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.
Discuss how we use water and alter freshwater systems.
Assess problems of water supply and propose solutions to address depletion of fresh water.
Describe the major classes of water pollution and propose solutions to address water pollution.
Explain how we treat drinking water and wastewater.
Freshwater Systems
Overview of Freshwater
Of all the water on Earth, only 2.5 ext{%} is considered fresh water, which is relatively pure with few dissolved salts.
Most of this fresh water is tied up in glaciers, ice caps, and underground aquifers.
Water is renewed and recycled as it moves through the water cycle.
Groundwater plays key roles in the hydrologic cycle.
Definitions of Water Types
A. Surface water refers to water located atop Earth’s surface.
B. Groundwater refers to water beneath the surface that resides within pores in soil or rock.
C. Groundwater is contained within aquifers:
Aquifers: Porous, spongelike formations of rock, sand, or gravel that can hold water.
The upper layer of aquifers contains pore spaces partly filled with water.
The lower layer has spaces that are completely filled with water. The boundary between these two zones is known as the water table.
Areas where water infiltrates Earth’s surface to reach the aquifer below are known as recharge zones.
Types of Aquifers
Confined aquifer (Artesian aquifer): A porous, water-bearing layer trapped between upper and lower layers of less permeable substrate (often clay). Water in these aquifers is under great pressure.
Unconfined aquifer: Has no impermeable layer above it, water is under less pressure and can be readily recharged by surface water.
River and Stream Ecosystems
B. Surface water converges in river and stream ecosystems.
Runoff: Water that falls from the sky as rain, emerges from springs, or melts from snow or glacier and flows over land.
The watershed (drainage basin) is the area of land drained by a river system, which consists of a river and all its tributaries.
Landscapes determine river flow; conversely, rivers shape the landscapes they traverse.
Floodplain: Areas nearest a river’s course that flood periodically, fostering diverse ecological communities.
Lakes and Ponds
C. Lakes and ponds are ecologically diverse systems.
Bodies of standing surface water classified into:
Littoral zone: Region ringing the edge of a water body.
Benthic zone: Bottom of a lake or pond.
Limnetic zone: Shallow waters away from shore that support phytoplankton and zooplankton due to light availability.
Profundal zone: Deeper areas where sunlight does not reach and lacks plant life.
Ponds and lakes change over time, transitioning from oligotrophic (low in nutrients, high in oxygen) to eutrophic (high-nutrient, low-oxygen) conditions.
Freshwater Wetlands
D. Freshwater wetlands include marshes, swamps, bogs, and vernal pools.
Wetlands: Systems with saturated soil generally featuring shallow standing water and ample vegetation.
Some wetlands are seasonal, wet only during certain times of the year.
Wetlands provide valuable habitats for wildlife and important ecosystem services.
Human activity has considerably drained and filled wetlands, often for agricultural purposes.
Wetlands and aquatic systems suffer from altered flow due to pollution, impacting water’s chemical, biological, and physical properties.
Human Impact on Waterways
E. Human activities significantly affect waterways, leading to a problematic distribution of freshwater and influencing human populations.
Water Use and Allocation
G. Water supplies served households, industry, and particularly agriculture:
Globally, about 70 ext{%} of fresh water is used for agriculture, 20 ext{%} for industry, and 10 ext{%} for residential and municipal uses.
Consumptive use: Water removed from a source without being returned.
Nonconsumptive use: Does not permanently remove or only temporarily removes water from a source.
Challenges and Impacts
H. Excessive withdrawals of freshwater can drain rivers and lakes, and groundwater can also be depleted.
Groundwater depletes more easily than surface water, as aquifers often recharge slowly.
Mining aquifers results in lowering water tables.
In coastal areas, over-extraction may lead to saltwater intrusion, compromising water quality.
Sudden land subsidence can lead to sinkholes, which can effectively swallow structures.
I. Groundwater is extensively used for bottled water supply, leading to substantial ecological impacts.
Approximately three out of every four plastic water bottles used in the U.S. are not recycled, creating around 1.5 million tons of plastic waste annually.
Flood Control and Water Management
K. Communities build levees to manage flood risks.
Flooding: A natural process that occurs when rivers swell due to snowmelt or heavy rain, causing overflow.
Levees (or dikes): Structures built to hold water within main channels and prevent flooding.
L. Surface water is diverted for human needs; thousands of dams have been erected.
A dam: Any obstruction in a river to block its flow, creating reservoirs (artificial lakes) for water storage.
Some dams are being removed to restore natural river ecosystems, enhance fisheries, and promote river recreation.
