Natural Attenuation Processes Research Articles

A Review of Numerical Models for Oil Penetration, Retention, and Attenuation on Shorelines

Oil spills that reach shorelines greatly increase risks to coastal resources. Understanding how long oil is likely to remain on a shoreline is important in deciding response priorities, areas to clean, and the degree of intervention recommended. Wave action, tides, and currents can relocate oil laterally along the beach, cause oil to penetrate vertically into the sediments, and remove oil from the shoreline. Physico-chemical processes transfer some hydrocarbons to the atmosphere and to the adjacent water column resulting in diminished oil on the shoreline. Oil dispersion, through formation of oil-particulate aggregates, and microbial degradation processes can break down a large fraction of the residual oil remaining on and within shorelines. A comprehensive review of the scientific literature reveals that although there are many models that describe and predict oil transport, behavior, and fate in the sea, few numerical models have been developed for oil stranded on shorelines. Canada’s Multi-Partner Research Initiative Program aims to develop a model-based “Decision Support Tool” that can predict the rates of oil loss that can be achieved from natural attenuation processes and the application of active spill response strategies. This model is built on the understanding of factors controlling: penetration, holding capacity, retention, and the residual capacity (persistence) of oil stranded on shorelines derived from the results of case histories, laboratory, meso-scale tests and field trials. Output from the model is intended to support spill response decision-making by allowing spill responders and the public to visualize the results achieved by natural attenuation versus remedial strategies.

A Review of Numerical Models for Oil Penetration, Retention, and Attenuation on Shorelines

Oil spills that reach shorelines greatly increase risks to coastal resources. Understanding how long oil is likely to remain on a shoreline is important in deciding response priorities, areas to clean, and the degree of intervention recommended. Wave action, tides, and currents can relocate oil laterally along the beach, cause oil to penetrate vertically into the sediments, and remove oil from the shoreline. Physico-chemical processes transfer some hydrocarbons to the atmosphere and to the adjacent water column resulting in diminished oil on the shoreline. Oil dispersion, through formation of oil-particulate aggregates, and microbial degradation processes can break down a large fraction of the residual oil remaining on and within shorelines. A comprehensive review of the scientific literature reveals that although there are many models that describe and predict oil transport, behavior, and fate in the sea, few numerical models have been developed for oil stranded on shorelines. Canada’s Multi-Partner Research Initiative Program aims to develop a model-based “Decision Support Tool” that can predict the rates of oil loss that can be achieved from natural attenuation processes and the application of active spill response strategies. This model is built on the understanding of factors controlling: penetration, holding capacity, retention, and the residual capacity (persistence) of oil stranded on shorelines derived from the results of case histories, laboratory, meso-scale tests and field trials. Output from the model is intended to support spill response decision-making by allowing spill responders and the public to visualize the results achieved by natural attenuation versus remedial strategies.

Bioremediation of toluene by bioaugmentation, biostimulation and natural attenuation

Contamination in subsurface environment is a serious environmental hazard. Main sources of the contamination are petrol, diesel fuel, gasoline at oil refineries, underground storage tanks, transmission pipelines and different industries. Permeable reactive barriers (PRBs), which is a promising technology to remediate groundwater in-situ, are filled with reactive materials for the removal of the contaminants present in groundwater. In this study, groundwater contaminated with toluene is treated in reactor columns by biological processes. This study was conducted to assess the impact of bioaugmentation (BA) and biostimulation (BS) on toluene degradation efficiency. After 44 days of treatment, toluene concentrations were decreased from 5 mg/l to 4.304 mg/l by the natural attenuation treatment (Reactor 2), which represents a 13.9% removal efficiency. Toluene was reduced to 0.0239 mg/l in the biostimulation and bioaugmentation treatment (Reactor 1), which represents a toluene removal efficiency of 99.5%. This study showed that the toluene removal efficiency in the combined BA and BS process was much higher than in natural attenuation (NA) process tested.

