Description
The Okavango Delta in Botswana, Africa has a unique hydrological setting. The massive geologic feature termed the Okavango "Delta" is actually a low gradient alluvial fan system spread out over 65,000 km2. This fan is subject to annual flood events from the discharge of the Okavango River. In this semi-arid environment, surface water volume is decreased by 98% from surface inflow at the head of the fan to surface outflow, over 200 km away, near the town of Maun. With increasing population, the need for sustainable water is unyielding. This has prompted the exploration of groundwater systems in the region. Three river valleys near Maun were chosen for the purpose of groundwater chronological studies using noble gases. Two of the river valleys have experienced recent surface flooding, which is the primary recharge mechanism of the region. The third system, which has not received surface flooding for the past 13 years, is the main groundwater supply to Maun. The groundwater systems of the Thamalakane, Boro, and Shashe river valleys are evaluated using helium isotopes and abundances. The 4He concentrations within the study area are found to be in excess of the solubility equilibrium with the atmosphere (4Hemeasured > 4Heair), implying subsurface radiogenic production of 4He. Corrections for air-derived helium are made using measured neon concentrations, which are assumed to have no subsurface production. Simple age calculations are then made using estimated values of U and Th concentrations in the aquifer rocks, their bulk density (p) and porosity (cj>), and the corrected 4He concentrations. The helium-derived ages are compared to age estimations made using Darcy's Law and based on the hydraulic properties of the aquifer systems. In general, the helium-derived ages range from 1 to 2 orders of magnitude greater than the hydraulic ages. Many critical assumptions are made with regard to the variables in the age equations (both for helium and hydraulic), such as: (l) a homogenous distribution of: U and Th, effective porosity, and bulk density; (2) a constant hydraulic gradient and hydraulic conductivity; and (3) a relatively good approximation of the recharge areas (distance away from recharge zones). In the Thamalakane system, helium is found to be highly scattered and does not show an expected trend. The Boro system has only two sampling boreholes, which do not conform to predicted values. The Shashe system has the best helium data and the most samples taken from one aquifer (the middle semi-confined aquifer). The helium concentrations increase down gradient from the recharge area and have a nearly constant slope. Helium derived ages in the Shashe Valley are greater than the expected hydraulic ages by a factor of 10. The age difference can be explained in terms of a crustal flux of helium degassing from the underlying basement rocks. This crustal flux, termed the solution rate factor, effectively reduces the apparent age of the groundwater by taking into account the basement lux and the distance from the borehole screen to the bottom of the aquifer. The solution rate factor often reduces the apparent age up to 3 orders of magnitude. Helium can and has been used to help characterize aquifers. Unfortunately the lack of quantitative measurements in the region prevents the complete characterization of these systems. Future studies in the area should focus towards obtaining more samples along the various valleys, and collecting the samples in specific aquifers. Measurements of 14C as well as U and Th, bulk density, porosity, and other hydraulic parameters need to be addressed.
