Neus Otero

Sulfur and Strontium Isotope Composition of the Llobregat River (Ne Spain): Tracers of Natural and Anthropogenic Chemicals in Stream Waters

The use of sulfur and strontium isotopes as tracers forthe source/s of water contaminants have been applied to thewater of the Llobregat River system (NE Spain). Surfacewater samples from June 1997 were collected from theLlobregat River and its main tributaries and creeks. Thechemistry of most stream waters are controlled mainly bythe weathering of Tertiary chemical sediments within thedrainage basin. The largest variation in δ34Svalues were found in the small creeks with values rangingfrom –9.9 to 15‰, whilst in the main river channels valuesranged from 6.3 to 12.4‰. The 87Sr/86Sr ratio fordissolved strontium ranged from 0.70795 for a non-pollutedsite to 0.70882 for a polluted one. Most of the waters withhigh NO3 and low Ca/Na ratio converge to the same87Sr/86Sr value, pointing to dominant pollutantend member contribution or a mixing of pollutants with anisotopic composition around 0.7083–0.7085. Although theconcentration of the natural inputs in the river forsulfate and strontium are high, as a result of the sulfateoutcrops within the geology of the basin, their isotopiccharacteristics suggest that they can be used as adiscriminating device in water pollution problems. Howeverto establish the detailed characteristics of the isotopesas geochemical tools, specific high-resolution case studiesare necessary in small areas, where the inputs are well known.

Nitrate pollution of groundwater; all right. . ., but nothing else?

Contamination from agricultural sources and, in particular, nitrate pollution, is one of the main concerns in groundwater management. However, this type of pollution entails the entrance of other substances into the aquifer, as well as it may promote other processes. In this study, we deal with hydrochemical and isotopic analysis of groundwater samples from four distinct zones in Catalonia (NE Spain), which include 5 different aquifer types, to investigate the influence of fertilization on the overall hydrochemical composition of groundwater. Results indicate that intense fertilizer application, causing high nitrate pollution in aquifers, also homogenize the contents of the major dissolved ions (i.e.; Cl(-), SO4(2-), Ca(2+), Na(+), K(+), and Mg(2+)). Thus, when groundwater in igneous and sedimentary aquifers is compared, significant differences are observed under natural conditions for Cl(-), Na(+) and Ca(2+) (with p-values ranging from <0.001 to 0.038), and when high nitrate concentrations occur, these differences are reduced (most p-values ranged between 0.054 and 0.978). Moreover, positive linear relationships between nitrate and some ions are found indicating the magnitude of the fertilization impact on groundwater hydrochemistry (with R(2) values of 0.490, 0.609 and 0.470, for SO4(2-), Ca(2+) and Cl(-), respectively). Nevertheless, the increasing concentration of specific ions is not only attributed to agricultural pollution, but to their enhancing effect upon the biogeochemical processes that control water-rock interactions. Such results raise awareness that these processes should be evaluated in advance in order to assess an adequate groundwater resources management.

Sulphur isotopes as tracers of the influence of potash mining in groundwater salinisation in the Llobregat Basin (NE Spain)

Conventional chemical data for spring and river waters are presented together with sulphur isotopic data for dissolved sulphate to elucidate the source of water salinisation in the middle section of the Llobregat River. As dilution processes do not affect sulphur isotopic composition, the analysis of delta34S of dissolved sulphate in waters provides an excellent tool for quantifying the environmental impact caused by the mining activity existing in the area. The delta34S of dissolved sulphate from mining effluents and saline springs unrelated to mining activity was analysed. The results obtained range from + 18 per thousand to + 20 per thousand (VCDT) for mining effluents and from + 10 per thousand to + 14 per thousand (VCDT) for natural saline springs. These values are in accordance with the pattern of sulphur isotopic composition of sulphates from the evaporite materials of this area. This distinctive isotopic composition has allowed us to determine the origin of salinity in those cases in which chemical features are not conclusive. In addition, two fertilisers widely used in the studied area are chemically and isotopically characterised and their contribution to groundwater salinisation is assessed.

