Hydrochemical and isotopic studies providing a new functional model for the coastal aquifers in Douala Coastal Sedimentary Basin (DCSB)/Cameroon.

A new conceptual model of the hydrogeological systems in Cameroon's Douala Coastal Sedimentary Basin (DCSB) was constructed. The model is based upon the basin's known geology, plus data from recent field campaigns that allowed the collection of rainwater and groundwater samples for analyses of stable isotopes (δ2H, δ18O, δ13C), radiogenic isotopes (3H, 14C), and water chemistry. Aquifer characteristics that were thereby deciphered include recharge, isotopic distributions, residence times, and mixing processes. Rainfall samples (mean δ18O = -2.0 ‰; mean δ2H = -6.80 ‰; weighted mean = -2.4 ‰ δ18O, -9.85 ‰ δ2H) scatter along two distinct lines, thus indicating that local rainfall events undergo processes during convective events, variability in humidity, amount effects, and seasonal variations. Stable isotope values of river water samples are close to the weighted mean of local precipitation, with some downstream enrichment. The Quaternary/Mio-Pliocene superficial aquifer system (depth < 70 m) and the intermediate Oligocene/Upper Eocene aquifer system (depth: 70 to 200 m) exhibit evidence of similar fractionation processes through an enrichment gradient of δ-values. The enrichment is more pronounced at the top of the superficial aquifer, which is very exposed to direct rainfall water infiltration, evaporation, and amount effects. The depth profiles of δ-values coupled to water chemistry and tritium contents, evidence leakage between (i) the superficial system's Quaternary alluvium sands and Mio-Pliocene sands; and (ii) the superficial and intermediate systems. Thus, the aquifers that contain modern, post nuclear groundwater are characterized by flow exchanges and direct recharge from rainfall events. In contrast, the Upper Eocene system has depleted δ-values and lower bicarbonate contents, suggesting not only that this system was recharged by rapid infiltration (with limited effect of evaporation), but that this recharge occurred during a cooler time in the past. The residence times (computed from 14C dates) indicate uncorrected ages ranging from hundreds to thousands of years.

Naturally dissolved arsenic concentrations in the Alpine/Mediterranean Var River watershed (France).

A detailed study on arsenic (As) in rocks and water from the Var River watershed was undertaken aiming at identifying (i) the origin and the distribution of As in this typical Alpine/Mediterranean basin, and (ii) As input into the Mediterranean Sea. Dissolved As concentrations in the Var River range from 0.1 to 4.5 µg⋅L(-1), due to high hydrological variability and the draining through different geological formations. In the upper part of the Var drainage basin, in the Tinée and the Vésubie valleys, high levels of dissolved As concentrations occur (up to 263 µg⋅L(-1)). The two main sources of As in rocks are the Hercynian metamorphic rocks and the Permian argilites. Highly heterogeneous distribution of As in waters draining through metamorphic rocks is probably related to ore deposits containing arsenopyrite. As, U, W and Mo concentrations in water and rocks correspond to the formation of As-rich ore deposits around Argentera granite by hydrothermal fluids deposited at the end of the Hercynian chain formation, which occurred about 300 My ago. In 2009, weekly monitoring was performed on the Var River (15 km upstream of the mouth), highlighting an average dissolved As concentration (<0.45 µm) of 2.7 ± 0.9 µg⋅L(-1), which is significantly higher than the world-average baseline for river water (0.83 µg⋅L(-1)). Taking the average annual discharge (49.4 m(3)⋅s(-1)) into account and the As levels in the dissolved phase and in deposits of the Var River, dissolved As input into the Mediterranean Sea would be 4. 2± 1.4 tons⋅year(-1) which represents 59% of the total As flux. This study also reveals a probable non-conservative As behaviour, i.e., possible transfer between aqueous and solid phases, during the mixing of the Var River with a tributary.

Soil water flow is a source of the plant pathogen Pseudomonas syringae in subalpine headwaters.

