Chapter Three: Water Sources Analysis

1 Chapter 3: Analysis of Water Sources

Water source is the single most important element and is key to the proper functioning and thus the sustainability of any water supply project. Evidence shows that whenever proper water source analysis has not been adequately conducted, most water supply projects fall into dysfunction. This chapter presents the analysis of water sources. It includes analysis of both surface and groundwater. Further, the chapter gets into the ‘nitty gritty’ of each water source.

1.1 3.1 AVAILABILITY OF WATER RESOURCES IN TANZANIA MAINLAND

Tanzania mainland is endowed with a wide range of water resources that include the main drainage systems, river basins and natural wetlands. All such sources are identified and discussed in the next paragraphs. The drainage systems of water resources in Tanzania mainland, is divided into five drainage systems that include:

• The Indian Ocean drainage system,
• The Internal drainage system to Lake Eyasi, Natron and Bubu depression,
• The Internal drainage systems to Lake Rukwa,
• The Atlantic Ocean drainage system through Lake Tanganyika,
• The Mediterranean Sea drainage system through Lake Victoria.

The drainage systems in turn comprise nine river basins with some bearing names resembling the drainage systems. These nine basins are indicated in Figure 3.1.

From the geographical point of view, Tanzania is party to at least eleven transboundary water resources in the form of lakes and rivers (NWSDS, 2008). These include the following:

• Lake Victoria,
  
• Lake Tanganyika,
  
• Lake Nyasa,
  
• Lake Chala,
  
• Lake Jipe,
  
• Kagera River,
  
• Mara River
  
• Pangani River,
  
• Umba River,
  
• Ruvuma River and
  
• Songwe River.
  

Figure 3.1: The River Basins of Tanzania (Source: NWSDS, 2008) Legend for Figure 3.1 (I) Pangani River Basin
(V) Lakes Nyasa Basin (II) Wami/Ruvu River Basin (VI) Internal Drainage Basin (III) Rufiji River Basin (VII) Lake Rukwa Basin (IV) Ruvuma and South Coastal River Basin (VIII) Lake Tanganyika Basin (IX) Lake Victoria Basin

With its numerous water bodies, Tanzania is perceived to have abundant surface and groundwater resources for meeting its present consumptive and non-consumptive needs. However, the reality is that severe and widespread water shortages exist in many areas of Tanzania because of climate variability, poor distribution of the resource in terms of time and space, and inadequate management of the water resources (NWSDS, 2008). As a result, Tanzania experiences frequent and intense water shortages and some water use conflicts. Furthermore, Tanzania is relatively dry with more than half of the country receiving, on average, less than 800 mm of rainfall per year depending upon air circulation patterns and the movement of the convergence zones in the region. The semiarid Central and Northern parts of the country, including areas immediately South of Lake Victoria receive less than 700 mm of rainfall per annum and are dry for an average of seven consecutive months a year. River flows in these areas are intermittent. In the Southern, Western and Northern highlands, which receive more than 1,000 mm/year of rainfall, rivers are perennial, and some of these experience frequent floods.

As an example, in 1999 the availability of renewable freshwater resources, both surface and groundwater were estimated1 to be about 2,700 m3/capita/year. By 2018, this estimate was reduced to 2,330 m3/capita/year due to increased population alone. The average figure is significantly above the level of 1,700 m3/ capita/year set by the United Nations as denoting water stress, or 1,000 m3/capita/ year denoting water scarcity. Furthermore, due to the projected population growth alone, Tanzania’s annual freshwater renewal rate is projected to drop to 1,500 m3/capita/year by 2025, thus categorising the country as being water stressed by then.

On the whole, Tanzania has sufficient surface and ground water resource potential to meet most of her present needs. However, differences in topography, rainfall patterns and climate account for the existing variation in the availability of water in different parts of the country. In the densely populated Pangani and Rufiji Basins, these variations have already resulted into water stress. It is estimated that the annual surface runoff from Tanzania to the world’s oceans is about 74 x 109 m3. The Rufiji, which drains a 177,000 km2 area, contributes over 50% of the runoff. Typical annual runoffs are shown in the Table 3.1 for some of the major rivers of Tanzania.

