The district area is formed by two main structural elements - The Aravalli hills, built up by the Precambrian basement rocks in the northeast, and the Cambay sedimentary basin in the rest of the district. Figure 1 is the geological map of the project area. The sedimentary formations are un-consolidated alluvial derived by fluvial transport from north-eastern catchments of Aravalli hills and are partly mixed with blown sands from southwest. The deposition took place during last 200 kyr (late Quaternary). The site of deposition was the Cambay basin which was formed during Tertiary due to block faulting and has been a tectonically active region ever since. The earlier stages of deposition (until about 65-70 kyr ago) experienced sea level fluctuations, while the later stages witnessed a rather arid environment with large scale contribution of aeolian sands. Neo-tectonic movements also accompanied the sedimentation process (Patel, 1986). Thus the deeper layers have fluvio-marine characters, while the upper layers have fluvio-aeolian characters. The thickness of sediments is more than 1,000m thick and comprises alternate layers of clay, silt, sand and gravel that show pinching and swelling structure of layers.
Due to their high permeability, the coarser layers comprising of sandy and gravelly horizons formed the aquifers which are separated by semi-permeable silty and clayey horizons. Two major aquifer units have been identified. The upper unit is mostly phreatic but becomes semi-confined to confined to some parts. It is designated aquifer ëAí. The lower unit comprises of a few hundred metres of alternating sandy and argillaceous beds from the confined aquifer system. It is subdivided in to aquifers ëBí, ëCí, ëDí, and ëEí within post-Miocene sediments and aquifers ëFí and ëGí in the Miocene sediments. Figures 2 to 5 are the various subsurface cross sections
across the lines AAí, BBí, CCí and DDí marked in Fig. 1. Brackish to saline condition are observed in deeper aquifers which are generally found below 250m depth but occasionally even at much shallower depths. The continuity of aquifers occasionally gets disrupted due to pinching of layers or fault displacement. According to Patel (1986), the lower aquifers are hydro-statically under artesian conditions. The general gradient of aquifers is towards west and these merge in eastern direction where, along the foothill zone of Palanpur-Kheralu, these are exposed and receive natural recharge. Towards their western extension, the deeper aquifers abut against a thrust plane near Radhanpur-Viramgam belt. Along this belt, tubewells tapping the deeper artesian aquifers show free flow at ground, have high temperature and saline water (see Figure 6). Recently excess dissolved helium has also been recorded from groundwater of those wells. Recent work (Prasad et al, 1997) has shown that the mainland of Gujarat was separated from the Saurashtra by a narrow, shallow sea-link joining the Gulf of Kachchh with the Gulf of Khambhat (see Figure 7) until about 65-70 kyr ago. The sea then receded during the period of last major glaciation and the sedimentation front advanced westwards filling the former sea corridor.
The depth to water table, below ground level (bgl), ranges from less than 5 metres to more than 20 metres for the unconfined aquifers (Figure 8). Whereas in terms of elevation the water table ranges from more than 180 metres in the Northeast to less than 20 metres above mean sea level (amsl) in the western and southern parts bordering the Little Rann of Kachchh. Water table elevation contours (amsl) for the period May 1995 are shown in Figure 9.
The upper aquifers are under semi-confined condition. They receive recharge (i) directly by seepage from the shallow unconfined aquifer, (ii) by lateral flow from the recharge zone of Palanpur-Kheralu foothill region in the east. The shallow unconfined aquifer receives direct recharge from (a) rainfall infiltration, (b) nearby stream flow and, (c) by return flow from the irrigation. As one moves westwards the groundwater progressively becomes saline.
The areas on the banks of the rivers draining the region, namely Saraswati, Rupen, Banas and other smaller streams, get recharged during the rainy season (June to September). At such locations, groundwater levels are relatively higher and shallow aquifers supply good quality water.
As discussed, post Miocene alluvium and older Quaternary sedimentary formations form multi-layered confined aquifer systems designated as ëBí, ëCí and ëDí in the post Miocene, and ëFí and ëGí within the Miocene sediments. The depth to piezometric surface in the tubewells in the region varies from less then 20 metres bgl in extreme west and northeast to more than 120 metres bgl in the central and southern parts (Figure 10), The piezometric surface elevation varies from +10 metres (amsl) in the extreme west to -20 metres in the central , to -50 metres in the southern to +70 metres in northeast (Figure 11).
Radiocarbon dating of groundwater from deeper aquifers showed that the groundwaters were 15-20 kyr old (Bhandari et al, 1986). This means that the water being pumped was recharged by rain 15-20 kyr ago and has been in contact with sediment grains for such a long period. It is hypothesised (Patel, 1986) that high fluoride levels in groundwater are a result of leaching of this element (along with others) from the sediments which have their provenance (source area) in the metamorphic rocks of Aravalli hills. In a groundwater mining situation, as it prevails in the Mehsana and neighbouring districts of Banaskantha, Gandhinagar and Ahmedabad, pumped water with a larger residence time in the aquifer is, therefore, likely to have higher salinity including fluoride. The high total dissolved solids (TDS) contour appear to run NE-SW and there is general decrease towards the NE end (Figure 6).
At places, the shallow groundwater also shows poor quality on local scale. This is attributed either to local stagnation or high concentration of toxic elements in the host sediments and, in some cases, due to industrial pollution.
Based on the study of hydro-chemical nature of groundwater, the district can be divided in to three distinct areas as shown in Figure 12.
Areas with fresh groundwater at all levels
In this region groundwater at all levels both in phreatic (unconfined ) as well as confined aquifers within 300 metres depth is fresh and can be used for irrigation and the domestic use. This include the entire area underlain by hard rocks and the piedmont plain and alluvial plains forming major parts of Kheralu, Visnagar, Vijapur, Kalol, Mehsana and Siddhpur talukas.
Areas with fresh groundwater overlain by saline groundwater
The central southern and south-western parts of the districts forming parts of Kadi, Kalol, Mehsana, Visnagar and Patan talukas and entire areas of Chanasma, Harij, and Sami talukas constitutes the region with fresh groundwater overlain by saline groundwater. In this region the TDS in the Phreatic groundwater is more than 3000 ppm whereas the TDS in the confined aquifers below 80 to 120 metres is less than 2000 ppm.
Areas with saline groundwater at all levels
Small areas in the extreme Southwest in Sami talukas has saline groundwater at all levels down to 400 to 450 depth with TDS more than 3000 ppm . The Phreatic aquifers in this area is highly saline with TDS more than 10,000 ppm. At dipper levels however, localised pockets of brackish groundwater with TDS between 2000 to 3000 ppm are observed at places.