Estimating nitrate and pesticide transfer mode within the unsaturated zone of a fluvioglacial aquifer and its implication on spatial and temporal concentration variability

International audienceThe Meyzieu fluvioglacial aquifer of 113 km 2 located at the eastern part of Lyon is intensively exploited and subject to multiple pressures (urban, industrial, agriculture). Nitrate concentrations are quite high while pesticides contamination is low considering the past and today's pressure. A project was initiated in 2011 with the objective of determining the role of the unsaturated zone in the transfer of NO 3 and pesticides from soil to groundwater. Monthly monitoring of major elements, some pesticides, and stable water isotopes is carried out at three sampling points located along the flow lines. Additional information such as hydrogeological functioning, pressure inventory was gathered and additional data from three water sampling campaigns was collected. The temporal variation of NO 3 , metolachlor and metabolites and atrazine and metabolites is quite different at the three monitoring points. At the sampling point located upstream, where the unsaturated zone has a thickness around 40m, the nitrate concentrations are comprised between 20 and 40 mg.l-1 from 2004 to mid-2013. From mid-2013 nitrate concentrations are increasing regularly and are now reaching 70 mg.l-1. This point presents the highest level in atrazine metabolites (DEA) of the area but with concentrations lower than 0.05 µg.l-1 and very low detection of metolachlor or its metabolites. In the centre of the basin, where agriculture pressure is the highest and unsaturated zone of up to 30m in depth, nitrate concentrations are above 50 mg.l-1 with low seasonal variations. Metolachlor is lower than 0.03 µg.l-1 but values of 0,39 µg.l-1 max. of ESA metolachlor (MESA) were reached. Downstream, just before the discharge area, the sampling point shows the lowest and more stable nitrate concentrations (around 40 mg.l-1). The water level in this sector is at less than 20 m deep. Metolachlor have been detected only once while MESA is nearly always detected and can reach 0.24 µg.l-1

Dissolved matter fluxes in the inner delta of the River Niger

Abstract This study is part of the EQUANIS programme, the objectives of which are (a) estimating inputs from the River Niger to the inner Delta; (b) computing the hydrological balance; and (c) monitoring the quality of water in the central lacustrine basin. Eleven sampling locations were selected near gauging stations, both on the Niger River and its main tributary, the Bani River. Those sites have been sampled weekly since July 1990. The inner delta of the Niger River is a system particularly subject to sahelian and sub-desert climatic conditions and characterized by large flood plains. Time series of input water volumes in the inner delta, and of the water losses within it, show that the water losses are high, due to the intense evaporation, and vary from 6 to 40 km3. The water .losses reach their maximum during the wettest years-up to 47%, and minimum during the driest years-only 32%, due to the reduction of the flooded area. The surface of the flooded area is inferred from the hydrological balance. The preliminary results of this study indicate that the Niger and Bani rivers have low levels of dissolved element concentrations. The mean conductivity values, ranging from 50-80 pS cm-', increase regularly during the low water stage and decrease drastically with rising water. The pH values are slightly basic, ranging from 7.1 to 8. Silica and bicarbonates are the main dissolved species; they always represent more than 75% of TDS. In May, when the rising water stage begins, the waters are poorly mineralized at the input of the delta, while they have been enriched during the dry season in the delta. A good mass balance is found between inputs and outputs through the delta. However, a disequilibrium appears at the sampling sites within the basin, which could be partially linked to poor mixing between the Niger and Bani river waters. The first results from the upper basin and below the inner delta, show low concentrations of matter. The specific dissolved loads vary between 10-12 t km-' year-' for the Niger River and 2.5 t km-2 year-' for the Bani River. The annual input in the inner delta was about 2.2 X lo6 t in 1992-1993. Chemical budgets show a saline deposit of 0.3 x lo6 t in the inner delta. Seasonal variations of the dissolved matter fluxes are very different between the upper and lower parts of the inner delta, due to the breaking of the annual flood and to the more important flood plains in the upper delta. Los flujos de materiales disueltos en el delta inerior del Niger Resumen Es presente estudio forma parte del programa EQUANIS, cuyos objetivos son (a) estimación de los aportes del río Niger al delta interior, (b) cómputo del balance gídrico, y (c) monitoreo de la calidad del agua en la cuenca lacustre central. Se seleccionaron once sitios de muestre0 próximos a las estaciones de aforo tanto en el Niger como en su principal afluente, el Bani. Desde julio de 1990 se llevaron a cabo allí muestreos semanales. EI delta interior del Niger is us sistema particular sometido a condiciones interior, y de pérdidas de agua en is mismo, muestran que las pérdidas son elevadas debido a la evaporación intensa, y fluctúan entre 40 y 6 km3. Las pérdidas de agua son máximas durante los años más húmedos (hasta un 47%) y mínimas durante los años más socos (solo 32%) debido a la reducción de las Breas inundadas. La superficie inundada se deduce del balance hídrico. Los resultados preliminares del estudio indican que los ríos Niger y Bani tienen un bajo nivel de concentración de elementos en disolución. Los valores medios de conductividad que van de 50 a 80 pS cm" se incrementan regularmente durante el período de estiaje y disminuyen drásticamente en períodos de avenidas y variacíon de niveles. Los valores de pH son ligeramente básicos, de 7.1 a 8. Los principales materiales dusueltos son el sílice y los bicarbonatos; siempre representan más del 75% del TDS. En mayo, cuando se inicia el período de crecida, las aguas contienen pocos minerales en la entrada del delta mientras que se han concentrado durante la estación seca del delta. Se observa un buen balance de masa en el delta entre los aportes y descargas, pero aparece un desequilibrio en los sitios de muestre0 dentro de la cuenca, que puede atribuirse parcialmente a una mala mezcla entre las aguals del Niger y del Bani. Los primeros resultados en la cuenca superior y baja del delta interior muestran bajas concentraciones de material. Las cargas disueltas específicas fluctúan entre 10 y 12 t km-2 año-' para el río Niger y 2.5 t km-* año-' para el río Bani. El aporte anual en el delta interior fue de unos 2.2 X lo6 t en 1992-1993. Los balances químicos presentan un depósito salino de 0.3 X lo6 t en el delta interior. Las variaciones estacionales de los flujos de materiales son muy diferentes entre las partes alta y baja del delta interior, debido al inicio de la crecida anual y a las más importantes planicies de inundación en la parte superior del delta

