Stéphane Guédron

Atmospheric mercury incorporation in soils of an area impacted by a chlor-alkali plant (Grenoble, France): Contribution of canopy uptake

This study focused on the fluxes of mercury (Hg) and mechanisms of incorporation into soils surrounding a chlor-alkali plant suspected to have emitted up to ~600 kg Hg year(-1) for decades into the atmosphere. Comparison of vertical Hg soil profiles with As, Cu, Ni and Zn (which were not emitted by the plant) support Hg enrichment in surface horizons due to atmospheric Hg inputs from the chlor-alkali plant. Based on chemical extractions and elemental correlations, Hg was found to be weakly leachable and bio-available for plants, and most probably strongly bound to organic matter. In contrast, other trace elements were probably associated with phyllosilicates, iron oxides or with primary minerals. Hg stocks in the surface horizon of a forested soil (1255 mg Hg m(-3)) were two-fold higher than in an agricultural soil (636 mg Hg m(-3)) at a similar distance to the plant. The difference was attributed to the interception of atmospheric Hg by the canopy (most likely gaseous elemental Hg and reactive gaseous Hg) and subsequent litterfall incorporation. Some differences in the ability to trap atmospheric Hg were observed between tree species. The characterization of the litter showed an increasing Hg concentration in the plant material proportional to their degradation stage. In agricultural soils, very low Hg concentrations found in corn leaves and grains suggested a limited uptake via both the foliar and root pathways. Thus, the short-term risk of Hg transfer to agricultural crops and higher levels of the trophic chain appeared limited. A possible risk which remains to be evaluated is the possible transfer of Hg-rich particles from the forest topsoil to downstream aquatic ecosystems during rain and snowmelt events.

(Methyl)Mercury, Arsenic, and Lead Contamination of the World’s Largest Wastewater Irrigation System: the Mezquital Valley (Hidalgo State—Mexico)

In the Mezquital valley, untreated wastewater (45xa0m3xa0s−1) from Mexico City is used for the irrigation of around 900xa0km2 of agricultural soil. High concentrations of metals including methylmercury (3.8u2009±u20092.5xa0ngxa0l−1) and lead (0.16u2009±u20090.05xa0mgxa0l−1) were measured in anoxic wastewater canals. Downstream, dissolved, and particulate polymetallic (Hg, Pb, Cr…) concentrations decreased by factors 10 to 1,000 in the Tula River (which received a mix of fresh and wastewater) due to the dilution and oxidation of surface water, and to the decrease of contaminants concentration in wastewater downstream irrigated soils. However, dissolved and particulate methylmercury concentrations (0.06 to 0.33xa0ngxa0l−1 and 1.6 to 4.5xa0μgxa0kg−1, respectively) remained elevated in comparison to other natural hydrosystems. The monitoring of an irrigation event and the distribution of metals in a soil profile irrigated for more than 80xa0years showed that metals were retained in the draining tilled layer. The oxic conditions and slightly acidic pH (~6.5) in this layer were found favorable for metal adsorption and co-precipitation with redox-sensitive elements (Fe, Mn) and suggestively for mercury demethylation. In the downstream Tula River and groundwater, almost all metallic concentrations remained below guideline thresholds. Only, dissolved As and Pb concentrations remained two to five times above thresholds for drinking water, highlighting a potential health risk for approximately 500,000 people who use groundwater as water supply.

Mercury tissue residue approach in Chironomus riparius: Involvement of toxicokinetics and comparison of subcellular fractionation methods

Along with the growing body of evidence that total internal concentration is not a good indicator of toxicity, the Critical Body Residue (CBR) approach recently evolved into the Tissue Residue Approach (TRA) which considers the biologically active portion of metal that is available to contribute to the toxicity at sites of toxic action. For that purpose, we examined total mercury (Hg) bioaccumulation and subcellular fractionation kinetics in fourth stage larvae of the midge Chironomus riparius during a four-day laboratory exposure to Hg-spiked sediments and water. The debris (including exoskeleton, gut contents and cellular debris), granule and organelle fractions accounted only for about 10% of the Hg taken up, whereas Hg concentrations in the entire cytosolic fraction rapidly increased to approach steady-state. Within this fraction, Hg compartmentalization to metallothionein-like proteins (MTLP) and heat-sensitive proteins (HSP), consisting mostly of enzymes, was assessed in a comparative manner by two methodologies based on heat-treatment and centrifugation (HT&C method) or size exclusion chromatography separation (SECS method). The low Hg recoveries obtained with the HT&C method prevented accurate analysis of the cytosolic Hg fractionation by this approach. According to the SECS methodology, the Hg-bound MTLP fraction increased linearly over the exposure duration and sequestered a third of the Hg flux entering the cytosol. In contrast, the HSP fraction progressively saturated leading to Hg excretion and physiological impairments. This work highlights several methodological and biological aspects to improve our understanding of Hg toxicological bioavailability in aquatic invertebrates.

