Decrease In Contaminant Concentration Research Articles

Evaluation of Phytoremediation in the Ogoni Wasteland of Southern Nigeria

Phytoremediation offers an effective strategy for managing environmental degradation in the Ogoni wastelands of southern Nigeria, but its benefits and socio-economic implications have not been adequately studied. This study aims to investigate the effectiveness of phytoremediation techniques on soil contamination and community perception and participation in environmental restoration efforts. Soil samples were collected from several sites in Ogoni wasteland, and pollutant concentrations were analyzed by gas chromatography and mass spectrometry. Selected plant species were used for phytoremediation interventions, and microbial activity was assessed using CO2 production rates. Household surveys and interviews were conducted to measure perceptions of community membership and involvement in correctional programs. Analysis of soil samples revealed a significant decrease in contaminant concentrations after phytoremediation, with average initial concentrations decreasing to 23.456 mg/kg and plant biomass accumulation rates of 0.567 kg/sq m returning to 4.901 kg/m², an indication of differences in activity of various organisms in pollutant uptake. After phytoremediation, the changes showed an increasing trend, with CO2 emissions increasing from 0.567 mg CO2/g/hr to 1.345 mg CO2/g/hr. The household survey showed a positive attitude towards the effectiveness of phytoremediation, with an average score of 4.012 on a scale of 1 to 5. Research findings show that phytoremediation can be a sustainable solution for soil pollution in the Ogoni wastelands. These results contribute to the existing body of knowledge by providing empirical evidence and insights into the environmental, social, and economic dynamics of ecological restoration in conflict zones on the background.

Fifty-Year Review of Effluent Discharge Trends to the Southern California Bight from Large Municipal Wastewater Treatment Facilities from 1971 to 2020

Historically, Southern California’s four largest publicly owned treatment works (POTWs) were major sources of contaminants in the Southern California Bight (SCB), home to over 22 million residents and diverse marine life. This study conducted a fifty-year review (1971–2020) to assess these facilities’ performance in protecting human and aquatic life. We analyzed effluent discharge flow, total annual mass emissions into SCB, and the rate of change of mass emissions in five ten-year timeframes. Results show a significant decrease in contaminant concentrations (43% to 99%) from 1971 to 2020, with most reaching historically low levels. Despite a growing population, effluent volume decreased by 23%, reflecting successful local water conservation and recycling efforts. Some constituents experienced temporary increases that later decreased. Overall, the decline in mass emissions for all constituents over fifty years highlights the POTWs’ efforts to treat effluent and reduce their impact on the SCB’s ecosystem.

Seasonal pollutant levels in littoral high-Arctic amphipods in relation to food sources and terrestrial run-off

Increasing terrestrial run-off from melting glaciers and thawing permafrost to Arctic coastal areas is expected to facilitate re-mobilization of stored legacy persistent organic pollutants (POPs) and mercury (Hg), potentially increasing exposure to these contaminants for coastal benthic organisms. We quantified chlorinated POPs and Hg concentrations, lipid content and multiple dietary markers, in a littoral deposit-feeding amphipod Gammarus setosus and sediments during the melting period from April to August in Adventelva river estuary in Svalbard, a Norwegian Arctic Aarchipelago. There was an overall decrease in concentrations of ∑POPs from April to August (from 58 ± 23 to 13 ± 4 ng/g lipid weight; lw), Hg (from 5.6 ± 0.7 to 4.1 ± 0.5 ng/g dry weight; dw) and Methyl Hg (MeHg) (from 5 ± 1 to 0.8 ± 0.7 ng/g dw) in G. setosus. However, we observed a seasonal peak in penta- and hexachlorobenzene (PeCB and HCB) in May (2.44 ± 0.3 and 23.6 ± 1.7 ng/g lw). Sediment concentrations of POPs and Hg (dw) only partly correlated with the contaminant concentrations in G. setosus. Dietary markers, including fatty acids and carbon and nitrogen stable isotopes, indicated a diet of settled phytoplankton in May–July and a broader range of carbon sources after the spring bloom. Phytoplankton utilization and chlorobenzene concentrations in G. setosus exhibited similar seasonal patterns, suggesting a dietary uptake of chlorobenzenes that is delivered to the aquatic environment during spring snowmelt. The seasonal decrease in contaminant concentrations in G. setosus could be related to seasonal changes in dietary contaminant exposure and amphipod ecology. Furthermore, this decrease implies that terrestrial run-off is not a significant source of re-mobilized Hg and legacy POPs to littoral amphipods in the Adventelva river estuary during the melt season.

