**2. Geological and hydrogeological setting**

In this phase of the research work, the aquifer type and water-bearing formations of borehole sites are generalized on the basis of lithological logs developed from well cutting logs collected. For the study, 29 boreholes were used to understand the Hawassa City aquifer.

The main water-bearing geological formation in the Hawassa City ground water system is classified along with the respective water-bearing thickness. The northeast part of the city has water-bearing formation of pumice ash and sand at depth of 14–20 m, weathered and fractured basalt formations at a depth 23–40 m, weathered pumice and rhyolite (42–54 m), and black and red scoria (60–66 m).

The north part of the city has sand (25.52–29 m), trachyte (29–39 m), and the volcanic origins' rocks (39–42 m) and (39–50.5 m). This shows that the area is likely dominated by volcanic rocks for depth below 39 m.

The lake shore (east and southeast of the Hawassa city) covers slightly rhyolite (7.15–9 m), scoria (9–12 m), dominant weathered basalt (12–27 m) and course sand scoriaceous basalt that covers up to 39 m depth of the area. As the well site goes apart to the west direction, the hydrogeology appears different for the whole lake shore; the water striking depth is increasing, and ash with sand formation (23.58– 31 m), fractured basalt (49.43–57 m), and ash with scoriaceous basalt (60–69.24 m) are recurrently reported. The far southern part of Hawassa city (*Gara Riqata area*) where relatively the deepest wells of this study are located, the water striking point gets deeper and the major aquifers recognized are pumice type of fractured, weathered and course-grained (18–32 m), highly weathered pumice (32–58 m), fine-to- medium-grained sand (66–72 m), silty sand and weathered rhyolite (79–94 m), fractured pumice (94–102 m), weathered pumice (102–120 m), fine-to-mediumgrained sand (120–166 m), weathered pumice (166–172 m) and fine-grained sand (184–196 m) are dominant of which sand covers the largest formation.

### *Aquifer Characterization: The Case of Hawassa City Aquifer DOI: http://dx.doi.org/10.5772/intechopen.91211*

coefficient. The study made use of the Theis-type [14] curve method in AquiferTest applicable to both partially and fully penetrating wells. This was used to calculate dimensional drawdowns that are compared with time-drawdown data from 23 observation points to estimate the hydraulic properties of a finite, layered unconfined aquifer situated in the harbor area of Antwerp. The study concluded that AquiferTest and WTAQ form an excellent pair for the analyses of single or

An assessment was made for the hydraulic properties of the Ethiopian Ashange formations applying AquiferTest software. In the study, a total of 70 wells raw pumping test data were analyzed and used besides their respective lithological log to determine hydraulic property of Ashange formation. This study has done identification, analysis, and interpretation of aquifer system hydraulic properties of the geologic formation using the secondary well pump test data, lithological log, and data of hydro geological field observations. Among the different stages of pumping tests, constant rate pumping tests lasting between 5 and 72 h and recovery tests were used to determine transmissivity, hydraulic conductivity, and storativity values. The study analyzed single pumping test data mainly using Theis time-drawdown graphic method by which aquifer properties have been calculated. The pump test data including measured and calculated ones have been organized and processed using the Aquifer test software version 3.5. Arc GIS 9.2 and Global mapper 11 were also used for mapping in that study. As a result, the study finally identified the aquifer characteristics of the Ashange formation with respect to depth of the boreholes, age, and

In this phase of the research work, the aquifer type and water-bearing formations of borehole sites are generalized on the basis of lithological logs developed from well cutting logs collected. For the study, 29 boreholes were used to under-

The main water-bearing geological formation in the Hawassa City ground water system is classified along with the respective water-bearing thickness. The northeast part of the city has water-bearing formation of pumice ash and sand at depth of 14–20 m, weathered and fractured basalt formations at a depth 23–40 m, weathered pumice and rhyolite (42–54 m), and black and red scoria (60–66 m). The north part of the city has sand (25.52–29 m), trachyte (29–39 m), and the volcanic origins' rocks (39–42 m) and (39–50.5 m). This shows that the area is likely

