values are discussed and illustrated with data sets from a number of large industrial buildings at various sites. An alternative approach to using maximum soil-gas values is proposed that serves to reduce the number of false positive results while controlling the number of false negatives to an acceptable level.