Substantial Decrease in Contaminant Concentrations in the Sediments of the Venice (Italy) Canal Network in the Last Two Decades—Implications for Sediment Management

The Venice canal network requires periodic intervention to remove sediments that progressively accumulate. The most recent dredging operation was carried out in the second half of the 1990s and early 2000s. These sediments had accumulated over a period of more than 30 years and were highly contaminated with Cd, Cu, Hg, Pb, Zn and PAHs. Sediments deposited after the dredging work were investigated in 2005, 2009, 2014 and 2017 by analysing sediment cores collected from three sites in the canal network. Arsenic, heavy metal and PAH concentrations were observed to be much lower than past values, although Cu, Hg and PAH levels were still relatively high. The high Cu concentrations (mean 161 mg kg−1) are partly due to the widespread use of Cu-based antifouling paint. Current Italian regulations forbid the disposal of dredged sediments with these concentrations inside the lagoon, thereby increasing the cost of canal network maintenance.

Three-year field experiment on the risk reduction, environmental merit, and cost assessment of four in situ remediation technologies for metal(loid)-contaminated agricultural soil

The traditional assessment of agricultural soil remediation technologies pay limited attention to sustainability and only considers the decrease in contaminant concentrations and cost, even though the sustainability of these technologies has been prioritized. This 3-year field study comprehensively assessed the sustainability of four commonly used agricultural soil remediation technologies in terms of metal(loid) removal efficiency, environmental merit, and cost. The farmland was contaminated by previous sewage irrigation with excessive amounts of As, Cd, and Pb. The four selected remediation technologies used were phytoextraction, intercropping of hyperaccumulators and cash crops, chemical immobilization, and turnover and attenuation (T&A). A risk reduction–environmental merit–cost model was utilized to compare these four technologies. Results showed that T&A reduced the health risks posed by excess metal(loid)s by ∼47% and yielded the highest risk reduction and lowest cost. Phytoextraction achieved the highest environmental merit because it produced the least interruption to the environment. A simplified assessment frame for soil remediation technology was established from a retrospective aspect using data from a real soil remediation project. Environmental merit is a less considered factor and more difficult to quantify than risk reduction or cost, thus requiring increased attention.

Pore-scale investigation of the use of reactive nanoparticles for in situ remediation of contaminated groundwater source

Nanoscale zero-valent iron (nZVI) particles have excellent capacity for in situ remediation of groundwater resources contaminated by a range of organic and inorganic contaminants. Chlorinated solvents are by far the most treated compounds. Studies at column, pilot, and field scales have reported successful decrease in contaminant concentration upon injection of nZVI suspensions in the contaminated zones. However, the field application is far from optimized, particularly for treatments at-or close to-the source, in the presence of residual nonaqueous liquid (NAPL). The knowledge gaps surrounding the processes that occur within the pores of the sediments hosting those contaminants at microscale limit our ability to design nanoremediation processes that are optimized at larger scales. This contribution provides a pore-scale picture of the nanoremediation process. Our results reveal how the distribution of the trapped contaminant evolves as a result of contaminant degradation and generation of gaseous products. We have used state-of-the-art four-dimensional (4D) imaging (time-resolved three-dimensional [3D]) experiments to understand the details of this degradation reaction at the micrometer scale. This contribution shows that the gas released (from the reduction reaction) remobilizes the trapped contaminant by overcoming the capillary forces. Our results show that the secondary sources of NAPL contaminations can be effectively treated by nZVI, not only by in situ degradation, but also through pore-scale remobilization (induced by the evolved gas phase). The produced gas reduces the water relative permeability to less than 1% and, therefore, significantly limits the extent of plume migration in the short term.

