Major ion chemistry of the lower Boro River, Okavango Delta, Botswana

Abstract

1. The composition of surface waters of the Boro River, Okavango Delta, Botswana, was determined on five occasions during [1989][1990]. The waters could be characterized as calcium-sodiumbicarbonate, with moderate alkalinity, and moderate to high amounts of silica. Mean values of the major ions, in mgl"', were: 4.8, 1.3, Na^ 3.9, K^ 2.7, HCOJ 27, CP 1.0 and SiO^ 38; pH c. 7.2. The relative ionic composition of the waters changed gradually from the inlet in Seronga to the outlet at Boro/Thamalakane junction. There was a gradient of increasing concentrations of solutes resulting from the evaporative concentration of the waters in this semi-arid region with an almost permanent water deficit, and the total dissolved solutes increased fromc. 30to95mgr'.3. There were seasonal differences in solute concentrations, connected with the rainfall pattern and climatic cycle and an ill-defined relation with discharge. The chemical spectrum of the Boro, and the freshness of its waters, can be attributed to the major influence of precipitation within the basin and selective removal of solutes.4. Chemical equilibrium models indicate that amorphous silica and sepiolite are likely to precipitate from the Boro waters. Carbonate and silicate systems are responsible for the selective removal of solutes in the swamps, islands, or flood plains, as proposed by others in recent studies, but not in the Boro River waters.

Variability in chemistry of surface and soil waters of an evapotranspiration-dominated flood-pulsed wetland: solute processing in the Okavango Delta, Botswana

“…Some of these cations, for example Ca, are essential for the growth and development of prokaryotes, fungi, and plants, and these organisms may sequester them in biomass. The order of solutes in concentration was Ca > Na > K > Mg, consistent with the findings of Sawula and Martins (1991) and Masamba and Muzila (2005). This was, however, contradictory to results of Mackay et al, (2011) who found a greater concentration of potassium than sodium.…”

“…This implies that there must be some differentiation in the processes affecting individual ions. The concentrations of major cations (Ca, K, Mg and Na) increased generally throughout the Delta for surface waters, similar to the trends reported by Sawula and Martins (1991) and Mackay et al (2011). This correlates well with a general increase in conductivity along the Delta distributaries.…”

“…This was, however, contradictory to results of Mackay et al, (2011) who found a greater concentration of potassium than sodium. There has been little or no systematic change in water chemistry (major ions) over the years, with calcium the dominant cation in the Delta (Sawula and Martins, 1991;Mackay et al, 2011).…”

“…The processes dominating the cycling of solutes in surface-and soil waters of the Delta, before they eventually precipitate or are sequestered in highly saline groundwater beneath island centres, are not fully understood, however. While there have been studies on water chemistry in the Okavango Delta (Sawula and Martins, 1991;McCarthy and Ellery, 1995;Cronberg et al, 1996a;Cronberg et al, 1996b;Mladenov et al, 2005;Bauer et al, 2007;Mackay et al, 2011;Cawley et al, 2012), the scope was primarily focused on concentrations of solutes in surface water and groundwater;…”

Spatial and Seasonal Variability in Surface Water Chemistry in the Okavango Delta, Botswana: A Multivariate Approach

“…However, while the hydrology of the Delta has been comprehensively studied (see Milzow et al 2009 for a review), there have been considerably fewer investigations into how the flood pulse in the Delta impacts nutrients (Mubyana et al 2003;Lindholm et al 2007) and inorganic chemistry (Sawula and Martins 1991). Previous studies focussed only on very short time periods (Ashton et al 2003), or were restricted to the Jao / Boro river system (Sawula and Martins 1991;Cronberg et al 1996a,b). Here we report on a large, comprehensive, surface water chemistry dataset that covers four important regions in the Delta, sampled over a complete flood cycle.…”