🔆Environmental Chemistry I Unit 5 – Aquatic Chemistry: Water Quality Issues

Key Concepts in Aquatic Chemistry

• Aquatic chemistry focuses on the chemical processes and interactions that occur in water environments such as oceans, lakes, rivers, and groundwater
• Water is a unique solvent due to its polarity and ability to dissolve a wide range of substances (salts, gases, organic compounds)
• pH is a crucial parameter in aquatic systems, measuring the concentration of hydrogen ions and indicating the acidity or alkalinity of water
• Dissolved oxygen (DO) is essential for aquatic life and is influenced by factors such as temperature, salinity, and biological activity
• Redox reactions involve the transfer of electrons and play a significant role in nutrient cycling, metal speciation, and organic matter decomposition in aquatic environments
• Eutrophication is the excessive growth of algae and aquatic plants due to high nutrient levels (nitrogen and phosphorus), leading to oxygen depletion and ecosystem imbalances
• Biogeochemical cycles, such as the carbon, nitrogen, and phosphorus cycles, are closely linked to aquatic chemistry and influence the distribution and transformation of elements in water bodies
• Aquatic ecosystems are sensitive to changes in water chemistry, and alterations can have cascading effects on biodiversity, food webs, and ecosystem services

Water Quality Parameters and Measurements

• Physical parameters include temperature, turbidity, and total suspended solids (TSS), which affect light penetration, heat transfer, and habitat quality in aquatic systems
• Chemical parameters encompass pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), nutrients (nitrogen and phosphorus), and trace metals
• Biological parameters involve the presence and abundance of indicator organisms (coliform bacteria) and the assessment of aquatic biodiversity and ecosystem health
• Water quality monitoring involves regular sampling and analysis of water bodies to track changes, identify pollution sources, and assess compliance with regulatory standards
• In-situ measurements using probes and sensors provide real-time data on parameters such as temperature, pH, DO, and conductivity
• Laboratory analysis is necessary for more detailed chemical and biological assessments, including nutrient concentrations, trace metals, and microbial contamination
• Water quality indices (WQI) integrate multiple parameters into a single score, facilitating the communication of water quality status to stakeholders and decision-makers
• Remote sensing techniques, such as satellite imagery and drone surveys, offer cost-effective and large-scale monitoring of water quality parameters (chlorophyll-a, turbidity)

Sources of Water Pollution

• Point sources are identifiable and localized, such as industrial discharges, wastewater treatment plant effluents, and accidental spills
• Non-point sources are diffuse and widespread, including agricultural runoff (pesticides, fertilizers), urban stormwater, and atmospheric deposition
• Domestic sewage contains organic matter, nutrients, and pathogens that can degrade water quality and pose health risks if not properly treated
• Industrial effluents may contain toxic chemicals, heavy metals, and persistent organic pollutants (POPs) that bioaccumulate in aquatic food chains
• Agricultural activities contribute to nutrient loading (nitrogen and phosphorus) from fertilizer use and animal waste, leading to eutrophication and harmful algal blooms
• Urban runoff carries pollutants such as oil, grease, heavy metals, and microplastics from roads, parking lots, and construction sites into nearby water bodies
• Atmospheric deposition of pollutants (acid rain, mercury) can originate from distant sources and have long-term impacts on aquatic ecosystems
• Emerging contaminants, such as pharmaceuticals, personal care products, and endocrine-disrupting compounds, are increasingly detected in water bodies and may have subtle but significant ecological effects

Chemical Reactions in Aquatic Systems

• Acid-base reactions involve the transfer of protons (H+) and influence the pH of water, affecting the solubility and speciation of compounds
• Precipitation reactions occur when the concentration of dissolved ions exceeds their solubility product, leading to the formation of solid particles (calcium carbonate, iron hydroxides)
• Complexation reactions involve the binding of metal ions to organic or inorganic ligands, altering their bioavailability and toxicity
• Photochemical reactions are driven by sunlight and can lead to the degradation of organic pollutants or the production of reactive oxygen species (ROS)
• Redox reactions couple the oxidation and reduction of chemical species, influencing the mobility and speciation of nutrients and contaminants (iron, manganese, arsenic)
• Adsorption processes involve the attachment of dissolved substances to the surface of particles or sediments, affecting their transport and fate in aquatic systems
• Biodegradation is mediated by microorganisms and plays a crucial role in the breakdown of organic matter and the removal of pollutants from water
• Abiotic degradation processes, such as hydrolysis and photolysis, can also contribute to the transformation and removal of contaminants in aquatic environments

