Keywords

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Volcanic rocks are formed by the solidification of magma at or near the ground surface. The most common volcanic rocks are basalts, which are of basic composition. Acidic and intermediate rock types such as rhyolites and andesites have comparatively very limited occurrences. Basalts are formed due to the eruption of lava either on the ground surface (subaerial eruption) or on the sea floor (submarine eruption). Eruption on the land surface could be of fissure type (plateau basalts), covering large areas on the continents or of central type, which is of limited distribution mostly forming volcanic cones (Fig. 14.1).

Fig. 14.1
figure 1

World distribution of major Mesozoic and younger plateau basalts and active volcanic arcs. 1 Siberian Traps; 2 Karoo Province; 3 Parana Volcanics; 4 Deccan Traps; 5 Thulean Province; 6 Ethiopian Basalts; 7 Columbia River Basalts

Full size image

14.1 Weathering, Landform and Drainage

Volcanic rocks possess characteristic landforms, weathering and drainage patterns. Generally, they are highly susceptible to weathering. Older extrusive rocks exhibit deeper weathering implying a thicker soil and vegetation cover; younger volcanic rocks are generally barren of vegetation. In the tropical climate, basalt weathers into ‘chernozem’, also known as ‘black cotton soil’ which being rich in clay (montmorillonite) is quite impervious but has high fertility. Under good leaching conditions, the ultimate weathering products of basalts are laterite and bauxite.

A variety of landforms develop in volcanic rock terrains which are linked to the composition of lava and type of eruption. They also govern the hydrogeological characteristics of the terrain. Acidic lava is viscous, and therefore restricted in extent, often forming steep-sided bulbous domes. Basic lava has a relatively lower viscosity and is commonly of a ropy type. It exhibits flow structures and possesses an oblate outline with flat topography.

The central type of eruption is marked by sloping cones, calderas, vents and conical landforms. Eruptions of the fissure type are flatter near the center and become serrated along the periphery, the flows generally have a rough surface topography and discordant contacts with the bedrock. The lava flows are often interbedded with weathered and pyroclastic material and other neo-volcanic sediments. From a distance, on scarp faces, a number of flows may collectively impart the impression of rough sub-horizontal bedding. The landscape looks like a series of giant steps, each flow producing a steep escarpment and a flat top (mesa) (Fig. 14.2). This step-like topography is known as trappen in German from which the word trap is derived. Due to such a landscape, the basaltic flows which cover large parts in Central India are known as Deccan Traps (Fig. 14.3). These lava flows are associated with dykes which acted as feeding channels and now occur as extensive ridges (Fig. 14.4), which may form barriers for surface water and groundwater flow.

Fig. 14.2
figure 2

Formation of ‘mesa’ and dykes in volcanic terrain

Full size image
Fig. 14.3
figure 3

Extensive layering produced by successive eruptions in Deccan Traps, Western India. (Courtesy H. Kulkarni)

Full size image
Fig. 14.4
figure 4

Dolerite dykes striking nearly ENE–WSW and running for several tens of kilometre in the Deccan Trap region; the fissures corresponding to the dykes formed feeding channels for eruption of the plateau basalts. (Image source: GoogleEarth)

Full size image

Lava tubes and tunnels are locally important subsurface features. A lava tube or tunnel is formed when the surface of a lava flow has cooled and hardened, but the interior remains more fluid and happens to drain out from beneath the solidified crust, leaving behind a tunnel-like void.

Basalts typically exhibit columnar jointing (Fig. 14.5). Some of the columns have 4, 5, 8 or 10 sides, but the majority of them are near-perfect hexagons, about 30–50 cm in diameter. They are formed due to contraction of the cooling lava, forming prismatic patterns in the solidifying rock. As shrinkage continues, the joints extend through the rock mass resulting in the network of vertical joints causing anisotropy and strong vertical hydraulic conductivity in basalts.

Fig. 14.5
figure 5

Columnar joints in basalts, N. Ireland. (After Reader’s Digest Association 1980)

Full size image

The lava flows may obliterate the pre-existing water divides, resulting in changes in surface and groundwater flow regimes, as is exhibited in the Snake River Plain in Idaho, USA (Stearns 1942). Lakes may be formed either due to closed depressions, viz. crater lakes, and also due to damming of river courses by lava flows. In areas of tectonic activity, volcano-tectonic lakes are formed as in New Zealand and Indonesia (Ollier 1969). The drainage pattern is influenced by the type of eruption and rock characteristics; radial and annular drainage patterns are associated with a central type of eruption.

