Columbus Air Force Base Research Articles

Nucleic acid analytical approaches in bioremediation: site assessment and characterization

Bioremediation, the removal of environmental pollutants by living organisms, has become a viable and promising means of restoring contaminated sites. Gene probing techniques have enhanced our ability to assess the efficacy of microbial-based remediation efforts. DNA probes targeting specific genetic sequences, i.e. those genes responsible for the degradative ability of the microorganism, can be used to characterize a contaminated site throughout the bioremediation program to determine overall community structure and catabolic activity. To do so, however, requires efficient techniques for recovering nucleic acids from environmental sites as well as methods for generating probes to the specific genetic sequences desired. This review discusses procedures for isolating DNA, messenger RNA, and ribosomal RNA from environmental samples, the utilization of polymerase chain reactions to construct gene probes, and hybridization methods to genetically match the probe to the environmental sample. The use of these methods and advancement of techniques at several bioremediation sites is also presented along with typical problems and limitations encountered. The first case study involves monitoring the effects of nutrient addition to stimulate microbial degradation of chlorinated solvents at the DOE Westinghouse Savannah River Site. The next case study describes the bioremediation of chlorinated solvents and low levels of BTEX at Dover Air Force Base, Delaware. The final study is a field-scale natural attenuation project currently underway at Columbus Air Force Base, Mississippi.

Impacts of Positive Skin Effects on Borehole Flowmeter Tests in a Heterogeneous Granular Aquifer

AbstractIn heterogeneous granular aquifers, an important issue affecting the analysis of pump test data is whether or not positive skin effects exist. Positive skin effects include head losses and changes in radial flow patterns caused by a local reduction of aquifer permeability near a well. In order to investigate the impact and formation of positive skins, field tests were performed at seven pairs of closely‐spaced wells located in the vicinity of a paleochannel at Columbus Air Force Base (CAFB), Mississippi. In the paleochannel a 1–2 m thick zone of high‐K channel lag deposits lies beneath a 2–3 m thick zone of fine‐grained channel fill deposits. Each well pair consisted of a well with and without a gravel pack. At the well pairs outside of the paleochannel boundaries, borehole flowmeter tests produce similar field results. At the well pairs inside of the paleochannel boundaries, however, borehole flowmeter tests produced very different results. Examination of the test results indicate that positive skins exist at wells without gravel packs that intersect the paleochannel. During well installations, positive skins appear to be produced by the downward mixing of the fine‐grained deposits into the channel lag deposits. Notable positive skin effects include increased specific capacities, increased total drawdowns, and changes in the distribution of horizontal flow to a pumped well. Among the key implications regarding borehole flowmeter tests in heterogeneous aquifers similar to that at CAFB are: (1) wells should be installed with gravel packs; and (2) transmissivity values should be calculated using the Cooper‐Jacob straight‐line method rather than the Cooper‐Jacob equation.

Correction to the Letter entitled “Further evidence of fractal structure in hydraulic conductivity distributions” by F. J. Molz and G. K. Boman

A format incompatibility between a data set and a computer code led to an erroneous plot and conclusion in Figure 2 of the above-referenced letter [lVlolz and Boman, 1995]. Based on rescaled range analysis it was concluded that the log(K) measurements made at the Columbus Air Force Base displayed an fGn structure in the vertical with a Hurst (H) coefficient of 0.67. Reanalysis of the correct data yielded an fBm structure with H = 0.54. The corrected Figure 2 is given below. All the other figures were re-analyzed using the modified range analysis procedure of Liu and Molz [1996] and found to be correct. If anything, the correction strengthens the conclusions of the letter and moves the analysis into more conformity with that of Newman [1990, 1994], in that