Benzene degradation in contaminated aquifers: Enhancing natural attenuation by injecting nitrate

Natural attenuation processes depend on the availability of suitable electron acceptors. At the megasite Zeitz, concentrations of the main contaminant benzene were observed to increase constantly in the lower aquifer to levels of more than 2.5 mM. This was accompanied by decreasing concentrations of sulphate (SO42−), which has been previously shown to be the main electron acceptor for benzene oxidation at this site, resulting in an electron acceptor-limited, sulphidic benzene plume. Therefore, a field experiment was conducted to stimulate benzene biodegradation by injecting nitrate (NO3−) into the sulphidic benzene plume aiming (i) to recycle sulphate by nitrate-dependent sulphide oxidation, and (ii) to serve as direct electron acceptor for benzene oxidation. Within 60 days, 6.74 tons sodium nitrate (NaNO3) were injected into the lower aquifer, and the resulting biogeochemical effects within the benzene plume were monitored for more than one year by chemical and microbiological analyses of groundwater samples taken from various depths of ten monitoring wells located in three observation lines downstream of nitrate injection. Nitrate was microbiologically consumed, as shown by changes in δ15N-NO3− and δ18O-NO3− values, partial nitrite accumulation, and changing ratios of Na+/NO3−. Main electron donors for nitrate reduction were reduced sulphur compounds, verified by changing δ34S-SO42− and δ18O-SO42− values, partially increasing sulphate concentrations, and strongly increasing abundances of typical sulphur-oxidizing, nitrate-reducing bacterial taxa within the nitrate plume. The general absent hydrogen isotope fractionation of benzene, also in the sulphidic, nitrate-free part of the plume, indicates that benzene was not biodegraded by sulphate-reducing consortia. However, detected small carbon isotope fractionation of benzene points to in situ benzene biodegradation processes in the plume, probably supported by nitrate. In conclusion, nitrate injection resulted in changing redox conditions and recycling of sulphate in the sulphidic, sulphate-depleted benzene plume due to microbial oxidation of reduced sulphur species, leading to presumably favored conditions for in situ benzene biodegradation.

Improving Long-term Monitoring of Contaminated Groundwater at Sites where Attenuation-based Remedies are Deployed.

This study presents an effective approach to tackle the challenge of long-term monitoring of contaminated groundwater sites where remediation leaves residual contamination in the subsurface. Traditional long-term monitoring of contaminated groundwater sites focuses on measuring contaminant concentrations and is applicable to sites where contaminant mass is removed or degraded to a level below the regulatory standard. The traditional approach is less effective at sites where risk from metals or radionuclides continues to exist in the subsurface after remedial goals are achieved. We propose a long-term monitoring strategy for this type of waste site that focuses on measuring the hydrological and geochemical parameters that control attenuation or remobilization of contaminants while de-emphasizing contaminant-concentration measurements. We demonstrate how this approach would be more effective than traditional long-term monitoring, using a site in South Carolina, USA, where groundwater is contaminated by several radionuclides. A comprehensive enhanced attenuation remedy has been implemented at the site to minimize discharge of contamination to surface water. The immobilization of contaminants occurs in three locations by manipulation of hydrological and geochemical parameters, as well as by natural attenuation processes. Deployment of our proposed long-term monitoring strategy will combine subsurface and surface measurements using spectroscopic tools, geophysical tools, and sensors to monitor the parameters controlling contaminant attenuation. The advantage of this approach is that it will detect the potential for contaminant remobilization from engineered and natural attenuation zones, allowing potential adverse changes to be mitigated before contaminant attenuation is reversed.

Taxonomic profiling of bacteria and fungi in freshwater sewer receiving hospital wastewater