The most abundant surface water resources exist in Lakes Victoria, Tanganyika, Nyasa, Chala and Jipe, as well as the Kagera, Mara and Songwe rivers, which are trans-boundary waters. The use of these abundant surface water resources for water supply, irrigation and other purposes is still very limited even today. Tanzania is also rich in wetland systems that are areas which, for part of the year, have enough water to enable the development of different types of plants

and animals adapted to these conditions. These include the lakes of the Western and Eastern Rift Valley system, Lake Victoria, numerous small lakes, river in flood plains and permanent swamps, coastal mangrove and deltaic systems, and a number of artificial impoundments and reservoirs and fish ponds. There are numerous permanent and seasonal freshwater swamps and flood plains distributed in almost all of the country’s major drainage basins, which account for some 2.7 million hectares. The largest in this category are found in the Rufiji/ Ruaha river system and in the Malagarasi/Moyowosi system, while other river systems are the Kagera River, along with Ugalla River, Suiwe River, Mara River, Pangani, Wami and Ruvu Rivers. The principal wetlands of Tanzania constitute one of the country’s richest and most durable resources.

1.2 3.2 WATER SOURCES AVAILABLE IN TANZANIA MAINLAND

In Tanzania, there are three main categories of water sources available, namely: rainwater, surface and groundwater.

1.2.1 3.2.1 Rainwater and fog harvesting

One of sources of water include rainwater and fog that can generate limited amounts of very clean water if they are properly collected and stored. In an area where other water sources are not available, consideration should be given to harvesting rainwater and fog.

1.2.2 3.2.2 Surface Water

For design purposes, the surface water sources that can be considered include;

• Rivers or streams,
• Impoundments (Reservoirs and ponds),
• Springs,
• Lakes,
• Dams (charco, sand, earth etc).

A brief description of each water source is provided below.

1.2.2.1 3.2.2.1 Rivers or Streams

Rivers and streams are water sources that originate from springs located in highlands which flow down to the end of the respective drainage basin which can be lakes, seas or oceans as depicted on the map of Tanzania in Figure 3.1.

1.2.2.2 3.2.2.2 Impoundments

Impoundments includes all types of reservoirs that emanate from road borrow pits, mining, human or natural activities that are utilised as sources of water for a formal water supply project.

1.2.2.3 3.2.2.3 Springs

Springs include artesian or freely flowing spring water that has been tapped by an intake structure to facilitate supply of water to a designated community. A spring is a point where groundwater flows out of the ground, and is thus where the aquifer surface meets the ground surface. The spring may be ephemeral (intermittent) or perennial (continuous). Springs can be developed by enlarging the water outlet and constructing an intake structure for water catchment and storage.

1.2.2.4 3.2.2.4 Lakes

Lakes found in Tanzania are either located at the end of drainage basins or are highland lakes and some of them are volcanic lakes. Tanzania is endowed with many small inland lakes in addition to the third biggest lake in the world (Lake Victoria) as well as Lake Tanganyika which is the Africa’s deepest lake and the world’s longest lake. Both lakes supply water to various localities around their respective catchments.

1.2.2.5 3.2.2.5 Dams

Dams are classified based on the availability of construction materials. Various types of dams can be built ranging from earth fill dams, concrete dams, sand dams and charco dams. These are purposely built structures that allow impoundment of river and/or rain water for various end uses.