Impacts on Ecosystems and Human Health

• Eutrophication leads to algal blooms, oxygen depletion, fish kills, and the loss of aquatic biodiversity and ecosystem services
• Harmful algal blooms (HABs) can produce toxins that pose risks to human health through direct exposure or consumption of contaminated seafood
• Bioaccumulation of persistent pollutants (mercury, PCBs) in aquatic food chains can reach high levels in top predators and pose health risks to humans consuming contaminated fish
• Waterborne diseases, such as cholera, typhoid, and hepatitis A, are caused by pathogens in contaminated water and can lead to outbreaks and public health emergencies
• Exposure to toxic chemicals and heavy metals in water can cause acute poisoning or chronic health effects (cancer, neurological disorders, reproductive issues)
• Degradation of aquatic habitats, such as coral reefs, wetlands, and seagrass beds, affects biodiversity, coastal protection, and the livelihoods of communities dependent on these ecosystems
• Ocean acidification, caused by the absorption of atmospheric CO2, threatens calcifying organisms (corals, shellfish) and alters marine food webs and biogeochemical cycles
• Water scarcity and quality issues disproportionately affect vulnerable populations, exacerbating social inequalities and hindering sustainable development goals

Water Treatment Technologies

• Coagulation and flocculation involve the addition of chemicals (alum, ferric chloride) to destabilize suspended particles and promote their aggregation into larger flocs for easier removal
• Sedimentation allows the settling of suspended solids and flocs by gravity, clarifying the water and reducing the load on subsequent treatment steps
• Filtration removes remaining particles and microorganisms by passing water through porous media (sand, activated carbon, membranes)
  • Slow sand filtration is a low-cost and effective method for removing pathogens and turbidity in small-scale water treatment systems
  • Membrane filtration (microfiltration, ultrafiltration, nanofiltration, reverse osmosis) offers high removal efficiencies for a wide range of contaminants based on size exclusion and charge interactions
• Disinfection inactivates pathogenic microorganisms using chemical agents (chlorine, chloramine, ozone) or physical processes (UV irradiation)
  • Chlorination is widely used for its residual disinfection capacity in distribution systems but can lead to the formation of disinfection by-products (DBPs) when reacting with organic matter
• Advanced oxidation processes (AOPs) generate highly reactive hydroxyl radicals to degrade recalcitrant organic pollutants and emerging contaminants
• Biological treatment, such as activated sludge and constructed wetlands, harnesses the metabolic activities of microorganisms to remove organic matter, nutrients, and some pollutants from water
• Adsorption processes, using materials like activated carbon or zeolites, can effectively remove dissolved organic compounds, taste and odor causing substances, and some micropollutants

Regulatory Framework and Standards

• The Clean Water Act (CWA) in the United States establishes the basic structure for regulating discharges of pollutants into waters and setting water quality standards
• The Safe Drinking Water Act (SDWA) sets national standards for drinking water quality and requires regular monitoring and reporting by public water systems
• The World Health Organization (WHO) provides guidelines for drinking water quality, which serve as a basis for national standards and regulations in many countries
• The European Water Framework Directive (WFD) aims to achieve good ecological and chemical status for all water bodies in the European Union through integrated river basin management
• Effluent guidelines and standards set industry-specific limits on the discharge of pollutants from point sources into water bodies
• Total Maximum Daily Load (TMDL) programs establish the maximum amount of a pollutant that a water body can receive while still meeting water quality standards
• Water quality criteria define the levels of contaminants that are safe for human health and aquatic life, guiding the development of discharge permits and cleanup goals
• International treaties and conventions, such as the Stockholm Convention on Persistent Organic Pollutants and the Minamata Convention on Mercury, address the global management of toxic substances in the aquatic environment

Case Studies and Real-World Applications

• The Chesapeake Bay restoration efforts involve multi-state collaboration to reduce nutrient and sediment loads from agricultural and urban sources, aiming to improve water quality and ecosystem health
• The Flint water crisis in Michigan, USA, highlighted the importance of proper water treatment and infrastructure maintenance to prevent lead contamination and protect public health
• The Gulf of Mexico dead zone is a hypoxic area caused by excessive nutrient loading from the Mississippi River Basin, affecting marine biodiversity and fisheries
• The Minamata disease in Japan was caused by the consumption of seafood contaminated with methylmercury discharged from a chemical factory, leading to severe neurological disorders in the local population
• The Great Barrier Reef in Australia is facing threats from ocean acidification, coral bleaching, and water quality degradation due to agricultural runoff and climate change
• The Ganges River in India is heavily polluted by untreated sewage, industrial effluents, and religious offerings, posing risks to human health and aquatic life
• The successful restoration of Lake Erie in North America through phosphorus load reductions and invasive species control demonstrates the effectiveness of coordinated management efforts
• The Catskill Mountains watershed protection program in New York, USA, showcases the benefits of investing in natural infrastructure and source water protection to ensure high-quality drinking water supply