Fate Pollutants in Subsurface Environments - Lecture 1

CEE 4622/5622: Pollutant Fate in Subsurface Environments - Lecture 1 Notes

Course Overview

This lecture introduces the course CEE 4622/5622, focusing on the fate of pollutants in subsurface environments. Key terms for the course include Fate, Pollutants, Subsurface, and Environments, which will be explored in depth.

Lecture 1 Agenda

Topics covered in this lecture:

  • Pre-quiz discussion

  • Considering the subsurface

  • Considering subsurface contamination

  • Syllabus review

  • Discussion of groundwater flow

Pre-Quiz - Key Concepts

These questions serve as an initial assessment and highlight core concepts for the course. Answers derived from the lecture content are provided.

1. Define porosity

Porosity ($\eta$) is the ratio of the volume of voids (empty spaces) to the total volume of a material.
\eta = V{voids} / V{total} = Vv / V It can also be expressed in terms of bulk density ($\rhob$) and grain density ($\rhom$): \eta = 1 - (\rhob / \rho_m)

2. Explain non-reactive tracer

A non-reactive tracer is a substance introduced into a system (e.g., groundwater) that moves with the flow without undergoing significant chemical reactions, sorption, or biodegradation. It is used to observe advective and dispersive transport mechanisms solely.

3. Explain and define Kd

Kd, or the distribution coefficient, is a measure of the extent to which a contaminant sorbs (sticks) onto solid phases (e.g., soil particles) relative to its concentration in the aqueous phase. A higher Kd indicates greater sorption and thus slower movement (retardation) of the contaminant compared to the groundwater flow.

4. What is the capillary zone?

The capillary zone is a region located above the water table within the vadose zone where water is held by capillary forces (surface tension) in the pore spaces, against the force of gravity. While not fully saturated, it can contain significant amounts of water. It is part of the zone of aeration.

5. Are kinetics or equilibrium more important?

Both kinetics and equilibrium are important, depending on the specific contaminant, medium, and timeframe. Equilibrium describes the final state where forward and reverse reaction rates are equal, while kinetics describes the rate at which that equilibrium is approached. For rapid processes or long travel times, equilibrium may be a reasonable approximation. For slow processes or short travel times, kinetics are crucial for accurately modeling contaminant fate.

6. What is NAPL?

NAPL stands for Non-Aqueous Phase Liquid. These are organic liquids (e.g., gasoline, diesel fuel, dry cleaner solvents) that are not miscible with water. They exist as a separate phase in the subsurface. While not fully miscible, they can still be sparingly soluble in water, leading to aqueous contamination plumes that can be toxic.

7. Name one remediation approach

One remediation approach is pump and treat, where contaminated groundwater is pumped to the surface, treated to remove pollutants, and then discharged or reinjected.

Groundwater and the Subsurface Environment

What is Groundwater?

Groundwater is water that originates from rain and snowmelt. It seeps into the ground, pulled downwards by gravity through the spaces between soil particles or cracks in rocks. Eventually, it reaches a depth where all interconnected openings in the soil or rock are completely filled with water. This region is called the saturated zone. The water within this saturated zone is groundwater. The upper surface of the saturated zone is known as the water table, which fluctuates seasonally and with precipitation.

Groundwater-Surface Water Interactions

Groundwater plays a crucial role in surface water bodies. During dry periods, groundwater discharge, known as baseflow, can account for more than half of the total flow in some streams.

Losing Stream

A losing stream occurs when the water table in the adjacent aquifer is below the level of the stream. In this scenario, water from the stream seeps downward into the groundwater aquifer.

Human Influence on Groundwater

Human activities significantly impact groundwater dynamics:

  • Pumping Wells: Pumping from a well, especially in an unconfined (water table) aquifer, lowers the water table near the well, creating a cone of depression. The land area above this cone is the area of influence. Within this area, groundwater flow naturally shifts towards the well.

  • Impact on Surface Water: A cone of depression can extend to nearby streams or lakes, lowering the water table below the surface water level. This causes the stream or lake to lose water to the groundwater aquifer. The Oregon Water Resources Department considers wells within 0.25 miles (0.40 km) of a stream to have a potential effect on stream flow.

Fundamentals of Groundwater Flow

Hydrologic Cycle

The hydrologic cycle describes the continuous movement of water on, above, and below the Earth's surface. Key components include:

  • Atmosphere: Water vapor and clouds.

  • Evaporation: Water turning into vapor.

  • Interception: Precipitation caught by vegetation.

