II.**Hard Rock Deccan Trap Basalt**

a. Aquifer – I (Shallow Aquifer): 8 to 32 m

b. Aquifer – II(Deeper Aquifer): 30 to 200 m

The aquifer system characteristics are described in detail in this chapter as the management options mainly depend on these characteristics [1, 2].

*Management of Hard Rock Basaltic Aquifer through Aquifer Mapping: A Case Study of Nashik… DOI: http://dx.doi.org/10.5772/intechopen.111644*

**Figure 3.** *Major aquifers.*

#### **4.1 Soft rock aquifer - alluvium**

Alluvium occurs in small areas in the form of discontinuous patches along the banks and flood plains of major rivers such as Godavari, Girna, and their tributaries. In alluvium, the granular detrital material, such as sand and gravel, usually occurs as a thin layer in the district that yields water. In the district, alluvium occupies an area of 1500 sq. km consisting of reddish and brownish clays with intercalations of sand, gravel, and Kanker. The loosely cemented coarse sands and gravels form 3–4 meters thick lower most horizons at the bottom of these alluvial pockets. Groundwater in alluvium occurs under unconfined conditions. The dug wells constructed in alluvium are ranging in depth from 8 to 15 m, whereas the borewells range in depth from 15 to 25 m and the yield of both the dug wells and bore wells ranges from 13 to 22 m<sup>3</sup> /day.

In the alluvium aquifer, the narrow deposits along river courses are observed in the northern part of 15 to 20 m thickness, whereas in the southern part near Godavari

#### **Figure 4.**

*Thickness and depth of occurrence of aquifer – I.*

River, it is observed in 20 to 25 m depth range with a thickness of granular material being 5 to 8 m (**Figure 4**).

#### **4.2 Hard rock aquifer – deccan trap basalt**

Basaltic lava flows are normally horizontally disposed over a wide stretch and give rise to a plateau. These flows occur in layered sequences and are represented by a massive unit at the bottom and a vesicular unit at the top of the flow. Flows are separated from each other by a marker bed known as a 'bole bed,' which is formed due to weathering of the top part of the flow and represents a hiatus in volcanic activity. The bole beds indicate the change in basaltic flows.

The groundwater in the Deccan Trap occurs mostly in the upper weathered and fractured parts down to 8 to 32 m depth. At places, potential zones are encountered at deeper levels in the form of fractures and inter-flow zones. The upper weathered and fractured parts form a phreatic aquifer and groundwater occurs under water table (unconfined) conditions. The water levels range from 1.2 to 15 m bgl and yield varies

#### *Management of Hard Rock Basaltic Aquifer through Aquifer Mapping: A Case Study of Nashik… DOI: http://dx.doi.org/10.5772/intechopen.111644*

from 10 to 100 m<sup>3</sup> /day depending upon the local hydrogeological conditions. At deeper levels, the groundwater occurs under semi-confined to confined conditions. Borewells drilled down to 200 m depth, tapping weathered, vesicular, and fractured basalt yielded negligible to 2.5 liters per second (lps), whereas the water levels varied from 8 to 55 m bgl.

Deccan basalts are hydrogeologically heterogeneous rocks. The weathered, jointed, and fractured parts of the rock constitute the zone of groundwater storage and flow. The existence of multiple aquifers is characteristic of basalt and is indicative of wide variation in the joint/fracture pattern and intensity. The yield of wells is a function of the permeability and transmissivity of the aquifer, and it depends upon the degree of weathering, the intensity of joints and fractures, and the topographic setting of the aquifer. Due to wide variations in secondary openings, the potential areas for groundwater are generally localized. In general, groundwater occurs under phreatic/ unconfined to semi-confined conditions in basalts.

The perusal of the Aquifer-I map (**Figure 4**) indicates the major part of the shallow basaltic aquifer is observed in the 10 to 15 m range, and the thickness of the aquifer is 8 to 12 m range. The depth of occurrence of 15 to 20 m with 12 to 16 m weathered and jointed thickness in the basaltic aquifer is observed in discontinued patches in the entire district; however, in the western fringe part of the district it is observed prominently and it coincides with highly dissected plateau and denudational slopes. The less than 10 m depth of occurrence is observed in limited and small areas. The Aquifer- I map indicates that a moderate depth of aquifer is available in the major part of the district with sufficient aquifer thickness.