1.2.3 3.2.3 Groundwater

Groundwater is that portion of rainwater which has percolated beneath the ground surface to form an underground reservoir referred to as aquifer water. The upper surface of groundwater is the water table. Groundwater is often clear, free from organic matter and bacteria due to the filtering effect of the soil on water percolating through it. However, groundwater almost always contains dissolved minerals from the soil. Groundwater is often better in terms of quality than surface waters. It is less expensive to develop for use, and usually provides more adequate supply in many areas in the country. In semi-arid and the drier parts of the country, groundwater has played and will continue to play a major role as the sole water source for various uses especially in the central and northern parts of the country and the drier regions of Dodoma, Singida, Shinyanga, Tabora, Mwanza, Mara, Arusha, Coast and Southern Kilimanjaro.

Groundwater can be considered as either spring water or well (or borehole) water. Springs, offer excellent water supply opportunities, but are generally found in hilly or mountainous areas only. They may require long pipelines to bring the water to the demand area. This is a feasible source for larger and concentrated settlements but rarely for dispersed populations. For rural water supply systems, groundwater is generally preferred as a water source.

The main sub-types of groundwater and extraction methods are as follows:

1.2.3.1 3.2.3.1 Infiltration Galleries/Wells

Infiltration galleries are horizontal wells, constructed by digging a trench into the water-bearing sand and installing perforated pipes in it. Water collected in these pipes converges into a “well” from which it is pumped out.

1.2.3.2 3.2.3.2 Well

This is a hole constructed by any method such as digging, driving, boring, or drilling for the purpose of extracting water from underground aquifers. Wells can vary greatly in depth, water volume and water quality. Well water typically contains more minerals in solution than surface water and may require treatment to soften the water by removing minerals such as arsenic, iron and manganese. Well water may be drawn by pumping from a source below the surface of the earth. Alternatively, it could be drawn up using containers, such as buckets that are raised mechanically or by hand.

Wells are various types of artificially constructed water production wells that are designated as shallow wells (up to 20 metres deep) or deep wells (more than 20 metres deep) as designated by the Ministry responsible for water from time to time. Water is pumped out of the well into the end user containers or a storage tank using various types of pumps that can be driven manually or using various energies. Typical cross sections through such wells are given in Section 3.6.5.

1.2.3.3 3.2.3.3 Classification of Wells Based on the Aquifer Tapped

As mentioned, an aquifer contains a considerable amount of groundwater underground beneath layers of permeable soil material like sand or gravel. Aside from their water storage capacity, aquifers allow the underground flow of groundwater. Aquifers are recharged with rainwater that seeps down to the soil and through the permeable layers.

1.2.3.3.1 3.2.3.3.1 Shallow wells

Generally, a well is considered shallow if it is less than 20 metres deep. Shallow wells tap the upper water-bearing layer underground. This permeable layer, however, usually has limited safe yield due to its great dependence on seasonal rainfalls. Therefore, the supply capacity of shallow wells could be unreliable and are sometimes intermittent. Also, the water extracted from the upper strata is usually more affected by contamination since the aquifer being tapped is near the ground surface where possible sources of contamination are abundant. Protection against contamination is therefore one of the main considerations in constructing a shallow well.

1.2.3.3.2 3.2.3.3.2 Deep wells

Deep wells, which are over 20 metres deep, tap the deeper unconfined aquifer. This aquifer is not confined by an overlying impermeable layer and is characterized by the presence of a water table. A deep well is less susceptible to surface contamination because of the deeper aquifer. Also, its yield tends to be more reliable since it is less affected by seasonal precipitation.

1.2.3.3.3 3.2.3.3.3 Artesian wells

Artesian wells are much like deep wells except that the water extracted is from a confined aquifer. The confining impermeable layers are above and below the aquifer. Groundwater recharge enters the aquifer through permeable layers at high elevations causing the confined groundwater at the lower elevations to be under pressure. In some cases, the hydraulic pressure of the aquifer is sufficient for a well to flow freely at the wellhead.

