Evaluation of bioremediation strategies of a controlled oil release in a wetland

A controlled petroleum release was conducted to evaluate bioremediation in a wetland near Houston, Texas. The 140-day study was conducted using a randomized, complete block design to test three treatments with six replicates per treatment. The three treatment strategies were inorganic nutrients, inorganic nutrients with an alternative electron acceptor, and a no-action oiled control. Samples were analyzed for petroleum chemistry and inorganic nutrients. These results are discussed in the context of our related research involving toxicology and microbiology at the site during the experiment. To evaluate biodegradation, the targeted compounds were normalized to the conservative compound C3017alpha, 21beta-[H]hopane, thus reducing the effects of spatial heterogeneity and physical transport. The two biostimulation treatments demonstrated statistically-higher rates of biodegradation than the oiled no-action control. For the majority of the experiment, target nutrient levels were maintained. Further research may be warranted to optimize these bioremediation strategies as well as evaluating additional treatment strategies for wetlands and other shoreline systems.     

Chemical dispersant effectiveness testing: influence of droplet coalescence

Thermodynamic and kinetic investigations were performed to determine the influence of coalescence of chemically dispersed crude oil droplets in saline waters. For the range of pH (4-10) and salinity (10 per thousand, 30 per thousand, 50 per thousand ) values studied, zeta-potential values ranged from -3 to -10 mV. As the interaction potential values calculated using Derjaguin-Landau-Verway-Overbeek (DLVO) theory were negative, the electrostatic barrier did not produce significant resistance to droplet coalescence. Coalescence kinetics of premixed crude oil and chemical dispersant were determined within a range of mean shear rates (Gm = 5, 10, 15, 20 s(-1)) and salinity (10 per thousand, 30 per thousand ) values. Coalescence reaction rates were modeled using Smoluchowski reaction kinetics. Measured collision efficiency values (alpha = 0.25) suggest insignificant resistance to coalescence in shear systems. Experimentally determined dispersant efficiencies (alpha = 0.35) were 10-50% lower than that predicted using a non-interacting droplet model (alpha = 0.0). Unlike other protocols in which the crude oil and dispersant are not premixed, salinity effects were not significant in this protocol. This approach allowed the effects of dispersant-oil contact efficiency eta(contact) to be separated from those of water column transport efficiency (eta(transport)) and coalescence efficiency (eta(coalescence)).     

Characterizing aquatic sediment-oil aggregates using in situ instruments

The effects of emulsified crude oil and salinity (15, 30 per thousand ) on the steady state aggregate volume distributions and fractal dimensions were determined for a range of mean velocity gradients, (G(m) =5-50 s(-1)). Aggregation was performed in a 40-L cylindrical tank with a 4-blade paddle mixer. Three-dimensional fractal dimensions (D3) and volume distributions were determined using a procedure integrating data from an electrozone and an in situ light scattering instrument. Two-dimensional fractal dimensions (D2) and derived volume distributions were determined using a recently developed submersible flow cytometer equipped with a digital camera and image analysis software. For latex beads or emulsified crude oil systems, the above listed instruments yielded consistent size distributions and fractal dimensions (D2=1.92 +/- 0.16, D3=2.94 +/- 0.12). Mean volume aggregate diameters determined using the FlowCAM were consistently larger that those determined using the LISST-100 or Coulter Multisizer due to aggregate orientations during measurements. With increasing G(m) values, all colloidal aggregates showed increasing D3 values due to reduced aggregate length. Because of the compactness of all the aggregates (D3 >2), D2 values remained constant at 2. Neither salinity nor sediment type significantly affected D2 values calculated for sediment-crude oil aggregates. However, clay-oil aggregates showed higher D3 values than clay aggregates. This suggests that colloidal oil and mixing shear are the more dominant factors influencing aggregate morphology in nearshore waters. Overall, the data suggests that the analysis methods provide consistent size distribution results. However, because of the shear and salinity of coastal waters, resulting aggregates are too compact to estimate their D3 values using image analysis alone.     

Redox dynamics of a tidally-influenced wetland on the San Jacinto River

The stage height, dissolved oxygen, and redox potential of the surface water and sediments of an intertidal Texas wetland were evaluated using continuous monitoring techniques. Surface water oxygenation was most drastically affected by photosynthesis, which served to elevate daytime oxygen concentrations compared to those measured at night. Colder temperatures affected dissolved oxygen concentrations by increasing solubility and decreasing heterotrophic consumption. Waters of the San Jacinto River were generally more oxygenated than those in the wetland, and incoming tides typically elevated dissolved oxygen levels unless the tide was in direct conflict with the daily photosynthetic cycle. The redox potentials in the sediments could not be correlated with a daily cycle as was found in the surface waters. Redox potentials in the intertidal zone fluctuated between −150 mV and 300 mV throughout normal cycles of periodic inundation and exhibited decreasing potentials with depth. Sediments further from the waterline had denser vegetation and exhibited higher redox potentials under similar hydrodynamic loading. During extremely dry periods, redox potentials varied from 300 mV to 600 mV, and deeper sediments were often more oxygenated than sediments closer to the sediment-air interface, due to the higher rates of metabolism in the upper stratum. Rates of oxidation and reduction in the sediments were lognormal and evenly distributed about the steady state, with maximum rates of 300 mV min⁻¹ during intermittent periods of inundation. The results show that biological carbon oxidation in sediments can entirely deplete oxygen from surface waters overlying the mud flats, that vegetation serves to oxygenate the rhizosphere, and although redox potential changes can be large in the dynamic intertidal zone, large changes are infrequent.     

Intrinsic bioremediation of a petroleum-impacted wetland

Following the 1994 San Jacinto River flood and oil spill in southeast Texas, a petroleum-contaminated wetland was reserved for a long-term research program to evaluate bioremediation as a viable spill response tool. The first phase of this program, presented in this paper, evaluated the intrinsic biodegradation of petroleum in the contaminated wetland. Sediment samples from six test plots were collected 11 times over an 11-month period to assess the temporal and spatial petroleum concentrations. Petroleum concentrations were evaluated using gas chromatography-mass spectrometer analyses of specific target compounds normalized to the conservative biological marker, C(30)17alpha,21beta(H)-hopane. The analyses of specific target compounds were able to characterize that significant petroleum biodegradation had occurred at the site over the one-year period. Total resolved saturate and total resolved aromatic hydrocarbon data indicated the petroleum was degraded more than 95%. In addition, first-order biodegradation rate constants were calculated for the hopane-normalized target compounds and supported expected biodegradation patterns. The rapid degradation rates of the petroleum hydrocarbons are attributed to conditions favorable to biodegradation. Elevated nutrient levels from the flood deposition and the unconsolidated nature of the freshly deposited sediment possibly provided a nutrient rich, oxic environment. Additionally, it is suggested that an active and capable microbial community was present due to prior exposure to petroleum. These factors provided an environment conducive for the rapid bioremediation of the petroleum in the contaminated wetland.