🍂Environmental Chemistry II Unit 5 – Aquatic Chemistry: Groundwater Processes

Key Concepts in Aquatic Chemistry

• Aquatic chemistry studies the chemical processes and reactions that occur in water environments (groundwater, surface water, oceans)
• Involves understanding the interactions between water, dissolved substances, and geological materials
• Includes concepts such as pH, alkalinity, redox reactions, and solubility
• Considers the influence of biological processes on water chemistry
• Applies principles of physical chemistry, inorganic chemistry, and organic chemistry to aquatic systems
• Helps in assessing water quality, managing water resources, and understanding the fate and transport of contaminants
• Provides a foundation for understanding the biogeochemical cycles of elements in the environment

Groundwater Basics and Formation

• Groundwater is water that fills the pores and fractures in soil and rock beneath the Earth's surface
• Forms when precipitation infiltrates the ground and percolates through the unsaturated zone to the water table
• The unsaturated zone, also known as the vadose zone, is the layer above the water table where pores contain both air and water
• The saturated zone, located below the water table, has all pores and fractures filled with water
• Aquifers are geologic formations that can store and transmit significant quantities of water
  • Confined aquifers are bounded above and below by impermeable layers (aquitards or aquicludes)
  • Unconfined aquifers have a water table as their upper boundary and are more susceptible to contamination
• Groundwater flow is driven by hydraulic head differences and is influenced by the hydraulic conductivity of the geologic materials

Chemical Composition of Groundwater

• The chemical composition of groundwater is determined by the interactions between water and the geologic materials it contacts
• Dissolved ions in groundwater originate from the dissolution of minerals, ion exchange reactions, and anthropogenic sources
• Major cations in groundwater include calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), and potassium (K⁺)
• Major anions include bicarbonate (HCO₃⁻), sulfate (SO₄²⁻), chloride (Cl⁻), and nitrate (NO₃⁻)
• The concentration of dissolved solids in groundwater can vary widely, from fresh water (<1,000 mg/L) to brine (>35,000 mg/L)
• Groundwater pH is influenced by the dissolution of minerals, such as calcite (CaCO₃), and the presence of dissolved carbon dioxide (CO₂)
• Trace elements, such as iron (Fe), manganese (Mn), and arsenic (As), can be present in groundwater and may pose health risks at elevated concentrations

Redox Processes in Groundwater

• Redox (reduction-oxidation) processes involve the transfer of electrons between chemical species
• Redox conditions in groundwater are characterized by the presence or absence of dissolved oxygen (DO) and other electron acceptors
• In oxic groundwater (high DO), oxygen is the primary electron acceptor, and aerobic respiration dominates
• As oxygen is depleted, anaerobic processes become dominant, using electron acceptors in the following sequence: nitrate (NO₃⁻), manganese (Mn⁴⁺), iron (Fe³⁺), sulfate (SO₄²⁻), and carbon dioxide (CO₂)
• Redox reactions can influence the mobility and fate of contaminants in groundwater
  • For example, the reduction of Fe³⁺ to Fe²⁺ can release arsenic bound to iron oxides, increasing its concentration in groundwater
• Microbial activity plays a crucial role in mediating redox reactions in groundwater environments

Contaminant Transport in Aquifers

• Contaminant transport in aquifers is governed by physical, chemical, and biological processes
• Advection is the movement of contaminants along with the bulk flow of groundwater, driven by hydraulic head differences
• Dispersion is the spreading of contaminants due to velocity variations at the pore scale and larger scales
  • Mechanical dispersion is caused by variations in flow paths and velocities
  • Molecular diffusion is the movement of contaminants from high to low concentration areas
• Retardation is the slowing of contaminant transport relative to the average groundwater velocity due to sorption onto aquifer materials
• Decay processes, such as radioactive decay or biodegradation, can reduce contaminant concentrations over time
• Contaminant plumes can develop downgradient of a source area, with the shape and extent determined by hydrogeologic conditions and transport processes

Groundwater-Surface Water Interactions

• Groundwater and surface water are interconnected components of the hydrologic cycle
• Streams can gain water from groundwater discharge (gaining streams) or lose water to groundwater recharge (losing streams)
• The direction and magnitude of water exchange between groundwater and surface water depend on the hydraulic head differences and the hydraulic conductivity of the streambed sediments
• Riparian zones, the areas adjacent to streams and rivers, are important transition zones where groundwater and surface water interact
• Groundwater discharge to streams can help maintain baseflow and moderate stream temperatures
• Contaminated groundwater can discharge to surface water bodies, leading to water quality impairments
• Pumping groundwater near streams can alter the natural groundwater-surface water interactions and potentially cause streamflow depletion

Analytical Techniques for Groundwater Studies

• Various analytical techniques are used to study groundwater chemistry, flow, and contaminant transport
• Water quality sampling involves collecting groundwater samples from wells or springs and analyzing them for chemical constituents
  • Proper sampling protocols, such as purging wells and using appropriate sample containers, are essential for accurate results
• Hydrogeologic characterization techniques, such as well logging and aquifer tests, provide information on aquifer properties and groundwater flow
• Geophysical methods, like electrical resistivity and ground-penetrating radar, can help delineate aquifer geometry and identify preferential flow paths
• Tracer tests involve introducing a conservative substance (e.g., dye or bromide) into groundwater and monitoring its transport to estimate flow velocities and dispersion
• Numerical modeling using software packages (MODFLOW, MT3DMS) can simulate groundwater flow and contaminant transport based on hydrogeologic data and governing equations
• Isotope analysis (stable isotopes, radioactive isotopes) can provide insights into groundwater age, recharge sources, and geochemical processes

Environmental Impacts and Management

• Groundwater is a vital resource for drinking water, irrigation, and industrial uses, but it is susceptible to contamination and overexploitation
• Common groundwater contaminants include nitrates from agricultural runoff, industrial solvents, fuel hydrocarbons, and pathogens from septic systems
• Groundwater contamination can pose risks to human health and the environment, and remediation can be costly and time-consuming
• Saltwater intrusion can occur in coastal aquifers when groundwater pumping induces the landward movement of saline water, rendering the aquifer unusable
• Land subsidence can result from excessive groundwater withdrawal, causing the compaction of aquifer materials and surface elevation loss
• Groundwater management strategies aim to balance the competing demands for groundwater while protecting its quality and quantity
  • Strategies include regulating groundwater pumping, implementing water conservation measures, and establishing groundwater protection zones
• Managed aquifer recharge (MAR) involves intentionally recharging aquifers with surface water or treated wastewater to store water for future use and mitigate declining water levels
• Effective groundwater management requires a sound understanding of the hydrogeologic system, monitoring networks, and stakeholder engagement