The lake shore (east and southeast of the Hawassa city) covers slightly rhyolite (7.15–9 m), scoria (9–12 m), dominant weathered basalt (12–27 m) and course sand scoriaceous basalt that covers up to 39 m depth of the area. As the well site goes apart to the west direction, the hydrogeology appears different for the whole lake shore; the water striking depth is increasing, and ash with sand formation (23.58– 31 m), fractured basalt (49.43–57 m), and ash with scoriaceous basalt (60–69.24 m) are recurrently reported. The far southern part of Hawassa city (*Gara Riqata area*) where relatively the deepest wells of this study are located, the water striking point gets deeper and the major aquifers recognized are pumice type of fractured, weathered and course-grained (18–32 m), highly weathered pumice (32–58 m), fine-to- medium-grained sand (66–72 m), silty sand and weathered rhyolite (79–94 m), fractured pumice (94–102 m), weathered pumice (102–120 m), fine-to-mediumgrained sand (120–166 m), weathered pumice (166–172 m) and fine-grained sand

(184–196 m) are dominant of which sand covers the largest formation.

multiple pumping tests in unconfined aquifers.

*Resources of Water*

variation of its spatial distribution and groundwater potential.

**2. Geological and hydrogeological setting**

dominated by volcanic rocks for depth below 39 m.

stand the Hawassa City aquifer.

**40**

Around the western part of the city (the industry zone), the water striking point is the deepest of the study area. Highly fractured and weathered scoriaceous formation dominates the water-bearing strata (52–84 m). The central areas generally fractured basalt (12–21.5 m), sand and ignimbrite (22.64–33.54 m), scoria and pumice (27.38–38.20 m), and highly weathered ignimbrite (39.27–45 m).

About 30–60 m ignimbrites and pumice are dominant in large area of the central part. These ignimbrite and pumice of the rift floor are well jointed while in some cases, it is massive and pumiceous. Where it is well jointed, it has a high or moderate permeability, but in the other part, it has low permeability.

The relationship between lithology and aquifer characteristics is used to understand the qualitative and quantitative aspects of the hydrogeology in these areas. The study by Glenn and Duffield [17] established the estimate of the representative range of hydraulic properties (horizontal and vertical hydraulic conductivity, storativity, specific yield, and porosity) of aquifers and aquitards in relation to the formation type using values reported in different literatures. These tabulated values are used to understand the hydraulic properties of the study area.

Therefore, the dominant water-bearing formations (weathered pumice, scoria, fractured basalt, and sand of different types) possess large pores. Pumice and fractured basalts strata, which are common relatively in the shallower formations, are devoid of primary openings but possess secondary openings in the form of fractures and joints. These features aid in the infiltration of surface water. Besides, pores and fractures in laterites and fractures and joints in basalts act as reservoirs of groundwater.

Highly fractured and weathered scoriaceous formation dominates the waterbearing strata (52–84 m), and the fine-to-course-grained sand is the main water source in depth beyond 100 m. Furthermore, lack of confining rocks like clay in the area studied indicates that groundwater occurs in phreatic, unconfined conditions in the weathered basalts that outcrop at the surface.

Looking into representative values, aquifers in the area are high hydraulic conductivity units and large porosity which will produce higher and more sustained well yields than an aquifer where the clean sands and gravels are compartmentalized by interbedding with clay and other low hydraulic conductivity units.

#### **2.1 Aquifer physical properties of Hawassa City**

The results show (**Table 1**) the depth ranges from 25 m to 200 m below the surface. The pumping phase of the tests had a duration of 1440 min; the recovery phase of the tests had a duration of 45–240 min. Constant rate of discharge was applied for each of the wells. These constant discharge rates are from 3.0 l/s (for Gara Riqita 6) to 66 l/s (for Gara Riqita 1). Total drawdown varied from 0.03 m (for Zewdu Village) to 12.36 m (for HU Techno Village) and average of 2.53 m.

#### *2.1.1 Specific capacity*

The specific capacity of the wells as the ratio of the yield to the total drawdown is determined using Eq. (1). These two parameters (TDDw and Sc) along with the discharge rate are calculated and tabulated for 29 wells as shown in **Table 1**.