Remediation of Organically Contaminated Soil Through the Combination of Assisted Phytoremediation and Bioaugmentation

Here, we aimed to bioremediate organically contaminated soil with Brassica napus and a bacterial consortium. The bioaugmentation consortium consisted of four endophyte strains that showed plant growth-promoting traits (three Pseudomonas and one Microbacterium) plus three strains with the capacity to degrade organic compounds (Burkholderia xenovorans LB400, Paenibacillus sp. and Lysinibacillus sp.). The organically contaminated soil was supplemented with rhamnolipid biosurfactant and sodium dodecyl benzenesulfonate to increase the degradability of the sorbed contaminants. Soils were treated with organic amendments (composted horse manure vs. dried cow slurry) to promote plant growth and stimulate soil microbial activity. Apart from quantification of the expected decrease in contaminant concentrations (total petroleum hydrocarbons, polycyclic aromatic hydrocarbons), the effectiveness of our approach was assessed in terms of the recovery of soil health, as reflected by the values of different microbial indicators of soil health. Although the applied treatments did not achieve a significant decrease in contaminant concentrations, a significant improvement of soil health was observed in our amended soils (especially in soils amended with dried cow slurry), pointing out a not-so-uncommon situation in which remediation efforts fail from the point of view of the reduction in contaminant concentrations while succeeding to recover soil health.

Distribution and migration of antimony and other trace elements in a Karstic river system, Southwest China.

Antimony (Sb) usually occurs associated with arsenic, lead, and other metal elements in sulfide deposits, and most is currently being extracted from the Karst areas, southwest China. In these areas, the acid generated from the oxidative dissolution of sulfide minerals is neutralized by the abundant carbonates but little is known about the effect of this process on the geochemical behavior of Sb and other contaminants. In this study, physicochemical properties (pH, EC) and the trace elements (Sb, As, Cu, Pb, Zn, Sr, etc.) concentrations in waters from mining-impacted Karstic environments were determined in order to determine their distribution and migration potential. It was found that pH values ranged from 6.51 to 9.82, and EC values varied from 369 to 1705μs·cm-1 in river water samples. Waters of various types such as adit waters, flotation drainage, leaching waters, and river waters all contained high concentrations of dissolved trace elements, reaching up to 5475μg·L-1Sb, 1877μg·L-1As, 10,371μg·L-1Zn, 1309μg·L-1 Pb, 46μg·L-1 Cu, and 1757μg·L-1 Sr. The elevated concentrations of dissolved Sr indicated that Sr could be considered as an indicator of oxidative dissolution of sulfide minerals in the Karst rivers. A proportion of the trace elements were removed in the streams in the vicinity of the mine due to adsorption onto particulate matter, whereas migration of trace elements in the downstream of mine area was attributed to dispersion in dissolved forms. Strontium and Sb have a strong hydrophilicity compared to the other elements analyzed; in contrast, Pb had a high affinity for suspended particulate matter (SPM). It was also found that downstream sediments had elevated concentrations of mining-derived trace elements, but there was a significant decrease in concentration of contaminants in aqueous phase, suggesting that contaminant behavior was conservative in the water-sediment systems under the oxic conditions prevailing in these waters. There was a good correlation between Sb and As in water-SPM-sediment system, indicating that Sb and As are homologous in water environment of the study area.

Effect of Temporal Changes in Air Injection Rate on Air Sparging Performance Groundwater Remediation.

Air sparging (AS) is a commonly applied method for treating groundwater contaminated with volatile organic compounds (VOCs). When using a constant injection of air (continuous mode), a decline in remediation efficiency is often observed, resulting from insufficient mixing of contaminants at the pore scale. It is well known that turning the injection on and off (pulsed mode) may lead to a better remediation performance. In this article, we investigate groundwater mixing and contaminant removal efficiency in different injection modes (i.e., continuous and pulsed), and compare them to those achieved in a third mode, which we denote as "rate changing." In this mode, injection is always on, and its rate is varying with time by abrupt changes. For the purpose of this investigation, we conducted two separate sets of experiments in a laboratory tank. In the first set of experiments, we used dye plume tracing to characterize the mixing induced by AS. In the second set of experiments, we contaminated the tank with a VOC and compared the remediation efficiency between the different injection modes. As expected, we observed that time-variable injection modes led to enhanced mixing and contaminant removal. The decrease in contaminant concentrations during the experiment was found to be double for the "rate changing" and "pulsed" modes compared to the continuous mode, with a slightly preferable performance for the "rate changing" mode. These results highlight the critical role that mixing plays in AS, and support the need for further investigation of the proposed "rate changing" injection mode.