Amazonian former gold mined soils as a source of methylmercury: Evidence from a small scale watershed in French Guiana

Total mercury (HgT) and monomethylmercury (MMHg) were investigated in a tropical head watershed (1 km(2)) of French Guiana. The watershed includes a pristine area on the hill slopes and a former gold mined flat in the bottomland. Concentrations of dissolved and particulate HgT and MMHg were measured in rain, throughfall, soil water and at three points along the stream. Samples were taken in-between and during 14 storm events at the beginning and middle of the 2005 and 2006 rainy seasons. Dissolved and particulate HgT concentrations in the stream slightly increased downstream, while dissolved and particulate MMHg concentrations were low at the pristine sub-watershed outlet (median = 0.006 ng L(-1) and 1.84 ng g(-1), respectively) and sharply increased at the gold mined flat outlet (median = 0.056 ng L(-1) and 6.80 ng g(-1), respectively). Oxisols, which are dominant in the pristine area act as a sink of HgT and MMHg from rain and throughfall inputs. Hydromorphic soils in the flat are strongly contaminated with Hg (including Hg(0) droplets) and their structure has been disturbed by former gold-mining processes, leading to multiple stagnant water areas where biogeochemical conditions are favorable for methylation. In the former gold mined flat high dissolved MMHg concentrations (up to 0.8 ng L(-1)) were measured in puddles or suboxic soil pore waters, whereas high dissolved HgT concentrations were found in lower Eh conditions. Iron-reducing bacteria were suggested as the main methylators since highest concentrations for dissolved MMHg were associated with high dissolved ferrous iron concentrations. The connection between saturated areas and stagnant waters with the hydrographic network during rain events leads to the export of dissolved MMHg and HgT in stream waters, especially at the beginning of the rainy season. As both legal and illegal gold-mining continues to expand in French Guiana, an increase in dissolved and particulate MMHg emissions in the hydrographic network is expected. This will enhance MMHg bio-amplification and present a threat to local populations, whose diet relies mainly on fish.

Baseline seasonal investigation of nutrients and trace metals in surface waters and sediments along the Saigon River basin impacted by the megacity of Ho Chi Minh (Vietnam).

The Saigon River, Southern Vietnam, crosses one of the most dynamic developing Megacity in Southeast Asia: Ho Chi Minh City (HCMC). The increased economic, industrial, and domestic developments may affect the environmental quality of water and halieutic resources. In this study, we evaluated the seasonal (dry and wet seasons) biogeochemical state of the Saigon River during two snapshot campaigns conducted along the river basin upstream from HCMC; the Saigon River was characterized by slightly acidic (pH 5.7–7.7) and oxygen-depleted water (dissolved oxygen (DO), 0.36–5.18xa0mgxa0l−1). Nutrients (N–NH4xa0=xa00.01–2.41, N–NO3xa0=xa00.14–2.72, and P–PO4xa0=xa0~0–0.42xa0mgxa0l−1), DOC (2.2–8.0xa0mgxa0l−1), POC, and trace metal(oid) (As, Cd, Cr, Cu, Zn, and Hg) concentrations were low showing a good quality of the upstream river. In the urban center area, DO dropped to 0.03xa0mgxa0l−1 accompanied with a rise of nutrient concentrations (e.g., N–NH4, up to 17.7xa0mgxa0l−1) likely originating from wastewater discharges. Trace metal concentrations also rose sharply (e.g., Cr and Hg rose up to 10-fold higher) in both water and sediments but remained under the World Health Organization (WHO) and Vietnamese concentration guidelines. In the downstream estuarine area, the intrusion of marine waters diluted water flowing from HCMC, leading water quality to return close to the state observed upstream from HCMC. In general, levels of nutrient and metal contaminations along the Saigon River during both seasons appear moderate regarding to Vietnamese and WHO guidelines although the urban area is highlighted as the major contributor for metal(oid) emissions. Finally, we showed that apart from wastewater and industrial discharges that affect the river quality, metal(oid) partitioning between solid and solution is controlled by the change in water geochemistry along the continuum during both seasons, such as DO (e.g., for As and Cr) and pH (e.g., for Pb) which drives their sorption/dissolution dynamics.

