Introduction to Hydrology and Water Resources

Posted on Aug 23, 2024 in Geology

The Hydrological Cycle

Water is evaporated from the oceans and continents by the direct action of solar energy, or indirectly through transpiration from plants. Evaporated water condenses and forms clouds. Due to gravity, they release the water as precipitation. Water falling over the land surface can take two primary paths: a portion flows over the ground surface (surface runoff) and the other infiltrates the ground and becomes part of the groundwater. Groundwater generally moves very slowly and is fed by springs (groundwater runoff).

Influence of Light, Temperature, and Salinity in Ocean Waters

Light

Light only penetrates the outermost layer of the oceans (100m to 200m), known as the photic zone. The area below, the aphotic zone, is vast since the average depth of the ocean is about 4,000m. Plant life can exist in the photic zone due to the presence of light, while the absence of light in the aphotic zone prevents their development.

Salinity

Salinity is the amount of dissolved salts in seawater. Seawater contains an average of 3.5% salts. This percentage varies between different seas. Rain, continental water contributions, and melting ice reduce salinity by dilution (e.g., Baltic Sea). Conversely, ice formation, volcanism, and evaporation tend to increase it (e.g., Red Sea and Dead Sea).

Temperature

Temperature varies with depth and latitude. Salinity, along with temperature, determines the density of seawater, which is critical in the distribution of ocean currents.

Numerical Patterns in Paragraphs 1 and 2

There are no numerical patterns present in the original paragraphs 1 and 2. The request to explain a numerical pattern is therefore not applicable.

Exploitation of Coastal Aquifers: An Example

Well 1 draws water from an aquifer where the water table has dropped. The aquifer is still significant because, although extraction is still active, its lower end remains within the saturated zone of water.

Well 2 exemplifies the exploitation of a coastal aquifer. Since groundwater extraction exceeds its recharge rate, the resulting vacuum is filled by seawater (seawater intrusion), increasing its salinity. The water becomes unusable for domestic and agricultural purposes and may even alter ecosystems such as wetlands.

Eutrophication: Causes, Consequences, and Prevention

Causes

Eutrophication occurs in stagnant waters such as lakes, ponds, and reservoirs. An excess of nutrients, particularly nitrates and phosphates, leads to excessive growth of algae and aquatic plants. A large accumulation of their remains at the bottom then fuels the development of decomposer microorganisms.

Consequences

The decomposition of organic matter by aerobic bacteria depletes the oxygen present in the deeper zones. This phenomenon is exacerbated by thermal stratification, which favors the growth of anaerobic bacteria responsible for fermenting the excess organic matter. Fermentation processes emit substances such as H2S, NH3, and CH4, responsible for the characteristic odor. The waters become turbid, lose oxygen, and consequently, aquatic life.

Prevention

The way to prevent eutrophication is to reduce the supply of nutrients, particularly phosphorus, by:

  • Using detergents without phosphates.
  • Employing rational fertilizer use.
  • Pretreating water before discharge.
  • Limiting the growth of algae.

Wastewater Treatment Plant (WWTP) Processes

Wastewater treatment is a set of physical, chemical, and biological processes that take place in a wastewater treatment plant (WWTP) to return water characteristics as close as possible to clean water. The treatment process involves three lines: water line, sludge line, and gas line.

Water Line

  1. Pretreatment: Removal of solids using screens and sieves.
  2. Primary Treatment: Separation of suspended solid materials by various physical-chemical processes: sedimentation, coagulation, flotation, etc.
  3. Secondary Treatment: Removal of organic matter and remaining suspended solids not removed in earlier treatments through microbial processes. These processes are performed in digestion tanks, stabilization ponds, filters, and drip systems.
  4. Tertiary Treatment: Elimination of viruses, heavy metals, and dissolved organic matter. Processes used include centrifugation and reverse osmosis.

Sludge Line

Sewage sludge originates from the previous treatments. Its treatment involves the following processes:

  • Thickening: Reducing the volume.
  • Stabilization: Reducing organic matter through aerobic digestion, which produces biogas (methane) that can be used as fuel for the treatment plant.
  • Dehydration: Can be mechanical or thermal.

Gas Line

The biogas produced can be reused as energy in the sewage plant itself. Unused gas is usually burned in a flare.

