Hydrochemical properties of the Aral Sea water in summer 2002

Abstract

In August 2002, new hydrochemical and hydrophysical data were collected in the Aral Sea. The survey includes crosssections in three locations: within the Small Aral close to Tastubek Bay, in the Large Aral at the northern tip that is Tschebas Bay and within the western basin at Chernishov Bay. All three locations represent different stages in the hydrochemical evolution of the Aral Sea.Depth profiles of pressure, temperature, conductivity, pH and dissolved oxygen were measured with a YSI 6600 profiler. Water samples were taken with a Niskin bottle and analyzed for dissolved oxygen and nutrients by standard photometric methods. Major anions and cations were analyzed by ion chromatography and ICP-OES, respectively. Benthic flux experiments were carried out with sediment cores in a batch mode assay on-site.In the Small Aral, the changes in the hydrochemical properties are not as dramatic as in the Large Aral. The Small Aral represents a brackish inland water body with salinities of 17 -18 g kg À1 . The wind-mixed layer reached 8 m during the survey. The salinity is vertically and horizontally almost uniform. Below 8 m, a temporally hypoxic layer forms during summer. Salt redissolution was found to be an important source of salt in the water. About 33.5 g SO 4 2À m À2 day À1 and about 30.7 g Cl À m À2 day À1 are released from the sediment in summer.In the Large Aral, the salinity distribution is uniform in shallow waters (less than 5 m) but varies strongly in deeper water. Tschebas Bay at the northern tip of the large Aral represents a shallow lagoon with a maximum depth of about 6 m. The water column was well mixed down to the bottom (f6 m) having salt concentrations of 82 g kg À1 on average. Almost no gradients in dissolved substances were observed. It is suspected that salinity is balanced by fresher water inflow originating from the Syr Darya flowing south and by groundwater exfiltration. Chernishov Bay in the north of the western basin is meromictic. Below a windmixed layer, a very strong pycnocline of 20 g kg À1 per m at 5 m depth isolates the water below from exchanges with the water above and led to the formation of huge anoxic water body down to the bottom at about 25 m depth. Below 10 m, the water contains hydrogen sulphide. The salt concentration increases from 82 g kg À1 in the surface water to 110 g kg À1 at depth. The salt release from the sediment is as high as 1143 g SO 4 2À m À2 day À1 and 1626 g Cl À m À2 day À1 . Benthic release of salt is considered as an important source for salt in the bottom water of the western basin and in sustaining the stable density stratification. D

Ongoing Changes of Ionic Composition and Dissolved Gases in the Aral Sea

“…The reduction of the organic matter in the absence of oxygen results in H 2 S production. Hydrogen sulfide in the Aral Sea water was first discovered in 2002, almost simultaneously by a German-RussianKazakh expedition in the Chernyshev Bay (the northernmost tip of the western basin) (Friedrich and Oberhansli 2004) and a Russian-Uzbek expedition of the authors in the open western basin ). This does not mean, however, that H 2 S was not there before 2002, given that there were virtually no soundings or sampling of the bottom layer since the early 1990 s. Therefore, we can only affirm that hydrogen sulfide first appeared sometime between 1992 and 2002.…”

“…The zone tends to be confined to the deepest portion of the western basin. We know that, at least sometimes, there exists also a separate pool of H 2 S in the relatively deep Chernyshev Bay (Friedrich and Oberhansli 2004), disconnected from the main western basin by a sill, but this location is undersampled. The spatial extent characteristic for the fall of 2003 is sketched in Fig.…”

The late Holocene mollusc fauna of the Aral Sea and its biogeographical and ecological interpretation

“…In 2003, its maximum depth was around 30 m and thus had decreased to less than half compared to 1961. Salinity increased correspondingly to about 90 g/l in the western lake basin (Zavialov et al 2003;Friedrich and Oberha¨nsli 2004). Values differ extremely between places.…”

“…Values differ extremely between places. For example, the Small Aral Sea into which the Syr Darya River flows is still brackish (Friedrich and Oberha¨nsli 2004). The species composition of Aral Sea habitats has changed extremely since the 1960s.…”

Geochemistry of sediments in the modern Aral Basin

“…Studies on the sedimentation and hydrologic processes in the Aral Sea were conducted in the Soviet era [8,9,[21][22][23][24][28][29][30][31] and were resumed in the last years on a new basis [14-20, 25-27, 32, 33, 36-38]. Our knowledge of the trace element composi tions of the bottom sediments is essentially limited to only a few elements with the exception of uranium, for which geochemical data are available from previous studies [3,23,32,33].…”