X.L. Otero

Variation with depth and season in metal sulfides in salt marsh soils

Seasonal monitoring of metal sulfides was carried out in four soils ofthe Ría de Ortigueira salt marshes. Soils from the high salt marsh (withsuboxic redox conditions at the surface), had low concentrations of ironsulfides (AVS and pyrite fraction) and thus a low degree of trace metalpyritization (DTMP) in surface layers (0–10 cm), butconcentrations of metals associated with the pyrite fraction increasedconsiderably at depth (27.5 cm). In the low salt marsh soils (withanoxic conditions at the surface) maximum concentrations of metal sulfides werefound in the surface layers of soils colonized by Spartinamaritima. These results are explained by the double effectexerted by roots in strongly reduced soils. On the one hand, they stimulate theactivity of sulfate-reducing bacteria and on the other, they favour the partialoxidation of the soil, thus generating polysulfides with which Fe2+immediately precipitates as pyrite, whereas in the deepest, permanently anoxiclayers, pyrite must be formed in a reaction in which FeS is an intermediate, asfollows: FeS + H2S→ FeS2 + H2.Concentrations of metal sulfides also varied greatly with the season, with twopatterns being distinguished. In soils colonized by S.maritima in both high and low salt marshes, the lowestconcentrations were found in summer. At this time of the year there is a netloss of metal sulfides throughout the profile, presumably due to physiologicalactivity of plants (evapotranspiration and release of oxygen from roots). Incontrast, maximum concentrations of AVS and pyritic metals were found in thesummer in the low salt marsh soils not colonized by vascular plants (creekbottom). In this case, the higher temperatures increased the activity ofsulfur-reducing bacteria leading to synthesis and accumulation of metalsulfidesin the soil.

Relationships between vegetation and environmental characteristics in a salt-marsh system on the coast of Northwest Spain

This study reports an investigation of relationships between environmental variables (electrical conductivity of groundwater, soil redox potential, water-table depth, and high-tide flooding depth) on vegetation zonation in a salt-marsh system on the coast of northwest Spain. Discriminant analysis indicated that conductivity (a measure of salinity) and redox potential are correlated with vegetation type within the study area. Conductivity declines with increasing altitude and distance from the sea, whereas redox potential does not vary along well-defined large-scale gradients. Soils with the most strongly oxidizing conditions (i.e. moderate salinity, with Eh greater than 200 mV and thus subtoxic levels of Mn2+, Fe2+ and S2-) are occupied by the Halimione portulacoides community. Communities dominated by Juncus maritimus, and Phragmites australis reedbeds, occur at more strongly reducing sites (Eh between 100 and 200 mV, with possibly toxic levels of Mn2+ but not of Fe2+); the presence of these communities may thus be limited by Fe2+ toxicity. The most strongly reducing sites (with Eh low enough for the reduction of Fe3+ to Fe2+) are occupied by Spartina maritima and Scirpus maritimus communities. These communities appear to be tolerant of Fe2+, and even of low concentrations of S2-.

A New Perspective in the Estimation of Postmortem Interval (PMI) Based on Vitreous [ K + ]

The relation between the potassium concentration in the vitreous humor, [K+], and the postmortem interval has been studied by several authors. Many formulae are available and they are based on a correlation test and linear regression using the PMI as the independent variable and [K+] as the dependent variable. The estimation of the confidence interval is based on this formulation. However, in forensic work, it is necessary to use [K+] as the independent variable to estimate the PMI. Although all authors have obtained the PMI by direct use of these formulae, it is, nevertheless, an inexact approach, which leads to false estimations. What is required is to change the variables, obtaining a new equation in which [K+] is considered as the independent variable and the PMI as the dependent. The regression line obtained from our data is [K+] = 5.35 + 0.22 PMI, by changing the variables we get PMI = 2.58[K+] - 9.30. When only nonhospital deaths are considered, the results are considerably improved. In this case, we get [K+] = 5.60 + 0.17 PMI and, consequently, PMI = 3.92[K+] - 19.04.