The Aquifer – II is observed only in Deccan Trap basalt formation. The map showing spatial disposition and the vertical extent of Aquifer-II indicating its depth of occurrence and fractured/granular rock thickness has been generated and shown in **Figure 5**. The Aquifer-II map indicates the deeper basaltic aquifer in the major part (100 m to 140 m range and thickness 1 m to 3 m). The depth of occurrence of 140 to 178 m is observed in the eastern part of the district in Nandgaon and Yeolablocks, in the small central part with 9 to 12 m fractured thickness. The south-western and northeastern parts of the district are having a shallower depth of occurrence of 30 to 80 m and fractured thickness of 0.50 to 1.00 m in south-western parts and fractured thickness of less than 0.50 m in north-eastern parts of the district. The Aquifer II map indicates that the deeper aquifer depth of occurrence in the major part of the district is 80 to 140 m bgl and the fracture thickness of 1 to 3 m is most vastly spread in the district.

#### **4.3 Yield potential of aquifer-I and II**

The yield potential of the aquifer is the capacity of the aquifer to yield groundwater. The yield potential of Aquifer-I is plotted in **Figure 6** and it indicates that in the entire basaltic terrain, the yield is less than 15 m3 /day. However, if we correlate the yield potential of Aquifer-I with the depth of occurrence and weathered thickness, it indicates that even though the aquifer occurs down to a moderate depth of 10 to 15 m with a sufficient thickness of 8 to 12 m, and it does not guarantee adequate yield in hard rock basaltic aquifer. Thus, it can be concluded that the yield potential of the basaltic Aquifer - I is independent of aquifer thickness.

The yield potential of Aquifer-II is plotted in **Figure 7** and it proves that the major area is characterized by low yields approximately ranging between 0.5 to 1 lps,

#### **Figure 5.**

*Thickness and depth of occurrence of aquifer – II.*

whereas moderate yields of 1.00 to 1.50 lps are observed in around 25% of the area. In a very small part of Niphad taluka near Godavari River moderate to high yield of 1.50 to 2.50 lps is recorded. However, if we correlate the yield potential of Aquifer-II with a depth of occurrence and fractured thickness, it indicates that even though the aquifer occurs down to moderate to deep depths of 80 to 140 m and is present in a major part of the area, but the yield is not dependent on the depth of occurrence of the aquifer. Thus, the common tendency of the borewell culture of going deep for getting more groundwater is not true in hard rock areas, especially basaltic aquifers.

#### **4.4 3-D aquifer disposition**

Based on the existing data, Mapinfo software 8.5 was used to prepare aquifer disposition in 3D, fence diagram, and 3D Lithological disposition diagram, and several hydrogeological sections have been prepared along section lines to understand the subsurface disposition of the aquifer system.

*Management of Hard Rock Basaltic Aquifer through Aquifer Mapping: A Case Study of Nashik… DOI: http://dx.doi.org/10.5772/intechopen.111644*

**Figure 6.** *Yield potential of aquifer – I.*

The 3D aquifer disposition is presented in **Figure 8**, which helps in visualizing the disposition of the aquifers in the three-dimensional model with topographic elevation and drainage. The 3D aquifer fence diagram and the aquifer bar diagram are shown in **Figures 9** and **10**. Both the fence and bar diagram covers the entire district and they reassert the factual position of the aquifer -I being shallow in depth with less thickness as compared to the aquifer -II. Both these figures also show that the massive part is the most dominant formation in vertical disposition limiting the occurrence of the waterbearing aquifers above it. The fence diagram gives a clear picture regarding the prospective depth of drilling the borewell at any given location in the district.

#### **4.5 2-D hydrogeological cross section**

To study the aquifer disposition in detail, four hydrogeological cross sections indicating aquifer geometry in different directions have been prepared, and the section lines are plotted in **Figure 9**. These sections indicate the disposition of Aquifers – I and II along with fracture and water levels.

#### *4.5.1 Hydrogeological cross section A-A'*

Hydrogeological cross section A-A' (**Figure 11**) represents data from seven exploratory wells for 136 km in the N-S direction with the depth of cross section

varying from 90 m bgl to 202 m bgl. The thickness of the Aquifer-I is almost uniform along the section line, whereas the Aquifer–II is having more thickness near Washala in the northern part and Kankapur in the central part of the section with the discharge/yield of 8.77 lps at Kankapur. Southwards from Kankapur along the section line, it can be seen that the thickness of Aquifer-II (deeper aquifer) is decreasing. The

**Figure 8.** *3D aquifer disposition.*

*Management of Hard Rock Basaltic Aquifer through Aquifer Mapping: A Case Study of Nashik… DOI: http://dx.doi.org/10.5772/intechopen.111644*

**Figure 9.** *Fence diagram.*

**Figure 10.** *Point aquifer disposition by Bar diagram.*

**Figure 11.** *Lithological section AA'.*

water levels of Aquifer-I and Aquifer-II have also been depicted in the section and a close observation of the water level indicates that the water table of Aquifer-II is almost the same at Gilane and Shirpurwadi, whereas at other places it is below the water level of Aquifer-I.