Consistent discharges of hospital wastewaters (HWWs) pose ecological risk to the biome of the receiving environment with cumulative effect on its healthiness. Understanding the taxonomic profile of microorganisms in the impacted systems is required to establish taxa that are bio-indicators of toxicants, and provide possible taxa for mitigating ecotoxicity of the HWWs. Geochemistry, pollution status and ecotoxicity of heavy metals (HMs) in HWW-impacted sewer (LU) were assessed. The microbiome profiling was based on 16S rDNA and ITS of 18S rDNA metagenomes. The degree of HMs contamination exceeded 50 and HMs pollution load index of LU was severe (1,084), which consequently exerted severe risk (1,411,575 toxic response factors) with very high toxic responses of Co, Cu, Pb, and Cd. Eco-toxicological impact of the HMs on LU skewed microbiome towards Proteobacteria (43%), Actinobacteria (18%), and about 5% apiece for Chloroflexi, Acidobacteria, Plantomycetes, and Bacteroidetes. Likewise, the relative abundance of in LU inclined towards Ascomycota (59%), Basidiomycota (17%) and unclassified Eukarya_uc_p (16%). Exclusively found in LU sediments were 44,862 bacterial species and 42,881 fungi taxa, while 72,877 and 53,971 species of bacteria and fungi, respectively, were found missing. Extinction and emergence of bacteria and fungi taxa in LU were in response to HMs ecotoxicity and the need for natural attenuation processes respectively. The profiled taxa in LU may be plausible in bioremediation strategies of the impacted system, and in designing knowledge-based bioreactor system for the treatment of HWWs before discharge into the environment.

Avaliação das características e impactos do etanol nas contaminações por combustíveis no Estado de São Paulo

Postos de combustíveis são uma das principais fontes de contaminação de solos e águas subterrâneas, estando presente de forma distribuída por todo o Estado de São Paulo, principalmente nas áreas urbanas. Os principais contaminantes associados a essas fontes são hidrocarbonetos aromáticos (benzeno, tolueno, etilbenzeno e xilenos), hidrocarbonetos aromáticos policíclicos e etanol, utilizado por décadas como aditivo na gasolina e combustível. Essa pesquisa buscou avaliar as principais características e fatores condicionantes das contaminações de solo e água subterrânea provenientes dos postos de combustível no Estado de São Paulo. Baseado na análise de 516 áreas contaminadas foi observada baixa correlação da localização regional (geologia e pedologia) com as características da contaminação. Metade das áreas apresentaram plumas de benzeno menor que 29 m, valor inferior ao observado em estudos nos Estados Unidos. A condutividade hidráulica se destacou como um fator condicionante para a extensão das plumas, assim como a ocorrência de processos de atenuação natural. A presença de etanol em concentrações elevadas (> 1000 mg L-1) apareceu associada a plumas de maiores extensões e maiores concentrações de hidrocarbonetos. No entanto, de forma geral as contaminações provocadas por postos de combustíveis apresentaram extensão relativamente pequena (percentil 90 e 75 menores que 73 e 43 m, respectivamente) e persistência limitada pela ocorrência de processos de atenuação natural.

Influence of grain size distribution towards improvements of turbidity, colour and suspended particles in a riverbank filtration process - a column study

Riverbank filtration (RBF) system is an efficient and natural treatment technology which, consists of high capacity pumping wells, located adjacent to river. It takes advantage of natural attenuation processes that occur as surface water infiltrate through the riverbed to produce potable water. This research determined the influence of media size distribution, and its hydraulic conductivity and porosity as filter media, of sand and gravel. The efficiency of column model for riverbank filtration system to remove turbidity, colour and suspended solids was also determined. The best condition of column model to remove contaminants with different flowrates and different types filter media used was identified through this research. The column study was conducted with three different types of filter media; C1 (sand), C2 (gravel) and C3 (sand + gravel) and with three different flowrates; 20 mL/min, 25 mL/min and 40 mL/min. Based on the results, C3 (sand + gravel) shows the highest percentage removal for turbidity, colour and suspended solids, which were 28.5%, 59.1%, 71.4% and 29.4%, respectively, although at difference flowrates. The highest percentage of removal of suspended solids (71.4%) occurs at 25 mL/min. The highest percentage of removal of colour (59.1%) occurs at 40 mL/min. Longer contact time might improve the removal of these contaminants.