Spatial variation in pyritization of trace metals in salt-marsh soils

In the present study, the pyritization of trace metals was studied in 12soils from 3 salt marshes in the Ría of Ortigueira (NW Spain). Theconcentrations of trace metals in the pyrite fraction were related tophysicochemical conditions, physiographical position in the salt marsh, and thepresence or absence and type of vegetation. Redox conditions in soils from thelow-salt marsh and from the creek bottom were strongly reducing throughout theprofile, and there were higher concentrations of Fe and some trace metals (Cuand Mn) in the pyrite fraction (soluble in HNO3) than in thereactivefraction (soluble in 1N HCl). In contrast, the trace-metal content in pyritefraction in the surface layers of the high-salt marsh was low. In some of thesoils, there was a significant increase in the pyrite content below 25cm, and levels of Fe, Mn and Cu incorporated into this fractionwere similar to those in the reactive fraction. The degree of pyritizationvaried greatly among metals in the order: Cu∼Fe-1NHCl>dithionite-Fe>Ni∼Mn>>Zn>Cr, although when we consideredonly the amorphous forms (ascorbate-Fe) as reactive-Fe, the order was:ascorbate-Fe>>Cu>Ni∼Mn>>Zn>Cr. These differences appearedto be a consequence of the different geochemical behaviour of each metal(mainlyin terms of the thermodynamic stability of sulfides and reaction kinetics),except for Zn. The low concentrations of Zn obtained may have been due to thesolubility of ZnS in 1N HCl, which meant that it was extracted with thereactivefraction. Finally, we observed a direct relationship between DOP and DTMP,whichwas independent of the geochemical behaviour of each metal and of itsconcentration in the soil. Thus, the strong correlation between pyrite-Fe andthe metals associated with this fraction appears to indicate that these metalscoprecipitate with pyrite rather than form metal sulfides.

Heavy metal geochemistry of saltmarsh soils from the Rı́a of Ortigueira (mafic and ultramafic areas, NW Iberian Peninsula)

Concentrations of Fe, Mn, Ni, Cu, Cr and Zn in their total, silicate, organic, reactive and pyrite fractions were determined in soils collected from the Ortigueira saltmarshes (Esteiro, Ladrido and Mera, NW Iberian Peninsula), from sediments of the Landoi and Esteiro Rivers, and from sludge generated by a nearby dunite mine. The Esteiro saltmarsh presented clear enrichments of the four metals studied (first 10 cm), especially of Ni and Cr, whose concentrations were among the highest. It is proposed that the elevated Cr and Ni levels found in the Esteiro saltmarsh were derived from recent contributions of the mine, which are partially discharged into the Landoi River. Total Cu and Zn concentrations were lower than the corresponding ones for Cr and Ni. Under suboxic conditions, Ni, Cr, Cu and Zn were mainly associated with the reactive fraction. Under anoxic conditions, Cu and Ni were associated mainly with the pyrite phase. Pyritic Zn and Cr concentrations were relatively low and similar in all three saltmarshes; however, where anoxic-sulfidic conditions prevailed these two metals were mainly associated with the reactive and organic fractions.

Spatial and seasonal variation in heavy metals in interstitial water of salt marsh soils

The composition of interstitial water collected from a salt marsh in NW Spain showed clear seasonal and spatial variations associated with redox cycles of Fe and S. In the summer, salinity increased in all soils as a consequence of the increase in evapotranspiration. The pH and concentrations of heavy metals also differed with season, but not all environments showed the same variations. Soils not colonized by plants had the highest pH and lowest heavy metal concentrations in the summer. These results support the idea that higher temperatures lead to an increase in the activity of sulfate-reducing bacteria, which in turn leads to an increase in alkalinity and concentration of sulfides in the water. Trace metals tend to precipitate with sulfides under these conditions and are removed from the interstitial water. In contrast, in the soils colonized by Spartina maritima, the oxidation of metal sulfides during the summer led to a decrease in pH and an increase in the metal concentrations in the interstitial water. The results obtained concur with those found for seasonal variations in metal sulfides in soils from the same salt marsh.

Improved estimation of postmortem interval based on differential behaviour of vitreous potassium and hypoxantine in death by hanging

Many formulae are available to estimate the relation between the potassium ([K+]) and hypoxantine ([Hx]) concentration in the vitreous humour and the postmortem interval (PMI). Typically these have been based on a correlation test and linear regression using the postmortal interval as the independent variable and [K+] or [Hx] as the dependent variable in order to estimate the confidence interval. However, a recent study has shown that a more precise measurement of PMI can be obtained if [K+] is used as the independent variable. The regression lines obtained from the most recent deceased subjects with forensic relevance received for autopsy in the Institute of Legal Medicine are [K+] = 5.589 + 0.174PMI and [Hx] = 26.459 + 3.017PMI, by changing the variables, we obtain PMI=3.967[K+] - 19.186 (R2 = 0.688, P < 0.001) and PMI = 0.172 [Hx] + 0.170 (R2 = 0.518, P < 0.001). In this paper we propose the cause of death as an extra factor which modifies the relationship and gives even greater precision in estimating PMI. In cases of death by hanging the results are considerably improved with [K+] = 5.224 + 0.225PMI and [Hx] = 15.161+4.957PMI, respectively, and consequently, PMI = 3.631[K+] - 17.334 (R2 = 0.818, P< 0.001) and PMI = 0.153[Hx] - 0.368 (R2 = 0.757, P < 0.001): the slope is less and the precision is obviously enhanced.

