Preliminary analysis of risks from a former manufactured gas plant (MGP) site revealed six media associated with potential exposure pathways: soils, air, surface water, groundwater, estuarine sediments, and aquatic biota. Contaminants of concern (COCs) include polycyclic aromatic hydrocarbons, volatile organic hydrocarbons, metals, cyanide, and PCBs. Available chemical data, including site-specific measurements and existing data from other sources (e.g., agency monitoring programs, Chesapeake Bay Program), were evaluated for potential utility in risk assessment. Where sufficient data existed, risk calculations were performed using central tendency and reasonable maximum exposure estimates. Where site-specific data were not available, risks were estimated using conservatively high default assumptions for dose and/or exposure duration. Because of the large number of potential exposure pathways and COCs, a sensitivity analysis was conducted to determine which information most influences risk assessment outcome so that any additional data collection to reduce uncertainty can be cost-effectively targeted. The sensitivity analysis utilized two types of information: (1) the impact that uncertainty in risk input values has on output risk estimates, and (2) the potential improvement in key risk input values, and consequently output values, if better site-specific data were available. A decision matrix using both quantitative and qualitative information was developed to prioritizemore » sampling strategies to minimize uncertainty in the final risk assessment.« less
Vertical flowmeter testing of a fully penetrating well, screened in a confined sand and gravel<br>aquifer, was performed to characterize the vertical variation of aquifer hydraulic conductivity.<br>Acquired data were used to design multi-level sampling without the use of packers or nested wells.<br>While slow pumping the well from above the screened interval, a Mount Sopris Instrument<br>Company, Inc. Heat Pulse vertical flowmeter was used to log the cumulative contribution to<br>vertical flow in the well at I-ft intervals. An impeller flowmeter was also tested at higher purge<br>rates, but could not measure low flows near the bottom of the well. An equation derived by Molz,<br>et al. (1989) was applied to the data to solve for hydraulic conductivity at each interval using an<br>average hydraulic conductivity calculated from slug tests. Variations of hydraulic conductivity<br>correlated closely with sedimentary strata.<br>Changes in the rate of cumulative flow versus depth were used to select discrete well sampling<br>intervals for simultaneous, multi-level sampling of the well without causing in-well mixing.<br>Peristaltic pumps and stainless steel tubing were used. Ground-water samples were analyzed for<br>organic compounds to evaluate if contaminant concentrations resulted either from the dissolution<br>of a bottom dense non-aqueous phase liquid (DNAPL) layer or preferential flow of contaminants<br>from a distal DNAPL source through coarser sediment layers.
Cyanide, an EPA priority pollutant and target analyte, is typically measured as total. However, cyanide complexation, information which is not acquired through total cyanide analysis, is often a driver of cyanide toxicity and treatability. A case study of a former manufacture gas plant (MGP) is used to demonstrate the usability of various cyanide analytical methods for risk and treatability assessments. Several analytical methods, including cyanide amenable to chlorination and weak acid dissociable cyanide help test the degree of cyanide complexation. Generally, free or uncomplexed cyanide is more biologically available, toxic, and reactive than complexed cyanide. Extensive site testing has shown that free and weakly dissociable cyanide composes only a small fraction of total cyanide as would be expected from the literature, and that risk assessment will be more realistic considering cyanide form. Likewise, aqueous treatment for cyanide can be properly tested if cyanide form is accounted for. Weak acid dissociable cyanide analyses proved to be the most reliable (and potentially acceptable) cyanide method, as well as represent the most toxic and reactive cyanide forms.
This report presents the results of an extensive literature review that was performed to assess the overall effectiveness, applicability, and limitations of the various in situ technologies currently being applied to remediate sites contaminated by petroleum hydrocarbons. Of 17 technologies that were identified in an initial review and database search, nine were selected as widely used or promising for increased future use: soil vapor extraction, bioventing, pump and treat, aquifer air sparging, biosparging, in situ enhanced aquifer bioremediation, natural attenuation, in-well aeration, and dual-phase extraction. Following a general discussion of in situ technology, the report devotes one chapter to each of these nine technologies, presenting in each chapter a description of the technology; criteria to be used in considering applicability of the technology at a site; a discussion of design criteria and operating conditions; a strategy for monitoring remediation and determining when clean-up criteria are met; a discussion of performance-related issues; documented case studies; and a hypothetical application of the technology. Report appendices provide an overview of petroleum hydrocarbon constituents and their properties, and a glossary of terms.
