In situ bioremediation, as introduced by Richard L. Raymond, was essentially a method to enhance natural attenuation (Norris, 1994). The process adds to aquifers those species (nitrogen, phosphorus, and oxygen) thought to be potentially rate limiting with respect to aerobic biodegradation of petroleum hydrocarbons. It was often found that the addition of an oxygen source was sufficient to achieve acceleration of the degradation process. There was little chance of negative impacts to the naturally occurring processes. In fact, non-aerobic processes such as those occurring in conjunction with nitrate, sulfate, and iron reduction were seldom considered important because these anaerobic processes were regarded as slow. The long 'retention time' of the hydrocarbon constituents in most aquifers, however, provides sufficient reaction time.
We now understand that the natural biodegradation of petroleum hydrocarbons and their degradation intermediates also results from the actions of denitrifying, manganese reducing, iron reducing, and sulfate reducing bacteria, as well as through fermentation. Some studies have indicated that these non-aerobic processes account for 90 percent or more of the biodegradation of monoaromatic hydrocarbons in some aquifers. This estimate may even be low based on the work of Kennedy and coworkers (Kennedy, et al. 1998).
Increasing dissolved oxygen and oxidation/reduction potential may result in contributions from non-aerobic bacteria being diminished. Fortunately, most of these microorganisms are facultative and will revive once oxygen levels and Eh are allowed to decrease. It is worth noting that the overall increase in degradation rates resulting from the addition of oxygen is the net of the aerobic degradation rate minus the decrease in the anaerobic degradation rate(s).
The natural biodegradation of chlorinated compounds is far more complex than that of petroleum hydrocarbons, and because the densities of most chlorinated compounds are greater than the density of water, their distribution in aquifers is typically far more complex. Biodegradation of many chlorinated ethenes and ethanes does not occur or is quite slow under aerobic conditions. Other chlorinated compounds do degrade under aerobic conditions whereby the chlorinated compound serves as a substrate or as a cometabolite. Under anaerobic conditions most chlorinated solvents undergo reductive dechlorination. Reductive dechlorination results in the sequential removal of chlorine atoms, generating a series of intermediate degradation products. Depending upon the geochemical conditions and specific microorganisms responsible for degradation, the process may proceed to completely dechlorinated compounds or terminate after removal of only some of the chlorine atoms. Some partially dechlorinated compounds may be degraded under aerobic or iron reducing conditions.
The reductive dechlorination processes in which the chlorinated compounds serve as electron acceptors require the presence of electron donors. It is now believed that degradation of many electron donors produces hydrogen that serves as the terminal electron donor for reductive dechlorination. This sequential process requires electron acceptors as well as electron donors. Further, the use of electron acceptors other than oxygen requires not only the absence of oxygen but a relatively low oxidation/reduction potential. Reductive dechlorination of vinyl chloride is favored by especially low redox potentials(e.g., less that -100mv).
In situ remediation technologies are designed to remove the constituents of concern (COCs) from soils and groundwater and/or to change the geochemical conditions so that the constituents are immobilized or, preferably, converted to non-toxic compounds. The reduction in COC mass reduces the long-term dependence upon natural attenuation mechanisms. This aspect thus improves the probability that monitored natural attenuation either downgradient of the treatment area or subsequent to active remediation will be adequate to meet the needs of the client and the agencies.
In the process of removing the COCs from the subsurface, other constituents with similar or greater solubility in water or volatility (Henry’s Law Constant) will also be removed. The removal of non-target constituents that are beneficial to degradation processes such as reductive dechlorination will impede natural attenuation within the zone of capture or radius of influence. For instance, around landfills very soluble compounds (including ketones, alcohols, and domestic waste related compounds such as sucrose) are important to reductive dechlorination processes. Their presence contributes to the utilization of dissolved oxygen, and many compounds can undergo hydrogen-producing fermentation. Because these classes of compounds are very soluble in groundwater, they are preferentially removed within the capture zone of a pump and treat system.
Implementation of a remediation technology that incorporates groundwater recovery of the source area reduces the gradient and thus the rate of COC migration within a portion of the plume. Within this portion of the plume, the slower movement of the COC(s) provides longer times for degradation to occur, contributing positively to the effectiveness of natural attenuation.
Processes such as air sparging/biosparging and chemical oxidation drive up dissolved oxygen levels and the ORP. Additionally, reduced forms of iron and manganese may be oxidized. These geochemical changes are detrimental to reductive dechlorination but may be reversible. It is not clear whether or not the microorganisms that can carry out reductive dechlorination will survive such treatment. Some experts within the industry are optimistic about this point, some of us are not optimistic. Removal and/or oxidation of anthropogenic and naturally occurring electron donors are not reversible without active intervention. Thus such processes might result in conversion of Type I (anthropogenic carbon sources) or Type II (natural carbon sources) sites to Type III (no carbon source) sites under which reductive dechlorination does not occur.
Table 1 provides a summary of the mechanisms, effects on geochemistry, and potential changes on electron donor availability. The level of concern that one should have for these effects will depend upon the design of a particular system, the mass and types of anthropogenic constituents, the hydrogeology, location and sensitivity of receptors relative to the current extent of the plume, and site specific cleanup factors. Potential effects are not limited to the microbiology (biodegradation). Acceleration of solubilization and mobilization of constituents may occur. Where groundwater gradients are altered, transport mechanisms are affected. Seepage velocity (advection) is altered and thus so may dispersion be affected. Changes in seepage velocity also alter the amount of time during which biodegradation may occur. Natural attenuation of metal concentrations in groundwater can occur as a result of variations in redox potential and/or pH along the plume. Reducing conditions and a near-neutral ph can convert highly mobile Cr (VI) into virtually insoluble hydrous Cr2O3. Changes in the oxidation states of Mn, Fe, and U can drastically affect the mobilities of these elements. Changes in pH can change the solubilities of metal carbonates, hydroxides, and sulfides. A decrease in ORP sufficient to form sulfide from sulfate will immobilize most toxic metals as insoluble sulfides. On the other hand, immobile metal sulfides will be mobilized by an increase in ORP. Most active remedies can affect these processes, with favorable or unfavorable results.
TABLE 1. Effects of active remediation in addition to mass reduction.
Mechanism(s) of Treatment
Changes to Geochemistry
|Pump and Treat||Extraction||Minimal||Removes some electron donors and electron acceptors. Alters (reduces) seepage velocity).|
|Surfactant Flush and Solvent Extraction||Extraction||May decrease DO and ORP||Removes some electron donors. May add electron donors. May be toxic to microorganisms. May affect seepage velocity.|
Degradation of some compounds
|Remove or degrade electron donors.|
|Chemical Oxidation||Degradation (some methods also result in extraction)||Increase DO
|Degrade and possibly remove electron donors. May be toxic to microorganisms|
|Enhanced Reductive Dechlorination||Degradation||Decrease DO
|Add electron donors, utilize electron acceptors.|
|Free Product Removal||Extraction||Minimal||May remove electron donors, impede reductive dehalogenation|
|Thermal Treatment||Heating of soil/volatilization||Oxidation||Sterilization, potential spreading of contaminants|
|Excavation||Mass removal||Oxidation||Alter flow fields, remove electron donors, enhance DNAPL spreading|
|Capping||Reduced flux to GW||Enhances reduction, reduces
|Reduces fermentative creation of substrates, reduces O2 input for VC oxidation, enhanced vapor spreading|
|Zero Valent Iron||Reductive dehalogenation||Decreased ORP||Adds dissolved iron to water, slow for less chlorinated compounds|