No abstract

This work considered the environmental impact of artisanal mining gold activity in the Migori-Transmara area (Kenya). From artisanal gold mining, mercury is released to the environment, thus contributing to degradation of soil and water bodies. High mercury contents have been quantified in soil (140 μg kg(-1)), sediment (430 μg kg(-1)) and tailings (8,900 μg kg(-1)), as expected. The results reveal that the mechanism for transporting mercury to the terrestrial ecosystem is associated with wet and dry depositions. Lichens and mosses, used as bioindicators of pollution, are related to the proximity to mining areas. The further the distance from mining areas, the lower the mercury levels. This study also provides risk maps to evaluate potential negative repercussions. We conclude that the Migori-Transmara region can be considered a strongly polluted area with high mercury contents. The technology used to extract gold throughout amalgamation processes causes a high degree of mercury pollution around this gold mining area. Thus, alternative gold extraction methods should be considered to reduce mercury levels that can be released to the environment.

SUMMARY In August 1985 the crustal structure underlying the southern part of the Kenya Rift Valley was investigated by long‐range explosion seismology. the experiment (KRISP 85) consisted of two seismic lines in the central sector of the rift, one along the axis and the other across it. Interpretation of the data, including time‐term analysis and ray tracing has shown that the thickness of rift infill varies from about 6km below Lake Naivasha to about 2 and 1.5km below Lake Magadi and Lake Bogoria respectively. the underlying material has a P‐wave velocity of 6.05 ± 0.03 km s‐1 which suggests that the rift is underlain by Precambrian metamorphic basement. A localized high‐velocity zone identified to the east of Nakuru may be due to basic intrusive material. the P‐wave velocity increases discontinuously to 6.45 ± 0.2 km s‐1 at a depth of 12.5 ± 1.0 km below sea level. This depth is similar to that inferred for the brittle‐ductile transition zone from a study of local seismicity in the Lake Bogoria region. A high P‐wave velocity layer (7.1 ± 0.2 km s‐1) occurs at 22 ± 2 km depth below sea level which might be associated with a sill‐like basic intrusion in the lower crust. an upper mantle velocity of 7.5 ± 0.2 km s‐1 (unreversed) is reached at a depth of 34.0 ± 2.0 km below sea level. This implies that only moderate crustal thinning has occurred beneath the central sector of the rift. No evidence was obtained for the existence of a continuous‘axial intrusion’reaching to shallow levels below the rift and associated with crustal separation as suggested by previous studies.

[1] Lavas of Pliocene-Pleistocene age were sampled in two regions in Kenya: Mount Kenya on the equator and the Loiyangalani region, east of Lake Turkana, at about 3°N. We sampled 100 sites distributed around the Mount Kenya Massif and to the northeast along the Nyambini Range. The equator bisects Mount Kenya, and all sites were sampled within 40′ of the equator. Thirty-two sites were sampled in the Loiyangalani area, making a total of 132 sites. Many sites from the Mount Kenya study were severely affected by lightning; however, after progressive AF demagnetization 69 sites yielded directions with a 95 equal to or less than 10°. Normal polarity sites dominate (N = 58 and a mean of declination (dec) = 1.2°, inclination (inc) = −0.7°, and a 95 = 3.6°) with only 11 reverse polarity sites (mean of dec = 182.3°, inc = 0.6°, and a 95 = 7.2°); no transitional directions were identified. Inverting the reverse sites yields a combined mean direction of dec = l.4°, inc = −0.7°, and a 95 = 3.2°. This result is not significantly different from what is expected from the geocentric axial dipole for the mean locality (dec = 0°and inc = 0°); a quadrupole component was not resolved. The samples from the Loiyangalani region were not seriously affected by lightning, and all 32 sites gave satisfactory data with a 95 less than 10°(17 reverse sites, dec = 183.4°, inc = 0.8°, and a 95 = 6.7°; 15 normal sites, dec = 358.6°, inc = −1.1°, and a 95 = 4.7°); after inverting the reverse sites the combined mean was dec = 1.1°, inc = −1.0°, and a 95 = 4.1°. Altogether, we had a total of 101 successful sites. A virtual geomagnetic pole (VGP) was calculated from each site mean; the VGP dispersion is low, with Sb = 10.9°for Mount Kenya and 9.8°for the Loiyangalani region. This dispersion agrees with updated Model G of McElhinny and McFadden (1997) and model TK03 of Tauxe and Kent (2004) that was tuned to the compilation of McElhinny and McFadden (1997) but disagrees with the higher dispersion near the equator and the smaller latitudinal gradient in dispersion estimated by Johnson et al. (2008). A new database is presented, and the included studies support a systematic decrease of dispersion from high to low latitudes.

No abstract