Population and water supply

1 Introduction

1.1

Project objectives

The main aim of this thesis is to determine the mineral/water interactions that control fluoride levels in groundwaters in the Arusha region of North Tanzania (Fig. 1.1) The reasons why this area was chosen are:

1) Known incidences of high fluoride groundwaters in the area (Aswathanaranyana et al., 1985), and as a representative sample area for the East African Rift in which the problem of high fluoride groundwater is widespread.

2) Proximity to four geologically distinct regimes.

Excess fluoride is detrimental to health and despite the existence of a fluoride problem in this area, and several previous studies, controls on fluoride levels are poorly established and little investigation has been undertaken to identify the roles of mineralogy and hydrogeology. Once the mineralogy and hydrogeology is understood it is hoped that a mechanism can be suggested for the transfer of fluoride to groundwaters. In addition to this the spatial variability of fluoride in groundwater, which is related to the geological and hydrogeological regimes, needs to be determined. It is hoped that a better understanding of the processes controlling fluoride concentration will lead to improved techniques for siting boreholes and/or managing groundwater abstraction in areas prone to high fluoride levels.

1.2 Project planning and limitations

The original plan was to have two field seasons, the first being a short reconnaissance trip collecting around 50 water samples and identifying areas for future study. The second field season was intended to be more comprehensive, for collecting a larger number of water, rock and soil samples. It was also envisaged that during this second field season some soil column experiments could be carried out. The field seasons were planned such that one would occur during the dry season and the other during the wet season. In order to carry out these field visits a number of problems needed to be overcome; a new network of contacts in Tanzania needed to be established

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and areas needed to be identified in terms of problem high fluoride groundwater, accessibility and support and facilities requires e.g. a field laboratory and storage area.

In the event the second field season was cancelled was due to financial constraints. With only one field season the number of water and rock samples is far from ideal, but can still be considered representative of the area. The lack of the second field season meant that many of the experiments planned in the field were not possible and that the areas outside of the Arusha Town area have a poor coverage of data. This lead to a refocussing of available data and a greater emphasis being placed upon the laboratory experiments..

Other problems encountered include; the difficulties in moving from one area to another in the field. The field season was at the onset of the wet season and this meant that trips had to be planned on a day to day basis depending upon the weather conditions; the lack of core samples which were promised from the Tanzanian Ministry of Water, Energy and Minerals but could not be located when the time came to transport them to the UK and general problems associated with working and record keeping in developing world countries.

1.3 Study methodology

The study started with a review of the existing data on high fluoride occurrences in groundwater.

A field season was undertaken in order to investigate the occurrences of high fluoride in the Arusha Region of northern Tanzania. This field season took place in March and April 1995. During this field season 43 water samples were taken, the majority from boreholes although some spring and lake samples are present. In addition to the water samples, relevant rock samples and other pertinent information was collected. The water samples were analysed for major cations and anions to determine the water chemistry and suitability for potable water supplies and give an indication of the possible sources and controls on fluoride concentrations. The rock samples were analysed to identify rock types, fluoride sources and the reason for any variability seen in the fluoride contents of the waters. A mass balance was attempted using the computer code NETPATH in order to account for fluoride found in groundwaters.

Once a hypothesis for the mineral/water interactions most likely to be causing high fluoride groundwater was developed, laboratory experiments were undertaken in order to gain a better understanding of the mechanisms by which fluoride enters the groundwater.

In presenting this work this thesis has been split into three sections:

I) Introduction: This section gives details on the project objectives, the effects of high fluoride groundwater and the results and conclusions of previous studies on high fluoride groundwater. In addition a background to the geology and hydrogeology of the study area is given.

II) Hydrochemistry and petrography of the Arusha region: This section outlines the methods used in the field and in the analysis of the rock and water samples. Results of the rock and water analyses are given and these results are interpreted to give a model for the observed hydrochemistry, focusing particularly on the high fluoride groundwaters.

III) Laboratory studies: This section describes the objectives and background to the laboratory experiments examining the mechanisms of fluoride release, and the experiments themselves. At the end of this section conclusions are drawn from all three sections.

1.4 Study Area Location

All the localities sampled he in the Arusha region of North Tanzania shown on

Fig. 1.1. The region itself is close to the equator, lying between 2° and 6° south and 35°

and 36° east, covering an area of 82,428.5 km^ (ARWMP, 1994).

Samples were taken from four different areas (Fig. 1.2):

1) The volcanics of Mt. Mem around Amsha Town.

2) The metamorphic Masai Steppe to the south of Amsha.

The Arusha Region

Key National boundary District boundary Regional boundary River/stream Lakes Volcanic crater

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quartzite and marble) G eological boundary Fault

Fig. 1.2 A simplified geological map of the Arusha Region of northern Tanzania, showing the four sampling areas: 1) The Arusha Town area; 2) The Masai Steppe area, 3) The Ngorongoro Crater area and 4) The Lake Manyara area (from ARWMP, 1994)

4) Lake deposits and metamorphics around Lake Manyara.

1.5 Background to the study area

For the purposes of this study it is important to have an understanding of the climate for the hydrogeology, and the population distribution for the health aspect.

1.5.1 Climate

The location of Tanzania, just to the south of the equator, means that it is located in a tropical climate zone. This is characterised by two rainy seasons, one short and one long that last between October and December and mid-March and early June respectively. Between these wet seasons are dry seasons when little or no precipitation occurs.

The climate in Tanzania varies greatly across the country. The coastal and island areas enjoy a tropical climate with average daytime temperatures of 23°C from June to September and I T C from December to March with a rainfall of greater than 1000mm. The central plateau is much drier with average annual rainfall of 250mm. The highlands to the north of the country are in the transition between the tropical climate and a semi- temperate climate on peaks over 1500m.

The tropical and semi-temperate climate zones in Tanzania are present within the Arusha Region, with rainfall varying dependant on altitude, 400 mm per annum (p/a) or less in the lowland areas (such as the Masai Steppe) and up to 2000 mm p/a in the highland areas (such as the Ngorongoro plateau highlands and Mount Meru) (ARWMP,

1994).

Temperatures in the Arusha Region are fairly uniform throughout the year but are also affected by altitude. In the hot season between July and September temperatures can reach 38“C in the lowland areas, but below freezing on the mountains.

The average temperatures and rainfall for two weather stations near Arusha Town over a number of years are listed in Appendix 1. Arusha is considerably wetter than the plains to the south but dryer than Mt. Meru to the north. During the time of the

field study to Arusha (March and April) the weather changed from the dry season to the major rains, with up to 77 mm of rain falling in a day.

1.5.2 Population and water supply

For the purposes of this study it is important to bear in mind the centres of population which carry the greatest risk of wide scale health problems from the ingestion of high fluoride waters.

The main centre of population within the study area is Arusha Town just to the south of Mount Meru where most of the water samples have been taken. Other areas of population consist of towns at the junctions of major routes such as Makuyuni to the west of Arusha (Fig. 1.2), and isolated villages.

Chapter 2

Previous studies on fluoride in groundwater

2.1 Health implications of fluoride

In document Sources and controls on fluoride in groundwater of the Arusha Region, northern Tanzania (Page 30-38)