Geochemistry, stable isotopes and statistic tools to estimate threshold and source of nitrate in groundwater (Sardinia, Italy).

In the European Union, nitrate vulnerable zone (NVZ) should be designed for the mitigation of nitrate (NO3-) contamination caused by agricultural practices. Before establishing new NVZ, the sources of NO3- must be recognized. A geochemical and multiple stable isotopes approach (hydrogen, oxygen, nitrogen, sulfur and boron) and statistical tools were applied to define the geochemical characteristics of groundwater (60 samples), calculate the local NO3- threshold and assess potential sources of NO3- contamination in two study areas (hereafter Northern and Southern), located in a Mediterranean environment (Sardinia, Italy). Results of the integrated approach applied to two case study, permits to highlight the strengths of integrating geochemical and statistical methods to provide nitrate source identification as a reference by decision makers to remediate and mitigate nitrate contamination in groundwater. Hydrogeochemical features in the two study areas were similar: near neutral to slightly alkaline pH, electrical conductivity in the range of 0.3 to 3.9 mS/cm, and chemical composition ranging from Ca-HCO3- at low salinity to Na-Cl- at high salinity. Concentrations of NO3- in groundwater were in the range of 1 to 165 mg/L, whereas the nitrogen reduced species were negligible, except few samples having NH4+ up to 2 mg/L. Threshold values in the studied groundwater samples were between 4.3 and 6.6 mg/L NO3-, which was in agreement with previous estimates in Sardinian groundwater. Values of δ34S and δ18OSO4 of SO42- in groundwater samples indicated different sources of SO42-. Sulfur isotopic features attributed to marine SO42- were consistent with groundwater circulation in marine-derived sediments. Other source of SO42- were recognize due to the oxidation of sulfide minerals, to fertilizers, manure, sewage fields, and SO42- derived from a mix of different sources. Values of δ15N and δ18ONO3 of NO3- in groundwater samples indicated different biogeochemical processes and NO3- sources. Nitrification and volatilization processes might have occurred at very few sites, and denitrification was likely to occur at specific sites. Mixing among various NO3- sources in different proportions might account for the observed NO3- concentrations and the nitrogen isotopic compositions. The SIAR modeling results showed a prevalent NO3- source from sewage/manure. The δ11B signatures in groundwater indicated the manure to be the predominant NO3- source, whereas NO3- from sewage was recognized at few sites. Geographic areas showing either a predominant process or a defined NO3- source where not recognize in the studied groundwater. Results indicate widespread contamination of NO3- in the cultivated plain of both areas. Point sources of contamination, due to agricultural practices and/or inadequate management of livestock and urban wastes, were likely to occur at specific sites.

Determination of traces of Sb(III) using ASV in Sb-rich water samples affected by mining.

Chemical speciation [Sb(V) and Sb(III)] affects the mobility, bioavailability and toxicity of antimony. In oxygenated environments Sb(V) dominates whereas thermodynamically unstable Sb(III) may occur. In this study, a simple method for the determination of Sb(III) in non acidic, oxygenated water contaminated with antimony is proposed. The determination of Sb(III) was performed by anodic stripping voltammetry (ASV, 1-20 µg L(-1) working range), the total antimony, Sb(tot), was determined either by inductively coupled plasma mass spectrometry (ICP-MS, 1-100µgL(-1) working range) or inductively coupled plasma optical emission spectrometry (ICP-OES, 100-10,000 µg L(-1) working range) depending on concentration. Water samples were filtered on site through 0.45 µm pore size filters. The aliquot for determination of Sb(tot) was acidified with 1% (v/v) HNO3. Different preservatives, namely HCl, L(+) ascorbic acid or L(+) tartaric acid plus HNO3, were used to assess the stability of Sb(III) in synthetic solutions. The method was tested on groundwater and surface water draining the abandoned mine of Su Suergiu (Sardinia, Italy), an area heavily contaminated with Sb. The waters interacting with Sb-rich mining residues were non acidic, oxygenated, and showed extreme concentrations of Sb(tot) (up to 13,000 µg L(-1)), with Sb(III) <10% of total antimony. The stabilization with L(+) tartaric acid plus HNO3 appears useful for the determination of Sb(III) in oxygenated, Sb-rich waters. Due to the instability of Sb(III), analyses should be carried out within 7 days upon the water collection. The main advantage of the proposed method is that it does not require time-consuming preparation steps prior to analysis of Sb(III).

