Marisol Vega

TEMPORAL EVOLUTION OF GROUNDWATER COMPOSITION IN AN ALLUVIAL AQUIFER (PISUERGA RIVER, SPAIN) BY PRINCIPAL COMPONENT ANALYSIS

Abstract A set of quantitative analytical data from the alluvial aquifer of the Pisuerga river, located at the north-east of Valladolid (Spain), has been processed by multivariate statistical techniques in order to investigate the evolution of the groundwater composition between two surveys. The original matrix consisted of 16 physico-chemical variables, determined in 32 groundwater samples collected in 32 sampling sites of the aquifer. The surveys were carried out in October 1994, at the end of the irrigation and low water period just before autumn rains, and in April–May 1995 at the beginning of the irrigation and high water period and after the recharge of aquifer. The experimental 64×16 matrix was analysed by Principal Component Analysis (PCA), and the resulting Principal Components (PCs) and Varimax rotated PCs (VFs) analysed by means of box and bivariate plots. The exploration of the correlation matrix allowed to uncover strong associations between some variables (sulphate, chloride, hydrogencarbonate, sodium, potassium, magnesium, calcium and electrical conductivity) as well as a lack of association between the others (pH, nitrate, and trace metals). PCA showed the existence of up to five significant PCs which account for 71.39% of the variance. Two of them can be initially assigned to ‘mineralization’ whereas the other PCs are built from variables indicative of pollution. Box and bivariate plots of the two first PCs showed that ‘mineralization’ is maintained through surveys and that trace elements increase from the first to the second survey in all zones, as a possible consequence of soil leaching caused by the rise of the groundwater level. Varimax rotation allowed to ‘break up’ the ‘mineralization’ PC into two VFs, assigned respectively to ‘natural’ mineralization and to ‘saline’ man-made contamination (sodium and chloride). Box and bivariate plots of VFs confirmed the existence of a differential ‘natural’ mineralization pattern, now ascribed exclusively to dissolution of geological materials, as well as the presence of a zone with unusually high levels of ‘saline’ contamination attributed to leakages from sanitary systems. The plot of VFs also allowed to visualise seasonal variations, identifying a process of dilution of the ‘natural’ mineralization during the recharge of the aquifer, and an increase of the ‘saline’ contamination, assigned to a variety of non-natural causes.

Determination and speciation of heavy metals in sediments of the Pisuerga river

Abstract We have analysed the contents and speciation of Zn, Cd, Pb, Cu, Ni and Co in sediments taken in the Pisuerga river, that flows through the town of Valladolid and is polluted by industrial and municipal effluents. All heavy metals were determined by voltammetric methods, and their mean total contents were (in μgg−1): Zn, 245.49; Cd, 1.05; Pb, 18.77; Cu, 66.53; Ni, 46.51; and Co, 11.41. We also determined the % H2O, % loss at 800°C, % CaCO3, % organic matter and % organic N for each sediment. All the data were examinated by principal components analysis in order to explain the behaviour of each metal and sampling point. The speciation was carried out by Tessiers method to find five metal categories: (a) adsorptive and exchangeable, (b) bound to carbonates, (c) bound to reducible phases, (d) bound to organic matter and sulphides and (e) residual metals. Cd and Pb, and to a lesser degree Zn, appeared in fractions (a), (b) and (c), whereas Cu, Ni and Co were mainly found in fractions (c) and (d). The residual metal fractions, taken as a guide for pollution, were found to be similar to the ones obtained in rivers belonging to our same European zone.

