Residence Times of Major Ions in Lake Naivasha, Kenya, and Their Relationship to Lake Hydrology

Lake Naivasha is “distinctive” among Rift Valley lakes of Ethiopia, Kenya and Tanzania in that it is fresh. Outflow of lake waters into the groundwater system is the prevailing explanation for the lake’s freshness. Seepage outflow in the southern portion of the lake has been estimated to be as much as 20% of the total water loss (Gaudet and Melack, 1981). In order to establish a better hydrologic budget for the lake, residence times were calculated for the major elements in the lake water and fell in the range of 3–6 years. Chloride was assumed to be a conservative element. The “chloride age” of the lake was computed to be 5.4 years. This very young age can be compared to chloride ages for other lakes in the region such as 2500–5000 yrs for Lake Turkana (Yuretich and Cerling, 1983; Barton et al., 1987) and 100–200 yrs for Lake Baringo (Barton et al., 1987). The residence times of Na⁺, K⁺, Ca²⁺, Mg²⁺, and HCO 3 − https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203748978/e0c5d746-b4ce-40ec-9112-f83a17f1d4ad/content/eq180.tif"/> are all within ±1 yr of that of Cl^–, while those of SO 4 2 − https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203748978/e0c5d746-b4ce-40ec-9112-f83a17f1d4ad/content/eq181.tif"/> and H₄SiO₄ are even less. The loss of SO 4 2 − https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203748978/e0c5d746-b4ce-40ec-9112-f83a17f1d4ad/content/eq182.tif"/> through diffusion into the sediments and sulfate reduction and H₄SiO₄ through diatom uptake supports the earlier conclusions of Gaudet and Melack (1981). Ojiambo (1992) computed mean parameters of the hydrologic budget using data from four earlier reports and found evaporation loss to be 77%, irrigation 3% and seepage loss 20%. Using chemical mass approaches, our data suggest that about 55% of the annual inflow is evaporated, while another 15% is utilized for irrigation. If this is so, then about 30% of the total loss must be via seepage outflow. Therefore, the most recent estimates suggest that 20–30% of the annual water loss from Lake Naivasha is through groundwater outflow.