Envhydrogeology Francistown

Bundesanstalt für Geowissenschaften und Rohstoffe Environmental Hydrogeology of Francistown Effects of Mining and Urban Expansion on Groundwater Quality by Benjamin Mafa Edited by Dr. H. Vogel March 2003 TABLE OF CONTENTS Page 1 INTRODUCTION ........................................................................................................1 2 THE FRANCISTOWN STUDY AREA ............................................................................2 2.1 The climate in the study area ......................................................................................2 2.2 The topography and drainage pattern in the study area ...............................................3 2.3 The geology in the study area .....................................................................................3 2.4 The hydrogeology in the study area ............................................................................5 2.5 Potential groundwater hazards in the study area ..........................................................7 3 METHODS AND MATERIALS ....................................................................................10 4 RESULTS ..................................................................................................................12 6.1 Environmental hydrogeology maps .............................................................................12 6.2 Groundwater pollution zones .....................................................................................14 5 DISCUSSION .............................................................................................................18 5.1 Pit latrines – Zones I and II ........................................................................................18 5.2 Mine waste dumps – Zones IV, IVa, VII and Ia ........................................................18 5.3 Waste disposal sites – Zones V and VI ......................................................................20 6 CONCLUSIONS .........................................................................................................22 7 RECOMMENDATIONS ................................................................................................23 8 REFERENCES ............................................................................................................24 9 APPENDICES .............................................................................................................25 I Borehole location map ...............................................................................................26 II Groundwater contour map .........................................................................................27 III Groundwater hazards map .........................................................................................28 IV Groundwater quality map ..........................................................................................29 V Groundwater contamination map ...............................................................................30 VI Spatial distribution of important process-indicator elements ........................................31 VII Spatial distribution of heavy metals and other trace elements .......................................39 VIII Spatial distribution of additional groundwater parameters ............................................57 IX BGR ICP-OES results (ppm) .....................................................................................63 X BGR ICP-MS results (ppb) .......................................................................................69 XI DGS Ion-Chromatograph results (ppm) ......................................................................75 i LIST OF FIGURES Page Figure 1: Major urban and rural centers in Botswana .............................................................1 Figure 2: Annual rainfall in Francistown (1925–2000) ............................................................2 Figure 3: Mean monthly rainfall (a) and humidity (b) between 1970 and 1999 ........................3 Figure 4: Average maximum and minimum air temperatures (1970–1999) ..............................3 Figure 5: Geology of the Francistown study area ...................................................................4 Figure 6: Hydrograph at borehole FT103 ..............................................................................7 Figure 7: Monarch South tailings ...........................................................................................9 Figure 8: Waste oil disposal tank in Francistown ....................................................................10 Figure 9: Piper diagram of the Francistown groundwater samples ...........................................14 Figure 10: Nitrate concentration in groundwater and areas with pit latrines in mid 2000 ............15 Figure 11: Groundwater pollution zones in Francistown in mid 2000 ........................................17 LIST OF TABLES Table 1: Samples submitted to different labs for analysis .......................................................11 Table 2: Laboratory analyses carried out and methods applied ..............................................11 ii 1 INTRODUCTION This study was carried out as part of a technical co-operation project between the Department of Geological Survey (DGS) in Lobatse (cf. Fig. 1), and the Federal Institute for Geosciences and Natural Resources (BGR1) in Hannover, Germany. The overall objective of the project is to establish a fully functional Environmental Geology Division within the DGS. Amongst other tasks, the division addresses major environmental issues associated with water in Botswana such as groundwater pollution due to urbanization and mining. The aim of this particular study was to determine whether groundwater pollution had taken place in Francistown (Fig. 1) due to urban expansion and/or historic gold mining activities, and to delineate affected areas as well as potential groundwater hazards (areas with pit latrines, industrial sites, mine tailings etc.) on thematic maps. The thematic maps were designed in an easily readable form so as to enable urban planners to utilize them for future development planning. Figure 1: Major urban and rural centres in Botswana 1 BGR is the German acronym for „Bundesanstalt für Geowissenschaften und Rohstoffe“ 1 2 THE FRANCISTOWN STUDY AREA Francistown is the oldest established town in Botswana. Born during the late 19th century as a gold mining town at the confluence of the ephemeral Tati and Ntshe sand rivers, Francistown is the commercial hub in the NE of Botswana (cf. Fig. 1). The city's rapid economic development, in particular since the 1970s, has caused its population to triple over the last three decades to approximately 100,000 inhabitants. Today Francistown is the second largest city in Botswana. In the not too distant past, water demands were entirely met by groundwater locally available from shallow alluvial and fractured volcanic rock aquifers. However, in the 1970s it was found that groundwater produced from the city’s public wells contained elevated concentrations of nitrate. In addition, the available limited groundwater resources could no longer meet the steadily rising demand for water. For these reasons public water supply was shifted in 1982 to surface water from the Shashe dam, which is located at a distance of approximately 30 km to the SW of Francistown. The Shashe dam was built during the 1970s to supply the copper-nickel mine in Selebi-Phikwe (cf. Fig. 1). 2.1 The climate in the study area The climate in Francistown is semi-arid with an average annual rainfall of 460 mm (Fig. 2), the bulk of which falls between November and February (Fig. 3 (a)). Rainfall is highly variable, however, both annually and seasonally. A case in point is the rainfall season 1999- 2000, when approximately 2000 mm of rain fell, most of which in February and March 2000. Because of the prevailing semi-arid climate, air humidity is generally low. But there is a clearly visible difference between the dry and the wet season (Fig. 3 (b)). Similarly, the average maximum air temperatures are very high in summer, in particular in poor rainfall years, while the dry winter season reveals very low average minimum temperatures (Fig. 4). 1000 900 800 700 600 500 400 300 200 100 0 1925 1927 1929 1931 1933 1935 1937 1939 1941 1943 1945 1947 1949 1951 1953 1955 1957 1959 1961 1963 1965 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 Figure 2: Annual rainfall in Francistown (1925-2000) 2 100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 Humidity (%) 20 10 10 Precipitation (mm) 0 0 Jul JUL JAN JUN Apr Oct FEB SEP Jan Mar Jun APR OCT DEC Feb Aug Sep Nov Dec MAR MAY AUG NOV May Month Month Figure 3: Mean monthly rainfall (a) and humidity (b) between 1970 and 1999 Figure 4: Average maximum and minimum air temperatures (1970-1999) 2.2 The topography and drainage pattern in the study area The topography in the Francistown study area is relatively flat with isolated kopjes (inselbergs) outcropping within the geologic units less susceptible to weathering. The highest point within the study area si at 1096 m above sea level and the lowest at 958 m above sea level. The overall slope is at 0.002 m to the southwest. The prevailing dendritic drainage pattern consists of a system of irregularly branching tributaries and forms junctions at various acute angles. This is a manifestation of the complex folded and contorted metamorphosed rocks where lithological

Load more