Changes in hydrocarbon concentration
In Figure 1, the results of monitoring for hydrocarbon concentration are presented. All data presented are for core samples from 0-20 cm depth. During the first month of treatment, all of the processes registered a reduction in TPH concentration: ~20% for biostimulation and bioaugmentation and 12% for the chemico-biological stabilization. However, during the second month of treatment, an increase of 4–9% in TPH concentration was observed. It is possible that this observation is due to changes in the interaction between the hydrocarbons and soil particles that make the hydrocarbons more available and results in an increase in the extraction efficiency. If the soil microorganisms begin to degrade some of the organic material in the soil (humic substrates, water lily or cachasse), this could liberate part of the hydrocarbons from soil particles. An alternative explanation could be that biosurfactants are produced by oil-degrading microorganisms, also leading to greater availability and increased extraction efficiency. It is also possible that some of the intermediates of biodegradation (alcohols, for example) produced greater IR absorbance, or were more easily extracted from the soil matrix than the original hydrocarbons. Increases of this type have also been reported by other authors during the intermediate phase of biodegradation (Adams et al., 2008; Bartha and El-Din, 1993).
During the third month of treatment, this trend (increased TPH concentration) continued in the bioaugmentation treatment (increase of an additional 5%), but in both the biostimulation and chemicobiological stabilization treatments, important reductions were observed to be about 3.5% for biostimulation and 9% for the chemico-biological stabilization treatment. Overall reduction during the three month period was ~ 14–16% in each of the three treatments. Interestingly, the gas chromatography of oil from this site showed 14% of the oil to be in the C10-C15 range (with 86% C>15). This overall low rate of biodegradation of very weathered hydrocarbons in soil is not unusual (Adams, et al., 2006; Atlas, 1986; Jerger et al., 1991). It probably results not only from more complexity in their chemical structure, but due to their high viscosity and reduced bioavailabilty. Similar kinds of hydrocarbons in liquid broth or slurry conditions (where surface area and bioavailability problems are mechanically overcome) have been highly biodegraded in relatively short time periods (Pradhan et al., 1997; Jerger et al., 1991).
Reduction in toxicity
The results of the toxicity evaluation are shown in Figure 2 In this scale, toxicity is represented in toxicity units=1/EC50, where EC50 is the effective concentration-50, the concentration of the sample which reduces the activity (bioluminescence) of the test organism in the bioassay by 50% (Bulinc, 1990). In this case, the EC50 is calculated as a proportion (75 % = 0.75, for example). The scale used in this study is the quarter log scale proposed by Kross and Cherryholmes (1993) normalized to the background toxicity at the site (Cornelio, 2001; UJAT, 2006). Samples in which no toxicity pattern was observed were assigned a value equivalent to the background. As in Fig. 2 is shown, both the biostimulation treatment and the chemico-biological stabilization treatment reduced the toxicity to the background levels, whereas the bioaugmentation treatment in this study resulted in little if there is any change in the toxicity. Likewise, the control showed practically no change, when the variability of the data is taken into account. Both the bioaugmentation treatment and the control had the final toxicity values in the slightly toxic range. In the chemico-biological stabilization treatment, the final material not only was intoxic but produced stimulation of the test organism in the bioassay, of 103–109 % with respect to the blank.
pH variation in remediation treatments
The initial pH of the sediment material was slightly alkaline in the 7.5–8.0 range. During the biostimulation and bioaugmentation treatments in the first month, a slight increase was observed, raising the pH to 8.1– 8.2, possibly due to the addition of inorganic nutrients. Following this brief raise, the pH dropped into the 7.0– 7.8 range during the rest of the study.
In the chemico-biological stabilization treatment, a large increase in the pH was observed immediately after adding calcium hydroxide and reaching 11.8-11.9. However, over the next few weeks, and after the addition of cachasse, the pH was moderated and fell to ~ 8.2. Over the next couple of months, the pH continued to decline slightly to ~7.8.
Untreated sediment collected in this area has the potential to produce TCLP leachates of ~10 to 12 mg/L TPH. Following treatment by all three methods, biostimulation, bioaugmentation and chemico-biological stabilization, the TCLP leachates were reduced to below the detection limits (< 1 mg/L in this study).
Polycyclic Aromatic Hydrocarbons (PAHs)
Due to the lack of in-house analytical capacity and the relatively high costs of these analyses, composite samples were collected for analysis from two batches of sediment used in this study (untreated material for the biostimulation/bioaugmentation test, and untreated material for the chemico-biological stabilization test). Likewise, composite samples were collected for each of the treatments at the end of the study. These were analyzed for the six carcinogenic PAHs that are considered in the pertinent Mexican environmental regulation NOM-138-SEMARNAT/SS-2003 (SEMARNAT, 2005), namely benzo(a)anthracene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, dibenzo(a,h)anthracene and indeno(1,2,3-cd)pyrene. (Table 1). In both the biostimulation and chemico-biological stabilization treatments, an increase was observed in the concentration of carcinogenic PAHs. The overall increase was roughly 4 times for the biostimulation treatment and 6 times for the stabilization treatment (based on detection limits). This increase may be due to variability in sampling, but most likely is due to either changes in the matrix structure which increase extraction efficiencies or production of these compounds as intermediates in the decomposition of more complicated structures. In the bioaugmentation treatment, a decrease in carcinogenic PAH concentration was observed of more than 8 times.