The risk of river pollution due to washout from contaminated floodplain water bodies during periods of high magnitude floods

Summary The risk of river pollution due to washout (removal of pollutants) from contaminated floodplain water bodies (floodplain lakes and quarries whose origin is related to the large-scale mining of nonmetallic building materials in the floodplain zone) during high magnitude flood periods is analyzed using a combination of one-, two- and three-dimensional hydrodynamic modeling and in situ measurements. The modeling performed for the floodplain water bodies contaminated by N compounds shows that during large magnitude floods washout occurs. The washout process consists of two stages: an initial rapid stage lasting about two hours during which the upper (3–4 m thick) layer is washed out, followed by a second stage when the concentration of NH4-N in the floodplain water body remains nearly constant. The maximum contaminant concentration in the river in the vicinity of a water intake for drinking water located 21 km downstream is attained about 9 h from the beginning of the flood; concentration of NH4-N can reach values several times larger than acceptable concentration guidelines. The initial primary peak in contaminant concentration at the water intake is followed by a slight decrease in contaminant concentration; a second peak related to the contaminant transport through the inundated floodplain subsequently occurs, after which the concentration slowly decreases, reaching acceptable values after 30–40 h. Contaminated floodplain water bodies located near drinking water supply systems are not significant sources of contamination during small and moderate floods, but during high magnitude floods, they can become sources of water pollution. Operational measures that can decrease potential health risks are discussed.

Phenomenological study and application of the combined influence of iron concentration and irradiance on the photo-Fenton process to remove micropollutants

The presence of low concentrations of persistent pollutants in waters (μg/L or ng/L), also called micropollutants, brings as a consequence the need to apply advanced oxidation treatments for their removal. The successful application of solar-driven photo-Fenton to treat highly polluted wastewaters (g/L and mg/L of pollutants) has prompted its application to lowly polluted effluents. However, a decrease in contaminant concentration may involve an alteration in the intrinsic process phenomenon, which until now has only been widely studied at the milligram-per-litre level or higher with this process. The aim of this research was to study the combined influence of the operating variable (iron concentration) and the environmental variable (irradiance) and application on the photo-Fenton process at pH2.8 when removing micropollutants. For this purpose, experimentation was carried out at laboratory and pilot plant scales with a biocide mixture of acetamiprid (ACTM), thiabendazole (TBZ) and imazalil (IMZ) (100μg/L each) as the model pollutant. Results indicated that above 15WUV/m2 and a light path length of 5cm (the most commonly used path for this type of application) iron concentration limited the process and there was irradiance excess under these conditions. On the other hand, and given the circumstances of irradiance excess, a higher light path length (10cm) was assessed. Results showed that path lengths wider than 5cm are recommended since more wastewater volume could be treated with a higher process rate per surface unit.

Bioremediation of an aged diesel oil-contaminated Antarctic soil: Evaluation of the “on site” biostimulation strategy using different nutrient sources

Under Antarctic conditions, bioremediation processes are highly limited. Although chronically contaminated soils seem to require no bioaugmentation, biostimulation proved to be beneficial although diverse results have been reported in relation to the type of nutrient source and the best experimental design. In this work we evaluated, in “on site” land plots, the effect that on the hydrocarbon removal and bacterial community structure of a fuel contaminated soil have an inorganic salts mixture, a complex organic matrix (fish meal) and a commercial product listed by the EPA. Also the effect of a surface active compound (Brij700) on biodegradation process was studied. Brij700 did not improve biodegradation in any of the studied conditions but induced relevant changes on bacterial community of soil amended with fish meal. Although fish meal significantly enhanced bacterial counts, this effect was unspecific, drastically changed the bacterial community structure and did not improve hydrocarbon removal. Salts amended systems evidenced a non significant decrease in contaminant concentration. Commercial product caused the higher reduction (49.4%, p < 0.05) of hydrocarbons compared with the control system after 45 d of treatment and cause the minor changes in bacterial community, constituting a promising alternative for some hydrocarbon-contaminated Antarctic soil restoration.

Functional genes reveal the intrinsic PAH biodegradation potential in creosote-contaminated groundwater following in situ biostimulation

A small-scale functional gene array containing 15 functional gene probes targeting aliphatic and aromatic hydrocarbon biodegradation pathways was used to investigate the effect of a pilot-scale air sparging and nutrient infiltration treatment on hydrocarbon biodegradation in creosote-contaminated groundwater. Genes involved in the different phases of polycyclic aromatic hydrocarbon (PAH) biodegradation were detected with the functional gene array in the contaminant plume, thus indicating the presence of intrinsic biodegradation potential. However, the low aerobic fluorescein diacetate hydrolysis, the polymerase chain reaction (PCR) amplification of 16S rRNA genes closely similar to sulphate-reducing and denitrifying bacteria and the negligible decrease in contaminant concentrations showed that aerobic PAH biodegradation was limited in the anoxic groundwater. Increased abundance of PAH biodegradation genes was detected by functional gene array in the monitoring well located at the rear end of the biostimulated area, which indicated that air sparging and nutrient infiltration enhanced the intrinsic, aerobic PAH biodegradation. Furthermore, ten times higher naphthalene dioxygenase gene copy numbers were detected by real-time PCR in the biostimulated area, which was in good agreement with the functional gene array data. As a result, functional gene array analysis was demonstrated to provide a potential tool for evaluating the efficiency of the bioremediation treatment for enhancing hydrocarbon biodegradation in field-scale applications.

