Combining Geoelectrical Measurements and CO2 Analyses to Monitor the Enhanced Bioremediation of Hydrocarbon-Contaminated Soils: A Field Implementation

Hindawi Publishing Corporation Applied and Environmental Soil Science Volume 2016, Article ID 1480976, 15 pages http://dx.doi.org/10.1155/2016/1480976 Research Article Combining Geoelectrical Measurements and CO2 Analyses to Monitor the Enhanced Bioremediation of Hydrocarbon-Contaminated Soils: A Field Implementation Cécile Noel,1,2 Jean-Christophe Gourry,1 Jacques Deparis,1 Michaela Blessing,1 Ioannis Ignatiadis,1 and Christophe Guimbaud2 1 BRGM, 3 avenue Claude Guillemin, 45060 Orléans, France LPC2E, CNRS, 3 avenue de la Recherche Scientifique, 45071 Orléans, France 2 Correspondence should be addressed to Cécile Noel; Received 2 October 2015; Accepted 12 November 2015 Academic Editor: Pantelis Soupios Copyright © 2016 Cécile Noel et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Hydrocarbon-contaminated aquifers can be successfully remediated through enhanced biodegradation. However, in situ monitoring of the treatment by piezometers is expensive and invasive and might be insufficient as the information provided is restricted to vertical profiles at discrete locations. An alternative method was tested in order to improve the robustness of the monitoring. Geophysical methods, electrical resistivity (ER) and induced polarization (IP), were combined with gas analyses, CO2 concentration, and its carbon isotopic ratio, to develop a less invasive methodology for monitoring enhanced biodegradation of hydrocarbons. The field implementation of this monitoring methodology, which lasted from February 2014 until June 2015, was carried out at a BTEX-polluted site under aerobic biotreatment. Geophysical monitoring shows a more conductive and chargeable area which corresponds to the contaminated zone. In this area, high CO2 emissions have been measured with an isotopic signature demonstrating that the main source of CO2 on this site is the biodegradation of hydrocarbon fuels. Besides, the evolution of geochemical and geophysical data over a year seems to show the seasonal variation of bacterial activity. Combining geophysics with gas analyses is thus promising to provide a new methodology for in situ monitoring. 1. Introduction Petroleum hydrocarbon leaks and accidental spills happen commonly during the production, refining, transport, and storage of petroleum. Release of petroleum hydrocarbons into the environment causes damage to ecosystems [1] and to soil and water resources [2]. Increasing demand for drinking water and cropland with population growth requires effective remediation techniques to treat the contamination and to decrease hostile effects on health and environment. In situ remediation techniques such as enhanced bioremediation were shown to be effective in cleanup of the contamination [3, 4]. Enhanced bioremediation involves the addition of nutrients or electron acceptors to the subsurface environment to accelerate the natural biodegradation processes which degrade hydrocarbons [5]. Due to the potential of cost saving of in situ techniques compared to conventional ex situ techniques, there is an economical interest for commercial providers to use enhanced bioremediation [6]. However, these processes remain partially unexploited, mainly because their in situ monitoring, before, during, and after soil treatment operations, is often expensive and technically challenging. Indeed, where significant subsurface heterogeneity exists, conventional intrusive groundwater sampling campaigns can be insufficient to obtain relevant information as they are restricted to costly monitoring piezometers at discrete locations. New monitoring tools are needed to overcome these limitations and make the enhanced bioremediation more reliable and robust, as well as economically competitive. Previous studies suggest that geoelectrical techniques, especially electrical resistivity (ER) and induced polarization (IP), can be used to detect the presence of LNAPLs (Light Nonaqueous Phase Liquids) [7–10] as well as monitoring the effects of their biodegradation [11–14]. Biodegradation 2 processes modify ground electrical properties because they change biophysicochemical conditions in the subsurface: (i) bacteria modify local redox conditions, inducing changes in self-potential (SP) [15]; (ii) microbial activity can produce organic acids and/or carbonic acid that affect the pore water conductivity, modifying both the in-phase and the quadrature conductivity [16]; (iii) during microbial growth and formation of biofilms, biomass can clog pores and potentially change the porosity and hydraulic conductivity, increasing the storage of electrical charges [17, 18]. Thus, ER and IP are expected to be effective nonintrusive tools to monitor enhanced bioremediation. Some studies have implemented geoelectrical methods to characterize hydrocarbon-contaminated sites under biodegradation [19–22], but only few apply these methods for long-term monitoring or to prove the efficiency of enhanced bioremediation [23]. These previous field studies suggest that ER and IP are highly sensitive to the biophysicochemical processes associated with biodegradation. Nevertheless, the interpretation of geophysical data remains challenging partly because several factors may contribute to the observed electrical response (presence of metallic particles or clays, e.g.) and/or influence the electrical response in function of time (variation of water saturation and temperature). That is why geoelectrical methods are often used in conjunction with geochemical measurements (temperature, pH, redox potential, water conductivity, and dissolved oxygen content) to detect changes in the chemical and physical properties of the soil and groundwater. With the aim of minimizing the use of piezometers, microbial activity is followed by studying gas emissions at the ground surface. Indeed, aerobic degradation of hydrocarbons results in the production of CO2 [24]. Due to its limited solubility (in alkalinity saturated groundwaters), CO2 gas will tend to migrate toward the ground surface and surface CO2 fluxes can be directly linked to the biodegradation intensity. Meanwhile, this CO2 production occurs concurrently with natural root and microbial soil respiration. Thus, tools capable of quantifying CO2 sources are also needed. Carbon stable isotopic analyses (ratio of 13 C/12 C) use the fact that petroleum hydrocarbon isotopic signature is distinguishable (range from −18 to −34‰ versus Vienna Pee Dee Belemnite or VPDB [25]) from other aquifer components: for example, carbonate minerals have a signature between +2 and −12‰ versus VPDB [26] and the C3 plants (plants that use the Calvin or C3 cycle of carbon fixation) produce organic matter with isotopic signature near −25 ± 5‰ versus VPDB [25, 27, 28]. Moreover, bacteria can induce a carbon isotopic fractionation as they (...truncated)