  • Precipitation: Rain, snow, sleet, hail reaching the surface.

  • Depression Storage: Water held in surface depressions.

  • Overland Flow: Water flowing over the land surface.

  • Infiltration: Water seeping into the ground.

  • Interflow: Lateral flow of water within the vadose zone.

  • Channel Flow: Water flowing in rivers and streams.

  • Reservoir Storage: Water impounded in reservoirs.

  • Evapotranspiration: Water combining evaporation from surface and transpiration from plants.

  • Root Zone Storage: Water held in the soil where plant roots are located.

  • Channel Seepage: Water from channels seeping into groundwater.

  • Groundwater Storage: Water held in aquifers.

  • Groundwater Flow: Movement of water within aquifers.

  • Ocean: Major reservoir of water.

Subsurface Zones

Below the ground surface, the subsurface is divided into distinct vertical zones based on water saturation:

  • Zone of Aeration (Vadose Zone): The region above the water table where pore spaces are not completely filled with water; they contain both air and water. It includes sub-zones:

    • Soil-water zone: Uppermost zone, influenced by plant roots and infiltration.

    • Capillary zone: Above the water table, where water is held by capillary action.

  • Water Table: The boundary between the zone of aeration and the zone of saturation.

  • Zone of Saturation (Phreatic Zone): The region below the water table where all pore spaces are completely filled with water. This is where groundwater resides.

Aquifers: Types and Characteristics

An aquifer is a geological formation that can store and transmit significant quantities of groundwater. Aquifers are classified based on the presence of confining layers:

  • Unconfined Aquifer (Water Table Aquifer): An aquifer whose upper boundary is the water table. It is not overlain by a confining layer. The water level in a well drilled into an unconfined aquifer will be at the same elevation as the water table outside the well.

  • Confined Aquifer (Artesian Aquifer): An aquifer that is bounded above and below by impermeable or low-permeability layers called confining strata. The groundwater in a confined aquifer is typically under pressure. A well penetrating a confined aquifer is called an artesian well. If this pressure causes water to rise above the aquifer level but still within the well casing, it's an artesian well. If the pressure is sufficient to push the water level above the ground surface, it becomes a flowing artesian well.

  • Piezometric Surface: For confined aquifers, this represents the imaginary surface to which water would rise in wells freely communicating with the aquifer. It is analogous to the water table in unconfined aquifers.

  • Recharge Area: The surface area where water infiltrates and replenishes an aquifer.

Watersheds and Groundwater Movement

A watershed is the entire land area drained by a single river system. In unconfined aquifers, groundwater generally mimics the direction of surface water flow, staying within the same watershed where the original precipitation fell. However, in confined aquifers, water can exit the local watershed through regional flow systems or fractures in the confining layers.

Aquifer Properties

Properties such as porosity, grain size, and packing significantly influence groundwater flow and storage.

Porosity (\eta)

Porosity is the percentage of total rock or soil volume that consists of pore spaces. It is a critical factor determining the amount of water an aquifer can store.

  • Definition: Ratio of void volume to total volume. \eta = V{voids} / V{total} or \eta = V_v / V

  • Calculation using densities: \eta = 1 - (\rhob / \rhom) where:

    • \rho_b is the bulk density (dry soil mass / unit volume).

    • \rho_m is the grain density (dry soil mass / soil volume).

  • Representative Values: Porosity varies widely by material (e.g., gravel: 28-34\%+, sand: 39-43\%, silt: 46\%, clay: 42\%, shale: 6\%, peat: 92\%).

Grain Size and Packing

These characteristics, along with porosity, influence:

  • The speed of groundwater flow (velocity).

  • The storage capacity of groundwater.

  • The ability of the aquifer to release water to wells (yield).

Notably, subsurface environments are often highly heterogeneous, meaning properties like hydraulic conductivity can vary significantly from point to point and are dependent on the measurement scale (K1 \neq K2 \neq K_3).

Characterization of Groundwater Flow
Discharge, Velocity, and Flux

  • Discharge (Q): The volume of water flowing through a cross-section per unit time, typically measured in length^3/time.

  • Velocity (q): The average speed of water movement, measured in length/time. This can also be referred to as flux (J), which is discharge per unit area: J = Q/A.

  • Water flows due to gravity and pressure differences.

Average Linear Velocity (v)

The average linear velocity (or seepage velocity) accounts for the fact that water only moves through the pore spaces, not through the solid grains. It is higher than the flux velocity (q) because the flow occurs through a smaller effective area.

  • v = q / \eta = Q / (A \cdot \eta) where:

    • v is the average linear velocity (length/time).

