The Ethiopian Rift Valley lakes: chemical characteristics of a salinity-alkalinity series

Abstract

The study on 10 lakes within the Ethiopian Rift Valley during March-May 1991 covered a range of conductivity (K 25 ) between 286 and 49100 /S cm '. HCO 3 -CO32-and Na + were the dominant ions in all the lakes. Concentrations of K', Cl-and SO4-increased with increasing salinity and alkalinity, whereas Ca 2+ and Mg 2 + decreased. Comparison of these data with previous records showed that a ten-fold dilution of total ionic concentration occurred over 30 years in Lake Metahara and about three-fold increase occurred over 65 years in Lake Abijata. Concentrations of soluble silica were generally high (12-222 mg SiO 21 -1) and increased with increasing salinity, except for Lake Chamo which showed SiO 2 depletion (to < 1 mg SiO21 -1) over the past three decades.The relationship between ionic concentration and phosphorus was irregular although high phosphorus concentrations generally corresponded with increasing salinity. Fitting data to the Dillon & Rigler (1974) chlorophyll a -total phosphorus relationship suggested that lakes Zwai, Awassa and Chamo are phosphorus-limited, whereas others have surplus phosphorus.

Major ions composition of the groundwater and surface water systems and their geological and geochemical controls in the Ethiopian volcanic terrain

“…For the majority of the samples collected and analysed by the author the analytical error is less than 5%. Certainly there is limitation in data quality from literature works (UNDP, 1973; Elizabeth Kebede et al, 1996); none of the data sources indicate the accuracy and analytical errors. However, for such a regional work the analytical procedures and results are of acceptable quality.…”

“…The exception to this is the expanding L ake Beseka which underwent substantial dilution due to substantial groundwater flow from the surrounding over-irrigated mechanized farms (Elizabeth Kebede et al, 1996;Zenaw Tessema, 1996;WWDSE, 2000; due to the introduction of water from surrounding irrigation fields, which uses the Awash River. The most exceptional lake in the Afar is Lake Afdera, which is located at about 80 meters below sea level.…”

Recent changes in the level of Lake Abiyata, central main Ethiopian Rift

“…Compilation of the sparse chemical data available since 1926 (Kebede et al, 1996) and chemical analysis since 1995 (Ayenew, 1998) has revealed a considerable increase in the total dissolved solids. Between 1926 and 1998, the salinity fluctuated more than 2.6 times (from 8.1 to 26 mg F 1 ), the alkalinity changed from 80 to 326 mg F 1 , and pH varied between 9.5 and 10.1.…”

“…In the last few decades, the ever-growing utilization of water resources in the rift and adjacent highlands has induced salinization of irrigation fields and lake level changes (Hailu et al, 1996;Ayenew, 1998). The chemistry and ecological setting of some of the lakes has changed (UN, 1973;Makin et al, 1976;Kebede et al, 1996). The main controlling factors are not well understood, particularly the relative importance of natural and manmade factors.…”

The diatom Thalassiosira faurii (Gasse) Hasle in the Ziway–Shala lakes (Ethiopia) and implications for paleoclimatic reconstructions: Case study of the Glacial–Holocene transition in East Africa

“…Barker et al, , pH=8) to hyper-alkaline waters in the lowest lake Shala (>20,000 57 µS.cm -1 , pH=10) (Kebede et al, 1994, Legesse and Ayenew, 2006). In the middle of the system, lake…”