R. Devesa-Rey

Arsenic release from river sediments in a gold-mining area (Anllóns River basin, Spain): effect of time, pH and phosphorous concentration

The release of arsenic (As) from As-rich riverbed sediments (Anllons River, Spain) was studied. In this area As is associated with pyrite and arsenopyrite and its mobilization has been favoured by past gold-mining activities. The effect of pH and phosphorous (P) concentrations, as well as the kinetics of the As release process were studied. Also, As species in solution were determined as a function of pH and time. The release of As was greatly influenced by pH and it was between 10 and 45 times higher at pH stat 10 than at pH stat 4 for all the sediments studied. The percentages of As released were between 0.1 and 0.8 % of total As at pH stat 4 but increased to values between ~7 and 11 % of total As at pH stat 10. The sediment containing the highest concentration of As released the highest amount of As (~18 mg/kg at pH stat 10). The As release kinetics was also greatly influenced by pH, mostly at alkaline pH. At pH stat 10, the amount of released As increased progressively with time. In contrast, at pH stat 4 only small differences were observed after 24 h in the amounts of As released. At alkaline pH As was released along with Fe and Al, and accompanied by organic matter dissolution. This suggests a determining role of organic matter in the As release behaviour in alkaline media. In contrast, at weakly acid pH, the release of these components was very low, suggesting that As release is controlled mainly by desorption. The speciation of As was influenced by the pH and the release time. At pH stat 4 the major species in solution were As(III) and As(V) (representing 45–69 % and 51–29 % of As released, respectively). In contrast, at pH stat 10 almost all the released As was As(V) (~99 %). The amount of As released from sediments increased with the increase in initial P concentration in solution and it experienced a dramatic increase at added P concentrations above 1 mM. Namely, the percentage of As released by 10 mM P reached ~6 % of total As for the sediments richer in As. The results obtained let us conclude that changes in pH, especially alkalinization, and inputs of P to the river markedly enhanced As mobilization from the Anllons River sediments, thus increasing health risks.

Study of phytopigments in river bed sediments: effects of the organic matter, nutrients and metal composition.

Phytopigment estimation has a considerable interest in the evaluation of freshwater bodies’ quality, because it takes into account the synergistic effect of nutrients like phosphorus or nitrogen on algal growth producing eutrophication. Furthermore, their increasing concentration constitutes the first step in the formation of biofilms on the surface sediments, adding a new and very important element to the dynamic nature of the surface sediments. In this study the distribution of phytoplankton—in terms of chlorophyll-a, chlorophyll-b, phaeophytin-a, phaeophytin-b, total carotenoids, total chlorophyll, and total phaeophytin—was evaluated in river bed sediments. Samples collected at sites with low levels of nutrients (P,N) and metal concentrations showed lower phytopigment concentrations than those collected at the sampling sites affected by sources of pollution. Phytoplankton concentrations were directly and highly related to the organic matter concentrations—in particular to the humic fraction—as well as to the total nitrogen (N), total phosphorus (PT) and available phosphorus (PA) concentrations in sediments. In addition, phytoplankton also correlates positively with Cu, Zn, Fe and Al extracted in oxalate, being Cu the variable that most influences the phytopigment growth. These are essential metals for the metabolism of the phytoplankton, so therefore the increase in metal concentrations can increase algal growth, unless they reach toxic levels.

Allochthonous versus autochthonous naturally occurring organic matter in the Anllóns river bed sediments (Spain)