As per **Table 1**, values of specific capacity range from 0.54 l/s/m to 2200 l/s/m. Maximum values are toward the southwest part of the city (at Hawassa University Referral Hospital) and tend to decline toward the central and then to the northern corner. A decline in specific capacity may indicate declining S or T values due to declining water levels or piezometric surfaces, thus large water level drawdown for the specified discharge rate. It can also be used to determine the distribution of transmissivity in the aquifer. The spatial distribution of specific capacity reveals


area. Percentage of recovery is also estimated using TDDw and the maximum recovery of water level during pumping test using Eq. (3). The results have 95– 100% recovery percentage which reflects high recovery rate. The recovery of these wells is also fast that they recover up to the SWL within few minutes, even in 30 s. The results clearly show that the study area is of high potential of ground water with

The thickness values found in the study area range from 8 m to 136 m and average of 30.30 m. Within the study area, the spatial distribution of saturated thickness clearly indicates that the depth and the lakeshore have significant effect on it. As the cutting depth increases and the site getting nearer to the lake shore, the

This hydrogeological analysis method is done to determine the important hydrogeological parameters of the wells using the method selected. Along with the curve, the analysis result displays the important parameters, that is, transmissivity,

hydraulic conductivity, aquifer thickness, and storativity (specific yield for unconfined aquifers). The result of the hydraulic parameters determined for the

As presented in **Table 2**, the three parameters are discussed below.

fast recovery for dewatering of the aquifer.

*Contour map of the total drawdowns (UTM Coordinates at Zone 37).*

*Aquifer Characterization: The Case of Hawassa City Aquifer*

*DOI: http://dx.doi.org/10.5772/intechopen.91211*

water-bearing formation thickness increases.

analysis of all the wells are tabulated in **Table 2**.

*2.1.3 Analysis results using Moench method*

*2.1.2 Saturated aquifer thickness (b)*

**Figure 2.**

**43**

#### **Table 1.**

*Discharge rate, total drawdown, and specific capacity results.*

that the increase in values coincides with the storage coefficient or transmissivity value presented in **Table 1**. Higher specific capacity values were also found to coincide with areas where extension fracture systems occur.

Using the total drawdown of the wells, a contour map is developed (**Figure 2**) to see the response of the wells at the end of the pump test duration. This is important to conclude about the potential of the aquifer for discharging.

This map shows, at the final hours, the water level that has been nearly stabilized at the end at about water strike zone for those high potential wells of smaller drawdown. This fluctuation could be due to the difference in the rock type of that

*Aquifer Characterization: The Case of Hawassa City Aquifer DOI: http://dx.doi.org/10.5772/intechopen.91211*

#### **Figure 2.**

*Contour map of the total drawdowns (UTM Coordinates at Zone 37).*

area. Percentage of recovery is also estimated using TDDw and the maximum recovery of water level during pumping test using Eq. (3). The results have 95– 100% recovery percentage which reflects high recovery rate. The recovery of these wells is also fast that they recover up to the SWL within few minutes, even in 30 s. The results clearly show that the study area is of high potential of ground water with fast recovery for dewatering of the aquifer.

### *2.1.2 Saturated aquifer thickness (b)*

The thickness values found in the study area range from 8 m to 136 m and average of 30.30 m. Within the study area, the spatial distribution of saturated thickness clearly indicates that the depth and the lakeshore have significant effect on it. As the cutting depth increases and the site getting nearer to the lake shore, the water-bearing formation thickness increases.

#### *2.1.3 Analysis results using Moench method*

This hydrogeological analysis method is done to determine the important hydrogeological parameters of the wells using the method selected. Along with the curve, the analysis result displays the important parameters, that is, transmissivity, hydraulic conductivity, aquifer thickness, and storativity (specific yield for unconfined aquifers). The result of the hydraulic parameters determined for the analysis of all the wells are tabulated in **Table 2**.

As presented in **Table 2**, the three parameters are discussed below.

that the increase in values coincides with the storage coefficient or transmissivity value presented in **Table 1**. Higher specific capacity values were also found to

Using the total drawdown of the wells, a contour map is developed (**Figure 2**) to see the response of the wells at the end of the pump test duration. This is important

This map shows, at the final hours, the water level that has been nearly stabilized

at the end at about water strike zone for those high potential wells of smaller drawdown. This fluctuation could be due to the difference in the rock type of that

coincide with areas where extension fracture systems occur.

*Discharge rate, total drawdown, and specific capacity results.*

**Table 1.**

*Resources of Water*

**42**

to conclude about the potential of the aquifer for discharging.

#### *2.1.3.1 Specific yield and storativity*

If the aquifer is considered as a confined one, the storativity is determined, and if it is unconfined, its specific yield is determined. The results show that confined aquifers have very low storativity values (much less than 0.01, and as little as 10<sup>5</sup> ), which mean that the aquifer is storing water using the mechanisms of aquifer matrix expansion and the compressibility of water, which typically are both quite small quantities. Unconfined aquifers have specific yield greater than 0.01 which is about 0.517 for the Hawassa City subsurface.

