Spatial analysis of fluoride concentrations in drinking water and population at risk in Namibia

© 2017, South African Water Research Commission. All rights reserved. Namibia, the driest country in sub-Saharan Africa, is largely reliant on groundwater for its potable water demand and groundwater is a major source of naturally-occurring fluoride. This study assessed the spatial distribution of fluoride in potable water and appraised the population at risk for high fluoride intake. Analysis of fluoride levels used existing databases that include 28 000 borehole locations across the country, while the population data were based on the 2011 Census. Spatial analysis and spatial statistics methods employed included Moran’s I, local indicators of spatial association (LISA), basic Euclidian distance, analysis of variance (ANOVA) and spatial overlay in a GIS environment. Fluoride concentrations above the recommended limit of 1.5 mg/L occur in a fifth of the boreholes and as much as 8% of the population across the country is at risk. Although the number of people in Namibia who are exposed to high fluoride is relatively small at a global scale, it is significant at a national level. Preventative measures against high fluoride intake are thus necessary in the country

A deuterium-based labeling technique for the investigation of rooting depths, water uptake dynamics and unsaturated zone water transport in semiarid environments

© 2015 Elsevier B.V. Non- or minimum-invasive methods for the quantification of rooting depths of plants are rare, in particular in (semi-)arid regions; yet, this information is crucial for the parameterization of SVAT (Soil-Vegetation-Atmosphere Transfer) models and understanding of processes within the hydrological cycle. We present a technique utilizing the stable isotope deuterium (2H) applied as artificial tracer to investigate the vertical extent of the root zone, characterize water uptake dynamics of trees and shrubs at different depths and monitor transport of water through the unsaturated zone of dry environments.One liter of 35% deuterated water (2H2O) was punctually applied at several depths (0.5m, 1m, 2m, 2.5m and 4m) at six different plots at a natural forested site in the Cuvelai-Etosha Basin (CEB), Namibia/Angola. Subsequently, uptake of the tracer was monitored by collecting plant samples (xylem and transpired water) up to seven days after tracer injection. Soil profiles at the plots were taken after the campaign and again after six months in order to evaluate the transport and distribution of2H within the unsaturated zone.Of 162 plant samples taken, 31 samples showed clear signals of artificially introduced2H, of which all originate from the plots labeled up to 2m depth. No artificially injected2H was found in plants when tracer application occurred deeper than 2m. Results further indicate a sharing of water resources between the investigated shrubs and trees in the upper 1m whilst tree roots seem to have better access to deeper layers of the unsaturated zone. The soil profiles taken after six months reveal elevated2H-concentrations from depths as great as 4m up to 1m below surface indicating upward transport of water vapor. Purely diffuse transport towards the soil surface yielded an estimated 0.4mm over the dry season.Results are of particular significance for a more precise parameterization of SVAT models and the formulation of water balances in semiarid areas. The developed methodology is beneficial for site-specific investigations in complex and data scarce environments, where the root zone plays a major role for the overall water balance. For arid and semiarid environments experiencing low recharge rates, water transported in its vapor phase is found to play an important role for the overall soil water balance. The use of2H2O is cost-effective and provides the opportunity to investigate multiple effects along the soil-vegetation interface that have been difficult to deal with previously