Water Pollution: Definition and Origin

Water pollution is the addition of foreign matter to water, such as microorganisms, chemicals, waste, wastewater, or energy like heat and radioactivity. This impairs water quality and renders it unsuitable for its intended uses.

The origin of pollution can be:

  • Natural: Occurring without human intervention (e.g., plant residues, animal dung, volcanic activity).
  • Anthropogenic: Primarily due to industrial and mining activities, farms, livestock, urban agglomerations, and civil engineering works.

Water Quality Parameters: BOD and COD

A parameter is a numerical expression of a system variable. The state of water pollution can be expressed through a series of physical, chemical, or biological parameters.

Biological Oxygen Demand (BOD)

BOD measures the amount of oxygen microorganisms need to oxidize organic matter. It is expressed in ppm or mg/L of oxygen. The most common measurement refers to a five-day period (BOD5) at 20°C, measured by subtracting the initial oxygen from the oxygen present after five days of incubation. The higher the BOD5, the greater the organic pollution (e.g., unpolluted rivers: 3 mg/L; domestic wastewater: 300 mg/L).

Chemical Oxygen Demand (COD)

COD is the amount of dissolved oxygen in water consumed to oxidize matter by chemical agents in an acidic medium. The recommended value should be = 20 mg oxygen/L.

Consequences of Oil Spills at Sea

Oil spills in the sea form a viscous mass on the surface, a “black tide,” with particularly harmful effects on:

  • Fish living near the surface (respiratory dysfunctions).
  • Seabirds (loss of waterproofing and the insulating layer of air that protects them from the cold).
  • Marine invertebrates.
  • Marine mammals.
  • Planktonic organisms (unlit phytoplankton cannot photosynthesize).
  • Benthic organisms.

These oil spills, upon reaching the coast, cause severe disturbances in coastal ecosystems:

  • Cliffs and rocky areas.
  • Reef formations.
  • Wetlands.
  • Estuaries.

Additionally, oil contains carcinogenic substances that can accumulate in bivalve mollusks. Oil spills cause serious economic damage to fisheries, marine fish farms, aquaculture, the quality of beaches for public use, and tourism and recreation areas.

Water Balance

The water balance (Bh) represents the inputs and outputs of water on continents and oceans (Bh = water additions – water losses). Globally, considering the Earth’s oceans, water losses due to evaporation and evapotranspiration and inputs due to precipitation, which lead to surface runoff or groundwater, are in balance.

In a balanced water budget:

Global water balance = water inlets – water loss = 0

In other cases, the water balance is not balanced, mainly due to weather patterns. For example, in the Mediterranean Sea, water loss through evaporation exceeds water gains from river inputs and rainfall. This is referred to as a negative water balance.

Water Plans: Concept and Importance

The main problem with freshwater globally is its scarcity. In most countries, based on climatic, seasonal, and pollution criteria, there is a severe shortage of drinking water. Large areas of Africa, Asia, and the Middle East are prime examples.

Other regions experience a constant demand for water due to population growth, intensification of agricultural systems, irrigation, and industrial expansion. This is coupled with the progressive deterioration of watersheds due to various causes. Consequently, the need for Hydrological Plans is growing.

Hydrological Plans are a set of guiding principles for water management to ensure that agricultural, industrial, urban, and environmental interests coexist sustainably.

Measures for Rational and Efficient Water Use

General Measures

  • Protect streams, rivers, and aquifers from contamination.
  • Protect forests to stabilize water flows and the water cycle.
  • Regulate the exploitation of aquifers.
  • Encourage recycling.

Sector-Specific Savings Measures

Agriculture

  • Utilize efficient irrigation systems, such as sprinkler irrigation and drip irrigation, especially in arid regions.

Industry

  • Encourage industries with low water consumption and clean processes.
  • Promote the use of recycled water in closed circuits.

Urban Areas

  • Encourage savings through public awareness campaigns.
  • Promote the use of low-consumption appliances.
  • Treat wastewater for reuse in irrigating parks and gardens.

Technical Measures

  • Regulate watercourses by constructing dams and reservoirs.
  • Utilize groundwater through wells, drilling, and pumps.
  • Implement water transfers under the National Water Plan to move water from surplus river basins to deficient ones.