Multi-isotope (carbon and chlorine) analysis for fingerprinting and site characterization at a fractured bedrock aquifer contaminated by chlorinated ethenes

The use of compound specific multi-isotope approach (C and Cl) in the characterization of a chlorinated ethenes contaminated fractured aquifer allows the identification of several sources and contaminant plumes, as well as the occurrence of biodegradation and mixing processes. The study site is located in Spain with contamination resulting in groundwater concentrations of up to 50mg/L of trichloroethene (TCE), the most abundant chlorinated ethene, and 7 mg/L of tetrachloroethene (PCE). The potential sources of contamination including abandoned barrels, an underground tank, and a disposal lagoon, showed a wide range in δ(13)C values from -15.6 to -40.5‰ for TCE and from -18.5 to -32.4‰ for PCE, allowing the use of isotope fingerprinting for tracing of the origin and migration of these contaminants in the aquifer. In contrast, there is no difference between the δ(37)Cl values for TCE in the contaminant sources, ranging from +0.53 to +0.66‰. Variations of δ(37)Cl and δ(13)C in the different contaminant plumes were used to investigate the role of biodegradation in groundwater. Moreover, the isotopic data were incorporated into a reactive transport model for determination of whether the isotope pattern observed downstream from the tanks source could be explained by the simultaneous effect of mixing and biodegradation. The results demonstrate that a multi-isotope approach is a valuable tool for characterization of complex sites such as fractured bedrock aquifer contaminated by multiple sources, providing important information which can be used by consultants and site managers to prioritize and design more successful remediation strategies.

Carbon isotope fractionation of 1,1,1-trichloroethane during base-catalyzed persulfate treatment.

The extent of carbon isotope fractionation during degradation of 1,1,1-trichloroethane (1,1,1-TCA) by a base-catalyzed persulfate (S₂O₈(2-)) treatment system was investigated. Significant destruction of 1,1,1-TCA was observed at a pH of ∼12. An increase in the NaOH:S₂O₈(2-) molar ratio from 0.2:1 to 8:1 enhanced the reaction rate of 1,1,1-TCA by a factor of ∼5 to yield complete (>99.9%) destruction. An average carbon isotope enrichment fractionation factor which was independent of the NaOH:S₂O₈(2-) molar ratio of -7.0 ± 0.2‰ was obtained. This significant carbon isotope fractionation and the lack of dependence on changes in the NaOH:S₂O₈(2-) molar ratio demonstrates that carbon isotope analysis can potentially be used in situ as a performance assessment tool to estimate the degradation effectiveness of 1,1,1-TCA by a base-catalyzed persulfate system.

Nitrate as a tracer of groundwater flow in a fractured multilayered aquifer

Abstract Knowledge of the processes that control nitrate migration and its geochemical evolution in the subsurface are fundamental for the regional management of polluted aquifers. In this paper, the spatial distribution and transient variations of nitrate concentrations, associated with manure fertilization, are used to depict hydrogeological dynamics within the sedimentary aquifer system of la Plana de Vic in the Osona region of Spain. Flow systems are identified from geological, hydraulic head, hydrochemical and isotopic data, and by considering nitrate itself as a tracer that indicates how flow paths are modified by human pressures. In this area, nitrates move through fractured aquitards in flows induced by groundwater pumping. Moreover, the lack of casing in the boreholes permits a mixing of groundwater from distinct layers inside the wells, which negates any benefits from the low-nitrate groundwater found in the deepest aquifer layers. Therefore, impacts on groundwater quality are related to intensive manure fertilization as well as to inadequate well construction and exploitation strategies. Citation Menció, A., Mas-Pla, J., Otero, N. & Soler, A. (2011) Nitrate as a tracer of groundwater flow in a fractured multilayered aquifer. Hydrol. Sci. J. 56(1), 108–122.

Characterizing sources and natural attenuation of nitrate contamination in the Baix Ter aquifer system (NE Spain) using a multi-isotope approach

Nitrate pollution is a widespread issue affecting global water resources with significant economic and health effects. Knowledge of both the corresponding pollution sources and of processes naturally attenuating them is thus of crucial importance in assessing water management policies and the impact of anthropogenic activities. In this study, an approach combining hydrodynamic, hydrochemical and multi-isotope systematics (8 isotopes) is used to characterize the sources of nitrate pollution and potential natural attenuation processes in a polluted basin of NE Spain. δ2H and δ18O isotopes were used to further characterize the sources of recharge of the aquifers. Results show that NO3- is not homogeneously distributed and presents a large range of concentrations, from no NO3- to up to 480mgL-1. δ15N and δ18O of dissolved NO3- identified manure as the main source of nitrate, although sewage and mineral fertilizers can also be isotopically detected using boron isotopes (δ11B) and δ34S and δ18O of dissolved sulphate, respectively. The multi-isotope approach proved that natural denitrification is occurring, especially in near-river environments or in areas hydrologically related to fault zones. δ34S and δ18O indicated that denitrification is not driven by pyrite oxidation but rather by the oxidation of organic matter. This could not be confirmed by the study of δ13CHCO3 that was buffered by the entanglement of other processes and sources.