1.3 3.3 QUALITY SUITABILITY OF WATER SOURCES FOR WATER SUPPLY PROJECTS

When considering the different water sources for water supply projects, it is necessary to ensure that the quality of the water source expected to be utilised is monitored well preferably for a period of not less than three years consecutively prior to commencing design to ensure the variability of the quality is captured during the wet and dry seasons. When one looks at the list of the potential sources presented in the foregoing section, such a monitoring programme may not be necessary for rainwater and fog. Only short-term monitoring of the quality of these two sources should be undertaken.

1.4 3.4 PILOT TESTING OF WATER SOURCES FOR ESTABLISHMENT OF APPROPRIATE TREATMENT

A decision on whether the water source needs to be subjected to water treatment or otherwise will emanate from the results of the short term tests on the quality of the water which will in turn guide the decision of pilot testing of the recommended flow sheets particularly for river/streams, lakes, impoundments and dams. Groundwater will usually need only a few unit operations for removal of the identified elevated impurities that may include Iron, Manganese or Fluoride and the need to disinfect water from shallow wells in addition to maintaining residual disinfectants for the prevention of re-contamination. Rainwater and condensates from fog may not need to be pilot tested.

1.5 3.5GENERAL CONSIDERATIONS FOR SELECTION OF WATER SOURCES

In the selection of a source or sources of water supply, adequacy and reliability of the available supply can be considered as the overriding criteria. Without these, the water supply system cannot be considered viable.

Sources which require little or no treatment of raw water such as springs, wells and boreholes should be given the highest selection priority provided their yields are sufficient to meet the water demands of the water supply scheme. For large supplies, surface water will continue to be the most economical alternative water source. In selecting surface water sources, rivers with upland and mostly forested catchments should be given preference. Sub-surface water drawn from a riverbed or river bank can sometimes be a viable alternative in dry areas with only seasonal flows in the river, or in rivers with a high silt load.

Sources from which water can be supplied by a gravity system are particularly more favourable than those which require pumping with significant energy costs. For household and small community water supplies, rainwater harvesting will be the most appropriate in most medium and high potential areas in Tanzania that receive sufficient rains.
These, together with the other interdependent factors should be considered as follows:

1.5.1 3.5.1 Adequacy and reliability

Adequacy of water supply requires that the quantity of water flow from a water source be large enough to meet present and future water demand. On the other hand, source reliability can be expressed by how frequently a water system expects normal demand to go unmet, such as a one-in-25 year or even a one in-50 year drought. Safe yield is a 1-day low flow rate that is exceeded for 96 percent of the period of record and that can be related to the determined average daily water demand in order to establish the reliability of a water source. For a river/stream, safe yield represents the minimum flow rate that will guarantee no risk to the river hydrology and its surroundings. Safe yield is estimated to check whether the planned withdrawal for water supply purposes will be met. To determine the safe yield of a river or stream, a flow-frequency/probability analysis presented in section 3.6.2should be performed. From the analysis, the determined 96% low flow index should be taken as the safe yield of the river or stream and thus considered as the water source reliability.

1.6 3.5.2 Quality of water sources

1.6.1 3.5.2.1 Surface Water Quality

The assessment of water quality of a water source is important to establish the suitability of water source for human consumption. The quality of surface water is determined by the amount of pollutants and contaminants picked up by the water in the course of its travel. While flowing over the ground, surface water collects silt, decaying organic matter, bacteria and other micro-organisms from the soil. Sources which require little or no treatment of the water should be chosen in the first instance, provided the required quantity of water can be obtained. Hence springs and ground water resources should always be exploited in the first hand. Surface water from rivers, streams and lakes will almost always require some treatment to render it safe for human consumption. However, for large supplies, surface water will often still be the most economical alternative. Rivers which have the bulk of their catchments in forest areas should be preferred.

Thus, all surface water sources should be presumed to be unsafe for human consumption without some form of treatment. The option to treat surface water to make it safe for human consumption in compliance with the latest edition of Tanzania potable water standards (TBS, TZS 789) has to be evaluated to decide on the feasibility of the water supply project.