Despite of the importance of the aquatic ecosystems as organic matter pools within the global carbon cycle, specific studies in river sediments are still insufficient. This study was carried out in an Atlantic basin where nine sites were selected in the lowest middle stretch of the River. The analyses include total organic carbon, C/N ratios, humus composition (humic, fulvic and humin), A2/A4 and A4/A6 ratios, biologically active organic matter (BAOM), water soluble organic matter (WSOC) and coloured water soluble organic carbon (CWSOC). The results suggested a predominance of allochthonous sources to the organic matter, highly influenced by the land uses together with the river hydrodynamics, namely urban and forest at sites 1, 2 and 9, and agricultural at sites 7 and 8. The 38% of the total organic matter was humic and fulvic acids. The biologically active organic matter, which may act as a proxy of microbial activity, showed a mean value of 0.9% of the total organic matter, showing the lowest values at sites with the highest organic matter total, soluble or coloured. This coloured organic matter represented an average of 3.60% of the total organic matter, and showed the highest visible light absorption at those sites more affected by human activities, close to centres of population (1, 2 and 9). This coloured fraction showed also a high degree of photostability. C/N ratios were less effective to discriminate between allochthonous or autochthonous sources, whereas A4/A6 ratios were effective to estimate variations of autochthonous sources, as it measures the variations of phytopigments of the river bed sediments.

Arsenate Retention by Epipsammic Biofilms Developed on Streambed Sediments: Influence of Phosphate

Natural geological conditions together with the impact of human activities could produce environmental problems due to high As concentrations. The aim of this study was to assess the role of epipsammic biofilm-sediment systems onto As (V) sorption and to evaluate the effect of the presence of equimolar P concentrations on As retention. A natural biofilm was grown on sediment samples in the laboratory, using river water as nutrient supplier. Sorption experiments with initial As concentrations 0, 5, 25, 50, 100, 250, and 500 μg L−1 were performed. The average percentage of As sorbed was 78.9 ± 3.5 and 96.9 ± 6.6% for the sediment and biofilm-sediment systems, respectively. Phosphate decreased by 25% the As sorption capactity in the sediment devoid of biofilm, whereas no significant effect was observed in the systems with biofilm. Freundlich, Sips, and Toth models were the best to describe experimental data. The maximum As sorption capacity of the sediment and biofilm-sediment systems was, respectively, 6.6 and 6.8 μg g−1 and 4.5 and 7.8 μg g−1 in the presence of P. In conclusion, epipsammic biofilms play an important role in the environmental quality of river systems, increasing As retention by the system, especially in environments where both As and P occur simultaneously.

Evaluation of Phosphorus Species in the Bed Sediments of an Atlantic Basin: Bioavailability and Relation with Surface Active Components of the Sediment

In this study, total phosphorus (P) concentrations and P fractions in the bed sediments of the Anllóns River (NW Spain) have been examined. The Lowest Effect Level of the Ontario Sediment Quality Guidelines was trespassed for total phosphorus (600 mg kg−1) at three out of the five sampling sites which, in addition, showed an average of 43% of bioavailable phosphate. The inorganic P fractions (loosely bound P, redox-sensitive P, metal oxide-bound P, carbonate and apatite P, and residual P) showed that inorganic P was mostly found as metal oxide bound P. The difference between total phosphorus and the sum of the inorganic P forms determined in these extracts is attributed to P associated to organic compounds being solubilized in some of the extractants, and represented in average 58% of total phosphorus. With the exception of loosely bound P, all the P fractions showed significant positive correlations with each other and with total phosphorus.

Effect of a compost mulch on seed germination and plant growth in a burnt forest soil from NW Spain

Abstract In this work, a revegetation experiment was performed on a forest soil artificially burnt in the laboratory, and the results of the soil without treatment were compared to those of the soil capped with an organic mulch. Plant biomass, as well as the vari-ation of pH, electric conductivity (EC), and phytotoxicity with depth, were recorded. Only a very low plant growth was observed in the uncapped soil. The substrate-amended soil had a higher production than the uncapped soil, despite showing an increased EC due to the mulch. The increase of pH or salinity during burning can be discarded as the cause of the different plant growth, because the values reached were not high enough to justify the negative effect observed. The determination of phytotoxicity after burning the soil at temperatures between 100oC and 500oC sug-gested that the problems observed for seed germination in the burnt soil might be linked to the formation of undetermined phytotoxic substances after soil heating at a temperature near 200oC. Although polycyclic aromatic hydrocarbons followed the same trend as phytotoxicity with temperature, their concentrations in soil were too low to be the cause of the effects observed, so they can be discarded as playing an important role in phytotoxicity.