Temporal geochemical variation in Ethiopian Lakes Shala, Arenguade, Awasa, and Beseka: Possible environmental impacts from underwater and borehole detonations

We present chemical analyses of 25 major, minor, and trace elements in 59 water samples from four lakes and five streams in central Ethiopia. Our major-element data extend to 2003 the intermittent series of measurements that reach back 40–65 years for Lakes Shala, Arenguade, Awasa, and Beseka within or adjacent to the Main Ethiopian Rift. Our minor-element and trace-element data help establish baselines for future monitoring of these four lakes.Water chemistry was analyzed using samples taken in Lake Arenguade and Lake Shala both before and after detonation of submerged explosive charges as part of an active-source seismic survey of the Main Ethiopian Rift. Our data demonstrate no clear impact on the chemistry of Lake Shala from a 900-kg detonation suspended in the water column, whether from dispersal of the explosive charge in the body of water, or from mixing of the lake, or from stirring up of bottom mud into the lake water. In contrast, some changes in the chemistry of Lake Arenguade, most notably a decrease in Na and K concentration of 15–20% occurring between 1 and 11 days after detonation of a 1200-kg charge placed on the lake bottom, may possibly be ascribed to reaction between lake water and sediment stirred up by the detonation. However, these chemical changes that are potentially caused by our seismic detonation are significantly smaller than the natural variations in lake chemistry documented by long-term records. Additionally, we found no change in water chemistry of samples taken from Lakes Awasa and Beseka and from several streams both before and after nearby borehole detonations of 50–1775 kg.Detonating explosive charges underwater greatly enhances seismic data quality. Bottom charges stir lake-bottom sediments into the water column, perhaps resulting in temporary changes in lake chemistry. Our borehole and suspended lake charges had no measurable chemical or lasting environmental effects. These ‘negative’ results – the lack of alteration of lake habitats consequent on seismic detonations – are a positive outcome.