Iron and sulfur geochemistry in semi-arid mangrove soils (Ceará, Brazil) in relation to seasonal changes and shrimp farming effluents

Iron and sulfur are key elements in the biogeochemistry of estuarine soils, in which Fe and sulfate reduction (SR) pathways are important for organic matter decomposition. In the semi-arid coast of NE Brazil, mangroves are characterized by large seasonal variations in weather and the presence of numerous shrimp farms. The objective was to determine the impacts of shrimp farm effluents on iron and sulfur geochemistry in mangrove soils under the semi-arid climate of NE Brazil. A seasonal study was made of two mangrove forest soils (SF, a mangrove forest that directly receives wastewater from shrimp ponds and CS, a control site). Pyrite Fe, oxyhydroxides Fe, acid volatile sulfide, degree of pyritization (DOP), pH, Eh, total organic carbon (TOC) and total S were determined. There was a clear decrease in pyritic Fe and DOP in the SF soils, which may be related to the anaerobic oxidation of pyrite coupled with nitrate reduction, or to the dominance of denitrification over SR. Lower TOC contents in the SF site suggest that below ground decomposition increased in response to eutrophication. The seasonal variations led to important changes in the semi-arid mangrove soils. During the dry period, both soils experienced oxidizing conditions with remarkable loss of reduced and oxidized forms of Fe, which may have important environmental implications as Fe is biolimiting for marine primary production. The data show that both factors (seasonal weather variations and shrimp effluents) play important roles in the geochemical processes that occur in these soils and, thus, may affect their functioning and maintenance.

Quantity, composition and water contamination potential of ash produced under different wildfire severities

Wildfires frequently threaten water quality through the transfer of eroded ash and soil into rivers and reservoirs. The ability to anticipate risks for water resources from wildfires is fundamental for implementing effective fire preparedness plans and post-fire mitigation measures. Here we present a new approach that allows quantifying the amount and characteristics of ash generated under different wildfire severities and its respective water contamination potential. This approach is applied to a wildfire in an Australian dry sclerophyll eucalypt forest, but can be adapted for use in other environments. The Balmoral fire of October 2013 affected 12,694 ha of Sydneys forested water supply catchment. It produced substantial ash loads that increased with fire severity, with 6, 16 and 34 Mg ha(-1) found in areas affected by low, high and extreme fire severity, respectively. Ash bulk density was also positively related to fire severity. The increase with fire severity in the total load and bulk density of the ash generated is mainly attributed to a combination of associated increases in (i) total amount of fuel affected by fire and (ii) contribution of charred mineral soil to the ash layer. Total concentrations of pollutants and nutrients in ash were mostly unrelated to fire severity and relatively low compared to values reported for wildfire ash in other environments (e.g. 4.0-7.3mg As kg(-1); 2.3-4.1 B mg kg(-1); 136-154 P mg kg(-1)). Solubility of the elements analysed was also low, less than 10% of the total concentration for all elements except for B (6-14%) and Na (30-50%). This could be related to a partial loss of soluble components by leaching and/or wind erosion before the ash sampling (10 weeks after the fire and before major ash mobilisation by water erosion). Even with their relatively low concentrations of potential pollutants, the substantial total ash loads found here represent a water contamination risk if transported into the hydrological network during severe erosion events. For example, up to 4 Mg of ash-derived P could be delivered into a single water supply reservoir.

Influence of a turbidite deposit on the extent of pyritization of iron, manganese and trace metals in sediments from the Guaymas Basin, Gulf of California (Mexico)

Abstract A core collected in the Guaymas Basin contained an organic-poor, Mn oxide-rich and (relatively) Fe oxide-rich turbidite layer that affected the distribution of Fe, Mn, C, S and trace metals. Results indicate that sediments not influenced by the turbidite layer achieved a 100% degree of pyritization and, by extension, that pyrite production is Fe-limited in these sediments. In contrast, the mud slide layer apparently supplied enough reactive Fe to transfer essentially 98% of the total S present at the base of the turbidite (17–19 cm) to the pyrite reservoir. C/S ratios showed rapid decreases with depth, from a high of 38 close to the sediment-water interface, to minimum values of 2.8 at the lower limit of the turbidite layer, a ratio equal to the average C/S value of normal marine modern sediments, where concentrations of organic C and pyrite supposedly have attained quasi-steady values. A significant part of the reactive Mn was associated with carbonates (41±12%) and, to a much lower degree, with pyrite (2.7±1.2%). The turbidite layer is currently showing a depletion of Mn relative to the host sediment. It is possible that Mn, a major metal constituent in these sediments, was initially present in high concentrations in the mud slide, but was eventually mobilized and transferred either to the water column or to the sediments immediately below the turbidite layer. Metals associated with this element probably followed the same path, affecting their incorporation into pyrite. The turbidite layer apparently affected the distribution of most of the trace metals associated with pyrite, except maybe Cd, Pb and, to a certain, extent Cr. However, Cu, Cr, Zn, Ni and Co were all found to be highly pyritized (>80%) in the sediments of the Guaymas Basin.