Surface drainage, in volcanic rocks, increases with weathering and age; older lavas display high drainage density and a dendritic pattern. Surface drainage in young volcanic rocks, however, may be almost absent owing to high porosity and permeability. For example, an area of about 25 000 km2 in southern Idaho, USA, underlain by basalt, has no surface run-off (Stearns 1942). Also in tropical volcanic islands such as Hawaii, despite heavy rainfall, which may exceed 750 cm per year, most of the streams are flashy and are seasonal. This is primarily due to a high rainfall and peculiar watershed characteristics which favour a high rate of flashy surface run-off; and also the high permeability of the basalts and soil cover which favour a high infiltration rate. Here, drainage basins are of a small size, and watersheds are characterized by steep slopes and steep valley walls with little channel storage. Therefore, in such areas surface storage of water is difficult due to the high rate of infiltration (Peterson 1972). Stearns (1942) has highlighted the problems of construction of reservoirs and dams in basaltic terrains.

14.2 Hydrogeology

14.2.1 Plateau Basalts

Plateau basalts, also known as continental flood basalts, are widespread in various parts of the world. They are of different geological ages, but the most common ones on the continents are of late Mesozoic and younger ages. Some of the important occurrences of plateau basalts from the hydrogeological point of view are listed in Table 14.1.

Table 14.1 Major plateau basalt provinces on the continents
Full size table

The plateau basalts usually consist of a number of flows of varying thickness superimposed over each other. The thickness of the individual layer ranges from less than 1 m to more than 30 m, most being between 10 and 30 m. The individual layer is much thicker in continental food basalts as compared to basalts of oceanic islands (Fig. 14.6). This could be due to greater original surface gradients of oceanic islands, as compared to that of continental plateau areas (Davis 1974).

Fig. 14.6
figure 6

Log-probability plot of bed-thickness of basalt flows. Data for Washington, Hawaii and La Palma are from Davis (1974) and data for Deccen Traps are from Krishnan (1949)

Full size image

The lateral extent of individual flow units in plateau basalts depends upon the composition and viscosity of lava, rate of supply and loss of volatiles during cooling. Basaltic lava with low silica and low viscosity spreads over larger areas, forming thin and extensive sheets as compared to acidic lavas such as rhyolite and andesite which are more viscous (Fig. 14.7). The lateral extent of individual flow may vary from a few tens of meters to as much as hundreds of kilometers. Many basalt flows occupy ancient river valleys and therefore locally enclose alluvial sediments of varying lithologies and thicknesses, which are important sources of water supply.

Fig. 14.7
figure 7

Variation in thickness and lateral extent of lava flows depending on chemical composition (SiO2%). (After Walker 1973)

Full size image

Many continental lava flows develop an autobrecciated top surface which is also progressively pushed beneath the flow as it advances (Fig. 14.8). This tends to produce a relatively permeable blocky top and base to many lava flow units (Mathers and Zalasiewicz 1994). Description of some important plateau basalt provinces is given below.

Fig. 14.8
figure 8

Development of autobrecciated material on the top surface of lava flow and its burial due to downward sliding along the steep moving front

Full size image

Columbia-Snake River Basalts

These are of Miocene to Quaternary age and occupy an area of more than 200 000 km2 covering the states of Washington, Oregon and Idaho in the USA. The total thickness is more than 1500 m. The individual flows are very thick (50–150 m), which can be traced for as much as 250 km indicating high fluidity of lava. The basalt flows are either flat or slightly dipping with an angle of 1–2° to the southeast. The interflow surfaces and sedimentary layers are highly permeable (K = 10–10 000 md− 1); deeper aquifers are more productive than shallower ones (Fetter 1988).

Pillow structures formed due to rapid cooling under marine conditions have very high permeabilities supporting large springs on the banks of the Snake River Canyon at Thousand Springs, Idaho (Table 14.4). The Columbia Snake River basalts also exhibit typical karst features like other plateau basalts with openings being as wide as 3.5 m and extending to few kilometre (Krishnaswamy 2008).