Interplay of biofilm growth, NAPL biodegradation and micro-scale heterogeneity in natural attenuation of aquifers delineated by pore-network modelling

The biofilm plays an important role in NAPL bioremediation, natural attenuation and the flow behaviour of contaminant in porous media. Therefore, NAPL biodegradation in the presence of any bacteria in the porous media needs to be considered for more accurate prediction of contaminant remediation from the soil and groundwater. The complex interaction between the NAPL biodegradation/dissolution and physiochemical bacteria (biofilm) growth which depends on both time and space results in difficulties in simulation of NAPL bioremediation in the porous media. In this work, the natural attenuation process is simulated using three-dimensional pore network models to investigate natural bioremediation and dissolution of NAPL in presence of different heterogeneities arising from uncorrelated and correlated pore radii distribution. The impact of an additional phase (biofilm) on transport and NAPL dissolution is dynamically incorporated in the pore network model with simplifying assumptions such as fully available NAPL for biodegradation and dissolution. Under such assumptions, the results indicate that as pore scale heterogeneity (in the form of correlation length of pore radii of pore networks) increases, the biofilm growth decreases resulting in a reduction of NAPL bioremediation and dissolution. As such it is critical to consider pore scale heterogeneity in predicting bioremediation efficiency. Additionally, we found that Kozeny-Carman equation fail to predict the permeability changes due to biofilm growth/extinction.

Occurrence, fate, and mass balance of selected pharmaceutical and personal care products (PPCPs) in an urbanized river

The identification and quantification of pharmaceutical and personal care products (PPCPs) in aquatic ecosystems is critical to further studies and elucidation of their fate as well as the potential threats to aquatic ecology and human health. This study used mass balances to analyse the sources, transformation, and transport of PPCPs in rivers based on the population and consumption habits of residents, the removal level of sewage treatment, the persistence and partitioning mechanisms of PPCPs, hydrological conditions, and other natural factors. Our results suggested that in an urbanized river of Guangzhou City, China, the daily consumption of PPCPs was the main reason for the variety of species and concentrations of PPCPs. Through the determination of PPCPs in the river water samples and a central composite design (CCD) methodology, the dominant elimination mechanisms of caffeine and carbamazepine from river water were photolysis and biodegradation, but that of triclosan was sorption rather than biodegradation. The mass data of 3 PPCPs were estimated and corroborated using the measured data to evaluate the accuracy of the mass balance. Finally, caffeine, carbamazepine and triclosan discharged from the Shijing River into the Pearl River accounted for 97.81%, 99.52%, and 28.00%, respectively, of the total mass of these three compounds in the surface water of Shijing River. The results suggest that photolysis are the main process of natural attenuation for selected PPCPs in surface waters of river systems, and the transfer processes of PPCPs is mainly attributed to riverine advection. In addition, the low concentration of dissolved oxygen inhibited the degradation of PPCPs in the surface water of Shijing River.

Biodegradation of Vegetable Oil Factory Effluent Using Extracellular Lipase Obtained from Alcaligenes Spp.

This study determined the degradation of vegetable oil factory effluent by extracellular lipase obtained from Alcaligenes spp. The extracellular lipase produced by isolated Alcaligenes spp was obtained and concentrated using Glycerol before Gel Filtration Chromatography. The partially purified enzyme obtained from the Gel Filtration Chromatography purification showed optimum activity at a temperature of 55° C and pH 7. The enzyme was then concentrated using glycerol prior to treatment. The raw effluent samples (from the vegetable oil factory) were treated with the partially purified enzyme for 20 days. The degradation activity of the enzymes on contaminant concentrations was monitored at every 5 days. A control experiment was also set up for possible natural degradation of contaminants. Effluents were analyzed for metals (Fe, Mn and Zn), Chemical Oxygen Demand (COD) and Lipid Content using the standard method. Data were analysed for graphical presentation using the Microsoft Excel package. Results showed reduction in Fe, Mn, Zn, COD and Lipid Contents between day 5 and 20 as 46.28%, 68.71%, 62.53%, 71.45% and 72.57% respectively. The application of extracellular enzyme in the treatment of the effluent enhanced the degradation of the effluent at a higher percentage than the natural attenuation process of the effluent. This showed the promising bioremediation potential of Alcaligenes spp.

