Combined dynamics of mercury and terrigenous organic matter following impoundment of Churchill Falls Hydroelectric Reservoir, Labrador

Sediments from two recently (40 years) flooded lakes (Gabbro lake and Sandgirt lake) and an unflooded lake (Atikonak lake) were sampled to investigate the effects of reservoir impoundment on mercury (Hg) and terrigenous organic matter (TOM) loading in the Churchill Falls Hydroelectric complex in Labrador, Canada. Lignin biomarkers in TOM, which exclusively derive from terrestrial vegetation, were used as biomarkers for the presence and source origin of TOM—and for Hg due to their close associations—in sediments. In the two flooded Gabbro and Sandgirt lakes, we observed drastic increases in total mercury concentrations, T-[Hg], in sediments, which temporally coincided with the time of reservoir impoundment as assessed by 210Pb age dating. In the natural Atikonak lake sediments, on the other hand, T-[Hg] showed no such step-increase but gradually and slowly increased until present. T-[Hg] increases in lake sediments after flooding were also associated with a change in the nature of TOM: biomarker signatures changed to typical signatures of TOM from vegetated terrestrial landscape surrounding the lakes, and indicate a change to TOM that was much less degraded and typical of forest soil organic horizons. We conclude that T-[Hg] increase in the sediments of the two flooded reservoirs was the result of flooding of surrounding forests, whereby mainly surface organic horizons and upper soil horizons were prone to erosion and subsequent re-sedimentation in the reservoirs. The fact that T-[Hg] was still enriched 40 years after reservoir impoundment indicates prolonged response time of lake Hg and sediment loadings after reservoir impoundments

Caractérisation de la matière organique dissoute d'un site d'eau de surface (fleuve Saint-Laurent) et d'un site d'eau souterraine (aquifère de l'Astien, France) par l'utilisation des isotopes du carbone et des produits d'oxydation de la lignine

The organic matter cycle at the continental scale still remains partly misunderstood. Thus, DOC has been mostly studied under indistinct chemical aspect. Isotopic studies and the use of biomarkers allowed the clarification of certain aspects of its cycle. The present study is part of this research perspective. Two types of environments, the St. Lawrence River and the Astien's aquifer (France), were examined. For the St. Lawrence study, we were interested in the terrigenous organic carbon (TOC) cycle from 13C and 14C measurements on DOC and a refractory fraction of DOC to understand the origin and evolution of TOC during its export towards the ocean. In the Astien study, similar approaches were used to determine the nature, sources, degradation state and temporal evolution of DOC in groundwater. Results show that we keep a predominantly terrigenous origin and a globally recent age for DOC from the St. Lawrence hydrological system. However, a small fraction of this DOC appeared to be much more ancient. The present study indicates that total DOC in the St. Lawrence hydrological system is constituted of organic compounds easily biomineralizable and thus potentially large consumers of dissolved oxygen. TOC could play a more important role than previously estimated in the apparition of hypoxic conditions in the world's estuaries and costal areas. In the Astien's aquifer system, relative kinetics of degradation of organic compounds product by lignin macromolecule could be used for geochronometrical applications.Le cycle de la matière organique à l'échelle continentale reste encore en partie méconnu. Ainsi, le COD a surtout été étudié sous un aspect chimique indifférencié. Des études isotopiques et l'utilisation de biomarqueurs ont permis d'élucider certains aspects de son cycle. La présente étude s'inscrit dans cette perspective. Deux types de milieux distincts, le fleuve Saint-Laurent et l'aquifère de l'Astien (France), ont été examinés. Dans le volet "Saint-Laurent", nous nous sommes intéressés au cycle du carbone organique terrigène (COT) à partir de mesures 13C et 14C du COD et d'une composante réfractaire du COD afin de comprendre l'origine et l'évolution du COT lors de son transit vers l'océan. Dans le second volet, les mêmes approches ont été retenues afin de déterminer la nature, les sources, l'état de dégradation et l'évolution temporelle du COD dans une nappe d'eau souterraine. Nous retenons une origine majoritairement terrigène pour le COD du Saint-Laurent, ainsi que son âge globalement très récent. Cependant, une fraction de ce COD est apparue beaucoup plus ancienne. La présente étude indique que le COD total du système hydrologique du fleuve Saint-Laurent est en majorité constitué de composés organiques facilement biominéralisables et donc potentiellement grands consommateurs d'oxygène dissous. Le COT pourrait donc jouer un rôle plus important qu'estimé aujourd'hui dans l'apparition de conditions hypoxiques dans les estuaires et zones côtières du globe. Dans le système de l'aquifère de l'Astien, les cinétiques relatives de dégradation des composés organiques issus de la macromolécule ligneuse pourraient donner lieu à des applications géochronométriques

The influence of anthropogenic disturbances and watershed morphological characteristics on Hg dynamics in Northern Quebec large boreal lakes

International audienceMercury (Hg) dynamics in the boreal environment have been a subject of concern in recent decades, due to the exposure of local populations to the contaminant. Land use, because of its impact on mercury inputs, has been highlighted as a key player in the sources and eventual concentrations of the heavy metal. In order to evaluate the impact of watershed disturbances on Hg dynamics in frequently fished, large boreal lakes, we studied sediment cores retrieved at the focal point of eight large lakes of Québec (Canada), six with watersheds affected by land uses such as logging and/or mining, and two with pristine watersheds, considered as reference lakes. Using a Geographical Information System (GIS), we correlated the recent evolution of land uses (e.g., logging and mining activities) and morphological characteristics of the watershed (e.g., mean slope of the drainage area, vegetation cover) to total Hg concentrations (THg) in sedimentary records. In each core, THg gradually increased over recent years with maximum values between 70 and 370 ng/g, the lowest mercury concentrations corresponding to the pristine lake cores. The Hg Anthropogenic Sedimentary Enrichment Factor (ASEF) values range from 2 to 15. Surprisingly, we noticed that the presence of intense land uses in the watershed does not necessarily correspond to noticeable increases of THg in lake sediments, beyond the normal increment that can be attributed to Hg atmospheric deposition since the beginning of the industrial era. Rather, the terrestrial Hg inputs of boreal lakes appear to be influenced by watershed characteristics such as mean slopes and vegetation cover

Impact of Soil Characteristics and Weed Management Practices on Glyphosate and AMPA Persistence in Field Crops Soils from the St. Lawrence Lowlands (Quebec, Canada)

The use of glyphosate-based herbicides (GBHs) in industrial agriculture has intensified in the past decades, causing a growing concern about the occurrence and spatial distribution of glyphosate and its principal metabolite, aminomethylphosphonic acid (AMPA), in the environment. In 2014, glyphosate and AMPA content was measured in 45 soils from the St. Lawrence Lowlands (Quebec, Canada) before seeding and at harvest in soybean field crops using various weed management practices with or without GBH applications. At the same time, a recent history of agricultural practices and soil conditions was compiled for the sampled sites. The results of the study show that 91% of the samples contained detectable amount of either glyphosate or AMPA, with maximum values of 0.47 mg·kg−1 and 1.16 mg·kg−1 for glyphosate and AMPA, respectively. Surprisingly, detectable amounts of AMPA were measured in fields not treated with GBHs in 2014, whereas traces of both glyphosate and AMPA were detected in organic field crops, highlighting the potential spreading and/or persistence of both compounds in the environment. Glyphosate content was significantly higher in clay soils rich in exchangeable cations, such as Mg2+, K+ and Ca2+, which can contribute to the retention of glyphosate in soil via complexation processes