Discussion

A synopsis of significant site data is presented in Table 3. The maximum initial concentration encountered was approximately 5100 mg/kg, and the lowest initial concentration was 3940 mg/kg. The minimum concentration achieved at any site was 74 mg/kg of total petroleum hydrocarbons (TPH). Values less than 100 mg/kg represented less than 10% of the total number of observations, thereby suggesting that a threshold for effective biodégradation is between 50 and 125 mg/kg, accounting for measurement accuracy and precision.

Biodégradation flux or loss rates were calculated for the four sites collectively and partitioned by concentration increments as shown in Table 4. Loss rate declines generally fit a firstorder decay function, with acclimated soils containing over 3000 mg/kg TPH degrading substrate at a mean rate of 95 mg (kg • day). Dispersivity of data at concentrations above 500 mg/kg was low, typically less than 10% of the mean. As substrate declines and competition

Table 3 Landfarming Petroleum-Contaminated Soils—Summary Statistics for Four Sites

- - Duration Plot size Depth

Table 3 Landfarming Petroleum-Contaminated Soils—Summary Statistics for Four Sites

- - Duration Plot size Depth

No.

Location

Max.

Avg.

Max.

Avg.

Min.

(weeks)

(acres)

(in.)

1

Ohio

5100

3800

194

158

107

14-23"

2

8

2

California

4650

2611

96

87

74

10-42

1.4

8

3

Michigan

2150

1875

147

112

93

16-38

1.5

7

4

Texas

3940

1817

133

106

89

18

0.5

12

TPH = total petroleum hydrocarbons.

"Activity ceased at point where cleanup criteria were exceeded.

TPH = total petroleum hydrocarbons.

"Activity ceased at point where cleanup criteria were exceeded.

Table 4 Petroleum Loss Rates

Loss rate (mg/kg)

Table 4 Petroleum Loss Rates

Loss rate (mg/kg)

Concentration3 C0 (mg/kg)

Max.

Avg.

Min.

>4000

157

95

74

1000-4000

71

53

40

500-1000

57

38

26

250-500

23

9.5

4.0

100-250

8.0

5.4

1.9

<100

1.3

0.8

0

"Lowest observed value, 74 mg/kg.

"Lowest observed value, 74 mg/kg.

sites disappear, pockets of viable and inviable activity form, and dispersivity increases to as much as 80% of the mean value at concentrations less than 200 mg/kg. The effects of the several individual parameters contributing to this phenomenon are discussed in turn below.

Specific observations were made with respect to threshold values and inhibition points for four parameters—oxygen, biological population enhancement, soil moisture, and temperature. As noted previously, these factors do not constitute all of the potential constraints on biodégradation of petroleum in soils; however, they represent critical elements that can often be controlled in a applications environment.

1. Oxygen Limitations

The primary purpose of tilling in a landfarm is to optimize the contact of soil microbes with atmospheric oxygen. In soils, aeration depends on the total amount of air-filled pore space. Elimination of air-filled porosity through excess watering or compaction reduces oxygen transfer between the microbial colonies and the substrate. Large amounts of biodegradable organics in the top layer of soil will deplete oxygen reserves in the soil and slow down oxygen diffusion to the deeper layers.

Oxygen levels were measured at selected points in each of the four tests at 1-week intervals. More frequent oxygen monitoring was performed in the first 2-3 weeks for purposes of evaluating tilling and moisture requirements. At three sites—Ohio, California, and Michigan— tilling biweekly was optimal. At the Texas site, where temperatures were significantly higher, oxygen transfer was more efficient and frequent; weekly tilling produced markedly higher degradation rates. This observation is somewhat contrary to that expected. The maximum oxygen content measured in soil water was 6.5 mg/L, which is approximately 80% of saturation. The minimum amount of oxygen measured, 0.9 mg/L, was at 6 in. below the surface in a soil that had not been tilled for 4 weeks. Microbial growth was found to be inhibited at concentrations below 2.5 mg/L, and total cell loss was detected at oxygen concentrations below 1.4 mg/L, which were found in plots with very low soil moisture.

2. Microbial Density Enhancement

One way to enhance biodégradation of organic compounds is to inoculate the soil or groundwater (as appropriate) with microorganisms known to metabolize the chemical readily. This concept has been roundly debated for its performance by various researchers and commercial vendors of inoculum [60], Both successes and failures have been noted in the literature. It has been my experience that commercial bacteria cannot be relied upon as a sole source as their populations tend to die off quickly, thus requiring numerous reapplications. In the natural en-

Remaining

Control (Tilled) Nutrients

Horse Manure & Nutrients

Remaining

Figure 3 Effects of a proprietary inoculum to degrade TPH compared to a horse manure as a bacteria additive over time.

vironment the hybrid species face competition, prédation, or parasitism. Any of these interactions could account for the inability of introduced microbes to survive [61].

The potential of hydrocarbon mixtures to be eliminated at a higher rate from soil when hydrocarbon-degrading bacteria are added to the soil is also a much disputed matter. A general criticism of the seeding approach is that the use of an allochthonous microbial population may not be necessary or effective in most cases. Also, most isolates implicated in petroleum hydrocarbon biodégradation are gramnegative, non-spore-forming bacteria. These are difficult to store in large quantities in a manner that preserves their viability.