Performance and Kinetics of Bioaugmentation, Biostimulation, and Natural Attenuation Processes for Bioremediation of Crude Oil-Contaminated Soils

Bioremediation of contaminated sites is usually limited due to the inadequate availability of nutrients and microorganisms. This study was conducted to assess the impact of bioaugmentation (BA) and biostimulation (BS) on petroleum hydrocarbon degradation efficiency. In addition, treatment performance and kinetics of different remediation processes were investigated. For this purpose, four tanks containing oil-contaminated soils were tested. Tank 1 was operated as the natural attenuation process. Then, a microbial inoculum and nutrients were added to tank 2 to promote BA and BS. In tank 3, only the BA process was adopted, whereas in tank 4, only the BS process was adopted. After 63 days of operation, the total petroleum hydrocarbon (TPH) in tank 2 was reduced from 1674 to 430 mg/kg, with 74% reduction. Tank 1, tank 3, and tank 4 indicated TPH reductions of 35%, 41%, and 66%, respectively. Microbiological analysis of the inoculum indicated that Alcanivorax was the dominant bacterium. The population of TPH degrader bacteria in tank 2 soil was two orders of magnitude higher than in the control tank. Reaction rate data were fitted with a first-order reaction rate model. The Monod kinetic constants, maximum specific growth rate (µmax), and substrate concentration at half-velocity constant (Ks) were also estimated. This study showed that the TPH removal efficiency in the combined BA and BS process was higher than in other processes tested. The populations of TPH degrading microorganisms in soil tanks were positively related to TPH removal efficiency during bioremediation of petroleum-contaminated soils.

Heavy metal contamination trends in surface water and sediments of a river in a highly-industrialized region

Surface water and sediment samples from the most contaminated river in Taiwan, Houjing River, were collected for five years (2015 to 2019) and from five locations to study the contamination trends of eight heavy metals (As, Cd, Cr, Cu, Hg, Pb, Ni, and Zn). Five locations along the river were picked based on their potential of being contaminated by nearby industrial activities. The monitoring data, geo-accumulation index (Igeo), contamination factor (CF), modified degree of contamination (mCd), pollution load index (PLI), and metal index (MI) were used to provide a detailed assessment of the heavy metal contamination. Demin (L3) and Chuwai (L4), two sampling sites close to the Nanzih Export Processing Zone (NEPZ) discharge points, were the most contaminated. The most contaminated periods were 2015–2016 and 2018–2019 for surface water and sediment samples, respectively. The surface water showed signs of heavy metal contamination in 2016 with mCd > 1.5. For the sediment samples, the heavy metal concentration fingerprints for the five years were similar. This implied that the natural attenuation process was not adequate, and treatment technologies are required to improve sediment quality. This study provides a basis for comprehensive risk assessments and integrated environmental management measures for the river’s surface water and sediment quality. Also, it would contribute to a more inclusive evaluation of heavy metal contamination in global rivers.

Temporal behavior analysis of leachate contamination in a tropical landfill.

Leachate generation is one of the problems present in the landfills and dumps, needing monitoring and quantification for the environmental conservation. The objective of this paper is to evaluate temporal data of leachate contamination in a municipal solid waste (MSW) landfill, located in the midwest of São Paulo state, Brazil. The physical-chemical parameters and chemical compounds of the local groundwater were evaluated during 13years (2002 to 2015). The methodology included data processing and temporal behavior analysis by Mann-Kendall test. Physical-chemical parameters demonstrated a change from acid to basic medium and decrease of BOD, COD, and TOC concentrations. Chemical compounds demonstrated high concentrations close to the administration building of the landfill, despite the decrease of certain compounds (Al, Fe, Hg, K, Na, Pb, SO42-, total PO43-, Zn) over time in the entire landfill area. The results demonstrate that the landfill is in natural attenuation process, and the structures of the surrounding landscape and the location of the MSW disposal are fundamental to understand the presence and rate of each chemical compound.

