Environmental Geosciences III: Hydrology and Hydrogeology, the Basins and Groundwater Provinces of India, Darcy's Law, Hydraulic Conductivity, Fluctuations, Tracers, Subsidence, Over-Use and Pollution of Groundwater, the Ghyben-Herzberg Relation, Resource Exploitation and Conservation, and Natural Hazards

Henry Darcy, experimenting with sand filters for the fountains of Dijon in 1856, found that the discharge through a porous medium is proportional to the hydraulic gradient and the cross-sectional area: Q = K i A, where K is the hydraulic conductivity, i = Δh ÷ L the hydraulic gradient (the fall in head over the flow length) and A the area. The Darcy flux, q = Q ÷ A = K i, has units of velocity but is not the speed at which water moves, because water flows only through the pores; the average linear (seepage) velocity is v = K i ÷ nₑ, where nₑ is the effective porosity, and it is the seepage velocity that sets how fast a pollutant travels. For example, in an aquifer with K = 10 m/day, a head fall of 2 m over 1,000 m (i = 0.002) and a cross-section 50 m wide and 10 m thick (A = 500 m²), Q = 10 × 0.002 × 500 = 10 m³/day; with nₑ = 0.25, the seepage velocity is 10 × 0.002 ÷ 0.25 = 0.08 m/day, so a contaminant takes over 30 years to travel a kilometre.

Darcy's law is valid only for laminar flow, in which viscous forces dominate: in practice where the Reynolds number based on grain size is below about 1 to 10, which covers almost all natural groundwater flow through sands and silts. It breaks down in turbulent flow through coarse gravels, large fractures and karst conduits near wells, and it assumes a saturated, rigid medium; in very fine clays at low gradients the flow may also depart from strict proportionality. Hydraulic conductivity depends on both the medium and the fluid (its density and viscosity): typical values are 10⁻³ to 10⁻¹ m/s for clean gravel, 10⁻⁵ to 10⁻³ m/s for clean sand, 10⁻⁸ to 10⁻⁶ m/s for silt and below 10⁻⁹ m/s for unfractured clay, a range of more than ten orders of magnitude. It is measured in the laboratory with constant-head and falling-head permeameters and in the field by pumping tests, analysed by the Thiem and Theis (1935) methods, and by slug tests.

Groundwater levels fluctuate seasonally, rising after monsoon recharge and falling through the dry season as pumping and natural discharge continue, so levels are monitored before and after the monsoon; a long-term decline in pre-monsoon levels signals extraction beyond recharge. Tracers reveal the origin, age and paths of groundwater. Natural tracers include tritium and carbon-14 for age, the stable isotopes of oxygen and hydrogen (δ¹⁸O and δ²H) for the source and altitude of recharge, chlorofluorocarbons and chloride; artificial tracers include fluorescent dyes such as fluorescein and rhodamine and salts such as bromide. When heads fall in confined aquifers interbedded with clay, the clays compact and the land subsides, often irreversibly; Mexico City and parts of the Central Valley of California have sunk by several metres.

India is the world's largest user of groundwater, which supplies most of its irrigation and rural drinking water. Satellite gravity measurements from the GRACE mission showed rapid depletion across north-western India, in Punjab, Haryana, Rajasthan and Delhi. The effects of over-use are falling water tables, dry shallow wells, rising pumping costs, subsidence, the intrusion of saline water in coastal aquifers and, in places, the mobilisation of fluoride and arsenic. The Central Ground Water Board assesses each assessment unit by its stage of groundwater extraction, annual extraction as a percentage of the annual extractable resource: up to 70% is safe, 70 to 90% semi-critical, 90 to 100% critical and above 100% over-exploited. Groundwater quality problems are both geogenic, fluoride in much of the peninsula and Rajasthan, arsenic in the Ganga-Brahmaputra plains, iron in the north-east and salinity in arid and coastal areas, and anthropogenic, nitrate from fertilisers and sewage, pathogens from on-site sanitation, and leachate from landfills and industry.

In a coastal or island aquifer, fresh groundwater floats on denser sea water. The Ghyben-Herzberg relation, derived independently by Badon Ghyben (1888) and Herzberg (1901), balances the two columns hydrostatically: z = [ρf ÷ (ρs − ρf)] h, where h is the height of the water table above sea level, z the depth of the fresh-salt interface below sea level, and ρf and ρs the densities of fresh (1.000 g/cm³) and sea water (1.025 g/cm³). Then z = 1.000 ÷ 0.025 × h = 40h. If the water table stands 1.5 m above sea level, fresh water extends about 60 m below sea level. The corollary is the danger: lowering the water table by 1 m by pumping raises the interface by about 40 m, drawing salt water into wells, which is why coastal aquifers must be pumped with great care.

Mineral and fuel resources are found by geological mapping, geochemical sampling of soils, stream sediments and plants, geophysical surveys (gravity, magnetic, electrical and seismic) and remote sensing, and then proved by drilling. Their exploitation carries environmental costs at every stage. Open-cast mining, the commonest method for coal, iron ore and bauxite in India, removes forest and topsoil, creates overburden dumps and voids, and generates dust; underground mining can cause subsidence and fires, as in the Jharia coalfield. Acid mine drainage forms when sulphide minerals such as pyrite are exposed to air and water: pyrite oxidises to sulphuric acid and dissolved iron, with bacteria such as Acidithiobacillus ferrooxidans accelerating the reaction, and the acid leaches heavy metals into streams. Mining also lowers water tables, silts rivers and displaces communities; oil and gas extraction adds spills, gas flaring and produced water.

Historically, resource use grew with each industrial revolution, from charcoal and wind to coal, then oil and gas, and the worry of exhaustion is old: M. King Hubbert predicted in 1956 that United States oil production would peak around 1970, which it did for conventional oil, and Hotelling (1931) showed that the scarcity rent of an exhaustible resource should rise at the rate of interest. Conservation of non-renewable resources relies on using less (efficiency and demand management), using longer (durable design and repair), using again (reuse, recycling and recovery of metals from scrap and waste), substituting renewable or more abundant materials, and planning mine closure and reclamation so that land and water are restored.

Earthquake size is expressed as magnitude, a measure of the energy released, and intensity, a measure of the effects at a place. Richter's local magnitude (1935) is logarithmic: each whole unit is a tenfold increase in the amplitude of ground motion and about a 32-fold increase in energy, so a magnitude 7 earthquake has 100 times the amplitude and about 1,000 times the energy of a magnitude 5; seismologists now use the moment magnitude for large events. Intensity is graded on the Modified Mercalli scale from I to XII. Mitigation in India is organised under the Disaster Management Act 2005, which created the National Disaster Management Authority, and follows the global Sendai Framework for Disaster Risk Reduction 2015–2030, which shifts the emphasis from responding to disasters to reducing risk before they happen.