Deccan Traps

The Deccan Traps cover an area of about 500 000 km2 in the western and central parts of India (see Fig. 20.3). Originally they would have covered an area of more than 1 ´ 106 km2. The age of the Deccan Traps, based on argon data, is estimated to be 65–60 Ma (Duncan and Pyle 1998). Based on deep seismic-sounding surveys, the greatest thickness of basalt is reported to be about 1500 m near the western coast (Kaila 1988). The thickness decreases towards the east, and it becomes only a few tens of metres in the eastern and southeastern parts. The eruption took place through wide fissures as is evidenced by the presence of dolerite dykes. These dykes extend in length from a few hundred metres to 70 km or more (Fig. 14.4) with an average width of 1–10 m. The dykes have a NW–SE trend on the western coast and an E–W trend in the central region. Lava tubes having diameter of about 100 m and lava channels more than 300 m wide are also reported (Misra 2002).

In the Deccan Traps as many as 29 lava flows are reported from a borehole at Bhusawal in Maharashtra. The thickness of individual flow units varies from a few metres to 50 m. The individual flows are separated from each other by residual soil locally called bole and other interflow sedimentary deposits (intertrappeans). The lower part of an individual flow is usually massive, and it becomes vesicular and amygdaloidal towards the top (Fig. 14.9). The bole is usually of a red colour but occasionally it may be green coloured also, the thickness being usually less than 1 m. It is rich in clay minerals and is believed to be a product of atmospheric weathering and/or hydrothermal alteration of amygdaloidal basalt or of pyroclastic material (Inamdar and Kumar 1994; Wilkins et al. 1994). Being rich in clay, the bole layer usually serves as a confining or semi-confining layer. However, when fractured it forms an aquifer as is seen in many dug well sections. Natural gamma ray logging is useful in the detection of red boles (Versey and Singh 1982).

Fig. 14.9
figure 9

a Field photograph showing layering within a flow in Deccan Traps (courtesy H. Kulkarni), b interpreted profile

Full size image

14.2.2 Volcanic Islands—Submarine Eruptions

Submarine basalts show distinct hydrogeological characteristics as compared to subaerial continental types, due to differences in the physical conditions of solidification of lava. The submarine basalts are characterized by a pillow structure in which the intervening spaces between the pillows form easy channels for the movement of water. Pillow structures can also develop under continental conditions when lava is erupted into lakes or in water-saturated sediments. In comparison to plateau basalts, the submarine basaltic flows show greater heterogeneity and are commonly associated with pyroclastic materials with low porosity and permeability.

Volcanic islands are of two types: (1) oceanic islands, viz. Hawaii and French Polynesia in the Pacific Ocean and Mauritius in the Indian Ocean, which are of basaltic composition and therefore belong to the Basaltic Province; and (2) island-arc islands, viz. Indonesia, New Guinea and the Philippines, which are formed of mainly andesitic lavas and hence belong to the Andesitic Province. The basaltic oceanic islands can further be classified into two types—high islands and low islands. Some of the high islands, e.g. Hawaii islands rise to a height of more than 5000 m above the floor of the Pacific Ocean. They are occupied mainly by young basalts and may be partly flanked by marine and terrestrial sediments. Low islands e.g. the Cook islands in the Pacific and Laccadive (Lakshadweep) Islands in the Arabian Sea, are only a few metres above sea level which may be partly or fully covered with coralline limestone reefs forming atolls underlain by volcanic rocks (CSC 1984).

The basaltic flows on volcanic islands are usually thin (6 m or less) but form main aquifers due to their high permeability. The high permeability is primarily due to clinker zones in the aa type flows, lava tubes and gas vesicles in the pahoehoe flows, columnar vertical joints and the irregular openings between the lava flows (Peterson 1984). In places, the flows are interbedded with ash beds which form confining layers.

Pahoehoe lava generally grades into aa lava with increasing distance from the source. In aa lava flow, the clinker zones which have high permeabilities occur at the top and bottom of the flow while the central part is massive with low permeability. The hydraulic conductivity of clinker zone ranges from several hundred to several thousand md− 1 which is similar to that of coarse well sorted gravel (Schwartz and Zhang 2003).

The central part of the Hawaii islands is characterized by a swarm of closely spaced vertical or steeply inclined dykes which cut across the gently dipping lava flows. The dykes, being more or less impermeable, serve as barriers against lateral movement of groundwater and thereby form water compartments in which groundwater may occur at different heights (Fig. 14.10).

