DETERMINATION OF SELENIUM SPECIATION OF SALINE LAKES FROM SOAN-SAKESAR VALLEY SALT RANGE PAKISTAN

Abstract

Saline lakes are breeding and feeding habitat for many shorebirds and waterfowls from Soan-Sakesar Valley. Salt-Range, Pakistan. The selenium speciation studies were carried out on lake stream, spring and ground waters; saturation extracts of soil (0-20 cm depth); and lake sediments (0-5 cm depth). The studies were started in March 1994 and completed in October 1996. The studies have shown elevated concentrations of selenium and a number of other inorganic constituents. In 1994 water samples were collected from Uchhali, Khabbaki and Jahlar lakes and analyzed for a variety of parameters such as: secchi disk transparency, chlorophyll-total phosphorus, nitrate, alkalinity, pH, total dissolved substances (TDS), calcium, magnesium, sodium, potassium chloride and sulfate. Chemical changes observed ill 1994 were compared to those or the study conducted by Institute of Geology Punjab University, in 1986. The three lakes studied can be characterized as hypereutrophic. Their Chemistry is controlled mainly by an evaporation-crystallization process and dissolved salts are supplied by underlying sedimentary submerged springs. Uchhali Lake is saline while Khabbaki and Jahlar lakes are brackish. Major ion chemistry of tile ground stream and spring studied for both drinking and irrigation. The effect or geology and hydrological conditions on the chemistry and quality of water have been discussed. Determination of NO3 can provide valuable diagnostic evidence or the extent or water contamination from agriculture and sewage effluent or villages. The ground stream and spring waters had following median concentrations: NO3 27.5, 26 and 16.2 mg/L; B 0.52, 0.25 and 0.35 mg/L, respectively. Fourteen percent ground waters have nitrate concentrations above 50 mg/L and 82 percent have boron concentrations >0.3 mg/L, thus are unfit for drinking as prescribed by WHO (1993) guidelines. Plotting the chemical analyses of the ground waters on Piper's diagram revealed: 48% Na, 34% Mg and remaining non-dominant cations; 21% bicarbonate, 28% sulfate and remaining non-dominant anions. Some groundwater samples exhibited positive saturation index for calcite and dolomite, which indicates that the groundwater of the aquifer is saturated with respect to these minerals and under saturated with respect to gypsum and halite. The Chemical ratios of rMg/r (Ca+Mg) expressed in milliequivalent per million are >0.5 in all the samples, indicating the dissolution of Mg-rich rocks. According to FAO irrigation guidelines (1985), groundwater samples are mostly fit for traditional crops but approx. 45% are harmful to citrus plants due to boron concentrations being >1.0 mg/L. Streams and springs adjoining Uchhali lake are not fit for irrigation purposes due to high TDS (total dissolved solids) and SAR (sodium adsorption ratio), while Sodi springs and Kanhati, spring are fit for irrigation purposes according to FAO irrigation guidelines. Physical and chemical characterization or the soil samples (0-20 cm depth) of Soan-Sakesar Valley are presented. Soil samples were analyzed for variety of parameters: pH, electrical conductivity, alkalinity, Na, K, Ca, Mg, Cl, SO4, Li, B and NO3 in saturation extracts; organic C, NaHCO3-extractable P and total P in whole soil; mineralogical investigations in whole-soil and separated clay fraction. A Principal Component Analysis was carried out on the correlation matrix, i.e., on standardized attributes; and soil samples subdivided into four salinity groups. The mean ECe levels of group1 to 4 were 1.27, 2.75, 2.07 and 5.67 dS/m; the corresponding sodium adsorption ratio was 5.36, 7.9, 15.7 and 29.6 mmoleo0.5, respectively. In these soil groups appropriate crops are suggested on the basis of their salt tolerance and the salinity of the irrigation water. Tile low availability of P from phosphatic fertilizers and farmyard manure caused due to fixation by calcite which is present in these soils to