Hazard of petrochemical pollution of ponds of the “Olexandria” arboretum (Bila Tserkva)

Groundwater pollution in the territory of Bila Tserkva aircraft repair plant developed in a latent form for decades, in a similar fashion to the territory of any object of the supply of petroleum products. In the early 90s of the last century, the first signs of this pollution were detected in places of natural drainage of the groundwater flow directed to the ponds cascade of the Western Hollow. It should be noted that this cascade of ponds located on the border of the arboretum protects the rest of the park by absorbing pollutants that migrate from the plant territory. More than 10 years of research of the contaminated area with the goal of designing remediation measures began and continued after that. This project was not implemented in full because of a lack of funds. Based on a comparative analysis of the results of ecological and hydrogeological studies of the past years and a modern survey of the western part of the arboretum “Olexandria” and the adjacent territory of the former aircraft repair plant, it was established that pollution of the pond “Poterchata” continues to this day. Over the past 12 years, the scheme of pollutants incoming into ponds has changed. At first, the main stream of oil pollutants directed to the upper reaches of the beam was considered as very dangerous. Nowadays, perhaps due to the remediation measures taken, this flow seems to be exhausted. However, there were signs of discharge of a polluted underground stream in the lower reaches of the pond “Poterchata”. In the soil samples from wells drilled near the water edge, the oil content is 600-900 mg/kg, and in places of water sampling from ponds, the content of dissolved hydrocarbons varies from 2 to 3 mg/dm3, that is, 60 times higher than the standard for fish farms ponds. It is assumed that LNAPL and contaminated groundwater move towards the cascade of ponds of the Western Hollow not in a continuous stream, but in the form of narrow tongues in places of increased conductivity, which are consistent with the lateral shallow gullies crossing the slope of hollow. It is possible that over time, oil pollution will reach the lower “Rusalka” pond. However, one cannot exclude the assumption of natural attenuation processes, the significance of which increased after the closure of the plant and a decrease in the volume of LNAPL because of its extraction from the subsoil. In order to confirm or refute the assumptions made and decide on the need to protect the ponds of the arboretum it has been planned to equip the observation points in places where we should expect influent of pollutants.

Natural attenuation in marine sediments: investigation of the effect of chloride concentration on the mobility of metals

Metals can be mobilized from contaminated sediments under variable environmental conditions. This paper discusses the effects of specific ions of the water column in conjunction with natural attenuation processes on the leaching of metals from marine sediments. In particular, the effect of the salinity and the presence of ions in the seawater, especially the chlorides of the water column, in leaching of metals was examined. Sediment samples were collected from sampling stations in the inner port of Piraeus, Greece. Due to the fact that natural attenuation is a slow procedure which consists of natural, chemical, and biological processes and is influenced by many factors, it was approached with experiments taking place under quite aggressive conditions. Sequential leaching tests in cycles of seven repetitions were performed. The results of these experiments showed that leaching of metals from contaminated sediments to the water column was influenced by the concentration of dissolved constituents. Initially leaching was significant with maximum concentration of leachable copper (Cu) 0.25mg/kg, lead (Pb) 0.0048mg/kg, and zinc (Zn) 0.28mg/kg, and then fell in the last repetitions. The leaching of Cu and Zn from contaminated sediments to the water column was positively correlated to the concentration of chlorides.

Estimation of Self-Neutralisation Rates in a Lignite Pit Lake

Natural anaerobic biogeochemical processes used for passive treatment of AMD were observed in the extensive shallow water zone of a polymictic pit lake in the former German lignite district of Upper Palatinate. Although continuously fed by acidic metalliferous groundwater, lake-pH increased from 3.5 to circumneutral over a little more than 10 years. The natural attenuation processes were studied and quantified using a regional surface- and groundwater flow model linked with hydrochemical monitoring datasets to establish a simple mass balance. The acidity inflow was estimated at ≈ 5900 kmol over the period 2014–2018, which corresponds to an average inflow of ≈ 1190 kmol/a. This estimate is in very good accordance with an acidity inflow rate for the period 2000–2014 estimated from acidity deposition in the sediment based on sediment core analyses plus the calculated cumulative acidity outflow based on extrapolation of pre-neutralisation acidity levels in the pit lake, together yielding a total acidity of ≈ 15,000 kmol, which corresponds to an inflow rate of ≈ 960 kmol/a. The results strongly indicate that the pit lake self-neutralised due to beneficial environmental and ecological conditions, amplified and potentially initialised by the circumneutral discharge from a chemical mine water treatment plant, and that well-known biogeochemical mechanisms such as natural microbial sulfate reduction were the driving force. The results give rise to perspectives concerning the potential development of pit lakes if ecological considerations are considered.

