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Ground Water/Surface Water Interactions in Lake Naivasha, Kenya, Using δ 18 O, δD, and 3 H/ 3 He Age‐Dating
Author(s) -
Ojiambo Bwire S.,
Poreda Robert J.,
Lyons W. Berry
Publication year - 2001
Publication title -
groundwater
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.84
H-Index - 94
eISSN - 1745-6584
pISSN - 0017-467X
DOI - 10.1111/j.1745-6584.2001.tb02341.x
Subject(s) - geology , rift , groundwater , hydrology (agriculture) , rift valley , aquifer , surface water , escarpment , geothermal gradient , water well , east african rift , water level , geochemistry , environmental science , geomorphology , structural basin , paleontology , geography , geotechnical engineering , environmental engineering , cartography
Abstract We have analyzed a series of ground water samples from the Lake Naivasha region, Kenya, for their helium isotopic composition. Lake Naivasha is unique among the East Africa Rift Valley lakes in that it is fresh. It has long been thought that the low salinity of this lake is due, in part, to rapid water loss from the lake into the local ground water system. Our results show that the Olkaria geothermal waters, south of the lake, are devoid of tritium and, thus, are more than 50 years old. An important implication of these results is that even if Olkaria geothermal reservoir water originated from Lake Naivasha, it has been underground for a long time, (>50 years) and is not derived from present‐day Lake Naivasha water. This flow time is of the same order of magnitude as conservative major solutes, such as chloride, as determined through residence time calculations. On the north side of Lake Naivasha, deep wells (91 m) have water ∼ 20 years old. Water from these wells has stable isotopic values resembling those of nearby rivers, and high‐elevation eastern Rift water. This indicates that this water recharges from rains from high eastern Rift Valley escarpments. Many of the shallow wells on the south side of the lake have 3 H/ 3 He ages between four and 17 years. The young ages and the δ 18 O‐enriched signature of the water from these wells indicate that they are recharged by a mixture of water from the lake, Rift flanks, and water from deep pumping wells that is recharged during irrigation. Water mixing ratio calculations using δ 18 O and δD isotopes show that about 50% to 70% of the southern ground water system is derived from the lake, while the Olkaria geothermal reservoir water shows that 40% to 50% of this water is originally lake water. Calculated mean recharge rates range from 0.10 to 1.59 m/yr with a mean of 0.52±0.40 m/yr. Estimated horizontal velocity from 3 H/ 3 He age dating between Lake Naivasha and a well about 3 km to the south is 75 m/yr, giving average horizontal hydraulic conductivity of 6 m/day.