Antimony in the soil-water-plant system at the Su Suergiu abandoned mine (Sardinia, Italy): strategies to mitigate contamination.

This study was aimed to implement the understanding of the Sb behavior in near-surface environments, as a contribution to address appropriate mitigation actions at contaminated sites. For this purpose, geochemical data of soil (8 sites), water (29 sites), and plant (12 sites) samples were collected. The study area is located at Su Suergiu and surroundings in Sardinia (Italy), an abandoned mine area heavily contaminated with Sb, with relevant impact on water bodies that supply water for agriculture and domestic uses. Antimony in the soil horizons ranged from 19 to 4400 mg kg(-1), with highest concentrations in soils located close to the mining-related wastes, and concentrations in the topsoil much higher than in the bedrock. The Sb readily available fraction was about 2% of the total Sb in the soil. Antimony in the pore water ranged from 23 to 1700 µg L(-1), with highest values in the Sb-rich soils. The waters showed neutral to slightly alkaline pH, redox potential values indicating oxidizing conditions, electrical conductivity in the range of 0.2 to 3.7 mS cm(-1), and dissolved organic carbon ≤2 mg L(-1). The waters collected upstream of the mine have Ca-bicarbonate dominant composition, and median concentration of Sb(tot) of 1.7 µg L(-1) (that is total antimony determined in waters filtered through 0.45 µm), a value relatively high as compared with the background value (≤0.5 µg L(-1) Sb) estimated for Sardinian waters, but below the limits established by the European Union and the World Health Organization for drinking water (5 µg L(-1) Sb and 20 µg L(-1) Sb, respectively). The waters flowing in the mine area are characterized by Ca-sulfate dominant composition, and median concentrations of 7000 µg L(-1) Sb(tot). Extreme concentrations, up to 30,000 µg L(-1) Sb(tot), were observed in waters flowing out of the slag materials derived from the processing of Sb-ore. The Sb(III) was in the range of 0.8 to 760 µg L(-1) and represented up to 6% of Sb(tot). In the waters collected downstream of the mine, median Sb(tot) concentrations decreased as distance from the mine area increases: 1300 µg L(-1) Sb(tot) in the stream Rio Ciurixeda at 3 km distance, and 25 µg L(-1) Sb(tot) in the main River Flumendosa 15 km further downstream. Attenuation of Sb contamination was mainly due to dilution. Results of modeling, carried out by both EQ3 and Visual MINTEQ computer programs, suggest that sorption of dissolved Sb onto solid phases, and/or precipitation of Sb-bearing minerals, likely give a minor contribution to attenuation of Sb contamination. The slightly alkaline pH and oxidizing conditions might favor the persistence of inorganic Sb(V)-bearing species at long distance in the studied waters. Concentrations of Sb in the plants Pistacia lentiscus and Asparagus ranged from 0.1 to 22 mg kg(-1), with maximum values in plants growing very close to the mining-related wastes. The P. lentiscus grows well on the soils highly contaminated with Sb at Su Suergiu and might be used for revegetation of the Sb-rich heaps, thus contributing to reduce the dispersion of contaminated materials. Major effects of contamination were observed on the water bodies located downstream of the Su Suergiu abandoned mine. The maximum load (16.6 kg Sb per day) to the Flumendosa, the main aquatic recipient, was observed after heavy rain events. Therefore, priorities of mitigation actions should be focused on minimizing the contact of rain and runoff waters on the heaps of mining wastes.