(a) Discuss briefly how do chemical, physical and bacteriological properties determine the usability of ground-water. (20…

(a) How chemical, physical and bacteriological properties decide the usability of groundwater

Usability depends on the purpose, which may be drinking, irrigation or industry, and on the limits of the BIS standard IS 10500 and WHO.

(i) Chemical. This is the most important group. Total dissolved solids (TDS) are desirable up to 500 mg/L and permissible to 2000 mg/L. Total hardness is 200 and 600 mg/L as CaCO₃, and high values cause scale and wasted soap. Fluoride is 1.0 and 1.5 mg/L, and higher levels cause dental and skeletal fluorosis. Arsenic must be under 0.01 mg/L. Nitrate is limited to 45 mg/L, and more causes methaemoglobinaemia in infants. The pH range is 6.5–8.5, and iron above 0.3 mg/L stains. For irrigation, EC, sodium adsorption ratio, residual sodium carbonate and boron decide the suitability; high sodium damages soil structure.

(ii) Physical. Turbidity (desirable 1 NTU, permissible 5 NTU), colour, taste, odour and temperature affect acceptability and shield pathogens from disinfection. Electrical conductivity is a quick measure of salinity.

(iii) Bacteriological. Potable water must have no coliforms or E. coli in 100 mL, because their presence shows faecal contamination and the risk of cholera, typhoid, dysentery and hepatitis A. Shallow wells near latrines and sewers are most at risk.

Indian examples: arsenic in the Ganga–Brahmaputra alluvium of West Bengal, fluoride in Rajasthan, salinity in coastal aquifers, and nitrate in intensively farmed areas.

Industrial use needs low hardness, silica and iron, as for boilers. Drinking water must pass all three groups, while irrigation water is judged mainly by salinity and sodium.

(b) Palaeobathymetry from marine fossils

Palaeobathymetry is the reconstruction of the water depth of an ancient sea, by comparing its fossils with the depth zonation of living organisms.

The diagram should be a section from the shore to the deep ocean with the depth zones: littoral or intertidal, neritic (shelf) to about 200 m, bathyal to about 2000 m, abyssal below it, and the carbonate compensation depth (CCD) at about 4–5 km, with the typical fossils in each.

(i) Shallow (photic) zone. Reef corals with zooxanthellae, calcareous algae, and thick-shelled molluscs indicate depths of less than about 50–100 m.

(ii) Benthic foraminifera and ostracods have depth-restricted genera, and the diversity and test type change with depth.

(iii) Planktonic to benthic foraminifera ratio rises from shelf to slope, low on the shelf and high in deep water.

(iv) Trace fossils (ichnofacies): Skolithos for the shore, Cruziana for the shelf, Zoophycos for the deeper shelf and Nereites for the deep sea.

(v) Below the CCD calcareous shells dissolve, so siliceous radiolarians and agglutinated foraminifera mark abyssal depths.

Fossils must be used with lithology, since some forms were moved.

(c) Palaeozoic sequence of the Kumaun–Garhwal Tethyan Himalaya and its fossils

The Tethyan (Tibetan) Himalaya lies north of the Higher Himalayan Crystallines, separated from them by the South Tibetan Detachment. In Kumaun and Garhwal it has a nearly complete marine sequence of a passive margin on the northern edge of Gondwana. Its Palaeozoic part is, from older to younger:

(i) Martoli and Garbyang Formations (Cambrian to Early Ordovician): quartzite, shale and limestone. Trilobites and brachiopods.

(ii) Shiala Formation (Ordovician): shale and limestone, with chitinozoans.

(iii) Yong Limestone (Late Ordovician): limestone with brachiopods, cephalopods, bryozoans, crinoids and corals.

(iv) Muth Quartzite (Silurian–Devonian): white quartzite, a shallow marine unit, with brachiopods and other shelly fossils.

(v) Lipak and Po Formations (Carboniferous): shale, sandstone and limestone with brachiopods, bryozoans and corals.

(vi) Kuling Formation (Permian): shale with a marine fauna, at the top of the Palaeozoic. A Gondwana-type fauna points to the nearness of the Indian plate to Gondwana.

The Palaeozoic marine record is overlain by Triassic limestones and shales, with the end-Permian extinction between them.