Radon measurement in karstic waters with Lucas cell technique

Based on Lucas cell technique, the portable device named “PRASSI” (SILENA mod. 5S) was used to realise discrete radon measurements in water samples. The calibration factor is (64.766.9) cpmQBq21 and, with the present set-up for measuring radon in water, the detection and the determination limits are, respectively, 0.007 and 0.020 Bq. Thus, the device is well adapted for determining low radon content in karstic waters. Several discrete radon samplings were performed nearly every week in the three main springs of the Bastareny karstic system (Catalonian Pyrenees, N-E Spain). Thus, it was possible to estimate the reproducibility of the methodology as well as to characterise every spring of the aquifer in relation to their hydrogeological features. With the portable radon monitor, some useful measurements were achieved to estimate radon loss due to degassing process over short distances, with important consequences for radon studies in karstic waters. Lastly, it was possible to perform intensive sampling during a flood and the results point out the great variability of radon levels, closely related to the flow rate trend. That permitted also an intercomparison with other radon probes installed previously in every spring, for continuous radon measurements

Irreducibility and Asymptotics of Stochastic Burgers Equation Driven by α-stable Processes

The irreducibility, moderate deviation principle and $\psi$-uniformly exponential ergodicity with $\psi(x):=1+\|x\|_0$ are proved for stochastic Burgers equation driven by the $\aa$-stable processes for $\aa\in (1,2),$ where the first two are new for the present model, and the last strengthens the exponential ergodicity under total variational norm derived in \cite{Do-Xu-Zh-14}

Evolution spatio-temporelle des teneurs isotopiques (delta2H - delta18O) des eaux de la cuvette lacustre du fleuve Niger (Mali)

Le delta du Niger est une importante zone d'inondation en région sahélienne. La variation des surfaces inondées dépend fortement des apports des rivières. L'évaporation est forte toute l'année. Les pluies montrent, sur l'ensemble du bassin amont, une variabilité de quantité et de composition isotopique due à sa position géographique et à son extension ; les eaux d'écoulement superficiel, ne suivant pas l'évolution des précipitations, témoignent d'un apport en eau souterraine et de subsurface, ainsi que d'une forte évaporation durant l'étiage. Du fait du faible gradient hydraulique, les teneurs isotopiques du delta ont un caractère évaporé, évoluant peu spatialement. Une rééquilibration de la vapeur d'eau avec la surface des eaux réduirait le fractionnement isotopique. (Résumé d'auteur