Effectiveness of different oxidizing agents for removing sodium dodecylbenzenesulphonate in aqueous systems

The present study investigates the efficacy of various oxidizing treatments (ClO −, ClO 2, KMnO 4, O 3, O 3/H 2O 2, O 3/activated carbon) to remove from waters sodium dodecylbenzenesulphonate (SDBS), considered as model surfactant. Results obtained show that the use of ClO − and ClO 2 does not cause appreciable SDBS degradation. Additionally, in the case of ClO −, trihalomethanes are generated, increasing system toxicity. Because the reaction kinetics between SDBS and KMnO 4 is very slow, a decrease in contaminant concentration is not observed, even at very acid pH values. SDBS reactivity with ozone is very low, with a kinetic constant ( k O 3 ) of 3.68 M −1 s −1, but its reactivity with HO radicals is very high ( k OH = 1.16 × 10 10 M −1 s −1), therefore O 3/H 2O 2 and O 3/activated carbon, which can also generate HO , appear as promising advanced oxidation processes to remove this contaminant from waters. The method based on ozone and activated carbon was the only process studied that produced both an increase in SDBS removal rate (due to the generation of HO radicals in the O 3–PAC or O 3–GAC interaction) and a considerable reduction in the concentration of dissolved organic carbon in the system due to the PAC adsorbent properties.

Applicability of Stable Isotope Fractionation Analysis for the Characterization of Benzene Biodegradation in a BTEX-contaminated Aquifer

In recent years the analysis of stable isotope fractionation has increasingly been used for characterizing and quantifying biodegradation of contaminants in aquifers. The correlation of carbon and hydrogen isotope signatures of benzene in a BTEX-contaminated aquifer located in the area of a former hydrogenation plant gave indications that biodegradation mainly occurred under anoxic conditions. This finding was consistent with the investigation of hydrogeochemical conditions within the aquifer. Furthermore, the biodegradation of benzene was calculated by changes in carbon isotope signatures using the Rayleigh-equation-streamline approach. Since contaminant concentrations can be also affected by nonisotope-fractionating abiotic processes such as dilution, volatilization, or irreversible sorption to the aquifer matrix, the Rayleigh-equation-streamline approach was adjusted for scenarios assuming that biodegradation and abiotic processes occur either consecutively or simultaneously along a groundwater flow path between contaminant source and sampling well. The results of the scenarios differed significantly, indicating that an abiotic process (typically dilution) causes a decrease in benzene concentration within the investigated aquifer transect. This comparison of results derived from the different scenarios can help to identify whether biodegradation is the predominant process for decrease in contaminant concentration. However, for a proper quantification of biodegradation, the temporal sequence between biodegradation and dilution needs to be known. The uncertainty associated with the quantification of pollutant biodegradation by the Rayleigh-equation-streamline approach increases when nonisotope-fractionating abiotic processes cause a significant decrease in contaminant concentrations.

Re‐evaluation of treatment approach for a site contaminated with Freon‐113 and 1,1,1‐trichloroethane

