Freshwater Systems and Resources
Chapter Objectives
This chapter will help students:
Describe the distribution of fresh water on Earth and the major types of freshwater systems, including surface water, groundwater, aquifers, rivers, lakes, and wetlands.
Discuss how human societies utilize water for agriculture, industry, and residential needs, and the significant alterations made to freshwater systems through infrastructure like dams and levees.
Assess the pervasive problems of water supply, such as depletion of surface and groundwater, land subsidence, and saltwater intrusion, and propose comprehensive solutions including desalination, agricultural efficiency, residential conservation, and market-based strategies.
Describe the major classes of water pollution—toxic chemicals, pathogens, nutrient overload, biodegradable wastes, sediment, and thermal pollution—from both point and non-point sources, and propose solutions to address water pollution through legislation and technological advancements.
Explain the fundamental processes involved in treating drinking water to ensure safety and the multi-stage treatment of wastewater, including primary and secondary methods, as well as the role of constructed wetlands.
Freshwater Systems
Overview of Freshwater
Of all the water on Earth, only 2.5 ext{%} is considered fresh water, meaning it has relatively pure with few dissolved salts, typically less than 1,000 ppm total dissolved solids.
The vast majority of this fresh water is inaccessible, with approximately 79 ext{%} locked away in glaciers and ice caps, 20 ext{%} existing as underground aquifers, and less than 1 ext{%} found in surface waters like lakes, rivers, and soil moisture.
Water is continuously renewed and recycled as it moves through the planetary hydrologic cycle, driven by solar energy and gravity.
Groundwater plays critical, often unseen, roles in the hydrologic cycle by recharging surface bodies, maintaining stream flow during dry periods, and providing a significant reservoir for human use.
Definitions of Water Types
A. Surface water refers to water located atop Earth’s surface in bodies such as rivers, lakes, ponds, and wetlands. It is readily accessible but more vulnerable to direct pollution and evaporation.
B. Groundwater refers to water beneath the surface that resides within pores in soil or rock formations, constituting a vast underground reservoir. It is typically cleaner due to filtration through soil layers but is slower to replenish.
C. Groundwater is primarily contained within aquifers:
Aquifers: These are porous, spongelike formations of rock, sand, or gravel that are permeable enough to hold and transmit water. They act as natural underground storage tanks.
The upper layer of an aquifer, known as the zone of aeration, contains pore spaces that are partly filled with water and air.
Below this is the zone of saturation, where spaces are completely filled with water. The boundary between these two zones is known as the water table, which fluctuates based on precipitation and extraction.
Areas where water infiltrates Earth’s surface to seep down and replenish the aquifer below are known as recharge zones. These areas are critical for aquifer sustainability and are particularly vulnerable to surface pollution.
Types of Aquifers
Confined aquifer (Artesian aquifer): This is a porous, water-bearing layer trapped between upper and lower layers of less permeable substrate, such as clay or shale. Water in these aquifers is under great hydrostatic pressure, and can rise spontaneously without pumping in a well, forming an artesian well.
Unconfined aquifer: This type of aquifer has no impermeable layer above it, allowing its water to be under less pressure and to be readily recharged directly by surface water that percolates downwards. The water table in unconfined aquifers is directly influenced by local precipitation and surface conditions.
River and Stream Ecosystems
B. Surface water invariably converges in complex river and stream ecosystems, forming dendritic networks across landscapes.
Runoff: This is water that originates from precipitation (rain or snowmelt) or emerges from springs, flowing over the land surface, often picking up sediment and pollutants before entering streams and rivers.
The watershed (also known as a drainage basin or catchment) is the entire area of land drained by a river system, which consists of a main river and all its interconnected tributaries. Activities anywhere within the watershed can impact the water quality and quantity downstream.
The underlying geology and topography of landscapes determine river flow patterns and velocities; conversely, the erosive and depositional power of rivers actively shapes the landscapes they traverse, carving canyons and building floodplains.
Floodplain: These are incredibly fertile areas nearest a river’s course that are periodically inundated during floods, depositing nutrient-rich sediments. This natural disturbance fosters exceptionally diverse ecological communities and productive agricultural lands, but also puts human settlements at risk.
