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Untersuchung der Prozesse der Nitratanreicherung in Böden und Grundwasser der semi-ariden Kalahari, Botswana

Leitung:Prof. Dr. Jürgen Böttcher
Laufzeit:2001-2005

Enrichment of nitrate in groundwater is a worldwide occuring phenomenon, mostly due to anthropogenic activities. In southern African countries high nitrate concentrations are found in mostly uninhabited semi-arid areas as the Kalahari region, Botswana. There, groundwater
extracted from the Ntane-Sandstone aquifer locally exhibits concentrations up to 600 mg NO3 L-1. Groundwater with enhanced nitrate concentrations poses a serious threat to human health, and thus should not be used as drinking water. Furthermore, it also may cause serious problems if used for cattle watering, which is very disadvantageous for Botswana, as cattle raising is an economic key sector in this country. In the Kalahari, natural nitrate accumulation processes as well as the influences of cattle grazing (Fig. 1) may play a role in nitrate enrichment of the groundwater, but the enrichment processes are not yet fully understood. However, understanding these processes is significant to improve the management of the limited groundwater resources.

 

Fig. 1: Area with high cattle grazing intensity near a cattle post in the Kalahari


In a joint research project of the Federal Institute of Geosciences and Natural Resources (BGR) and the Institute of Soil Science of the University of Hannover, Germany, and the Department of Geological Survey (DGS), Botswana, soil scientific investigations are carried out in the Kalahari between Serowe and Orapa. In the first place, the investigations aim at identifying and quantifying processes and sources leading to nitrate enrichment in the sandy soils. Afterwards, the leaching of this nitrate down to the groundwater under the semi-arid climatic conditions is analysed. Main objectives are i) quantification of the soil nitrogen balance at typical sites of the Kalahari, and evaluation of stocks of mineralized nitrogen in soils and unsaturated zone with respect to vegetation and cattle grazing intensity, ii) identification of nitrogen dynamics in the root zone, and investigation of leaching processes and the transport and transformation of ammonia and nitrate in the unsaturated zone, and iii) developement of a model for prognoses of nitrate leaching into groundwater with respect to vegetation and land use (grazing) intensity.

The three years project started in September 2001, and a first reconnaissance of soils, vegetation and land use in the research area was done in Oct. – Dec. 2001. The soils in the Kalahari are mostly reddish or yellowish to brownish coulored Arenosols, i.e. soils with a sandy texture and only a weak accumulation of organic matter (Fig. 2 and 3). Despite of the semi-arid climatic conditions the Arenosols are often more or less acidic (Fig. 3). The vegetation in the Kalahari region is dominated by acacia trees and shrubs, and grass as ground cover (comp. to Fig. 4).

 

 Fig. 2: Arenosol in the Kalahari near               Fig. 3: Profile of an Arenosol with pH
Serowe                                                                   down to 40 cm
 

First results for nitrate stocks in the unsaturated zone were obtained from two sampling campaigns during the dry and the wet season in 2002 (Juni – August 2002, resp. October-December 2002). Soil samples were taken from profiles (Fig. 3) and by means of hand augering down to depths of > 200 cm (Fig. 4), and nitrate was directly extracted in the field camp (Fig. 5). Extraordinary high nitrate stocks were found in areas where cattle is watered (cattle posts or “kraals”). But also in soils more or less unaffected by cattle, nitrate stocks high enough to cause high nitrate concentrations in the soil solution were measured (Fig. 6).

  

Fig. 4: Taking soil samples in the Kalahari     Fig. 5: Extraction of soil samples in the
                                                                                  field camp

 

Fig. 6: Nitrate concentrations in soil solution of an Arenosol under natural vegetation
(concentrations estimated from nitrate stock and soil moisture)


Data from the sampling campaign of Oct-Dec 2002 from the Makhi test farm demonstrate the direct connection between cattle grazing intensity and nitrate concentration in the soil. The test farm is surrounded by an outer fence and has a radius of about 2600 m. Once a day all the animals come to the centre of  the test farm to drink water. This causes a high grazing intensity direct near the centre, while near the surrounding fence there is no sign of overgrazing. In Figure 7, the position of soil profiles drilled along a transect from the overgrazed centre to the surrounding fence of Makhi test farm are shown on the x- axis. The measured nitrate concentrations of these profiles are very high in the centre and decrease exponentially with increasing distance from the kraal. This fact is also confirmed by similar results from other, commercial cattle posts in the same area.
In a next sampling step the nitrate concentration in and between termite mounds was determined. Extremely high nitrate levels of about 4000 mg nitrate per kg of soil in the termite mound were measured (Fig. 8). Soil samples between the two neighboring termite mounds (105 m apart) show nitrate levels of about 10 mg per kg soil, which seems to be representative for grazed areas without overgrazing as already found on the Makhi test farm.

      

Fig. 7: Change of concentrations by                      Fig. 8: Change of concentrations by
distance from kraal center of Makhi                       distance between two termite
Test Farm (averages from 0 - 500 cm                  mounds 20 km north ofLethlakane
depth)                                                                          (averages from 0 - 70 cm depth)


During the sampling campaigns in 2003 soil samples will be taken and analysed to determine the spatial variability of nitrate in the soils, especially at sites with different cattle grazing intensities and different vegetation cover, and at depressions like pans which gather run-off water. For this purpose the evaluation of satellite images of the research area proved to be very helpful. Furthermore, sampling of vegetation litter for N analysis and acquisition of additional data (e.g. biomass production, cattle density) are important tasks in the next future to enable us to calculate site specific nitrogen balances.

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