The alteration and loss of wetlands due to human manipulations negatively impacts vital ecosystem services.
Solutions to Depletion of Fresh Water
Desalination
A. Desalination provides another avenue to increase fresh water supplies.
Desalination: The removal of salt from seawater or marginal quality water.
One method: Mimics the hydrologic cycle through heating and evaporating ocean water, then condensing vapor to distill fresh water.
Another method: Reverse osmosis, forcing water through membranes to filter out salts.
Challenges include high costs, reliance on fossil fuels, harmful impacts on aquatic life, and creation of concentrated salt waste.
Agricultural Improvements
B. Reducing agricultural demand is critical.
Farmers can enhance efficiency by:
Lining irrigation canals to prevent leaks.
Leveling fields to minimize runoff.
Adopting efficient irrigation methods.
Selecting crops that suit their environment to conserve water.
Utilizing selective breeding and genetic modification to produce high-yield, low-water-demand crops.
Reducing Residential Use
C. Lowering residential and industrial water use benefits overall conservation.
Xeriscaping: Landscaping with plants adapted to dry conditions gains popularity, particularly in arid regions like the U.S. Southwest.
Market-Based Approaches
D. Discussing market-based strategies for water conservation:
Ideas include eliminating governmental subsidies for inefficient practices and adjusting water pricing to reflect real costs to enhance sustainable use.
Concerns about equity arise when water pricing increases, potentially making it less accessible for poorer populations.
Industrial water use is significantly more profitable compared to agricultural use, risking prioritization for wealthier individuals.
Attempts to privatize water supplies aimed at increasing efficiency raise concerns over equitable access.
Decentralizing water control from national to local levels can enhance conservation efforts.
Shifting from supply-side to demand-side solutions shows promising results already.
International Cooperation
E. Nations frequently collaborate to address water disputes, crucial for resolving conflicts over shared resources.
Freshwater Pollution and Its Control
Pollution Sources
A. Water pollution stems from both point and non-point sources:
Water pollution: Changes in the chemical, physical, or biological properties of water due to human activities.
Point sources: Discrete pollution origins like factories and sewer pipes.
Non-point sources: Cumulative pollution from broader areas, including agricultural runoff and urban runoff.
Types of Water Pollution
B. Different forms of water pollution include:
Toxic chemicals
Pathogens and waterborne diseases
Nutrient pollution
Biodegradable wastes: Including wastewater, which could serve as a source of organic pollutants.
Sediment
Thermal pollution
Groundwater Pollution
C. Groundwater pollution presents complex challenges:
Toxic chemicals that naturally occur at high concentrations (aluminum, fluoride, nitrates, and sulfates) can contaminate groundwater.
Industrial, agricultural, and urban wastes can leach into aquifers, including heavy metals and pesticides.
The leakage of carcinogenic pollutants from underground tanks is a significant threat to groundwater quality.
Radioactive materials leaking from tanks also present groundwater risks.
Agriculture contributes via pesticides and nitrate fertilizers.
Pollution Control and Treatment
E. Legislative and regulatory measures have been implemented to reduce water pollution.
F. Drinking water treatment processes are in place to ensure safety.
G. Wastewater treatment involves:
Wastewater includes various sources, including sewage, industrial cleaning processes, and storm runoff.
Septic systems are commonly used in rural areas to manage wastewater:
Wastewater flows to a septic tank where solids and oils separate. Clarified water moves to a drain field, where microbes decompose pollutants.
Urban areas use centralized municipal sewer systems, from which wastewater goes to treatment facilities:
Primary treatment: Physical removal of contaminants using settling tanks, eliminating approximately 60 ext{%} of suspended solids.
Secondary treatment: Aerobic bacteria treat organic pollutants, removing about 90 ext{%} of suspended solids.
Final treatment involves chlorination and sometimes ultraviolet light to eliminate bacteria, ensuring water safety.
H. Constructed wetlands can aid in wastewater treatment, utilizing natural processes to filter pollutants.
Closing the Loop
Sustainable Water Management
A. Citizen actions, government regulation, technological advancements, economic incentives, and education are crucial for addressing the challenges related to freshwater quantity and quality.
B. There remains optimism for achieving sustainable practices in water utilization.
Key Terms
aquifers
artesian aquifer
confined aquifer
consumptive use
dam
desalination
desalinization
eutrophic
flooding
floodplain
fresh water
groundwater
nonconsumptive use
nonpoint source
oligotrophic
point source
primary treatment
recharge zone
reservoirs
runoff
secondary treatment
septic systems
sinkholes
surface water
unconfined aquifer
wastewater
water pollution
watershed
water table
wetlands
xeriscaping