Lakes and Ponds
C. Lakes and ponds are ecologically diverse systems of standing fresh water, varying greatly in size, depth, and nutrient status.
Bodies of standing surface water are typically classified into distinct zones based on light penetration and distance from shore:
Littoral zone: This is the region ringing the edge of a water body, extending from the high-water line down to where aquatic plants no longer grow. It is rich in emergent vegetation, diverse invertebrates, fish, and amphibians.
Benthic zone: This is the bottom of a lake or pond, extending from shore to the deepest point. It is inhabited by decomposers, detritivores, and various invertebrates adapted to low-oxygen conditions and minimal light.
Limnetic zone: This refers to the open, shallow waters away from shore, but still within the reach of sunlight. It is the primary zone for photosynthesis, supporting abundant phytoplankton (algae) and zooplankton, which form the base of the food web for fish.
Profundal zone: This comprises the deeper areas where sunlight does not penetrate sufficiently for photosynthesis. It is characterized by cooler temperatures, lower oxygen levels, and relies on organic matter drifting down from the limnetic zone; plant life is absent here.
Ponds and lakes undergo natural processes of change over geological timescales, gradually transitioning from oligotrophic conditions (characterized by being low in nutrients, exceptionally clear water, and high in oxygen, supporting cold-water fish) to eutrophic conditions (marked by high-nutrient concentrations, often turbid water, and low-oxygen levels due to decomposition of organic matter, leading to reduced biodiversity and warm-water fish species). This process can be greatly accelerated by human-induced nutrient runoff, a phenomenon known as cultural eutrophication.
Freshwater Wetlands
D. Freshwater wetlands encompass a variety of critical ecosystems, including marshes, swamps, bogs, and vernal pools, all sharing characteristics of saturated soil.
Wetlands: These are systems defined by saturated soil, generally featuring shallow standing water for at least part of the year, and ample vegetation adapted to waterlogged conditions (hydrophytes).
Some wetlands are seasonal, meaning they are wet only during certain times of the year, such as spring (vernal pools), playing vital roles in amphibian breeding and groundwater recharge.
Wetlands provide invaluable habitats for a vast array of wildlife, from migratory birds to fish and amphibians, and offer critical ecosystem services, including flood control (by absorbing excess water), groundwater recharge, water purification (by filtering pollutants), and shoreline stabilization.
Historically, human activity has considerably drained and filled extensive areas of wetlands, often for agricultural expansion, urban development, or disease control, leading to significant loss of biodiversity and ecosystem services.
Wetlands and other aquatic systems also suffer from altered flow regimes due to dams and diversions, and are highly susceptible to pollution, which impacts water’s chemical, biological, and physical properties, leading to habitat degradation and species loss.
Human Impact on Waterways
E. Human activities have profoundly affected waterways globally, leading to both a problematic distribution of freshwater resources and significant influences on human populations, often resulting in water scarcity and ecological damage.
Water Use and Allocation
G. Water supplies are primarily allocated for three major sectors: households, industry, and particularly agriculture, each with distinct consumption patterns:
Globally, approximately 70 ext{%} of all fresh water withdrawn is used for agriculture (primarily irrigation), 20 ext{%} for industrial processes (cooling, manufacturing), and 10 ext{%} for residential and municipal uses (drinking, cooking, gardening, sanitation).
Consumptive use: This refers to water that is removed from a source and not returned to that source or basin; it is instead evaporated, transpired by plants, incorporated into products, or consumed by humans/animals. For example, irrigation water that evaporates or is absorbed by crops is a consumptive use.
Nonconsumptive use: This type of use does not permanently remove or only temporarily removes water from a source, and the majority of it is returned. Examples include water used for hydroelectric power generation (where water flows through turbines and is returned to the river) or for certain industrial cooling processes where water is circulated and then discharged back into its source, often at a different temperature.
Challenges and Impacts
H. Excessive withdrawals of freshwater, particularly from surface water bodies like rivers and lakes, can lead to their shrinking or disappearance, causing widespread ecological and economic impacts. Groundwater, while less visible, can also be severely depleted.
Groundwater depletes more easily than surface water because aquifers often recharge at extremely slow rates, sometimes taking centuries or millennia to refill, making them essentially non-renewable on human timescales if over-extracted.
