Geophysical techniques identify fresh water for refugees
Geophysicist Paul Bauman was teaching a short course on finding groundwater sources at the UN’s Kakuma refugee camp in Kenya when he realized his skills could be put to better use.
Although a dozen wells were servicing the 163,000-person camp, the water supply was inadequate and fluoride concentrations exceeded Kenyan drinking water guidelines, posing a risk of fluorosis to both teeth and bones.
Bauman thought he could use geophysics to find a better source, so when he returned to Calgary he applied for and won a grant from the SEG’s Geoscientists Without Borders for groundwater exploration in the semi-arid region of northwestern Kenya.
Other than the few days of the year when the river is in flood, there is no surface water at Kakuma. And ever since the camp opened in 1992 to house the “Lost Boys of Sudan” fleeing civil war, drilling success for water wells has been variable. The further the distance from the river, the less the likelihood of hitting groundwater and the greater the chance the water would be brackish. All the wells were high in fluoride.
The complex geology and hydrogeology of the area – unconsolidated sediments interfingered with volcanics underlain by sedimentary rocks and finally by Precambrian basement consisting of gneisses and schists – called for a modern approach to finding the sand and gravel aquifers more likely to produce clean, fresh water.
Bauman and his team zeroed in on areas near the dry riverbed or “laga” where they might find side channels containing water from indirect recharge that takes place when the river flows. They ran the equivalent of 11.4 km lines of ERT (electrical resistivity tomography) to differentiate fine (clay and silt) from coarse (sand and gravel) materials and saline water from fresh. A simultaneous seismic refraction survey totalling 5.8 line kilometres helped to determine the top of the bedrock and the thickness of the overburden.
They compiled the resulting survey data, along with data from drone flights, Google Earth, and digital elevation models in Geosoft’s Oasis montaj mapping and analysis software to come up with high priority targets for drilling.
By combining the data, the team succeeded in identifying granular overburden saturated or partially saturated with fresh water at a resistivity range of 30-100 ohm-metres. The UN drilled three new holes based on their results, which produced sustainable water yields of 29-45 cubic metres per hour with acceptable fluoride concentrations, providing enough to supply 57,000 refugees with 20 litres of good quality water each per day.
The program also identified other thick sand and gravel aquifers that are likely to be low in fluoride and high in productivity that the UN can drill when necessary.
Read the full article on Earth Explorer.