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The Nabwal Hills, northeast of Lake Turkana, contain a record of magmatism associated with the initiation and early development of the East African Rift System in northernmost Kenya. The predominantly volcanic Asille Group, 1400 m thick, directly overlies metamorphic basement and comprises a sequence of basaltic lava flows with significant intervals of rhyolitic pyroclastic units, and minor intercalations of fluviatile sediments. The basement gneisses yield K-Ar cooling ages on biotite of 510 and 522 Ma, typically Pan-African. The 40 Ar-39 Ar ages on alkali feldspar crystals from the rhyolitic units are concordant and show that the Asille Group spans an interval from at least 34.3 to 15.8 Ma, continuing to at least as young as 13 Ma based on previous measurements. Vertebrate fossil sites, containing primate remains, at Irile and Nabwal are shown to be 17 ± 2 Ma old, Early Miocene, based upon K-Ar age measurements on immediately overlying basalts. Variably reliable whole rock KAr ages, determined on basalt samples from low in the sequence, indicate that volcanism commenced as early as 34.8 Ma ago. The overall geochronological results show that magmatism in the Nabwal Hills began about 35 Ma ago in Late Eocene times, interpreted as the time of initiation of crustal extension that led to the development of this segment of the East African Rift System. The Asille Group is tilted about 6 • to the SSW. This tilting occurred later than 13 Ma ago, and prior to the eruption of the flat-lying Gombe Group basalts. These basalts may have begun erupting about 6 Ma ago in Late Miocene times, although much of this volcanism occurred between about 3.9 and 4.2 Ma ago in Pliocene times. It is suggested that the main rifting, which continues today, commenced in Late Miocene times, less than 13 Ma ago, and is partly reflected in the tilting of the Asille Group.

A decline in the watertable due to a long period of low rainfall, and the disturbance of sulfidic peat soils by dewatering and excavation in the Perth suburb of Stirling, has led to widespread acidification of groundwater at the watertable in a residential area and contamination of groundwater by arsenic and metals. The acidification has been caused by the oxidation of sulfide minerals within the peat, which contains up to 15% by weight of oxidiseable sulfur. Groundwater of pH 1.9 has been measured in shallow monitoring bores in the area, as well as high arsenic (up to 7 mg/L), aluminium (up to 290 mg/L), and iron (up to 1300 mg/L) concentrations. Contaminated groundwater pumped from affected domestic garden bores caused plant deaths in gardens and has given rise to health concerns because of high arsenic and metal concentrations. Drilling has indicated that acidic groundwater generally extends 5-10 m below the watertable, and that deeper groundwater is currently unaffected by contamination. As groundwater forms 70% of Perth's total water usage and sulfide-rich peat soils are common in the region, acid sulfate soil risk maps and management policies need to be developed and implemented as a matter of urgency to prevent similar acidity problems occurring elsewhere in Perth.