Mining aquifers, or extracting water faster than it can be replenished, results in chronic lowering of water tables, making wells run dry and increasing pumping costs.
In coastal areas, over-extraction of groundwater can lead to saltwater intrusion, where the lower water table allows denser saltwater from the ocean to seep into freshwater aquifers, compromising drinking water quality and making land unsuitable for agriculture.
Drastic drops in water table levels can remove the structural support provided by the water in the ground, leading to sudden land subsidence. This can manifest as sinkholes, which can effectively swallow structures, roads, and agricultural fields as the overlying land collapses into the empty pore spaces.
I. Groundwater is extensively used for bottled water supply, a practice that leads to substantial ecological impacts and raises ethical concerns.
Approximately three out of every four plastic water bottles used in the U.S. are not recycled, exacerbating the global plastic pollution crisis and creating around 1.5 million tons of plastic waste annually. Beyond waste, bottled water production consumes significant fossil fuels for manufacturing, transportation, and refrigeration, contributing to greenhouse gas emissions, and depletes local water sources for profit.
Flood Control and Water Management
K. Communities frequently implement structural measures like levees and dams to manage flood risks and optimize water resources.
Flooding: While often viewed as a disaster, flooding is a natural process that occurs when rivers swell due to excessive snowmelt or heavy rain. This overflow can deposit nutrient-rich sediment, forming floodplains. However, human development in floodplains significantly increases damage and risk.
Levees (or dikes): These are long, raised embankments built along riverbanks or coastlines to hold water within main channels and prevent flooding of adjacent developed areas. While protecting specific areas, levees can disconnect rivers from their floodplains, reduce natural sediment deposition, and potentially worsen flooding downstream or create catastrophic failures if overtopped or breached.
L. Surface water is extensively diverted for human needs through vast engineering projects; thousands of dams have been erected worldwide, profoundly altering river ecosystems.
A dam: This is any obstruction placed in a river or stream to block its flow, creating an upstream reservoir (artificial lake) for water storage. Dams serve multiple purposes, including generating hydroelectric power, providing reliable water for irrigation and drinking, and controlling floods. However, they fragment rivers, alter downstream flow regimes, block fish migration, and change aquatic habitats.
Increasingly, some dams are being removed to restore natural river ecosystems, re-establish fish migration routes (e.g., for salmon), enhance fisheries, and promote river recreation, recognizing the ecological damage caused by impoundments.
The alteration and outright loss of wetlands due to human manipulations like draining, filling, and diversions negatively impacts their vital ecosystem services, including water purification, flood retention, and wildlife habitat provision.
Solutions to Depletion of Fresh Water
Desalination
A. Desalination provides another avenue to increase fresh water supplies, particularly in arid coastal regions, by converting saline water into potable water.
Desalination: This refers to the removal of salt from seawater or marginal quality brackish water. Two primary methods are used:
One method, distillation, mimics the hydrologic cycle through heating and evaporating ocean water, then condensing the vapor to distill pure fresh water. This process is energy-intensive and produces significant thermal pollution.
Another increasingly common method is reverse osmosis, which involves forcing seawater through semi-permeable membranes at high pressure to filter out salts and other impurities, allowing only fresh water to pass through. This method is generally more energy-efficient than distillation.
Despite its potential, desalination faces significant challenges, including extremely high construction and operational costs, substantial reliance on fossil fuels (leading to greenhouse gas emissions), harmful environmental impacts on aquatic life (due to intake pipes and concentrated brine discharge), and the creation of highly concentrated salt waste (brine) that must be carefully managed to avoid environmental damage.
Agricultural Improvements
B. Reducing agricultural demand for water is critical, as it accounts for the largest share of global freshwater use. Significant improvements in irrigation efficiency are possible:
Farmers can substantially enhance water use efficiency by:
Lining irrigation canals with impermeable materials (like concrete or plastic) to prevent leaks and seepage losses into the soil.
Leveling fields precisely (often using laser guidance) to ensure uniform water distribution and minimize runoff, improving infiltration.
Adopting efficient irrigation methods such as drip irrigation (which delivers water directly to plant roots through small tubes, minimizing evaporation and runoff) and center-pivot sprinkler systems (which spray water in a circular pattern, often low to the ground to reduce evaporation, though less efficient than drip).