The airborne plant pathogenic bacterium Pseudomonas syringae is ubiquitous in headwaters, snowpack and precipitation where its populations are genetically and phenotypically diverse. Here, we assessed its population dynamics during snowmelt in headwaters of the French Alps. We revealed a continuous and significant transport of P.syringae by these waters in which the population density is correlated with water chemistry. Via in situ observations and laboratory experiments, we validated that P.syringae is effectively transported with the snow melt and rain water infiltrating through the soil of subalpine grasslands, leading to the same range of concentrations as measured in headwaters (10(2) -10(5) CFU l(-1) ). A population structure analysis confirmed the relatedness between populations in percolated water and those above the ground (i.e. rain, leaf litter and snowpack). However, the transport study in porous media suggested that water percolation could have different efficiencies for different strains of P.syringae. Finally, leaching of soil cores incubated for up to 4 months at 8°C showed that indigenous populations of P.syringae were able to survive in subalpine soil under cold temperature. This study brings to light the underestimated role of hydrological processes involved in the long distance dissemination of P.syringae.

Estimating ungauged catchment flows from Lake Tana floodplains, Ethiopia: an isotope hydrological approach.

The isotope balance approach, which used (18)O content of waters, has been used as an independent tool to estimate inflow to Lake Tana of surface water flows from ungauged catchment of Lake Tana (50% of the total area) and evaporative water loss in the vast plains adjoining the lake. Sensitivity analysis has been conducted to investigate the effects of changes in the input parameters on the estimated flux. Surface water inflow from ungauged catchment is determined to be in the order of 1.698×10(9) m(3)a(-1). Unaccounted water loss from the lake has been estimated at 454×10(6) m(3)a(-1) (equivalent to 5% of the total via surface water). Since the lake is water tight to groundwater outflow, the major error introduced into the water balance computation is related to evaporative water loss in water from the flood plains. If drained, the water which is lost to evaporation can be used as an additional water resource for socio-economic development in the region (tourism, agriculture, hydropower, and navigation). Hydrological processes taking place in the vast flood plains of Lake Tana (origin of salinity, groundwater surface water interaction, origin of flood plain waters) have been investigated using isotopes of water and geochemistry as tracers. The salinity of shallow groundwaters in the flood plains is related to dissolution of salts accumulated in sediments covering former evaporation pools and migration of trace salt during recharge. The waters in the flood plains originate from local rainfall and river overflows and the effect of backwater flow from the lake is excluded. Minimum linkage exists between the surface waters in the flood plains and shallow groundwaters in alluvio lacustrine sediments suggesting the disappearance of flood waters following the rainy season, which is related to complete evaporation or drainage than seepage to the subsurface. There is no groundwater outflow from the lake. Inflow of groundwater cannot be ruled out. Discharge of groundwater to the lake is presumed to take place along rocky bottom in southern sector from Quaternary volcanics covering the southern sector of the catchment.

Organic matter as a potential complementary tool for delta(18)O data interpretation in heterogeneous aquifers.

Evaluating transit time by using delta(18)O seasonal variation is often difficult in a Mediterranean context due to the erratic rainfall signature added to the complexity of flow mixing. The present study aims to show that using organic matter can improve interpretations of the delta(18)O signal. The natural fluorescence of organic compounds and dissolved organic carbon (DOC) data were recorded in the underground low-noise laboratory, located within the catchment area of the Fontaine de Vaucluse karstic system, over a four-year period. By combining both total fluorescence and DOC, a systematic seasonal variation is observed only due to soil-water interaction. Therefore, when the recharge rate is enough at the time of the season concerned, a new specific seasonal tracer, independent of rainfall signature, is available. Besides, the DOC concentration is a tracer of rapid infiltration with short transit time associated with high DOC concentration, while long transit time waters are characterised by a low DOC concentration. Then this can also shed light on such a mixing, recent/old waters. A more sensitive interpretation of delta(18)O variations is then possible: if the rainfall amount and delta(18)O follow a seasonal trend, both tracers may be used in the same way; if the recharge is discontinuous due to discontinuous rainfall regime, delta(18)O tracing alone is usable; in case of erratic or smooth rainfall signature during the homogeneous rainy period, fluorescent organic matter tracing alone is then usable.