As per the results, the specific yield is high in the south, west, central, and southwest lake shore parts of the area and low in the east and northeast corner part of the area. The storativity or/and specific yield values generally range from 4.77 <sup>10</sup><sup>4</sup> to 5.17 <sup>10</sup><sup>1</sup> . Further, it could be seen that there is a decrease of specific yield in the eastern to southeastern parts and again an increase toward the west and central areas.

#### *2.1.3.2 Transmissivity*

Results show that the value of transmissivity varies from 4.77 <sup>10</sup><sup>4</sup> <sup>m</sup><sup>2</sup> /s to 1.75 <sup>10</sup><sup>1</sup> <sup>m</sup><sup>2</sup> /s. This follows the general pattern of increasing value from east to west (the lake shore), that is, the value increases from the upper part of the basin to the lower. This also shows a gradual increase of the hydraulic gradient. The high transmissivity coincides with areas where the fractured zone occurs.

This high transmissivity in the study area is a better indicator of the water production capacity of an aquifer than hydraulic conductivity. To see why, if we consider a thin aquifer, for example, a sand bed interbedded (sandwiched) between thick clay layers. The bed has a very high hydraulic conductivity because it consists of clean sand; however, if it is not thick, it will not sustain a large production well (its transmissivity is low).

#### **2.2 Ground water flow dynamics**

Annual ground water flow of the Hawassa City aquifer system is determined using wells that are tested at same year and similar season of that year. This is aimed to understand the spatial and temporal regional ground water flow pattern which is essential for managing local and regional groundwater resources, protecting groundwater quality, and delineating wellhead protection zones or drinking water supply source areas. To develop hydraulic head distributions contour map of the area, SWL and DWL data were used from **Table 3**.

From **Table 3**, five batches of wells are selected since their pumping test and completion are undertaken at nearly similar season of that year. In this respect, for 2006, 4 wells; for 2009, 6 wells, for 2012, 5 wells; for 2013, 4 wells; and for 2014, 4 wells are annually grouped along with their SWL and DWL data.

To understand the spatial trend of the flow, contour maps and vector maps are developed (**Figure 3A–J**). During vector map development, the vector orientation is reversed using the command in the Surfer software. Because the SWL and DWL readings are from the top surface downward and the software assumes the values as elevation points otherwise and then wrong flow direction will be identified.

From the above five sets of graphs (**Figure 3A–J**), groundwater moves from higher elevations to lower elevations and from locations of higher pressure to locations of lower pressure.

**Table 2.**

**45**

*Analysis result of the hydraulic parameters in Hawassa City.*

*Aquifer Characterization: The Case of Hawassa City Aquifer*

*DOI: http://dx.doi.org/10.5772/intechopen.91211*

*2.1.3.1 Specific yield and storativity*

*Resources of Water*

4.77 <sup>10</sup><sup>4</sup> to 5.17 <sup>10</sup><sup>1</sup>

(its transmissivity is low).

**2.2 Ground water flow dynamics**

area, SWL and DWL data were used from **Table 3**.

wells are annually grouped along with their SWL and DWL data.

west and central areas.

*2.1.3.2 Transmissivity*

1.75 <sup>10</sup><sup>1</sup> <sup>m</sup><sup>2</sup>

identified.

**44**

locations of lower pressure.

about 0.517 for the Hawassa City subsurface.

If the aquifer is considered as a confined one, the storativity is determined, and if it is unconfined, its specific yield is determined. The results show that confined aquifers have very low storativity values (much less than 0.01, and as little as 10<sup>5</sup>

which mean that the aquifer is storing water using the mechanisms of aquifer matrix expansion and the compressibility of water, which typically are both quite small quantities. Unconfined aquifers have specific yield greater than 0.01 which is

As per the results, the specific yield is high in the south, west, central, and southwest lake shore parts of the area and low in the east and northeast corner part of the area. The storativity or/and specific yield values generally range from

specific yield in the eastern to southeastern parts and again an increase toward the

Results show that the value of transmissivity varies from 4.77 <sup>10</sup><sup>4</sup> <sup>m</sup><sup>2</sup>

transmissivity coincides with areas where the fractured zone occurs.

west (the lake shore), that is, the value increases from the upper part of the basin to the lower. This also shows a gradual increase of the hydraulic gradient. The high

