1. Introduction to Surface and Ground Water
Water on Earth is broadly categorized into surface water and ground water (subsurface water). Both form integral components of the hydrological cycle and serve as primary sources for domestic, agricultural, and industrial needs.
Surface Water
Surface water refers to all natural freshwater located on the Earth's surface in liquid form. Examples include rivers, streams, lakes, ponds, reservoirs, and wetlands. It is primarily replenished through precipitation and surface runoff.
Ground Water
Ground water is water beneath the Earth's surface stored in soil pore spaces and fractures of rock formations. It originates from precipitation that infiltrates the soil layer and percolates downward under the influence of gravity.
| Feature | Surface Water | Ground Water |
|---|---|---|
| Location | Above the Earth's crust | Beneath the soil and rock layers |
| Accessibility | Easily accessible | Requires wells or pumps |
| Vulnerability to Pollution | High (direct exposure) | Lower, but hard to remediate once contaminated |
| Seasonal Variation | Fluctuates rapidly with precipitation | Relatively stable and buffer-protected |
2. Surface and Ground Water Pollution
Water pollution occurs when harmful biological, chemical, or physical substances contaminate water bodies, degrading quality and rendering water unsafe for consumption or ecological health.
Surface Water Pollution
Surface water bodies are vulnerable to point sources (e.g., sewage discharge pipes, industrial effluents) and non-point sources (e.g., agricultural runoff containing fertilizers and pesticides, urban storm runoff).
Ground Water Pollution
Groundwater contamination occurs when pollutants leach into the subsurface. Common sources include leaking underground storage tanks, septic tanks, landfill leachate, and excessive agricultural chemicals.
Surface water pollution is typically visible and immediate, whereas groundwater pollution is insidious, slow to spread, and extremely difficult to remediate.
3. Water Table
The water table is the upper boundary line of the saturated zone in subsurface rock or soil formations.
Water Table Definition: The surface below which all pore spaces and fractures in the soil or rock are completely saturated with water under atmospheric pressure.
The depth of the water table varies based on topography, seasonal rainfall, soil permeability, and pumping rate from nearby wells.
4. Vertical Distribution of Water
Subsurface water is vertically divided into two major zones separated by the water table:
1. Zone of Aeration (Vadose Zone)
Located above the water table, where soil pore spaces contain both air and water. It consists of three sub-zones:
- Soil Water Zone: Extends from the surface down through the root zone of plants.
- Intermediate Vadose Zone: Extends from the base of the soil water zone to the top of the capillary fringe.
- Capillary Fringe: Extends immediately above the water table where water is drawn upward by capillary force against gravity.
2. Zone of Saturation (Phreatic Zone)
Located beneath the water table, where all pore spaces and rock fractures are completely filled with water under pressure greater than atmospheric pressure.
5. Formation and Properties of Aquifers
An aquifer is an underground layer of water-bearing permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt) from which groundwater can be usefully extracted.
Types of Aquifers
- Unconfined Aquifer: Has a permeable layer above it; its upper boundary is the water table.
- Confined Aquifer: Trapped between two impermeable layers (aquitards or aquicludes). Water is under hydrostatic pressure.
- Perched Aquifer: Occurs when an isolated lens of impermeable material holds water above the main water table.
Key Properties of Aquifers
- Porosity (n): The ratio of void space volume to total volume of rock or soil. Formula: n = (Vv / Vt) × 100%
- Permeability (K): The ability of porous media to transmit fluids through interconnected pores.
- Specific Yield (Sy): The volume fraction of water that drains freely from a saturated aquifer due to gravity.
- Specific Retention (Sr): The volume fraction of water retained against gravity by capillary forces. Note: Porosity (n) = Sy + Sr.
- Transmissivity (T): The rate at which water flows through a unit width of aquifer under a unit hydraulic gradient (T = K × b, where b is aquifer thickness).
6. Techniques for Ground Water Recharge
Groundwater recharge is the process by which water moves downward from surface water to groundwater. Artificial recharge techniques enhance natural replenishment:
- Spreading Methods: Basin method, stream channel modification, flooding, and ditch/furrow systems allowing water to infiltrate large surface areas.
- Recharge Wells: Injection wells used to inject surface water directly into deep or confined aquifers.
- Induced Recharge: Pumping wells located near surface water bodies to draw river or lake water into the aquifer.
- Sub-surface Structures: Construction of check dams, percolation tanks, and subsurface dykes to slow runoff and encourage natural percolation.
7. River Structure and Patterns
Rivers shape terrestrial landscapes and transport water, sediment, and nutrients from high elevations to sea level.
River Zones / Structure
- Upper Course (Youthful Stage): Steep gradient, narrow V-shaped valleys, high energy causing vertical erosion, waterfalls, and rapids.
- Middle Course (Mature Stage): Moderate gradient, wider U-shaped valleys, lateral erosion dominates, formation of meanders and floodplains.
- Lower Course (Old Stage): Gentle gradient, wide floodplains, deposition dominates, formation of oxbow lakes, estuaries, and deltas.
Drainage Patterns
- Dendritic Pattern: Tree-like branching pattern forming on uniform geological strata.
- Trellis Pattern: Parallel principal streams with short right-angle tributaries, common in folded bedrock.
- Radial Pattern: Streams flow outward in all directions from a central high point (e.g., volcano or dome).
- Centripetal Pattern: Streams converge towards a central low area or depression.
- Rectangular Pattern: Streams exhibit right-angle bends along faulted or jointed rock networks.
8. Importance of Watershed and Watershed Management
A watershed (or drainage basin) is an area of land that drains all the streams and rainfall to a common outlet such as a reservoir, bay, or river mouth.
Importance of Watersheds
Watersheds collect and filter water, support terrestrial and aquatic ecosystems, regulate hydrologic flow, and reduce flood risk.
Watershed Management
Watershed Management: Integrated management of land, water, vegetation, and human activities within a watershed boundary to conserve natural resources and ensure sustainable water supply.
Key strategies include soil conservation (contour bunding, terracing), afforestation, check dam construction, and public participation.
9. Rain Water Harvesting in Urban Settings
Urban rainwater harvesting collects rainfall from impervious city surfaces (roofs, paved areas) to augment urban water supplies and alleviate municipal stormwater management issues.
Key Components
- Catchment Surface: Roofs, paved courtyards, terraces.
- Conveyance System: Gutters, downspouts, and piping network.
- First-Flush Device: Diverts initial rainfall containing atmospheric pollutants and roof dust away from storage tanks.
- Storage and Filtration: Cisterns, sand-gravel filters, and sumps.
Recharge Methods in Urban Areas
- Recharge pits and trenches filled with gravel and sand.
- Recharge wells attached to urban stormwater drainage.
- Permeable pavements allowing infiltration into soil.
10. Rain Water Harvesting in Rural Settings
Rural rainwater harvesting emphasizes agricultural water security, livestock management, and groundwater table elevation across broader landscapes.
Traditional and Modern Rural Techniques
- Percolation Tanks: Earthen dams constructed across seasonal streams to store runoff and promote percolation into the water table.
- Check Dams / Johads: Small barrier structures across stream beds to retard water flow, impound runoff, and recharge nearby wells.
- Contour Bunds and Trenches: Barriers built along elevation contours on sloping land to reduce runoff velocity and increase soil infiltration.
- Farm Ponds: Excavated structures to collect surface runoff for supplemental irrigation and livestock.
- Rooftop Harvesting in Rural Dwellings: Catchment collected in tanks for drinking and cooking needs during dry seasons.