Borehole Diffusive Flux Apparatus for Characterizing Diffusive Mass-transfer in Subsurface Systems.

The concept of the Borehole Diffusive Flux Apparatus (BDFA) is presented herein. The BDFA is an innovative apparatus designed to provide continuous direct access to an undisturbed column of sediment that can be monitored at multiple discrete vertical intervals to provide high-resolution characterization of local-scale mass transfer and attenuation. The conceptual basis and technical design of the device are presented, along with an example of borehole design and installation at a field site. Mathematical simulations are used to illustrate its application for two scenarios. The results of these simulations indicate that test periods of several weeks to a few months should be sufficient to obtain robust results. The device has the potential to improve our ability to characterize critical mass-transfer and attenuation processes and to quantify the associated rates. This information is key to the evaluation of remediation alternatives, for enhancing the accuracy of mathematical models, and to support more effective long-term management of large groundwater contaminant plumes present at many sites.

Transfer of Heavy Metals from Soils to Vegetables and Associated Human Health Risks at Selected Sites in Pakistan.

Contamination of the food chain with heavy metals is considered as one of the major environmental pathways of human exposure to metals leading to potential health risks. This study aimed to investigate the concentrations of heavy metals such as copper (Cu), zinc (Zn), chromium (Cr), nickel (Ni), and manganese (Mn) in agricultural soils and food crops (fruit, leaf, and root vegetables), and their associated health risks to the local population in selected southern districts of Khyber Pakhtunkhwa Province, Pakistan. The concentrations of the selected metals in soil varied over a wide range, in the following decreasing order: Mn > Zn > Cr > Ni > Cu. The bioaccumulation of metals in vegetables was within the permissible risk limits, except for Cr which showed higher contamination in all the tested food crops. The trend of metal transfer factors for different vegetables was in the order of Cu > Ni > Cr > Mn > Zn, while the calculated daily intake of metals (DIM) in adults and children through consumption of food crops was in the decreasing order of Mn > Zn > Ni > Cr > Cu. The health risk index (HRI) values for the heavy metals for both adults and children were less than 1. Therefore, no significant health risk is anticipated for the local consumers through ingestion of these food crops.

Application of phytoscreening to three hazardous waste sites in Arizona.

The great majority of prior phytoscreening applications have been conducted in humid and temperate environments wherein groundwater is relatively shallow (~1-6m deep). The objective of this research is to evaluate its use in semi-arid environments for sites with deeper groundwater (>10m). To that end, phytoscreening is applied to three chlorinated-solvent hazardous-waste sites in Arizona. Contaminant concentrations were quantifiable in tree-tissue samples collected from two of the sites (Nogales, Park-Euclid). Contaminant concentrations were detectable, but not quantifiable, for the third site. Tree-tissue concentrations of tetrachloroethene (PCE) ranged from approximately 400-5000ug/kg wet weight for burrobrush, cottonwood, palo verde, and velvet mesquite at the Nogales site. In addition to standard trunk-core samples, leaf samples were collected to test the effectiveness of a less invasive sampling method. Leaf-sample concentrations were quantifiable, but several times lower than the corresponding core-sample concentrations. Comparison of results obtained for the test sites to those reported in the literature suggest that tree species is a major factor mediating observed results. One constraint faced for the Arizona sites was the relative scarcity of mature trees available for sampling, particularly in areas adjacent to industrial zones. The results of this study illustrate that phytoscreening can be used effectively to characterize the presence of groundwater contamination for semi-arid sites with deeper groundwater.

Analysis of trichloroethene vapour in soil-gas samples using solid-sorbent tubes with gas chromatography/mass spectrometry.

A sampling method for the determination of chlorinated contaminant vapor concentrations present in the vadose zone, specifically TCE, has been developed, and was applied at the Tucson International Airport Authority (TIAA) Superfund site. The method, modified from the NIOSH Manual of Analytical Methods (NMAM) # 1022 for ambient-air sampling of TCE, is targeted to situations requiring cost effective sample collection, particularly for cases when concentrations are at or below maximum contaminant levels (MCLs). In the modified NIOSH method, TCE vapor is sampled using a solid sorbent tube. Gas Chromatography with Mass Spectrometry is used to confirm and quantify the presence of TCE. The results of laboratory tests demonstrate a maximum TCE vapor load of approximately 22 mg before breakthrough to the secondary sorbent tube section, and an extraction efficiency of approximately 97%. The results of a performance comparison test conducted in the field demonstrated that concentrations obtained with the sorbent tube samplers (~5 ug/L) were similar to those obtained with the use of standard Summa canisters (~3 ug/L). The quantitative detection limit for the new method was 0.03 ug/L under the operative conditions, a significant improvement to current regulatory analytical methods. The results indicate that use of the sorbent tube method will be effective for vapor sample collection at VOC-contaminated sites, particularly in characterizing low concentrations.

Measuring air-water interfacial area for soils using the mass balance surfactant-tracer method.

There are several methods for conducting interfacial partitioning tracer tests to measure air-water interfacial area in porous media. One such approach is the mass balance surfactant tracer method. An advantage of the mass-balance method compared to other tracer-based methods is that a single test can produce multiple interfacial area measurements over a wide range of water saturations. The mass-balance method has been used to date only for glass beads or treated quartz sand. The purpose of this research is to investigate the effectiveness and implementability of the mass-balance method for application to more complex porous media. The results indicate that interfacial areas measured with the mass-balance method are consistent with values obtained with the miscible-displacement method. This includes results for a soil, for which solid-phase adsorption was a significant component of total tracer retention.