Water Quality Status of Lake Tana, Ethiopia

The purpose of this study was to assess the physico-chemical water quality status of Lake Tana at 25 sampling sites from August 2011 to July 2012.A total of 150 water samples from 25 sites on Lake Tana were collected in the period August 2011 to July 2012. Water samples were collected from Lake Tana using acid–washed  polyethylene bottles for chemical analysis(nutrients).In-situ measurements of physical properties were determined using Multi-Maji parameter by dipping the probe about 3-5cm below the water surface. Lake Tana water is slightly alkaline of acceptable pH range ( p H range 6.98 to 9.97).The temperature varies between 16.4 o c to 31.3 o c. Lake Tana has high turbidity (ranges5.1 to 989 NTU) with high seasonal variation. The electrical Conductivity (EC) value ranges between60 mS/cm to 1000µS/cm. It has low TDS (20 to500mg/l).The alkalinity value varies between21 to 3630mg/l. The total hardness value ranges between 65 mg/l to 5300 mg/l. The nitrate value ranges between 0.003 mg/l to 4.7 mg/l. The nitrite value ranges between not detectable to 3.4 mg/l-N 0 to 3.25mg/l. The ammonia value varies between0 to 6.6mg/l. The phosphate value varies between 1.55 to 15.8mg/l. Trends for the Lake Tana water quality monitoring sites and overall trends for the period between 2011 and 2012 time period, there were generally a mixture of both increasing and decreasing trends for all analyses. There were increasing overall trends for electrical conductivity, ammonia, total alkalinity and total hardness and decreasing overall trends for PH, temperature, turbidity, total dissolved solid, nitrate and nitrite. No trend or stable overall trends for phosphate. Keywords : Lake Tana, physico-chemical properties, water quality, spatial variations, and trends