In both the bioaugmentation treatment and the chemico-biological stabilization treatment, the final concentration of each of the carcinogenic PAHs was below the Mexican norms (NOM-138-SEMARNAT/SS- 2003; 2 mg/kg for all PAHs except for benzo (k) fluoranthene – 8 mg/kg, for non-industrial sites). In the biostimulation treatment, only benzo (b) fluoranthene was slightly above the Mexican norms, 2.360 mg/kg in the treated sample vs. 2.0 mg/kg permitted. It is likely that with slightly longer treatment times all PAHs would be within the Mexican norms.
Considering the regional prices of machinery rental, fuel, labor and materials costs, the treatment alternatives were evaluated for cost at the site being considered for remediation. These calculations were based on an in-place volume of 3200 m2 and an in-situ superficial application of the remediation methods. In the regional market, the machinery rental prices are inflated with respect to national levels, due to the high demand in the petroleum industry for construction and remediation projects. These costs may be similar to other petroleum producing regions in developing economies, but fuel, materials and labor will probably be somewhat less than in very industrialized countries (Table 2). These costs do not include profit or taxes. The notable difference between the biostimulation/ bioaugmentation and chemico-biological stabilization is the cost for machinery and labor, being roughly 60% less. This is due primarily to the need for frequent tilling in the biostimulation/ bioaugmentation treatments that are avoided in the chemico-biological stabilization treatment. Likewise, there are savings reflected in the materials costs, which in this case include fuel for machinery. Fuel costs for the chemico-biological stabilization treatment were estimated to be only 2-3% of the fuel cost for biostimulation. Almost one-half of the overall cost for the chemico-biological stabilization is for the chemical reagent (hydrated lime). However, considering the savings in fuel, machinery rental and labor costs, this alternative was still estimated to be > 40 % more economical than biostimulation.
All of the three treatment methods evaluated produced similar results with respect to the reduction in hydrocarbon concentration, being ~ 14–16% over a three-month period. If the objective of the remediation is merely to reduce the concentration of hydrocarbons, these methods do not result in reductions sufficient to be accepted by environmental regulations in Mexico (NOM-138-SEMARNAT/SS-2003; SEMARNAT, 2003).
The TPH concentrations (after three months of treatment) were still ~ 4–5% (~ 40,000–50,000 mg/kg). However, in Mexico, as in many other countries, riskbased remediation, including stabilization, is considered to be an acceptable alternative (SEMARNAT, 2003; SEMARNAT, 2005). In these treatments, the overall results were evaluated in terms of soil functionality, based on toxicity, leachate potential, pH and carcinogenic PAHs.
All of the three evaluated treatments reduced the leachate potential to non-detectable levels. The biostimulation treatment and the chemico-biological stabilization treatment completely reduced acute toxicity. Furthermore, the stabilization treatment not only reduced the acute toxicity completely but also actually caused stimulation (increased bioluminescence) in the bioassay. This bioassay is a good tool to evaluate general toxicity in the environment and the results are consistent with toxicity to soil and sediment invertebrates (Doherty, 2001). With respect to other parameters, all of the three treatments resulted in a final pH of 7.0–7.8 in material with an initial pH of 7.5–8.0. In terms of carcinogenic PAHs, the bioaugmentation and stabilization treatments had final values that are below the permissible maximum limits in Mexico, and the biostimulation treatment only presented one PAH slightly above the permissible limit (2.36 mg/kg benzo (b) fluoranthene vs. 2.0 mg/kg permitted). It is very likely that with a little longer treatment time, the biostimulation treatment would also have reduced the PAH concentration to acceptable levels.
Considering these results, both the biostimulation and chemico-biological stabilization treatment methods are acceptable alternatives at least for the remediation of this site. The bioaugmentation, however, did not reduce the acute toxicity sufficiently during the period of this study. One advantage of the chemico-biological stabilization treatment, is the biological stimulation observed in the bioassays, which may reflect better growing conditions for microorganisms in the treated material and result in better recovery of the soil ecosystem and vegetative growth (Alexander, 1995; Overton, 1996).
However, the primary advantages of the chemicobiological stabilization are logistics and cost. Since frequent tilling is not required, fuel, labor and machinery rental costs are greatly reduced. The overall cost of the chemico-biological stabilization treatment in this evaluation was less than 60% of that for biostimulation.
Based on the results of this study, chemicobiological stabilization was proposed in the remedial action plan for this site and has been approved by Mexican authorities (Ministry of the Environment and Natural Resources, SEMARNAT for its abbreviation in Spanish). This may be an important alternative for the treatment of soils, muds and sediments highly contaminated with heavily weathered hydrocarbons in tropical and subtropical environments such as southeastern Mexico, eastern Venezuela, the Niger Delta, Sumatra, Louisiana, etc.