Nathalie Gassama

Geochemical study of surface waters in mountain granitic area. The Iskar upper watershed: Massif of Rila, Bulgaria

The aim of this study is to determine the characteristics of the Iskar upper watershed surface waters as a first step of a seasonal evolution study. The geochemical and petrochemical characteristics of the Rila massif are well-suited to study the relative significance of different weathering mechanisms occurring along the year in a mountain aquatic system. In such an area the atmospheric input contribution to the surface water quality could be assessed. A first sampling survey in summer-time allowed us to get some data on major and minor elements in the soluble phase. The results show that the Iskar upper watershed is well-suited because of no mixing with long residence time groundwaters and of the low population density which results in insignificant anthropic input.

Atmospheric, weathering and biological contributions in the chemical signature of stream water: the upper Iskar Reka watershed, Bulgaria

Abstract The chemical signature of stream water is influenced by several factors: atmospheric input, weathering of bedrock and soils, biological uptake, soil storage, and decomposition of organic matter. The importance of weathering and biological activity, according to season, can be assessed by subtracting atmospheric input. In the upper Iskar Reka watershed, Bulgaria, results show that Na and Ca are mainly exported out of the basin (weathering is dominant), while nitrate and sulphate are consumed/stored. The behaviour of K suggests that biological activity is dominated by vegetation uptake in spring and by bacterial activity in autumn. In summer, the weathering load of Mg is compensated by reactions with clays. The low nutrient input from weathering due to low reserves in the soil may lead to a biomass production closely linked to a rapid internal cycle and also to a relative sensitivity of the ecosystem to any change in vegetation cover and atmospheric input. Editor Z.W. Kundzewicz Citation Gassama, N. and Violette, S., 2012. Atmospheric, weathering and biological contributions in the chemical signature of stream water: the Upper Iskar Reka watershed, Bulgaria. Hydrological Sciences Journal, 57 (3), 535–546.

Chemical characterization of aquifers with the same geological origin

Physical characteristics of the Bicaz dammed lake are described as follows: water volume of 1.150x 109 m 3, 31.1 km in length, 2 km in width. The sluice is at 45 m above the maximal altitude of the lake. The Bistrita River is the main tributary of the lake: mean water contribution of 1.721x109 m3/a, mean suspended matter flow of 291000 t/a. This river drains a catchment rich in formerly mined mineralizations of polymetallic sulfides. The aim of this study is to determine if the lake can be considered as a sink toward trace elements. We analyzed major and minor elements, TOC, organic N, and some trace elements (e.g. As, Ba, Cd, Cr, Co, Cu, Mo, Ni, Pb, Sb, Sn, V, U, Zn) in filtered and bulk water samples. We worked on three locations, an upper site, a middle site, and a down site. Here we present the data for July and October 2005. Some trace elements have smaller dissolved concentrations in lake water than in the Bistrita River. Common associations can be defined in this lake such as: Co is associated with Mn; As is released in solution as Fe oxi-hydroxide is dissolved; V and U are following the same behaviour, etc. However, their distribution is not as regular as in a natural lake. The stratification of the water column at all 3 sampled sites is not as regular as expected. At the upper site (water depth of 33 m), the water column is homogeneous until 20 m, then the redox condition allows having Mn as Mn (+ II ) and Fe as Fe (+III). For some elements a perturbation can be noticed at about 10 m. At the middle site (water depth of 59 m), there is a water section richer in dissolved and particulate Fe and Mn, with a dominant particulate form. This can assess this section, in between 30 to 50 m it is more oxidizing. At the down site, the reverse figure can be seen between 30 and 50 m depth. This redox anomaly is more noticeable for elements with a high solubility difference according to their oxidation rate. Other parameters do not follow this figure. This section could be linked to the sluice drawing off. In the whole lake, the stratification is more and more visible from the up to the down stream but it is not so pronounced than in natural lakes. Water column structuring seems to be disturbed by a lateral flow, induced by the sluice drawing off.