1.7 3.5.2.2 Groundwater Quality

Generally, all groundwater contains salts in its solution that are derived from the location and past movement of the water. Groundwater quality parameters exhibit considerable spatial variability. The eight common groundwater parameters that are measured include:

• Dissolved oxygen DO
  
• Electrical conductivity EC
  
• Total dissolved solids TDS
  
• Salinity
  
• pH
  
• Turbidity
  
• Total suspended solids TSS
  
• Chloride
  

Industrial discharges, urban activities, agricultural, groundwater pump age, and disposal of wastewater all can affect groundwater quality. The quality required of groundwater supply depends on its purpose.

Natural groundwater quality
The groundwater in natural systems generally contains less than 100mg/l dissolved solids, unless it has:

• Encountered a highly soluble mineral, such as gypsum,
• Been concentrated by evapotranspiration,
• Been geothermally heated.

Sources of salinity in groundwater
All groundwater contains salts in solution ranging from less than 25 mg/l in quartzite spring to more than 300,000 mg/l in brines. The types and concentration of salts depend on the environment, movement, time in residence with a particular geological formation, and source of the groundwater. Generally, salinity increases with depth.

Graphic representation of groundwater quality
In the field of hydrogeology and groundwater analysis, piper plots (also known as tri-linear diagrams) are very powerful tools for visualizing the relative abundance of common ions in water samples. Although there are other plot types that can show abundance of ions in groundwater, this plot type is especially useful because it allows one to plot multiple samples on the same plot, thus allowing for grouping water samples by groundwater facies and other criteria. In this day and age, when groundwater is so closely monitored, it is especially important to have a plot type like the piper plot (Figure 3.2) that makes it easy to determine. Piper plot is comprised of three components: a ternary diagram in the lower left representing cations (magnesium, calcium, and sodium plus potassium), a ternary diagram in the lower right representing anions (chloride, sulfate, and carbonate plus bicarbonate), and a diamond plot in the middle which is a matrix transformation of the two ternary diagrams. Each sample is normalized (sum of cations = 100 and sum of anions = 100), so the relative concentrations are on a percentage basis.

Bottom left is a ternary plot of the cations, bottom right is a ternary plot of the anions, and top is a diamond plot of a projection from the other two plots.The diamond plot can then be analysed to tell you what kind of groundwater you’re looking at. Samples in the top quadrant are calcium sulfate waters, samples in the left quadrant are calcium bicarbonate waters, samples in the right quadrant are sodium chloride waters, and samples in the bottom quadrant are sodium bicarbonate waters. See example on Figure 3.2.

  

 Figure 3.2: The Three Components of the Piper Plot
 (Source: http://inside.mines.edu, Golden Software Support)

Interpretation of the diamond plot
Samples in the top quadrant are calcium sulphate waters, which are typical of gypsum ground water and mine drainage. Samples in the left quadrant are calcium bicarbonate waters, which are typical of shallow fresh ground water. Samples in the right quadrant are sodium chloride waters, which are typical of marine and deep ancient ground water. Samples in the bottom quadrant are sodium bicarbonate waters, which are typical of deep groundwater influenced by ion exchange. See Figure 3.3.

 

Figure 3.3: Interpretation of the Diamond Plot

(Source: http://inside.mines.edu, Golden Software Support)

1.7.1 3.5.3 Technical Requirements

The development of the source should be technically feasible, the operation and maintenance requirements for the source abstraction and supply system should be appropriate to the resources available.

1.7.2 3.5.4 Cost implications to develop a water source

The assessment of investment costs to develop a given water source including operation and maintenance costs has a bearing in the selection of the water source for development. Affordability of investment costs is an important factor to be considered in the selection of a water source.

1.7.3 3.5.5 Protection of water sources

The location of a water source is a key factor in securing the highest quality water source. In analysing a water source location, the design engineer should consider the measures necessary to protect the water source from human excreta, from industrial discharges and from agricultural run-off. In addition, measures to establish and maintain watershed control, physical protection and barriers to contamination must be considered to ensure sustainable quantity and quality of the raw water.

