Okavango Delta Management Plan - Ramsar Convention on Wetlands

numerous islands in the <strong>Delta</strong> have been formed through evapotransporative concentration
in groundwater of infiltrating solutes followed by precipitation and volume increase.
Evidence of this is the large amount of calcrete in island soils. These islands of 3-10 m
thickness with clayey soils are under-laid by fine Kalahari sand to a depth of 200-300 m,
which also indicates that they are formed through surface processes.
The infiltration rate of surface water from floodplains and streams into islands is very high
and predominantly a lateral process that is unidirectional. This flow is driven by
evapotranspiration in the riparian woodland zone and causes the ground-waters in the
central area of islands to have very high salinities as evidenced by halophyte species. By
use of chloride as a conservative element, the concentration factor between the channel and
the central island groundwater/surface water is calculated to be 500 – 1000. This
groundwater is completely depleted of calcium and magnesium supporting the early
deductions that they have precipitated as calcrete. There is however an even larger
depletion of silicate which probably has precipitated as well forming the clayey soils typical
of the islands. The central island groundwater is dominated by sodium, bicarbonate and
dissolved organic matter. The gradual increase of salinity in this central island groundwater
is causing it to become much heavier than the under-laying deep groundwater with lower
salinity and periodically some of this high-saline water is let off through a fingering process
to the deeper layers. Island growth through precipitation of solutes and salinity-density
fingering to deep groundwater are the two major sink processes of inflowing solutes and
explains how the <strong>Okavango</strong> <strong>Delta</strong> can be a freshwater system.
Table 2-1: The mass balance of dissolved elements entering the <strong>Okavango</strong> <strong>Delta</strong> through the
inlet (McCarthy et al., 1986).
Tonnes per year
29
Total solute load in inlet 381 000
Loss through outlet 21 000
Remains in <strong>Delta</strong> 360 000
Precipitation in islands 60% 216 000
Sink to deep groundwater 40% 144 000
The biogeochemical growth of islands is a unique process first described for the <strong>Okavango</strong>
<strong>Delta</strong> by McCarthy et al., 1991b, 1993; McCarthy and Ellery 1994. It has recently been
discovered that the islands in another wetland of global importance, the Everglades in
Florida, USA, are formed through the same process. In contrast, the density-driven salt
transport process into deep groundwater (fingering) that is functioning as a permanent sink
of solutes has probably not been described for any other wetland system.
2.2.6.7 Retention of nitrogen, phosphorus and carbon
About 90% oO the total nitrogen, phosporus and dissolved carbon coming into the <strong>Delta</strong>
through the inlet has disappeared before the water reaches the outlet. For nitrogen it is
probably due to de-nitrification in the infiltration zone formed by the floodplains/islands
where the groundwater has high numbers of bacteria and also a considerable methane
emission. About half of the phosphorus is probably precipitated as complexes in island
soils but a similar amount of dissolved phosphorus is deposited in deep groundwater
caused by the density fingering process. By this process about 20% of dissolved organic
carbon is also permanently deposited, while the rest probably is processed by bacteria and
returned to the atmosphere as carbon dioxide and methane. The importance of these
elimination and retention processes the establishment of low concetrations of nutrients in
the surface waters that in turn may contribute to the crystal clear waters in the <strong>Delta</strong>. The