Large-scale bioventing degradation rates of petroleum hydrocarbons and determination of scale-up factors

Bioventing is a cutting edge, nondestructive treatment method that uses indigenous soil microorganisms in situ to remediate petroleum hydrocarbons in the unsaturated soil zone. Transferring the application of this technology to a field environment still has some uncertainties due to scale-up challenges. In order to identify the scale-up factor, a 80-kg soil reactor system was developed, consisting of a custom-made reactor, climate chamber, low-flow venting system, and an off-gas capture device. Sandy and clayey soils were tested with known concentrations of spiked synthetic gasoline. Various environmental conditions were monitored, which included moisture levels, pH, microbial levels, and nutrient and oxygen levels. Results show a second-stage degradation rate similar to the degradation rate obtained from research conducted with a 4-kg reactor, giving an average scale-up factor of 2.3 ± 0.4. The completed research shows that working with a 80-kg laboratory reactor is feasible, yet not always necessary for the development of scale-up factors. A complimentary study with aged soil contaminants was performed and yielded degradation rates that were significantly reduced.     

Degradation Rates for Petroleum Hydrocarbons Undergoing Bioventing at the Meso-Scale

ABSTRACT

Bioventing can be effective for the remediation of soil contaminated with petroleum hydrocarbons. However, implementing laboratory results in field scenarios is difficult due to the lack of scale-up factors. Accordingly, laboratory bioventing experiments were undertaken at the meso-scale and then compared with previously completed micro-scale tests to evaluate the important scale-up factor. The developed meso-scale system holds 4 kg of soil, with bioventing conditions controlled from a nutrient, airflow, and water content perspective. Three soils were tested, and categorized as loamy sand, silt loam, and a mixture. Results over a 30-day period showed a two-stage degradation pattern that encompassed first-order degradation rates as compared with the single-stage first-order degradation rate determined in the micro-scale study. For the first stage (0–8 days), the degradation rate for loamy soil was 0.598 day^?1, with the silty soil at 0.460 day^?1, and mixed soil at 0.477 day^?1. After 8 days, the degradation rate constant for the loamy soil dropped to 0.123 day^?1, with the silty soil dropping to 0.075 day^?1, and the degradation rate for the mixed soil dropping to 0.093 day^?1. Comparison of the measured degradation rate values with the results from the micro-scale experiments gave scale-up factors varying from 1.9 to 2.7 for the types of soil considered in the current study. These differences in degradation rates between the two scales show the importance of scale-up factors when transferring feasibility study results to the field.     

Bioaccessibility of Mercury in Soils

The initial risk assessment for the East Fork Poplar Creek (EFPC) floodplain in Oak Ridge, Tennessee, a superfund site heavily contaminated with mercury, was based on a reference dose for mercuric chloride. Mercuric chloride, however, is a soluble mercury compound not expected to be present in the floodplain, which is frequently saturated with water. Previous investigations had suggested mercury in the EFPC floodplain was less soluble and therefore potentially less bioavailable than mercuric chloride, possibly making the results of the risk assessment unduly conservative. A bioaccessibility study, designed to measure the amount of mercury available for absorption in a child's digestive tract (the most critical risk pathway endpoint), was performed on 20 soils from the EFPC floodplain. The average bioac-cessible mercury for the 20 soils was 5.3%, compared with 100% of the mercuric chloride subjected to the same conditions. The alteration of the procedure to more closely mimic conditions in the digestive tract did not significantly change the results. Therefore, the use of a reference dose for mercuric chloride at EFPC, and potentially at other mercury-contaminated sites, without incorporating a corresponding bioavailability adjustment factor may overestimate the risk posed by the site.     

Development and Placement of a Sorbent-Amended Thin Layer Sediment Cap in the Anacostia River

Incorporating materials into sediment caps that can sequester contaminants will greatly improve their ability to isolate contaminants in the underlying sediments from the rest of the aquatic environment. For highly sorptive media a thin layer (cm) may be sufficient, but accurately placing a thin layer (cm) of material over submerged contaminated sediment is difficult. A reactive core mat (RCM) was designed to accurately place a 1.25 cm thick sorbent (coke) layer in an engineered sediment cap. In April 2004, twelve 3.1 m × 31 m sections of RCM were placed in the Anacostia River, Washington, D.C., and overlain with a 15 cm layer of sand to secure it and provide a habitat for benthic organisms to colonize without compromising the integrity of the cap. Placement of the RCM did not cause significant sediment re-suspension or impact site hydrology. The RCM is an inexpensive and effective method to accurately deliver thin layers of difficult to place, high value, sorptive media into sediment caps. The approach can also be used to place granular reactive media that can degrade or mineralize contaminants.     