This high transmissivity in the study area is a better indicator of the water production capacity of an aquifer than hydraulic conductivity. To see why, if we consider a thin aquifer, for example, a sand bed interbedded (sandwiched) between thick clay layers. The bed has a very high hydraulic conductivity because it consists of clean sand; however, if it is not thick, it will not sustain a large production well

Annual ground water flow of the Hawassa City aquifer system is determined using wells that are tested at same year and similar season of that year. This is aimed to understand the spatial and temporal regional ground water flow pattern which is

From **Table 3**, five batches of wells are selected since their pumping test and completion are undertaken at nearly similar season of that year. In this respect, for 2006, 4 wells; for 2009, 6 wells, for 2012, 5 wells; for 2013, 4 wells; and for 2014, 4

To understand the spatial trend of the flow, contour maps and vector maps

orientation is reversed using the command in the Surfer software. Because the SWL and DWL readings are from the top surface downward and the software assumes the values as elevation points otherwise and then wrong flow direction will be

From the above five sets of graphs (**Figure 3A–J**), groundwater moves from higher elevations to lower elevations and from locations of higher pressure to

are developed (**Figure 3A–J**). During vector map development, the vector

essential for managing local and regional groundwater resources, protecting groundwater quality, and delineating wellhead protection zones or drinking water supply source areas. To develop hydraulic head distributions contour map of the

/s. This follows the general pattern of increasing value from east to

. Further, it could be seen that there is a decrease of

),

/s to


**Table 2.**

*Analysis result of the hydraulic parameters in Hawassa City.*


In the vector maps shown, the arrow symbol points in the "downhill" direction of water table and the length of the arrow depends on the magnitude, or steepness,

skipped by changing the frequency setting for the better view of the contour. Since the grid contains dynamic water level data of wells, the direction arrows point in the direction of water flows—from high water elevation to low water elevation. Magnitude is indicated by arrow length. Therefore, the steeper slopes would have

Looking into the map of the study area, SWL and DWL vectors indicate similar trends for each of the years. The results show that for the year 2006, medium to high magnitude of water flowed from the northern and central parts to west and S-W direction. This indicates that there was high discharge from the western and

of the slope. A vector is drawn at each grid node; however, some nodes are

*Aquifer Characterization: The Case of Hawassa City Aquifer*

*DOI: http://dx.doi.org/10.5772/intechopen.91211*

longer arrows.

**47**

### **Table 3.**

*The ground water level data with space and time.*

*Aquifer Characterization: The Case of Hawassa City Aquifer DOI: http://dx.doi.org/10.5772/intechopen.91211*

In the vector maps shown, the arrow symbol points in the "downhill" direction of water table and the length of the arrow depends on the magnitude, or steepness, of the slope. A vector is drawn at each grid node; however, some nodes are skipped by changing the frequency setting for the better view of the contour. Since the grid contains dynamic water level data of wells, the direction arrows point in the direction of water flows—from high water elevation to low water elevation. Magnitude is indicated by arrow length. Therefore, the steeper slopes would have longer arrows.

Looking into the map of the study area, SWL and DWL vectors indicate similar trends for each of the years. The results show that for the year 2006, medium to high magnitude of water flowed from the northern and central parts to west and S-W direction. This indicates that there was high discharge from the western and

**Table 3.**

*Resources of Water*

**46**

*The ground water level data with space and time.*

S-W areas. For the year 2009, the map presented shows turbulence, so the flow direction has no clear trend except for the lake shore area that receives water from the nearby aquifer. For the year 2012, relatively medium magnitude water flows into the Gara Riqata area, which produces huge water discharge for the next year 2013. The two years 2013 and 2014 results clearly reveal that significant amount of

*(A) SWL contour map and vector map showing flow direction (2006) with color scale; (B) SWL contour map and vector map showing flow direction (2009) with color scale; (C) SWL contour map and vector map showing flow direction (2012) with color scale; (D) SWL contour map and vector map showing flow direction (2013) with color scale; (E) SWL contour map and vector map showing flow direction (2014) with color scale; (F) DWL contour map and vector map showing flow direction (2006) with color scale; (G) DWL contour map and vector map showing flow direction (2009) with color scale; (H) DWL contour map and vector map showing flow direction (2012) with color scale; (I) DWL contour map and vector map showing flow direction (2013) with color scale; and (J) DWL contour map and vector map showing flow direction (2014) with color scale.*

*Aquifer Characterization: The Case of Hawassa City Aquifer*

*DOI: http://dx.doi.org/10.5772/intechopen.91211*

**Figure 3.**

**49**