Modeling Cationic Surfactant Transport in Porous Media

AbstractLaboratory and field experiments have shown that cationic surfactants can be used to modify aquifer materials, and thereby form zones of enhanced sorption for hydrophobic organic contaminants (HOC's) migrating in ground water. Coupled to a contaminant degradation or removal process, this concept has potential as a remediation technology. In order to apply enhanced sorption in a remediation scheme, an ability to predict the transport and partitioning behavior of cationic surfactants in the subsurface is necessary. In this paper we present the results of a numerical modeling study in which the transport and partitioning behavior of the cationic surfactant hexadecyltrimethylammonium chloride (HDTMA) in porous media is investigated. Modeling of previously published batch and column HDTMA sorption experimental results for Columbus Air Force Base aquifer material indicates that, under certain conditions, kinetic effects will dominate the transport process with slow desorption of HDTMA being the likely rate‐controlling step. The results suggest that a significant departure from equilibrium will exist under natural gradient conditions for the Columbus aquifer material. Low aqueous surfactant concentrations can be expected to persist within a surfactant‐enhanced sorption zone, even after considerable flushing with surfactant‐free ground water. The low aqueous concentration may have implications in terms oftoxicityto microorganisms.

Degradation kinetics of aromatic organic solutes introduced into a heterogeneous aquifer

Degradation rates of benzene, p‐xylene, naphthalene, and o‐dichlorobenzene have been measured in a heterogeneous, unconfined aquifer during a pulse injection experiment at Columbus Air Force Base, Columbus, Mississippi. Dissolved oxygen in the pulse plume maintained aerobic conditions. Degradation kinetics calculated from the complete field data set were approximately first order with the following rate constants: benzene, 0.0070 d−1; p‐xylene, 0.0107 d−1; naphthalene, 0.0064 d−1; and o‐dichlorobenzene, 0.0046 d−1. Reaction rates were also calculated from a near‐field subset of the data using a model based on the hydrologic characteristics of the aquifer. Shapes of the degradation rate curves were consistent with microbial degradation processes. Maximum degradation rates obtained are presumed to be characteristic of the microbial population metabolism. Carbon 14‐labeled p‐xylene was included in the injection solution to permit detection of degradation products. This technique is suggested for future field experiments, because it distinguishes solute degradation from solute losses by sorption and evaporation and allows mass balance to be demonstrated throughout the course of the reaction in the aquifer.

Field study of dispersion in a heterogeneous aquifer: 3. Geostatistical analysis of hydraulic conductivity

Observations of the spatial variability of hydraulic conductivity at a tracer test site, located at Columbus Air Force Base in Mississippi, are presented. Direct measurements of hydraulic conductivity of the heterogeneous alluvial aquifer at the site were made using borehole flowmeter logging, slug tests, and a laboratory permeameter to test undisturbed soil cores. Indirect methods estimating hydraulic conductivity were also evaluated, including soil grain size analyses, surface geophysical surveys, and mapping of sediment facies. The spatial covariance of the 2187 hydraulic conductivity values obtained with the borehole flowmeter method was examined. The log hydraulic conductivity variance (σln2K) and the horizontal and vertical correlation scales (λh and λv) of 4.5, 12.8 m, and 1.6 m, respectively, were estimated assuming second‐order stationarity of the conductivity field. The covariance parameters are uncertain with bounding values that are 24–76% above or below the estimate. Covariance parameters estimated with more limited nonflowmeter data were within the same magnitude as those obtained using the extensive flowmeter data, suggesting that data from a variety of methods may be used to provide approximate values of the autocovariance parameters. Nonstationarity of the ln K field was examined by removing three‐dimensional polynomial trend surfaces and calculating variograms of the residuals. Significantly lower estimates for σln2K, λh and λv of 2.7, 4.8 m, and 0.8 m, respectively, were obtained from the third‐order log conductivity residuals. After trend removal, the bounding parameter values differ 15–44% from the estimated values. Accounting for unsteady flow and the uncertainty in the covariance parameters of the third‐order log conductivity residuals, the calculated longitudinal and horizontal transverse macrodispersivities ranged from 1.5 m to 1.8 m and 0.3 m to 0.6 m, respectively.