the extent of 7.8 to 15.2%, resulted in lower yield of grain crops (wheat and maize). On the otherhand higher application of nitrogen fertilizers along-with farmyard manure to irrigated crops and vegetables results in N leaching from the crop rooting zone into groundwater. The soils under discussion have mixed clay mineralogy with dominance of chlorite, illite, and kaolinite while major nonclay minerals are albite calcite and quartz. In order to determine trace quantities of selenium in environmental samples a simplified procedure of hydride generation atomic absorption spectrometric (HGAAS) method was developed and used to determine Se speciation (Selenate, selenite and organic Se) in the samples. Ground, stream and spring waters had medium Se concentrations of total Se 48, 404, and 82 ug/L; Se+VI 40, 350 and 51 ug/L; Se+IV 8, 48 and 29 ug/L; Se-11 0, 6.5 and 2.0 ug/L respectively. The concentration of Se exceeded the recommended water quality guidelines for drinking and irrigation water of 10 and 20ug/L, respectively. In saturation extracts median total Se, selenate, selenite and organic Se were 190, 146, 37 and 7.0ug/L, respectively. Uchhali, Khabbaki and Jahlar lake water samples had the mean concentration of: total Se 2103, 670 and 297ug/L; Se+VI 1777, 470 and 232 ugL; Se+IV 291, 166 and 39 ug/L; Se-11 35, 34 and 25ug/L respectively. The Sakesar limestone Formation of Eocene age rich in shales and fossils (median Se concentration 7.2 mg/kg) are investigated to be the source of selenium that has enriched the environments of Soan Valley. Se speciation results show the abundance of selenate in all the environmental samples. A linear relationship between Se+VI and pH (R2=0.84, 0.78, 0.88, 0.82, and 0.91; significant at the 0.05 level) for groundwater, saturation extract, stream, spring and lake water were noted, suggesting that the Se+VI in the samples were highly associated with pH. Linear relationship between Se+VI and Mg, NO3 and SO4 for groundwater (R2= 0.24, 0.16, and 0.64; significant at the 0.01 level) and surface water (R2 0.96. 0.14 and 0.91; significant at the 0.001 level), suggesting that the presence of high concentrations of additional anions (e.g. Mg, SO4) that strongly adsorb on and compete with selenium for surface sites increases the Se+VI fraction. The presence of selenate on other species in the analyzed samples indicates the oxidizing environments in the Valley. The chemical species of selenium were examined in the surface sediments in May 1995. Lake Uchhali, Khabbaki and Jahlar had concentration of total selenium 4.03, 1.55 and 1.4 mg/kg Se+VI 0.83, 0.37, and 0.53 mg/kg, respectively. Lake sediments have higher nitrogen (0.04-0.35% DW dry weight) and phosphorus (0.04-0.29% DW) levels due to agriculture drainage water. X-rays' diffraction analyses have shown that the sediments of these lakes are enriched in albite, calcite, chlorite, illite, kaolinite and quartz minerals. The organic C content also increased in the sequence of Uchhali Khabbaki and Jahlar lakes 2.2, 2.8 and 3.1% DW respectively. Linear relationships between the Se (-II) and organic C levels (significant at the 0.05 level) were observed for three lakes. It is resolved that greater percentage of Se in the Se+VI fraction in the analyzed samples may be related lo higher pH and the presence of high concentrations of additional anions (e.g., NO3) that strongly adsorb on and compete with selenium for surface sites of minerals and organic matter. The presence of more selenate as compared to other Se species in the analyzed samples indicates the oxidizing environments in the Valley. The higher percentage of selenium in shales and fossils of Sakesar limestone. Formation of Ecoene age were considered to be the source of selenium in lake, spring, stream, ground waters, saturation extracts of soils, and lake sediments of Soan-Sakesar Valley. Moreover the nutrient loads and organic C are responsible for the high microbial activity which reduces selenates to more insoluble Se species in the lake sediments.