Magnetic solid phase extraction based on phenyl silica adsorbent for the determination of tetracyclines in milk samples by capillary electrophoresis

A magnetic solid phase extraction method coupled to capillary electrophoresis is proposed for the determination of tetracycline, oxytetracycline, chlortetracycline and doxycycline in milk samples. Five different magnetic phenyl silica adsorbents covered with magnetite were synthesized by varying the molar ratio of phenyltrimethylsilane and tetramethylorthosilicate; these adsorbents were evaluated in terms of their pH and degree of hydrophobicity for tetracycline retention. The optimal, selected combination of conditions was a pH of 10.0 and a magnetic sorbent ratio of 4:1; under these conditions, the retention capacity ranged from 99.7% to 101.2% for the four tetracyclines analyzed. The elution conditions and initial sample volume of the proposed extraction method were also optimized, and the best results were obtained with 1×10(-3) M acetic acid in methanol as eluent and a 200 ml of sample volume. Under optimal conditions, average recoveries ranged from 94.2% to 99.8% and the limits of detection ranged from 2 to 9 μg l(-1) for the four tetracyclines. After the proposed method was optimized and validated, 25 milk samples of different brands were analyzed, oxytetracycline residues were detected in five samples, in concentrations ranging from 98 to 213 μg l(-1). Subsequent analysis of positive samples by SPE-CE and magnetic solid phase extraction-HPLC revealed than no significant differences were found from results obtained by the proposed methodology. Thus, the developed magnetic extraction is a robust pre-concentration technique that can be coupled to other analytical methods for the quantitative determination of tetracyclines.

A Case of Hydrochemical Characterization of an Alluvial Aquifer Influenced by Human Activities

Abstract21 physico-chemical parameters have been determined in 47 groundwater samples collected in October 1994 from the alluvial aquifer of the Pisuerga river, located at the north-east of Valladolid (north-western Spain). Conventional hydrochemical methods and multivariate statistical analysis have been used to discriminate natural phenomena and pollution processes in the area. It has been demonstrated the existence of a heterogeneity in groundwater mineral contents related to anthropic actions such as irrigation, source of irrigation water and location of polluting sources. This study demonstrates the existence of two sub-units in the alluvial aquifer (left and right river banks) with different hydrochemical characteristics, and identifies three different singular areas in the left bank. Groundwater from this bank showed higher levels of dissolved ions caused by concentration processes derived from the predominant use of groundwater for crop irrigation, whilst in the right bank of the aquifer less saline water from canals is used. Degradation of groundwater quality has been observed in some areas of the left aquifer sub-unit (north, Santovenia and south-central sites) where anomalous high concentrations of sulphate, sodium, chloride and nitrate were measured, and can be related to the presence of potentially polluting sources such as leachates from industrial waste disposals, sewage effluents and use of organic and inorganic fertilizers.

Study of the Contents and Speciation of Heavy Metals in River sediments By Factor Analysis

Abstract Heavy metals Zn, Cd, Pb, Cu, Ni and Co from two spanish river sediments have been determined by voltammetric techniques, along with % CaCO3 and % organic matter, and the whole data set examined by means of Factor Analysis, finding a different behaviour of the metals in each river. Heavy metal speciation in the sediments was carried out by using the Tessiers scheme: Zn and Pb were mainly associated to ‘reducible phases’, Cd and Cu bound to ‘organic matter and sulphides’, and Ni and Co were distributed between the above fractions. The ‘residual’ metals were used as pollution indicators from the rivers, finding a similar behaviour to other industrialized river systems.

Determination of nickel and cobalt in refined beet sugar by adsorptive cathodic stripping voltammetry without sample pretreatment

Abstract Nickel and cobalt have been analysed in refined beet sugar at the μg kg −1 level by adsorptive cathodic stripping voltammetry in the differential-pulse mode (DPCSV) at the hanging mercury drop electrode (HMDE) as their dimethylglyoxime complexes, M(DMG) 2 . Measurements were directly carried out on untreated sugar solutions. The performance of the procedure was compared with electrothermal atomic absorption spectrometry (ETAAS) applied to digested sugar samples, and showed in general better accuracy. The procedure was applied to the determination of these toxic elements in refined beet sugar samples, and concentrations below 50 μg kg −1 nickel and 10 μg kg −1 cobalt were found, which are much lower than the toxic levels of these elements.