Development of a CERCLA Multimedia Containment Remedy within an Active Stream Channel

Under an Administrative Consent Order between a Group of PRPs (the Group) and the United States Environmental Protection Agency (USEPA), the Group was required to install a pump and treat system to mitigate ongoing chemical discharges to a stream. The site, comprised of approximately 8 acres located largely within the flood plain of a local tributary to the Chesapeake Bay, was used as a solvent recovery facility from the 1960s through the 1980s. The facility managed large numbers of drums and tanks for the storage and processing of solvents. Solvents were released to the subsurface and site studies show they leached into the soil, ground-water, and stream sediments resulting in three roughly co-equal sources of mass contribution to surface water in the     

In situ remediation of aquifers contaminated with dense nonaqueous phase liquids by chemically enhanced solubilization

Chemically enhanced solubilization (CES) is an advanced variant of pump‐andtreat that results in more effective and more rapid remediation of groundwater contaminated with organic solvents and other dense nonaqueous‐phase liquids (DNAPLs). Attempts to remediate DNAPL‐contaminated groundwater by pump‐and‐treat have generally not been successful, due to the low aqueous solubility of most DNAPLs. Regions of undissolved, organic liquids slowly release additional contamination to surrounding groundwater, in effect acting as in situ sources of contamination and hindering the progress of remediation attempts. Cleaning up an aquifer can take many decades or more of pump‐and‐treat. CES accelerates pump‐and‐treat by using surfactants at low concentration to increase the solubility of organic contaminants by up to three orders of magnitude, while maintaining hydraulic control. The surfactants are chosen to maximize contaminant solubilization while minimizing decreases in the DNAPL/ water interfacial tension in order to prevent mobilization of DNAPL to uncontaminated regions. The surfactants are also selected to be nontoxic and biodegradable (many are U.S. Food and Drug Administration‐ (FDA‐) approved food additives). After the contaminants have been solubilized, they are pumped to the surface and treated by air stripping and other methods as in traditional pump‐and‐treat operations. CES has had extensive laboratory development and is now being field tested at three sites. The first field test is at Canadian Forces Base Borden, a military facility in Ontario, Canada. The field test involves the controlled contamination of a shallow sand aquifer with approximately 240 L of tetrachloroethylene (PCE). CES increased the contaminant concentration in the extracted water to over 10,000 ppm of PCE, compared with an aqueous solubility of 200 ppm. At latest report, more than 80% of the residual PCE has been removed. A second field test is currently in preparation at a chlorinated solvent manufacturing facility in Texas and a third at a DOE site with PCE, 1,1,1‐trichloroethane (TCA), and trichloroethylene (TCE) contamination.     

A perspective on metals in soils

Soil contaminated with metals from a variety of sources can be toxic to plants and animals, including humans. The extent of contamination is often determined by comparison with the total elemental composition of an uncontaminated soil, although some leaching tests have also been proposed. In any event, knowledge of the natural background concentrations of metals at the site is required. Analysis of a control sample from the site will provide some information, but soils are still inherently variable. This review summarizes the total elemental composition of soils not known to be contaminated from anthropogenic sources. The total concentrations of 50 metals reported for these soils were shown to be log‐normally distributed, making the geometric mean (GM) and geometric standard deviation (GS) appropriate. Thus, 99.7% of the data should fall between GM/GS³ and GM*GS³. Fifteen metals were above the upper 99.7% limit and eight were below the lower limit. Most fell between the 99.9% limits of GIWGS⁴ and GM*GS⁴. The exceptions are nine metals (At. Ca, Fe, Mg, K Na, S, Si, and Ti) present at high concentrations with large GS values of 7500 to 66,400. GSs exceeding 5.0 probably indicate distributions with multiple modes. For K and Si, the arithmetic mean may be more appropriate. Available information on the concentrations of metals in relation to soil taxonomy is included, along with some specific data for soils in the Northeast. Data for the EPA target analytes are tabulated to assist regulators and others in cases where the soil is contaminated and remediation is required.