Bioaugmentation was used at the landfarm in Ohio. The ability of a proprietary inoculum to degrade TPH was compared to horse manure as a bacteria additive and the simple addition of sufficient macronutrients (fertilizer). Results of this evaluation are given in Figure 3. Inoculum was applied on a biweekly basis as population density levels were found difficult to maintain. Manure was found to last at least 4 weeks before a reapplication was needed. While the commercial additive needed substantially less acclimatization time in the soil—4 days as compared to 3 weeks for the manure—its effectiveness diminished rapidly after the first month of testing. After 15 weeks of activity the horse manure subplot performed the best, followed by the commercial additive subplot and then the subplot with only macronutrient additions. Although the performance difference between horse manure and commercial organisms is statistically insignificant, the test indicated that continuous use of commercial bacteria was not beneficial. It was determined, however, that initial injection of commercial bacteria decreased acclimatization time, and ongoing injection of 5-15% of such bacteria with fertilization probably is valuable and economically sound.

3. Moisture Effects

Moisture is essential to the growth of soil microorganisms. Water provides the mechanism for the exchange of reactants and food absorption through the cell walls. Excessive amounts of moisture can be disruptive. Commensurately, when soil moisture drops to near the point of

Figure 4 Effects of moisture on petroleum degradation, plot 3.

specific retention, usually corresponding to 10% by volume or less, metabolic activity ceases. Bossert and Bartha [62] suggested that moisture contents be maintained in the range of 2080%. Biodégradation studies performed at the site in Michigan sought to define a more accurate or narrow range where moisture content is optimum.

Moisture data from three different subplots are shown on Figure 4. All other parameters such as temperature, soil type, and nutrient loadings were kept as constant as possible; only moisture content was deliberately varied. Values of 15%, 50%, and 85% of saturation were chosen for the test. These levels were checked once a day for a total test period of 12 weeks. Losses from the 15% moisture subplot were minor over the entire duration of the test. TPH loss that was observed is believed to be due almost exclusively to volatilization, not biodégradation. It was thus confirmed that moisture contents below 20% are threshold limiting to biodégradation.

Biodégradation was observed in the 85% subplot but was only about one-third the loss observed in the 50% moisture subplot. This evidence suggests that indeed inhibition above 80% is present but is not a threshold condition. It is further offered, based on the performance of this test, that optimum moisture levels are probably in the range of 40-60% of saturation. Additional study in this area is warranted.

4. Temperature Effects

Temperature affects the respiration rates of microorganisms, and, through controlling respiration, temperature affects metabolism. It also affects the solubility of the host hydrocarbons. Temperature is directly related to the amount of evaporation and volatilization from the soil, which in turn impacts microbial populations. Mesophilic organisms in general perform best at about 95°F. Huddleston and Cresswell [63] reported petroleum degradation at temperatures as low as 30°F. Kuhlmeier and Sunderland [64] found that inhibition became easily detectable at temperatures below 55°F.

Temperature was closely monitored at each of the four test locations. Mean TPH loss rates were calculated across a wide range of temperatures. Results of this analysis are presented in Figure 5. Beginning and ending TPH concentrations for a given period in which the average

40 50 60 70 80 90 Temperature ("f )

Figure 5 Effects of temperature on microbial activity, plots 1-4.

40 50 60 70 80 90 Temperature ("f )

Figure 5 Effects of temperature on microbial activity, plots 1-4.

daily temperature was in a designated temperature bracket (i.e., 40s, 50s, etc.) were summed, and the mean loss was derived from data in all tests. There was virtually no biodégradation associated with observations at temperatures under 50°F. Losses shown in Figure 5 represent volatilization losses and measurement inconsistencies.

There was a significant difference between degradation rates below 80°F from those above this reference value. Interestingly, biologically attributable losses above 95°F did not prove to be significantly greater than those at 85°. Although less than 2% of the total observations were at temperatures above 100°F, there was a noticeable difference from the balance of the data points. When ambient temperatures exceeded 100°F the mean percent TPH loss was observed to be 92%. The slight dropoff from the 80s-90s bracket is postulated to be attributable to high initial volatilization losses but subsequent cell mortality due to heat stress, rapid moisture loss, or nutrient constraints or possibly some combination of these factors.

A review of data collected at four independent petroleum landfarming operations has suggested that biodégradation of contaminated soils may be kinetically limited at concentrations below 50 mg/kg. Loss rates decline dramatically when remaining substrate dips below 150 mg/kg.

Oxygen levels become critical at 2 mg/L as measured in soil moisture. Ideally, soil moisture should be maintained in the 40-60% of saturation range. Ambient temperature hits a threshold limit for degradation at approximately 50°F and reduces microbes to cell maintenance levels once temperatures drop to 45°.

The addition of commercial microorganisms was not found to be highly beneficial. Natural additives such as sewage sludge or horse manure do enhance microbial activity by introducing large quantities (and diverse types) of microbes. Macronutrients, although not detailed specifically in the discussion, are important for cell activity. A carbon/nitrogen ratio of 160:1 as suggested by the American Petroleum Institute appears to be overly conservative.

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