Fig. 14.10
figure 10

Different types of groundwater structures used in volcanic islands and coastal areas. (Modified after UNESCO 1987)

Full size image

In the Indian Ocean, Mauritius is an important volcanic island. The island is totally composed of basalts varying in age from 7 Ma to Recent. Younger flows are vesicular and scoriaceous with high permeability (Rogbeer 1984).

14.2.3 Dykes and Sills

Dykes are vertical or steeply inclined intrusive igneous bodies which cut across the pre-existing rocks. They are usually of basic composition. Dykes vary in thickness from a few decimeters to hundreds of metres, but widths of 1–10 m are most common. In length they may be from a few metres to several kilometers long. They represent feeders for the lava flows. As dykes are usually more resistance to erosion than the country rock they stand out prominently as wall-like ridges. Massive and unweathered dykes form barriers for lateral movement of water, thereby confining large volumes of surface and groundwater (Fig. 14.11). On the other hand, fractured dykes may form good aquifers. One of the dykes in the Palaghat Gap (western coast of India) extends for a strike length of about 14 km and is highly fractured, forming a potential source of groundwater. The discharge from some of the wells in this dyke is of the order of 240–840 m3d− 1 (Kukillaya et al. 1992). Sometimes intrusion of dykes may cause fracturing of the adjacent country rock due to thermal effects and differences in the mechanical properties resulting in formation of effective conduits for groundwater flow parallel to the contact (Gudmundsson et al. 2003). In such cases, drilling into the country rock close to the dyke is recommended, e.g., in the Precambrian crystalline basement rocks of South Africa, Western Australia and north-east Brazil (Boehmer and Boonstra 1987). Pumping tests in dyke areas from Botswana indicate that dykes that are thicker than 10 m serve as groundwater barriers, but those of smaller width are permeable as they develop hydraulic continuity with the country rock through cooling joints and fractures (Bromley et al. 1994).

Fig. 14.11
figure 11

a LANDSAT TM image showing damming of surface streams by dykes, b interpretation map

Full size image

Sills are nearly horizontal tabular bodies which are commonly concordant and follow the bedding of enclosing sedimentary rocks or the older lava flows. Some of the sills are very thick and extend over large areas, as in the Karoo System of South Africa. Due to their low permeability, except when fractured, sills form perched water bodies, as in the Hawaiian Islands. However, studies in the Palisades sill in Newark Rift Basin, New York show that the main transmissive zones are located within the dolerite—sedimentary rock contact zones characterized by chilled dolerite. This is mainly a result of thermal cracking and fracturing of both formations resulting in higher permeability along the contact zone (Matter et al. 2006).

14.3 Groundwater Occurrence

Groundwater in volcanic rocks occurs under perched, unconfined and confined conditions. Perched water occurs above the regional water-table due to the presence of impervious formations, viz. sills, ash beds and dense basalt flows. Water under confined conditions is reported where the previous lava beds are confined between impervious sedimentary beds (Stearns 1942; Fetter 1988). Confined conditions are also created when vesicular or fractured basalt is sandwiched between massive units (Singhal 1973). Free-flowing artesian conditions are reported from some places, viz. Honolulu (Stearns 1942) and Java island in Indonesia (Krampe 1983). Water under artesian conditions is also reported from the Deccan Traps (Kittu 1990). Dykes also cause lateral confinement of water forming compartments as described earlier.

The water-table gradients are low (<1:1000) in areas where volcanic rocks have high permeabilities, as in Columbia-Snake River basalts. The depth to the water-table varies depending upon the recharge and permeability of volcanic rocks, and the depth to the water-table is considerable (>150 m) in areas where the basalts are highly permeable (Ollier 1969). In the Deccan Traps, which are less permeable than Columbia River basalts, the water-table is generally shallow (<10 m) and it follows the ground topography.

The general velocity of groundwater in basalts is high, but not as high as in carbonate aquifers. The velocities are reported in the range of about 3.6–255 md− 1 (Shelton 1982).

14.4 Hydraulic Characteristics

Primary porosity and permeability in volcanic rocks depend on the rate of cooling, viscosity of magma and degassing during cooling. Acidic volcanic rocks, like rhyolites and trachytes, are usually more massive than basalts and therefore have lower porosity and permeability, with some exceptions.

