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| Titel |
Water-Isotopes (2H, 3H, 18O) to trace the source and timing of recharge in a fractured granite aquifer in Western Kenya, Africa |
| VerfasserIn |
Martin Kralik, Stefan Whylidal, Francis Asunah, Jürgen Sültenfuß |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250100610
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| Publikation (Nr.) |
 EGU/EGU2014-16593.pdf |
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| Zusammenfassung |
| The Vihiga District in West Kenya North-West of Lake Victoria is one of the most densely
populated areas in Kenya with 1033 person per square kilometer. To find the most
suitable location of an own well for a Primary School in this district, springs, school
wells and creeks were sampled in the surroundings to get information about the
hydrological cycle in the area. The Waluka Primary school (0.02134°N, 34.64311°E) is
situated on the northern slope of the Maragoli Hills 20 km to the North-West of
the Nyanzan provincial capital of Kisumu at the eastern shore of Lake Victoria.
The hilly relief varies between 1535 - 1675m. The yearly precipitation is between
1200-1600 mm/a (23°C mean temperature) with biannual rainy seasons in which the
long rains are generally from March to May as the Inter-Tropical Convergence
Zone (ITCZ) moves northwards, and the short rains are typically from October to
December as the ITCZ retreats southwards. A lateritic soil covers a thin alteration
zone above the Precambrian Maragoli-Granite (Saggerson, 1952). Water circulates
either in the thin alteration zone or in fault zones cutting through the Precambrian
granite. From discharge measurements of two springs and a creek at the end of the
dry season (February 2012) a minimum discharge of ca. 10-20 L/s km2 (300-600
mm) can be estimated. The water is of the alkaline sulfate-nitrate type with low
mineralization (70-150 μ S/cm, 25°C) and a low pH of about 5 to 6. The delta
oxygen-18 and deuterium value ranges between -2.84 to -1.98 oand -8.5 to - 3.9
o(VSMOW). The deuterium excess ranges from 11.7-14.2 oThe water of one
spring and well close to the school have a tritium content of 1.42 - 1.62 TU. All
groundwater has a low arsenium, fluorine and uranium content, which had only a short soil
passage. The relatively elevated, but not problematic content in nitrate (10 - 16 mg/L)
probaly reflects the intensive agricultural activities in this area. As the mean δ 18O
values during the rainy seasons are significantly lower (-3 to -4 o) than in the mean
precipitation during the rest of the year (-2.5 to -1.9 o; Mwango, 2003) one can conclude
that the main spring ’Anzaya’ and the well in the Naboka Secondary School are
recharged from deeper faults with water supplied more during the rainy season. The
slightly higher d-excess of 13.4-14.2 ocompared to 11.7-12.6 oin the rest of the
samples, indicates a somewhat higher recharge area of this two sites with water
vapor recycled in the precipitation around the Liailhunuu peak (ca. 1675m). This
effect is also supported by spring-water measurements at the Kilimanjaro (d-excess
13.4-6.6 o) 400 km SE. Similarily, the tritium content of 1.42 - 1.62 TU indicate
that compared to a mean tritium content of 2 TU in the rain of this area (Mwango,
2003) the mean residence time can be in the range of recent to few years only. |
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