Selecting crops that are naturally suited to their local environment and climate (e.g., drought-resistant varieties in arid regions) to conserve water naturally without needing extensive irrigation.
Utilizing selective breeding and genetic modification techniques to produce new crop varieties that are inherently high-yield but require less water or are more tolerant to drought conditions.
Reducing Residential Use
C. Lowering residential and industrial water use, while representing a smaller percentage of overall global use, still offers significant benefits for overall conservation and local water availability.
Xeriscaping: This increasingly popular landscaping approach involves designing gardens with plants specifically adapted to dry conditions, thereby requiring minimal or no supplemental irrigation once established. It is gaining significant popularity, particularly in arid regions like the U.S. Southwest, and also includes practices like using efficient irrigation schedules, reducing turf areas, and applying mulch to retain soil moisture.
Other residential conservation efforts include installing low-flow toilets and showerheads, fixing leaky faucets, rainwater harvesting for gardening, and promoting water-efficient appliances.
Market-Based Approaches
D. Discussing market-based strategies for water conservation offers economic incentives and disincentives to influence water use behavior:
Ideas include eliminating governmental subsidies for inefficient water practices (e.g., cheap irrigation water) and adjusting water pricing to reflect the real costs of supply, treatment, and environmental impacts to enhance sustainable use. This aims to make water users more accountable for their consumption.
Concerns about equity frequently arise when water pricing increases, potentially making essential water less accessible for poorer populations and disproportionately impacting their livelihoods.
Industrial water use is often significantly more profitable compared to agricultural use, risking prioritization for wealthier individuals or industries over basic human needs or traditional farming communities if water becomes a commodity traded solely by economic value.
Attempts to privatize water supplies, where private corporations manage water utilities, are often aimed at increasing efficiency and investment but raise strong concerns over equitable access, affordability, and accountability to the public good.
Decentralizing water control and management from large national or regional bodies to local levels can enhance conservation efforts by allowing for more tailored, community-specific solutions and greater local ownership of water resources.
Shifting from traditional supply-side solutions (e.g., building more dams and pipelines) to demand-side solutions (e.g., conservation, efficiency improvements) shows promising results already, as it focuses on reducing the need for water rather than just increasing supply.
International Cooperation
E. Nations frequently collaborate to address water disputes, which are crucial for resolving conflicts over shared transboundary resources (rivers, lakes, aquifers) and promoting regional stability and equitable access, often through treaties and joint commissions.
Freshwater Pollution and Its Control
Pollution Sources
A. Water pollution stems from both readily identifiable point sources and diffuse non-point sources, requiring different control strategies:
Water pollution: Defined as any physical, chemical, or biological change in the quality of water that harms living organisms or makes water unsuitable for desired uses, primarily due to human activities.
Point sources: These are discrete, identifiable pollution origins, such as discharge pipes from factories, sewage treatment plants, or specific industrial outfalls. They are relatively easy to monitor and regulate.
Non-point sources: These represent cumulative pollution originating from broader, more diffuse areas. Examples include agricultural runoff (pesticides, fertilizers, animal waste), urban runoff (oil, chemicals, sediment from streets), atmospheric deposition, and acid rain. They are much harder to identify, monitor, and regulate effectively due to their widespread nature.
Types of Water Pollution
B. Different forms of water pollution each pose unique threats to aquatic ecosystems and human health:
Toxic chemicals: These include industrial solvents, pesticides, heavy metals (like lead, mercury, arsenic), and petroleum products. They can cause direct mortality, developmental defects, cancers, and bioaccumulate in food chains.
Pathogens and waterborne diseases: Contamination by disease-causing microorganisms (bacteria, viruses, protozoa) primarily from inadequately treated human and animal waste. Examples include cholera, typhoid, giardiasis, and dysentery, which are major public health threats in many parts of the world.
Nutrient pollution: Excessive input of nitrogen and phosphorus (often from fertilizers, detergents, and sewage) leading to eutrophication. This over-enriches water bodies, causing algal blooms, reduced light penetration, and depletion of oxygen (hypoxia or anoxia) as algae decompose, creating