The various openings which impart porosity and permeability to basaltic rocks are (a) scoariae, (b) breccia zones between flows, (c) cavities between pahoehoe lava flows, (d) shrinkage cracks, parallel to the flow surfaces or columnar joints, (e) gas vesicles, (f) lava tubes, and (g) fractures and lineaments (Stearns 1942; UNESCO 1975). The order of importance may, however, vary in different areas. Pahoehoe flows and pillow lavas are more permeable than the thick dense aa flows. The lower and upper brecciated parts of flows develop good permeability (Fig. 14.8). Other interflow spaces may also impart permeability. Sheet and columnar joints formed due to cooling, and other fractures and lineaments produced as a result of later tectonic activity, sometimes impart high permeability. Table 14.2 gives the range of values of porosity and hydraulic conductivity of volcanic rocks, indicating that the variation in permeability is almost nine orders of magnitude. Such a variation indicates their importance both from the point of view of water supply as well as for the selection of suitable sites for the disposal of radioactive waste (Dietz 1985; Evans and Nicholson 1987; Flint et al. 2002).

Table 14.2 Porosity and hydraulic conductivity values in volcanic rocks. (Based on data after Davis 1969; Freeze and Cherry 1979; UNESCO 1975; Sharp et al. 1993)
Full size table

The hydraulic conductivity of pyroclastic deposits depends on the degree of consolidation and welding. The unconsolidated pyroclastic deposits, especially those which are reworked, show higher conductivities, as in Indonesia, Japan and in central California, USA (Stearns 1942; Davis and De Weist 1966). Welded tuffs which are formed at high temperatures by the fusion of rock fragments have low porosity in the range of 5–20% and very low conductivity (10− 9–10− 7 ms− 1) depending on the degree of welding (Sharp et al. 1993).

Volcanic rocks also show anisotropy. Horizontal hydraulic conductivity is several times greater than vertical hydraulic conductivity due to the presence of interflow spaces and horizontal fractures. In Gran Canaria, Spain, the ratio of Kh/Kv is reported to be between 20 and 100 (Custodio 1985). Therefore, the rate of horizontal flow may be several orders of magnitude greater than the vertical flow. However, at places, columnar jointing may impart high vertical conductivity which may be of the order of 10− 1 m s− 1.

A decreasing trend in porosity and hydraulic conductivity of volcanic rocks with increasing geological age is reported from several places. This is attributed to the filling of vescicles and fractures by secondary minerals such as zeolites, calcite and secondary silica. For this reason pre-Tertiary basaltic rocks of Brazil, Deccan Traps of India and basalts of eastern United States form generally poor aquifers, as compared to the Tertiary basalts of Columbia-Snake River area and Hawaiian islands (USA), and Canary Islands (Spain). Pillow lavas of Triassic age in Connecticut (USA) and the island of Guam also show lower conductivity due to the infilling of interstices with secondary minerals, as compared with the younger Tertiary pillow lavas from Snake River Canyon, USA (Stearns 1942) and Pleistocene and Holocene basalts of Mexico and Mauritius (Niedzielski 1993; Rogbeer 1984) (Table 14.3).

Table 14.3 Hydraulic properties of volcanic rocks
Full size table

The above generalized relationship of hydraulic conductivity with age holds good mainly for primary porosity and permeability. However, at times the older basalts can aquire even higher secondary porosity and permeability due to fracturing, as is reported from some places in the Deccan traps, and even older Rajmahal Traps of Jurassic age, from India (Khan and Raja 1989).

The groundwater flow characteristics in basalts are similar to those in other hard rocks. Accordingly, basalts may have discrete fracture flow, or may form a continuum medium when fractures, lava tubes and vesicles are interconnected. Therefore, estimation of aquifer parameters in volcanic rocks is besieged with problems similar to those in other fractured rocks (Chap. 9). In literature there are many examples where the conventional methods of Theis, Jacob and Thiem were used to determine the aquifer properties from pumping test data (viz. Walton and Stewart 1961; Davis 1969; Adyalkar and Mani 1974; Jalludin and Razack 1993). Further, packer tests have been used for estimating permeability of discrete zones. The Papadopulos model has been considered for analyzing pumping test data from large diameter dug wells (Rao 1975). The double-porosity method has also been used for estimating transmissivity of matrix blocks and fractures (Singhal and Singhal 1990). Attempts are also made to estimate hydraulic conductivity from specific capacity data. viz. Rotzoll and El-Kadi (2008) estimated hydraulic conductivity ranging from 3 to 8200 md− 1 for the basalts of Hawaii islands. This helped in the preparation of hydraulic conductivity maps for use in groundwater management.