Optimisation of a purification method for metal-containing wastewater by use of a Taguchi experimental design

A procedure for purifying waters polluted with metal ions has been designed. The method is based on the precipitation of metals as magnetic ferrite from the alkalinised solution containing iron(II). The working conditions were optimised by using a Taguchi L9(34) experimental design in order to minimise the total residual concentration (TRC) of metal ions in solution. A statistical analysis of the experimental data revealed the most influential factor to be the Fe(II)/metal concentration ratio (F), with a 29.5% contribution, followed by pH (P, 5.2%) and time (H, 2.3%). On the other hand, temperature (T) had little effect on the purification efficiency (1.0%), whereas noise (N, KMnO4) was found to contribute by as much as 22.1%. Maximal purification efficiency (99.99%) is achieved when wastewater samples are treated for 3 h at 50°C and pH 10 in the presence of iron(II) in a ratio Fe(II)/total metal of 15. In these conditions, the process efficiency is also the least influenced by variability in the sample composition, which validates the proposed procedure.

Pseudopolarographic determination of stability constants of labile zinc complexes in fresh water

The formation and distribution of labile zinc species in fresh water were investigated by using a pseudopolarographic approach. Pseudopolarograms were obtained by plotting the anodic stripping voltammetric peak current versus the deposition potential for a series of solutions containing 100 μg/l Zn(II) and increasing amounts of ligand, at the pH of a typical river water sample. Stoichiometry and stability constants of the complexes were evaluated from the shift of the half-wave potential of the pseudopolarogram as a function of the ligand concentration. Four major inorganic ligands occurring in river waters were examined: chloride, hydrogenphosphate, hydrogencarbonate and sulphate, but no shift was obtained for chloride at the concentration levels tested. The species ZnHPO4, Zn(HPO4)4−3, ZnHCO3+, Zn(HCO3)2 and ZnSO4 were found to occur at ligand concentrations ≤ 0.25 M, and their stability constants calculated. The distribution diagram of Zn in the river water investigated showed that Zn species present in concentration ≥ 0.1% of the total amount of metal were: Zn(H2O)2+6 (aquo-ion), ZnHPO4, ZnHCO3+, Zn(HCO3)2, ZnSO4, ZnOH+ and Zn(OH)2. Species of higher coordination index only appear at a ligand concentration superior to that present in fresh water.

Determination of copper and arsenic in refined beet sugar by stripping voltammetry without sample pretreatment.

Copper and arsenic have been analysed in refined beet sugar at the microgram kg-1 level by anodic stripping voltammetry (copper) and cathodic stripping voltammetry (arsenic) in the differential-pulse mode (DPASV and DPCSV) at a hanging mercury drop electrode (HMDE). DPCSV measurements of arsenic are based on its accumulation onto the HMDE as an intermetallic Cu-As compound followed by the reduction of As0 to arsine in hydrochloric acid medium. Measurements were directly carried out on untreated sugar solutions. The performance of the procedures was compared with electrothermal atomic absorption spectrometry and stripping voltammetry applied to digested sugar samples, and showed in general better accuracy. The procedures were applied to the determination of these toxic elements in commercial beet sugar samples of concentrations below 50 micrograms kg-1 copper and 15 micrograms kg-1 arsenic were found, which are much lower that those allowed by the current regulations.

Determination of Zinc, Cadmium and Lead in Untreated Sugar Samples by Anodic Stripping Voltammetry

Trace amounts of zinc, cadmium and lead were determined in refined beet sugar by differential-pulse anodic stripping voltammetry (DPASV) at a hanging mercury drop electrode. Measurements were performed on samples without previous treatment. The optimised procedure allowed the direct determination of metals at the m g kg-1 level. The performance of the procedure was compared with electrothermal atomic absorption spectrometry and DPASV applied to digested samples, and showed better accuracy. The procedure was applied to the determination of toxic elements in commercial beet sugar samples and levels of metals below 35 m g kg-1 Pb, 80 m g kg-1 Zn and 10 m g kg-1 Cd were found.