C, Cl and H compound-specific isotope analysis to assess natural versus Fe(0) barrier-induced degradation of chlorinated ethenes at a contaminated site.

Compound-specific isotopic analysis of multiple elements (C, Cl, H) was tested to better assess the effect of a zero-valent iron-permeable reactive barrier (ZVI-PRB) installation at a site contaminated with tetrachloroethene (PCE) and trichloroethene (TCE). The focus was on (1) using (13)C to evaluate natural chlorinated ethene biodegradation and the ZVI-PRB efficiency; (2) using dual element (13)C-(37)Cl isotopic analysis to distinguish biotic from abiotic degradation of cis-dichloroethene (cis-DCE); and (3) using (13)C-(37)Cl-(2)H isotopic analysis of cis-DCE and TCE to elucidate different contaminant sources. Both biodegradation and degradation by ZVI-PRB were indicated by the metabolites that were detected and the (13)C data, with a quantitative estimate of the ZVI-PRB efficiency of less than 10% for PCE. Dual element (13)C-(37)Cl isotopic plots confirmed that biodegradation was the main process at the site including the ZVI-PRB area. Based on the carbon isotope data, approximately 45% and 71% of PCE and TCE, respectively, were estimated to be removed by biodegradation. (2)H combined with (13)C and (37)Cl seems to have identified two discrete sources contributing to the contaminant plume, indicating the potential of δ(2)H to discriminate whether a compound is of industrial origin, or whether a compound is formed as a daughter product during degradation.

Nitrate attenuation potential of hypersaline lake sediments in central Spain: flow-through and batch experiments.

Complex lacustrine systems, such as hypersaline lakes located in endorheic basins, are exposed to nitrate (NO3(-)) pollution. An excellent example of these lakes is the hypersaline lake located in the Pétrola basin (central Spain), where the lake acts as a sink for NO3(-) from agricultural activities and from sewage from the surrounding area. To better understand the role of the organic carbon (Corg) deposited in the bottom sediment in promoting denitrification, a four-stage flow-through experiment (FTR) and batch experiments using lake bottom sediment were performed. The chemical, multi-isotopic and kinetic characterization of the outflow showed that the intrinsic NO3(-) attenuation potential of the lake bottom sediment was able to remove 95% of the NO3(-) input over 296days under different flow conditions. The NO3(-) attenuation was mainly linked with denitrification but some dissimilatory nitrate reduction to ammonium was observed at early days favored by the high C/N ratio and salinity. Sulfate reduction could be neither confirmed nor discarded during the experiments because the sediment leaching masked the chemical and isotopic signatures of this reaction. The average nitrogen reduction rate (NRR) obtained was 1.25mmold(-1)kg(-1) and was independent of the flow rate employed. The amount of reactive Corg from the bottom sediment consumed during denitrification was 28.8mmol, representing approximately 10% of the total Corg of the sediment (1.2%). Denitrification was produced coupled with an increase in the isotopic composition of both δ(15)N and δ(18)O. The isotopic fractionations (ε of (15)N-NO3(-) and (18)O-NO3(-)) produced during denitrification were calculated using batch and vertical profile samples. The results were -14.7‰ for εN and -14.5‰ for εO.

Carbon isotope fractionation of chlorinated ethenes during oxidation by Fe2 + activated persulfate

The increased use of persulfate (S(2)O(8)(2-)) for in situ chemical oxidation to treat groundwater and soils contaminated by chlorinated hydrocarbon compounds (CHCs) requires unbiased methods to assess treatment performance. Stable carbon isotope analysis offers a potential tool for assessing the in situ treatment performance of persulfate at sites contaminated with CHCs. This study investigated the extent of C isotope fractionation during oxidation of tetrachloroethene (PCE), trichloroethene (TCE) and cis-dichloroethene (cis-DCE) by persulfate activated by ferrous ion (Fe(2+)). An average carbon isotope enrichment factor ε(bulk) of -4.9‰ for PCE, -3.6‰ for TCE and -7.6‰ for cis-DCE were obtained in batch experiments. Variations in the initial S(2)O(8)(2-)/Fe(2+)/CHC molar ratios did not result in any significant differences in carbon isotope fractionation. The occurrence of carbon isotope fractionation during oxidation and the lack of dependence of enrichment factors upon the S(2)O(8)(2-)/Fe(2+)/CHC molar ratio demonstrate that carbon isotope analysis can potentially be used at contaminated sites as an additional technique to estimate treatment efficacy during oxidation of CHCs by Fe(2+) activated persulfate.