AbstractThis study demonstrates a remedial approach for completing the remediation of an aquifer contaminated with 1,1,2‐trichlorotrifluoroethane (Freon‐113) and 1,1,1‐trichloroethane (TCA). In 1987, approximately 13,000 pounds of Freon‐113 were spilled from a tank at an industrial facility located in the state of New York. The groundwater remediation program consisted of an extraction system coupled with airstripping followed by natural attenuation of residual contaminants. In the first phase, five recovery wells and an airstripping tower were operational from April 1993 to August 1999. During this time period over 10,000 pounds of CFC‐13 and 200 pounds of TCA were removed from the groundwater and the contaminant concentrations decreased by several orders of magnitude. However, the efficiency of the remediation system to recover residual Freon and/or TCA reduced significantly. This was evidenced by: (1) low levels (&lt; 10 ppb) of Freon and TCA captured in the extraction wells and (2) a slight increase of Freon and/or TCA in off‐site monitoring wells. A detailed study was conducted to evaluate the alternative for the second‐phase remediation. Results of a two‐year groundwater monitoring program indicated the contaminant plume to be stable with no significant increase or decrease in contaminant concentrations. Monitored geochemical parameters suggest that biodegradation does not influence the fate and transport of these contaminants, but other mechanisms of natural attenuation (primarily sorption and dilution) appear to control the fate and transport of these contaminants. The contaminants appear to be bound to the soil matrix (silty and clay units) with limited desorption as indicated by the solid phase analyses of contaminant concentrations. Results of fate and transport modeling indicated that contaminant concentrations would not exceed the action levels in the wells that showed a slight increase in contaminant concentrations and in the downgradient wells (sentinel) during the modeled timeframe of 30 years. This feasibility study for natural attenuation led to the termination of the extraction system and a transaction of the property, resulting in a significant financial benefit for the original site owner. © 2003 Wiley Periodicals, Inc.

Contaminant Spread and Flushing in Fractured Rocks Near Oak Ridge, Tennessee

AbstractLiquid wastes, including metals dissolved in nitric acid, were discharged into the S‐3 Ponds from 1951 to 1983. During this period, contaminants in ground water spread along shallow fracture flow paths toward nearby streams. Also, a high concentration of nitrate in one well at a depth of 110 to 240 in shows that some contaminants may have moved downdip because of differences in fluid density. Neutralization of the ponds in June 1983 caused a dramatic decrease in the contaminant concentrations of Bear Creek about three months later. Since then, the contaminant concentrations of Bear Creek have decreased at a first‐order exponential rate. This average rate, which is the same for both more reactive and less reactive constituents, can be interpreted to show that the contaminant reservoir consists of the unfractured rock matrix.Flushing caused by the natural recharge and discharge of ground water is occurring at all locations, but contaminant concentrations are controlled by the relative rates of molecular diffusion from the rock matrix and advection along the fracture flow paths. Hushing has thus been most effective near the water table. If the exponential decrease in contaminant concentrations continues, water in Bear Creek will meet drinking water standards by 2012: regardless of any remedial action, contaminants will remain in the rocks for many years.

The organic geochemistry of a sanitary landfill leachate plume

Leachate from the North Bay municipal landfill has contaminated an unconfined, sandy aquifer throughout the 700 m flow system from the site to a discharge zone at a creek. The major organic contaminants identified are aromatic hydrocarbons, especially substituted benzenes. The high groundwater velocity of about 75 m yr −1 and the low organic sorption properties of the sand have permitted non-transformed contaminants to spread throughout the total flow system. There is considerable temporal and spatial variability in groundwater chemistry. Most of the aqueous organic carbon has a nominal molecular weight of <2000 and the general decrease in the mass of this fraction relative to Cl indicates it is being mineralized significantly during transport. IR spectra indicate a general trend of increased aromaticity and decreased OH content of organic matter along the flow system. The aqueous organic matter has a significant apparent complexing capacity and so it is somewhat surprising that toxic metal concentrations in leachate-impacted groundwaters are low. In the leachate plume, this complexing capacity is taken up by major cations and H + and to a far lesser extent by toxic metals such as Pb, Cd or Zn. Dispersion is clearly responsible for considerable decrease in contaminant concentration along the flow system. Biotransformation under strictly anaerobic conditions has probably caused 1,1,1-trichloroethane and trichloroethylene to be restricted to the immediate vicinity of the landfill. A simple method of comparing the concentrations of pairs of organics at points along the flow system provides relative transformation rates for pairs of organics even with variable inputs from the landfill and dispersive dilution. Relative to ethylbenzene, o-xylene is rapidly lost from this system. O-xylene may be less persistent than m- or p-xylene; a result unexpected from previous studies of these dimethylbenzenes. In the initial, strictly anarobic segment of the flow system 1,2,4-trimethylbenzene and 1,4-dichlorobenzene are equally persistent, but in the final, less anaerobic segment, the former appears to be degraded more rapidly than the latter. Contaminant distributions in aquifers reflect the results of a number of processes integrated in a complex manner and so are difficult to interpret in terms of specific processes. However, they do provide evidence for what processes are most significant in real groundwater systems and they will also provide critical tests of how well laboratory-derived information relates to real groundwater contamination situations.