1.8 3.5.6 Legal and management requirements

The ownership of the land and the legal requirements of obtaining permission to abstract are also factors to consider when selecting a source. Sources on private land may cause access problems.

1.9 3.5.7 Distance of water supply source

The source of the water supply must be situated as near to the demand area as possible. Hence, less length of pipes needs to be installed and thus economical transfer and supply of water. The source(s) nearest to the demand area is usually selected.

1.10 3.5.8 Topography of the project area and its surroundings

The area or land between the source and the area to be served by water supply system should not be highly uneven, i.e., it should not have steep slopes because the cost of construction or laying of pipes is very high in such areas.

1.11 3.5.9 Elevation of a source of water supply

The source of water should preferably be at a higher elevation than the demand area so as to provide sufficient residual pressure in the water for daily requirements. When the water is available at lower levels, then pumps are used to pressurize water. This requires excess developmental and operational tasks and costs.

1.11.1 3.6 DETERMINATION OF WATER SOURCE YIELD

1.11.1.1 3.6.1 Rainwater and Fog Harvesting

1.11.1.1.1 3.6.1.1 Rainwater Harvesting

Rainwater harvesting is a technique of collection and storage of rainwater into natural reservoirs or tanks, or the infiltration of surface water into subsurface aquifers (before it is lost as surface runoff). The types of rainwater harvesting systems are described in the sections below.

3.6.1.1.1 Types of Rainwater Harvesting
Two types of rainwater harvesting should be considered:

• Land catchment
• Roof catchment
  

Important data for the design of rainwater harvesting systems are:

• Rainfall data
• Catchment/Surface Area
• Run-off Coefficient

To accurately estimate the potential rainwater supply from a catchment, reliable rainfall data for a 10-year period is required2. The Hydrology Section, Tanzania Meteorology Agency, and Agriculture Departments should be contacted for rainfall data wherever rainwater-harvesting technology is proposed.

The amount of rainfall collected depends on the surfaces where rain falls and the runoff coefficient K of the surface. The runoff coefficient varies with topography, land use, vegetation cover, soil type and moisture content of the soil. In selecting run off coefficients the future characteristics of the water shed are considered. If land use varies within a watershed consider the segments individually and use a weighted coefficient value to determine the total runoff for the watershed. Since most of the rainfall occurs during the rainy seasons between October and May annually, design should be mostly based on rains received during this period.

(i) Run-off Coefficients Table 3:2 shows the runoff coefficients for various surfaces. They should be used for calculating the fraction of the rainfall which can be harvested.

Table 3.2: Run-Off Coefficients for Different Surfaces

3.6.1.1.2 Components of a Rainwater Harvesting System
(a) Catchments Area: The catchment of a water harvesting system is the surface which directly receives the rainfall and provides water to the system.
(b) Coarse mesh at the roof to prevent the passage of debris.
(c) Gutters to collect and transport rainwater to the storage tank. Gutters can be semi-circular or rectangular and could be made using:

• Locally available materials such as plain galvanised iron sheets (20 to 22gauge), folded to required shapes
  
• Semi-circular gutters of PVC material can be readily prepared by cutting those pipes into two equal semi-circular channels. Recently, there are commercially available gutters in standard sizes for simple home or community schemes. For larger schemes, the same may be ordered from pipe manufacturing companies in the right sizes,
  
• Bamboo trunks cut vertically in half.
  

The size of the gutter should be according to the flow during the highest intensity rain. It is advisable to make them 10 to 15 per cent oversize.
(d) Conduits/pipeline that carry rainwater from the catchment or rooftop area to the harvesting system. Conduits can be of any material like polyvinyl chloride (PVC) or galvanized iron (GI), materials that are commonly available.
(e) First Flush pipe

3.6.1.1.3 Estimation of the Yield A first estimate of the average yield of a catchments area can be found using the following expression.