Quantification of natural attenuation using analytical-chemical tools

Contamination of groundwater is a widespread environmental problem, which
particularly arises from various point sources such as leaking underground storage tanks, accidental spills, inappropriate use and disposal techniques, and industrial discharges. Thus, the development of efficient and cost-effective remediation approaches is urgently needed. The natural attenuation concept is a passive approach relying on the ability of the intrinsic microorganisms to degrade the contaminants. This approach implies the demonstration and verification of on-going biodegradation and a subsequent monitoring. However, the verification of biodegradation is not straightforward, since decreasing concentrations are ambiguous, and direct field scale mass reduction can be difficult to prove. Various in situ indicators have been suggested as supplemental evidence in the demonstration of biodegradation. A number of these in situ indicators are evaluated herein. The evaluation emphasized the importance of differentiating between bulk contamination and specific compounds. The term, bulk contamination, is used to describe a mixture of contaminants, which make up the majority of the plume and can be metabolically degraded. The degradation of bulk contamination may significantly alter the groundwater quality in terms of exhaustion of electron acceptors and production of DIC and alkalinity. This phenomenon is typically observed at hydrocarbon field sites and landfills. A specific compound is basically any compound, which is considered separately, but the term is especially used to describe specific contaminants of special environmental concern. The degradation of a specific compound is not reflected in the bulk changes of e.g. concentrations of electron acceptors, either because the concentration of the specific compound (e.g. pesticides or critical components of petroleum such as benzene) is possibly orders of magnitude lower than the overall concentration of bulk contamination, or because the specific compound (e.g. chlorinated solvents and other highly chlorinated compounds) is degraded by reduction and therefore is related to the consumption of an electron donor rather than the consumption of electron acceptors. The use of degradation products for the demonstration of biodegradation was investigated. The starting point was the traditional use of dechlorination products to verify the occurrence of reductive dechlorination of chlorinated solvents such as perchloroethene (PCE) and trichloroethene (TCE). The sequential and quantitative transformation of parent compounds into successively less chlorinated daughter compounds implies the possibility of quantification of biodegradation. More recently, the use of benzyl- and alkylbenzylsuccinates were suggested as specific indicator metabolites of the anaerobic degradation of alkylbenzenes. It was found that these metabolites were likely to occur at hydrocarbon field sites and even at landfill sites, but not in uncontaminated groundwater. However, the evaluation of existing field studies indicated that the succinate derivatives could only be used as qualitative indicators, while their possible use for quantification was questionable. Degradation products are known for a number of pesticides. The phenoxy acid herbicides are frequent groundwater contaminants with a well-described degradation pathway going through the corresponding chlorophenol. A few degradation studies also showed dechlorination as a possibility under anaerobic conditions. The chlorophenols and the possible dechlorination products are also frequently found in groundwater, which at first might suggest in situ degradation of phenoxy acids. However, a review of the history of the manufacture of these herbicides revealed that a range of chlorophenols and non-herbicide phenoxy acids are present as impurities in the herbicides. In the early years of production the impurities could account for more than 30% of the herbicide. Therefore, unless a direct increase in a metabolite is observed along a flowline, the presence of possible phenoxy acid degradation products in contaminated groundwater does not indicate the degradation of phenoxy acids. It was found, however, that the impurity/parent herbicide ratios could be used as in situ indicators. Indication of biodegradation is obtained when the impurity/parent compound ratio changes along a flowline, although in the case of chlorophenol impurities, the change needs to be an increase, since a decreasing ratio could be caused by sorption of the chlorophenol. Indication of biodegradation can also be obtained independently of the flowpath, if the impurity/parent herbicide ratio exceeds the worst-case ratio, which is the largest possible ratio in the original herbicide based on synthesis yields. The use of specific compound isotope analyses has gained much interest in recent years. The degradation of an organic compound may result in an increasing ¹³C/¹²C ratio of the residual fraction due to isotopic fractionation. Enrichment factors have been determined for a number of compounds, primarily hydrocarbons and chlorinated hydrocarbons, under different redox conditions. This means that when degradation of a specific compound is indicated by an increasing isotope ratio of the compound along a flowline, the relative biodegradation can be quantified. The carbon isotope fractionation seems to be somewhat larger for chlorinated solvents than for hydrocarbons, especially for the less chlorinated daughter compounds. But the concurrent formation and further degradation of these intermediate metabolites complicates the interpretation. For hydrocarbons the approach is more easily applied, and has been successful even in a complex matrix of leachate-contaminated groundwater with relatively low concentrations of specific hydrocarbons. Yet, at most field sites the approach only indicated degradation for some of the specific compounds, while no systematic trend could be identified in the isotope ratios of other compounds. This could be because the degradation of the latter compounds at the specific field site was not associated with isotope fractionation, or because the concentration dropped to below the detection limit of the isotope analysis. Specific compound isotope analysis is not feasible for phenoxy acids, because the analysis involves the separation of compounds by GC, and derivatization should be avoided as it might cause fractionation and increases the analytical uncertainty by adding a substituent to the molecule, which needs to then be corrected. Some phenoxy acids, however, are chiral molecules, which might be enantioselectively degraded, depending on the environmental conditions. In this way the ratio between the enantiomers, i.e. the two mirror image forms, changes. Much like the isotope ratios, a change in enantiomeric composition along a flowline will indicate degradation. An advantage of enantiomeric fractions compared to isotope ratios is a lower detection limit and that many sources can be assumed to have a racemic composition of the phenoxy acids. In that case, the observation in the plume of an enantiomeric fraction significantly different from racemic is indicative of degradation, independent of the flowpath. It was found that a site-specific enantioselectivity could be estimated in supportive microcosm degradation studies, and subsequently applied to obtain quantitative estimates of biodegradation at the field scale based on the observed changes in enantiomeric composition. The enantioselectivities probably needs to be determined for each field site, while isotope enrichment factors for a certain compound under certain redox conditions seems to be more generally valid. However, the successful application of isotope ratios for phenoxy acids is currently not very likely. Having quantified the degradation of some specific compounds based on the changes in isotope ratios or enantiomeric compositions, the degradation of some other compounds could be quantified as well by way of the use of compound ratios. A compound ratio is the ratio between two compounds with similar physical and chemical properties that can be related to the same part of the source, which is why the possible change in the compound ratio along a flowline indicates degradation. As stand-alone indicators, the compound ratios may have a relatively limited use as qualitative indicators, but in combination with other methods, they were found to be very useful tools. Overall, the evaluated in situ indicators all have their forces and limitations. Some apply to the degradation of bulk contamination, while others apply to a single or a group of specific compounds. Some are qualitative, while others are quantitative. Some are flowpath dependent, while others are absolute. Each in situ indicator may provide a piece of the puzzle, and the combined evidence from different in situ indicators along with other lines of evidence may eventually form the basis for a safe and efficient application of natural attenuation as a remedy.