A study of the hydrochemistry of brines and associated inflows of lake Manyara.

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University of Dar es Salaam
Lake Manyara has been previously poorly chemically researched prompting us to re-examine the hydrochemistry of brines and associated inflows. The concentrations of the major ions and trace elements were investigated. Temperature of the hot springs was recorded in the field. For most of the samples specific gravity and pH were similarly recorded but re-confirmed in the laboratory at Dar es Salaam. The field and laboratory measurements are in good agreement. Ionic concentrations were determined by standard laboratory methods. "Wet methods" were employed for the determination of anions and alkalinities While mrtailic constituents Were assayed by Atomic Absorption Spectrophotometry (AAS). fluoride has been determined by ion selective techniques (fluoride selective electrode). Seasonal and Iocational variations of the chemical constitution were examined. Electrical conductivity tends to correlate with Total Dissolved Solids (TDS). The in draining streams are dilute (TDS < O.12 g/I) and the lake brines show gradual increase in TDS (10-58.4 g/I) with increasing salinity as evaporative concentration progresses. Ionic proportions for the major ions are target}' invariant in brines generally indicating the non-existence of a selective elimination/enrichment mechanism in the natural steady state. The order of abundance of major ions is: Na+ > K+ > Ca2+ Mg2+ : (CO32- + HCO3- ) > Cl- > SO42- > PO43-> F- - This pattern generally reflects the geochemistry of rocks in the drainage basin which are often associated with carbonatitic volcanicity. The accumulated solutes in brines is the result of repetitive annual concentrative evaporative cycles stretching many thousands of years although a contribution from hot springs is acknowledged together with minor sources such as droppings from the migratory flamingo population. While Ca2+ and Mg2+ levels are significant upstream where CO32- concentrations are low, a reverse trend is observed in the lake brines as carbonate precipitates as MCO3 ( M = Ca2+ or Mg2+ ) and thus vanishes from the lake brines. One salient feature of Lake Manyara is the high level of phosphate and fluoride. The neighbouring Minjingu phosphate deposits represent the dried up bed of an ancient lake and largely explain the unusual phosphate levels. Heavy metal content is at trace level throughout. Solute concentrations in the lake varies with location and season. Chemical equilibrium parameters such as pH, carbonate and fluoride pKa values in the concentrated electrolyte environment have been evaluated by an approach which caters for the high ionic strength. Synthetically re-constituted lake samples at various ionic strengths Were made up by diluting calculated volumes of standardised electrolyte solutions With Water. Petentiometric pH of the synthetic brines made of NaCl(aq) and NaOH(aq) were evaluated by the usual laboratory procedure. The pKa values for carbonic acid were determined by potentiometric titration of a standard solution of hydrochloric acid against standard sodium carbonate ail at identical ionic strength values. The pKa for HF was determined potentiometrically by direct measurement of the concentration of free fluoride ions using Fluoride Selective Electrode (FSE). Often, the values obtained have shown marked departure from those measured in dilute aquatic media. The pH as measured using this provision is about 2 units higher than the conventional direct readout. The pKa values have shown to be influenced by ionic strength and therefore TDS. The pKal and pKa2 for carbonic acid at ionic strengths of 1.389, 1.024 and 0.584 are 9.23 vs 6.09, 9.49 vs 6.0, and 9.53 vs 6.13 respectively. The apparent pKa values for HF are 2.96, 2.92, 2.82 and 2.74 at 0.266, 0.584, 1.024 and 1.419 ionic strength values leading to an evaluated thermodynamic value of 3.15 which agrees well with the literature value of 3.45. These parameters have to be evaluated independently for individual cases since in each natural concentrated aquatic media the hydrochemical environment is unique to itself.
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Salt lakes, Water chemistry, Tanzania, Lake Manyara
Qulwi, Q. W. (1995). A study of the hydrochemistry of brines and associated inflows of lake Manyara. Master dissertation, University of Dar es Salaam. Available at (