Spatial variation and risk assessment of trace metals in water and sediment of the Mekong Delta

The Mekong Delta, is home to 17 million inhabitants and faces numerous challenges relating to climate change, environmental degradation and water issues. In this study, we assess trace metals concentrations (Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Mo, Cd, Hg, Pb) in the water, suspended particulate matter and surface sediments of the Tien River, the Northern branch of the Mekong Delta, during both dry and rainy seasons. Metal concentrations in the dissolved and suspended particle phases remain in the low concentration range of the main Asian Tropical River. During transportation in the riverine part, we evidenced that V, Cr, Co, As and Pb are dominant in the particulate phase while Mo, Ni and Cu dominate in the dissolved fraction. In the salinity gradient, dissolved U, V, Mo exhibit conservative behaviour while Ni, Cu, As, Co and Cd showed additive behaviour suggesting desorption processes. In the surface sediment, metal concentrations are controlled by the particle-size, POC contents and Fe, Al and Mn - oxy(hydr)oxides. Calculated Enrichment Factor and Geoaccumulation Index evidenced As enrichment while the calculated mean effect range median quotients evidenced a low to medium ecotoxicological potential effects range in the surface sediments.

How Uncontrolled Urban Expansion Increases the Contamination of the Titicaca Lake Basin (El Alto, La Paz, Bolivia)

Cities in developing countries encounter rapid waves of social transformation and economic development where the environment is mostly a neglected aspect. The Katari watershed encompasses mining areas, El Alto city (one of the fastest growing urban areas in South America and the biggest in the Altiplano) as well as agricultural areas. Its outlet is Cohana Bay, one of the most polluted areas of Lake Titicaca. Here we propose an integrative approach (hydrological, physicochemical, chemical and bacterial data) to understand the pollution problem of this developing area, in which a variety of anthropogenic activities takes place. Both mining and urban areas appear to be sources of metal pollution. Nutrient and bacterial contaminations are mainly related to urban and industrial discharges. These situations have impacts in the basin from the mining area down to Cohana Bay of Lake Titicaca. Pollutant concentration patterns are highly influenced by seasonal hydrology variations. The poor quality of surface waters in the basin represents a risk for human and animal populations, as well as for the quality of aquifers located underneath El Alto city.

Impact of gold mining on mercury contamination and soil degradation in Amazonian ecosystems of French Guiana

As early as the 16th century, major expeditions were searching for gold and other precious metals and ores in the ‘New World’ (Nriagu, 1994; Tandeter, 2006). Later, in the middle of the 19th century, gold rushes that started in California spread to South America, as well as Australia and South Africa (Nriagu, 1994; Ali, 2006). Their main goal was to extract alluvial gold, occurring as fine particles in sediments derived from soils and weathered rocks. Another more current strategy of artisanal mining groups is to extract eluvial gold concentrated in the soil as fine particles and nuggets, originating from the in situ weathering of rocks. The extent of such artisanal small-scale gold mining (ASGM) activities varies with the price of gold (Malm, 1998). According to the United Nations Industrial Development Organization, 10–15 million people worked on ASGM sites throughout the world in 2004 (Veiga and Baker, 2004) in more than 160 mainly developing countries (Telmer and Veiga, 2009). Unfortunately, there is little information about the size and type of ASGM operations (Telmer and Veiga, 2009) or the surface area of soil and sediment worked; thus, the exact spatial extent and magnitude of impacts are difficult to determine. Gold mining changes land use drastically, with a considerable impact on the entire ecosystem, including soil. First, extracting eluvial deposits requires removing vegetation and then using powerful water jets to strip off surface soil horizons and reach the gold-bearing layer (Fig. 9.1). Besides the large loss of topsoil, these practices affect the turbidity of watercourses (Fig. 9.1) and sometimes their drainage patterns (Telmer et al., 2006; Hammond et al., 2007) and thus have impacts far beyond the local site of operation. Second, fine gold particles in sediments and soils are recovered by amalgamating them with mercury (Hg) (Nriagu, 1994; Malm, 1998), which is subsequently burnt to evaporate the Hg. The amalgamation process releases 650–1350 t Hg into the global environment annually (mean annual release of 1000 t, of which 350 t are emitted to the atmosphere through burning of amalgam and 650 t are discharged into the hydrosphere as elemental and particulate Hg (Telmer and Veiga, 2009). Mercury contamination of aquatic trophic chains is the most alarming health risk for local populations, whose protein diet is largely provided by carnivorous fish and 9 Impact of Gold Mining on Mercury Contamination and Soil Degradation in Amazonian Ecosystems of French Guiana