Legislative Measures

  • Regulate water intake, waste control, economic systems, and aquifer exploitation.
  • Implement and enforce water laws, such as Spain’s Water Law of 1985.

Key Elements of an Aquifer

Water Table

The upper surface of the zone saturated by groundwater. It is not static; its height varies with the amount of water accumulated in the subsoil, influenced by seasons and heavy rainfall.

Well

A structure used to access water from a free aquifer, typically requiring pumping to extract the water.

Aquifer

A porous and permeable underground geological formation saturated with water, allowing its movement and extraction.

Influence of Ocean Currents on Climate: U.S. Atlantic Coast vs. Europe

The Gulf Stream, a warm surface current in the North Atlantic, significantly influences the thermal characteristics of Europe’s Atlantic coast. It brings milder temperatures, especially in winter, compared to areas at the same latitude on the Atlantic coast of North America, which experience lower temperatures.

The La Niña Phenomenon

The diagram represents the La Niña phenomenon. In the eastern tropical Pacific, trade winds blow westward, pushing the surface layer and thermocline off the coast. The displaced water is replaced by cold, nutrient-rich deep water, supporting large fish stocks. Consequently, these areas are (or were) among the most important fishing grounds globally.

1. Surface Layer

The less dense marine layer in terms of temperature and hydrological changes. It experiences significant energy exchange with the atmosphere. Its depth is not uniform and usually does not exceed 500m.

2. Thermocline

A layer characterized by a rapid temperature drop, separating the less dense and less saline surface waters from the cooler, denser, and more saline deep waters.

3. Deep Layer

A mass of cold water with temperatures ranging from 1°C to 5°C, depending on latitude. These temperatures remain relatively constant near the seabed, despite climate variations at the surface.

4. Upwelling Zone

Occurs when trade winds push surface water away from the coast, allowing the upwelling of cold, nutrient-rich deep water. Upwelling zones are often rich in marine life and support productive fisheries, as seen on the west coast of South America.

Water Flow in the Mediterranean Sea

The Mediterranean Sea’s waters have high salinity due to significant evaporation, making them denser. This denser water sinks and flows through the Strait of Gibraltar into the Atlantic Ocean. A surface current of less dense Atlantic water flows in the opposite direction.

Types of Aquifers and Wells

1. Piezometric Level

The height to which groundwater would rise in a confined aquifer under pressure when it emerges from a well or natural spring.

2. Confined Aquifer

An underground geological formation of porous and permeable material saturated with water, allowing its movement and extraction. In confined aquifers, the water pressure is higher than atmospheric pressure.

3. Artesian Well

A well drilled into a confined aquifer where the pressure is sufficient for the water to rise above the land surface without pumping.

4. Well Requiring Pumping

A well where water does not reach the surface because its opening is located above the water table, requiring pumping to extract water.

Causes of Aquifer Pollution

The primary sources of groundwater pollution are:

  • Point sources: Domestic and industrial wastewater, landfill leachate, mine drainage, etc.
  • Diffuse pollution: Nitrates and other products from agricultural fertilizers and pesticides.

Aquifer Components

An aquifer is a porous and permeable underground geological formation saturated with water, allowing its extraction and movement. The main elements of an aquifer are:

  • Aeration zone: The subsurface area where water infiltrates vertically.
  • Saturated zone: The zone where the rocks are saturated with water.
  • Water table: The top of the saturation zone. Impermeable strata often underlie the saturation zone.

Aquifer Types and Well Characteristics

Aquifer Types

  • A1: A free aquifer that feeds the river, making it an effluent or gaining river.
  • A2: A perched aquifer located on the hillside.
  • A3: A confined aquifer.

Well Characteristics

  • P1: An artesian well where water rises spontaneously to a height of NP.
  • P2: A dry well without water because it does not reach the water table. It needs to be deepened to access water.
  • P3: A well requiring a pump to extract water because its opening is below the water table.

River Self-Purification

Self-purification is the natural process by which pollutants in a river are processed, neutralized, or eliminated through a series of physical, chemical, and biological processes. This natural capacity of a river to cleanse itself contributes to maintaining water quality.

Consequences of Oil Spills at Sea (Repeated Section)

This section repeats the previously discussed consequences of oil spills at sea. Please refer to the earlier section for the information.