Step-drawdown test data have been analysed by Rao (1975), and Mishra et al. (1989) using Jacob’s and Rorabough methods for estimating well and formation loss coefficients as well as transmissivities. Methods of analysis of pumping test data from dykes and adjacent country rocks are described in Sect. 9.2.4. Acidic rocks, e.g. dacites and andesites, usually have low transmissivities as compared with basalts. Hydraulic characteristics of some volcanic rocks from different parts of the work, as estimated from pumping tests, are given in Table 14.3.

In recent years, a greater interest has developed in the hydrogeological characteristics of basalts and tuffs as they could be potential host rocks for the disposal of radioactive waste, due to their low permeability. Therefore, detailed in situ hydraulic tests have been conducted in Columbia River basalts at Hanford, USA (Dietz 1985) and in fractured tuffs at the Nevada test site in the USA (Russell et al. 1987), to assess their hydraulic parameters (see Sect. 12.3.3).

14.5 Groundwater Development

Springs are an important source of water in volcanic terrains. They usually show high discharge, quite comparable to those in carbonate rocks (Table 14.4). In the USA, out of the 66 first magnitude springs (discharge 2.83 m3 1 or more), 36 issue from basalts (Stearns 1942). Springs from the Quaternary basalts in Idaho, USA contribute more than 90% of the total flow of Snake River and are an important source of irrigation. In eastern Australia, some springs show a discharge as much as 5 m3s− 1(UNESCO 1975). In volcanic islands, dykes and ground topography control the location of springs (Fig. 14.10). The spring discharge depends on the size and interconnection of fractures and other passageways like volcanic tubes etc. Relatively constant discharge is expected where the fractures are narrow and the groundwater body is large, while greater variability is observed when flow is through wide fractures and other openings which are well-developed and abundant.

Table 14.4 Discharge values of some major springs in basalts
Full size table

Dug wells and borewells are constructed to tap groundwater. Horizontal boreholes, tunnels and galleries are used to tap perched water on hills or on impermeable ash or tuff layers between flows and interappeans in plateau basalts and also in volcanic islands (Fig. 14.10).

Yield of large-diameter dugwells in the Deccan Traps of India is reported to be 0.01–0.05 m3s− 1 for moderate drawdown (Adyalkar and Mani 1974). Specific capacity of these wills is in the range of 1 ´ 10− 4–5 ´ 10− 3 m2 s− 1 depending on the permeability of basalts (Deolankar 1980). Borewells in the Deccan Traps are usually of 10–20 cm diameter and 40–50 m depth having a yield of 1–2 ´ 10− 3 m3s− 1. The well productivity generally decreases beyond a depth of 70 m. Hydro-fracturing helps to increase well yield. Borewells located in intersections of lineaments have higher yields (0.02–0.04 m3s− 1) for a drawdown of 3–5 m (Kittu 1990; Subramanian 1993). Borewells in older Rajmahal traps of Jurassic age in eastern India have an yield of 2 ´ 10− 3 for a drawdown of less than 1 m from fractured horizons (Khan and Raja 1989). In Columbia River basalts of the USA, typical well yields from confined aquifers located at depths between 150 and 300 m are 0.06–0.12 m3s− 1 (Fetter 1988). Specific capacity of wells in Columbia basalts is reported to vary from 2 ´ 10− 6 to 8 ´ 10− 2 m2 s− 1 with an average value of 2 ´ 10− 3 m2 s− 1 (Maxey 1964).

In coastal areas and volcanic islands, basal groundwater which floats over sea-water, is mostly developed by radial wells (Maui type), tunnels, galleries, shafts and a combination of these to minimize the chances of sea-water intrusion (Fig. 14.10). The type of structure depends upon topography, rock structure and their permeabilities. Confined aquifers in coastal sediments are also an important source of water supply tapped by screened well. Such wells in Honolulu, Hawaii, have an yield of 0.025 m3s− 1 (UNESCO 1975).

The Maui type of tunnels may produce up to 2 m3s− 1 depending on their length and nature of rocks (Fetter 1988). In places, the tunnels dip slightly towards the entrance so that water can flow out by gravity. Sometimes they are also provided with vertical bores (well shafts) to tap simultaneously deeper aquifers, as in the Canary Islands. These tunnels or galleries are of a few kilometers in length and are located at a depth of 100 m below ground level (Custodio 1985). Water confined in the dyke compartments is either discharged as springs or is tapped by tunnels.

