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Bundesanstalt für Geowissenschaften und Rohstoffe Nitrate hotspots and salinity levels in groundwater in the Central District of Botswana by Horst Vogel, Kentlafetse Mokokwe, and Thato Setloboko November 2004 TABLE OF CONTENTS Page 1 INTRODUCTION ....................................................................................................... 1 2 OBJECTIVES ............................................................................................................ 2 3 GEOGRAPHY OF THE STUD Y AREA ......................................................................... 2 3.1 Mining ..................................................................................................................... 2 3.2 Physiography and geology ...................................................................................... 3 3. 3 Climate .................................................................................................................... 6 3.4 Groundwater and water use .................................................................................... 6 4 GROUNDWATER QUALITY INDICATORS .................................................................7 4.1 Salinity .................................................................................................................... 7 4.1.1 Total dissolved solids (TDS) .................................................................................. 8 4.1.2 Electrical conductivity (EC) ................................................................................... 8 4.1.3 Sodium (Na+) .......................................................................................................... 9 4.1.4 Chloride (Cl-) .......................................................................................................... 9 2- 4.1.5 Sulphate (SO4 ) ...................................................................................................... 10 - 4.2 Nitrate (NO3 ) .......................................................................................................... 11 4.2.1 Nitrate health problems ........................................................................................... 11 4.2.2 Nitrate cycle in dryland environments .................................................................... 12 4.2.2.1 Cattle posts .............................................................................................................. 13 4.2.2.2 Termites and ants .................................................................................................... 13 4.2.2.3 Biological soil crusts ............................................................................................... 14 5 METHODS AND MATERIALS .................................................................................... 14 6 RESULTS .................................................................................................................. 15 6.1 Groundwater nitrate hotspots .................................................................................. 16 6.2 Total dissolved solids in groundwater .................................................................... 19 6.3 Selected groundwater salinity indicators ................................................................ 19 7 DISCUSSION AND CONCLUSIONS ............................................................................. 20 8 REFERENCES ........................................................................................................... 22 9 APPENDICES ............................................................................................................ 27 I Map of groundwater nitrate hotspots ...................................................................... 28 II Map of total dissolved solids (TDS) in groundwater .............................................. 29 III Map of selected groundwater salinity indicators .................................................... 30 IV Guidelines for groundwater sampling .................................................................... 31 - i - LIST OF FIGURES Page Figure 1: Administrative district boundaries in Botswana ...................................................... 3 Figure 2: Physiographic units of Botswana ............................................................................ 4 Figure 3: The Makgadikgadi Pans complex ........................................................................ 5 Figure 4: Degraded landscape near Rakops, mid-Boteti area ............................................. 7 Figure 5: Fluctuation of nitrate concentrations in Kgagodi in 1998 ................................... 17 Figure 6: Development of nitrate concentrations in Mathathane between 1979 and 2002 . 18 Figure 7: Nitrate concentrations at police station in Martin’s Drift .................................... 18 Figure 8: Nitrate concentrations at Machaneng .................................................................. 18 Figure 9: Chloride versus sodium concentrations at 1396 sites in the Central District ...... 20 - ii - 1. Introduction The very first steps that eventually led to the preparation of this report date back to July 1999 when two of the authors visited the diamond mine in Orapa on a fact finding mission concerning environmental geology problems. While groundwater salinity was known to be conspicuous in many parts of Botswana, the reported high levels of nitrate were not clearly understood (Mokokwe, 1999). That is why the Environmental Geology Division in October 1999 proposed setting up a nitrate research project that eventually commenced in 2001. The prime objective of this project is to investigate the processes that lead to nitrate enrichment in the Ntane sandstone aquifer between Serowe and Orapa. The quintessential findings of this research project are that cattle posts, pans, and termite mounds are definitely locations of extreme nitrate enrichment in soils but the observed highly heterogeneous nitrate distribution in groundwater could not be attributed to obvious point sources (Stadler et al., 2004a). During the intervening time, that is prior to the commencement of the nitrate research project, the Environmental Geology Division embarked on a first groundwater sampling campaign in the area between Serowe (recharge area) and Orapa (discharge area). She also began to compile groundwater quality data from external sources, in particular from the Department of Lands. These activities led to the compilation of a digital draft map on groundwater quality in the Ntane sandstone aquifer between Serowe and Orapa (Mokokwe & Wolff, 2001). The map depicts the spatial distribution of nitrates and total dissolved solids (TDS) along with piezometric surfaces and water types, though based on a limited data pool. Because both staff moved on shortly after the draft map was made, both the study and the map were not finished. A parallel stepping stone in collecting data was an in-depth study into the environmental hydrogeology of Orapa (Matthes, 2002). No hard-and-fast conclusions could be drawn from this study with regards to the origin of nitrates. But there were indications that up-welling of groundwater (along fault lines and/or fractured zones in dolerite dykes ) from strata deeper than the Ntane sandstone could be a source of increased nitrate levels in the Ntane sandstone. Another possible source of nitrates in the immediate vicinity of the open pit was assumed to be explosives used in pit blasting. Similar assumptions emerged from a second in-depth study that was carried out at the neighbouring diamond mine and village of Letlhakane (Keipeile, 2004) . This study also confirmed that nitrate levels in the groundwater pumped from the Ntane sandstone aquifer were highly variably. Some had very low concentrations, while others featured high ones. An opportunity to collect data in an adjacent area arose through a collaborative exercise with the Department of Water Affairs (Setloboko, 2003). This study provided access to data from the area of the river Boteti, which until lately used to discharge remnant water from the Okavango delta into the Makgadikgadi Pan complex of the Central District. The study concluded that there was no clear-cut correlation between nitrate levels and land use, and that most nitrates occurring in the study area could be of biogenic origin. - 1 - 2. Objectives The objectives of this report are manifold. First and foremost it assesses important groundwater quality indicators against the national water quality guidelines for drinking water (BOS, 2000) with a view to provide guidance to groundwater users, policy makers, and those government agencies in charge in one way or the other of groundwater resources. This objective goes hand in hand with the attempt to provide guidance levels on these groundwater quality indicators for livestock watering. Second, the study intends to highlight the wealth of data that already exists within various government departments and private consultancy firms alike. Most often these data rest idle once the particular project, which had led to its collection and/or generation, has come to an end. Yet, such data may be collated into data bases, reports, and geographic information coverage for further regional and national elucidation. Third, the study aims to highlight the usefulness of maps in assisting with water quality planning and research efforts. If put into Geographical Information System (GIS) formats, such maps may easily and readily be updated and improved upon as new data becomes