Assessing methyl tertiary butyl ether residual contamination in groundwater using radon

About 15 years ago, a fuelling station in Roma (Italy) was dismissed. When underground tanks were removed, a subsoil NAPL (Non-Aqueous Phase Liquid) contamination came out, showing gasoline leakage from the reservoirs. Monitoring actions took place next and only recently radon dissolved in groundwater was measured for a year and used as tracer of NAPLs in view of its high solubility in these substances. The relative deficit of radon in polluted groundwater compared to radon in background “clean” water allowed us to detect areas where residual gasoline is still located. The source of pollution was identified in correspondence of former gasoline tanks, in agreement with direct measurements of dissolved NAPLs, mainly Methyl Tertiary Butyl Ether (MTBE), a resistant and water-soluble additive introduced in gasoline in place of lead. A short and transient plume of MTBE was occasionally recognized. We hypothesize that the rise of groundwater table enhances removal of MTBE, likely adsorbed onto soil minerals such as zeolites, thus increasing its concentration in water. MTBE levels are then progressively reduced by natural attenuation processes, with half-life of about 23 days. Estimates of MTBE saturation from radon-deficit equations were not reliable because the aquifer is not homogeneous in terms of 226Ra distribution, porosity and emanation power and no equilibrium is reached for radon partitioning between NAPL and water.

Identification of leachate infiltration and its flow pathway in landfill by means of electrical resistivity tomography (ERT).

Sanitary landfills are a well-planned engineering work for final disposal of municipal solid waste in order to minimize the environmental impacts in soil and groundwater. Therefore, several control systems are installed such as liners and leachate and biogas collectors. However, the establishment of landfill in vulnerable areas, the inadequate operations, and failures in collectors and liner systems can cause subsurface contamination. The discovery of eventual leachate leakage usually is based on chemical analyses of groundwater using monitoring wells; which may not be representative in spatial terms. This work involves a geophysical monitoring of the leachate percolation in a landfill waste cell closed in 2014, in which geomembrane boreholes and aquifer contamination have been proved. The DC resistivity method was applied by means of electrical resistivity tomography (ERT) in order to detect eventual contrasts in electrical properties. Twelve ERT lines have been carried out in a steady mesh during three years of study (2016, 2017, and 2018). The results of 2016 allowed a clear contrast between zones of leachate percolation into the aquifer and the natural environment. The comparative analyses of the three-year monitoring results reveal a gradual increase in resistivity values in the areas of leachate percolation. The absence of replacement of biodegradable organic matter after 2014 conditions the gradual decrease of leachate salinity. In this sense, the increase in resistivity is an indicator of the tendency for chemical stabilization of the organic waste and existence of natural attenuation processes.

Disturbance, carbon physicochemical structure, and soil microenvironment codetermine soil organic carbon stability in oilfields

The stability of soil organic carbon (SOC) is crucial for soil quality, fertility, and natural attenuation processes of pollutants. The physicochemical structures of SOC were believed to control its stability, yet has become controversial. Here we hypothesized that disturbance intensity and variations in the soil environment can also influence the SOC stability, and conducted a case study with oil contaminated soils to quantify the contributions to SOC stability of various factors including contamination level, carbon physicochemical structure, and soil properties. Oil contamination led to increased SOC stability, as suggested by appreciably decreased soil CO2 fluxes, the enrichment of the δ13C in the oil contaminated soils, as well as analysis of soil aggregates and humic substances. Redundancy analysis indicated that overall SOC stability were highly correlated to microaggregate (M2), HA/FA, Fe, soil porosity, EC, pH, and total petroleum hydrocarbon (TPH) in oilfields. Variance partitioning analysis showed that carbon physicochemical structure (S), soil properties (P), and oil contamination (O) could explain the variance of overall SOC stability by up to 90%, while 18% of the variation was explained by S × P and 43% by S × P × O. These results show that multiple factors of the disturbance, carbon physicochemical structure, and soil properties should be essential for future studies of SOC stability.