14.6 Groundwater Recharge

The recharge in volcanic rocks can be estimated by the same techniques as in other rocks (Chap. 20). The main source of recharge is the direct infiltration from precipitation, seepage from streams and return flow from irrigation. Fog-drip could also be an important source or recharge as reported from the Maui Island in Hawaii (Stearns 1942) and La Palma island of Spain (Veeger et al. 1989).

Recharge in old volcanic rocks is generally low which is attributed to weathering effects and low permeability of rocks resulting in high run-off. In the Deccan Traps, based on water balance and water-table fluctuation methods, groundwater recharge is estimated to vary from 10 to 20% of the rainfall (Adyalkar and Rao 1979). Tritium data also gave similar values of recharge. Even in high rainfall areas, recharge is low due to the high relief and impervious nature of the Deccan Traps. In arid regions of different parts of the world, the groundwater recharge in basaltic terrains is reported to be about 10% of the annual rainfall (UNESCO 1975). Recharge in the fractured tuffs at the Nevada nuclear test site, USA, located in an arid climate, was estimated to be approximately 8% of precipitation (Flint et al. 2002). These estimates are of importance in the selection of potential repository sites for high level waste disposal. At this site, a comparison of the travel time of water with the pre-nuclear testing period indicated increased permeability due to induced fracturing as a result of nuclear testing (Russell et al. 1987). In the Hawaiian islands, where annual rainfall varies from 500 to 6300 mm, the recharge in older basalts is 6–10% and in younger volcanics 30–36% of the annual rainfall (Wright 1984). In Mauritius, where basalts are of Quaternary to Recent age, and average annual rainfall is 1200 mm, the recharge is computed to be 24% of the rainfall (Rogbeer 1984).

Deeper confined and semi-confined aquifers may be recharged by leakage from shallow horizons through vertical fractures, and partly by lateral subsurface flow from outcrop areas. In Columbia River basalts, good recovery of water-levels after pumping from deeper aquifers, and other hydraulic data indicate high vertical hydraulic conductivity and interconnection between aquifers. On the other hand, the age of groundwater as determined by the 14C method ranges from modern to as old as 32 000 year BP; age increasing with increasing depth. This indicates lack of interconnection between shallow and deeper aquifers (Fetter 1988). Low tritium content and greater salinity of water in the deeper aquifers of the Deccan Traps also indicates very limited or negligible vertical movement of water (Versey and Singh 1982).

As in carbonate rock aquifers, spring discharge data are also used to determine the recharge pattern and the groundwater potentiality of volcanic rocks. Analysis of spring flow data from fissured basalts of Mexico shows a double recharge mechanism as in karstic terrains (Sect. 15.5). In this case, the main recharge takes place immediately within a period of 1–3 days after rainfall through open fractures, but the recharge due to diffused flow is slow through fine fissures and is delayed by 1–3 months with respect to a rainy period. This diffused flow is mainly responsible for dry weather discharge (Niedzielski 1993).

14.7 Groundwater Quality

Basalts mainly consist of plagioclase and ferromagnesian minerals rich is Ca and Mg which weather rapidly. Release of Mg from olivine is several times faster than the release of Ca and Na from plagioclases. Therefore, groundwater in basalts has a higher amount of alkaline earths (Ca2+ and Mg2+) and a lower amount of alkalis (Na+ and k+). In olivine bearing rocks, Mg may be a dominant cation. The TDS is usually less than 500 mg l− 1 (Table 14.5). Among the anions, HCO3 is dominant over Cl− and SO4 2 −. The pH is usually in the range of 6.7–8.5 and the SiO2 content is high (>30 mg l− 1). In general, the groundwater in basalts is of the Ca–Mg–HCO3 type. In the case of acidic volcanic rocks (rhyolties), groundwater has a higher concentration of SiO2 and Na+ and the waters are of the Na–HCO3 type (Table 14.5).

Table 14.5 Chemical composition of groundwater (in mg 1− 1) from volcanic rocks
Full size table

In areas where volcanic racks have high permeability, there are greater chances of groundwater contamination. Pollution of groundwater in volcanic islands, due to sewage and increased use of pesticides, has been reported from some places (CSC 1984). However, in areas where permeable basalts are covered with some impervious material, the chances of downward migration of contaminants are minimized, as in the Columbia-Snake River area (Davis and DeWiest 1966). Overexploitation of groundwater in oceanic islands can also lead to sea-water intrusion.