    • q is the Darcy flux velocity (length/time).

    • \eta is the porosity (dimensionless).

    • Q is the discharge (length^3/time).

    • A is the cross-sectional area (length^2).

Darcy's Law

Darcy's Law describes the flow of fluids through porous media. It states that the specific discharge (or Darcy flux) is proportional to the hydraulic gradient.

  • Formula: Q = -KA \frac{\Delta H}{\Delta x} where:

    • Q is the volumetric discharge (length^3/time).

    • K is the hydraulic conductivity (length/time), a measure of how easily water can pass through a porous medium.

    • A is the cross-sectional area perpendicular to the flow (length^2).

    • \Delta H/\Delta x is the hydraulic gradient (unitless), representing the change in hydraulic head (total energy of water) over a given distance.

    • The negative sign indicates that flow occurs in the direction of decreasing hydraulic head.

Subsurface Contamination

Goals of Studying Contamination

Studying subsurface contamination aims to:

  • Describe or characterize the contamination and its sources.

  • Consider means to avoid or remediate contaminated sites.

Sources of Contamination

Subsurface contamination can arise from six primary categories of sources:

  1. Designed to Discharge Substances: Activities explicitly intended to release substances into the subsurface.

    • Examples: Septic tanks and cesspools, injection wells, land application of wastewater or biosolids.

  2. Store, Treat, and/or Dispose of Substances: Facilities or practices designed for containment but where leakage or spillage can occur.

    • Examples: Landfills, open dumps/residential disposal areas, surface impoundments (ponds, lagoons), mine wastes/material stockpiles, graveyards/animal burials, above and underground storage tanks (ASTs/USTs), open incineration and detonation sites.

  3. Retain Substances During Transport: Systems for moving substances that can fail and release contaminants.

    • Examples: Pipelines, transport trucks and trains.

  4. Discharge as a Consequence of Other Planned Activities: Contamination resulting as an unintended byproduct of primary operations.

    • Examples: Irrigation (leaching chemicals), pesticide/fertilizer application, farm animal waste, road salt/deicers, home water softeners, urban runoff, atmospheric pollutants (dry and wet deposition), mine drainage/excursion (acid mine drainage), fracking.

  5. Conduit for Aquifer Contamination: Structures that provide pathways for contaminants to reach aquifers.

    • Examples: Production wells, monitoring wells and exploratory borings, infiltration galleries, construction excavation.

  6. Naturally Occurring Sources - Mobilized by Activity: Background constituents that become mobilized or concentrated due to human influence.

    • Examples: Groundwater-surface water interactions (e.g., induced infiltration of poor-quality surface water), natural leaching (enhanced by human activities), saltwater intrusion (over-pumping near coastlines).

Typical Groundwater Constituents

Clean groundwater naturally contains dissolved constituents, classified by abundance:

  • Major Constituents (greater than 5 mg/L): Bicarbonate, Calcium, Carbonic Acid, Chloride, Magnesium, Silicon, Sodium, Sulfate.

  • Minor Constituents (0.1-5.0 mg/L): Boron, Carbonate, Fluoride, Iron, Nitrate, Potassium, Strontium.

  • Trace Constituents (less than 0.1 mg/L): Aluminum, Antimony, Arsenic, Barium, Beryllium, Bromide, Cadmium, Cesium, Chromium, Copper, Iodine, Lead, Lithium, Manganese, Molybdenum, Nickel, Phosphate, Radium, Selenium, Silver, Tin, Uranium, Zinc.

  • Important Note: Most organic contaminants are not detectable in clean, uncontaminated groundwater, highlighting their anthropogenic origin when found.

Challenges of Subsurface Contamination

Understanding subsurface contamination is challenging primarily because the subsurface is:

  • Hidden: Contamination is not visible on the surface.

  • Heterogeneous: Geologic formations vary greatly, impacting flow paths and contaminant behavior.

  • Complex: Interplay of physical, chemical, and biological processes makes prediction difficult.

Regulatory Framework

Cleanup efforts for contaminated sites are largely driven by federal regulations in the U.S.:

  • CERCLA: Comprehensive Environmental Response, Compensation, and Liability Act (also known as Superfund), addresses hazardous waste sites.

  • RCRA: Resource Conservation and Recovery Act, governs the generation, transport, treatment, storage, and disposal of hazardous waste.
    These acts deal with Department of Energy (DOE), Department of Defense (DOD), state, and private facilities, primarily administered by the Environmental Protection Agency (EPA), with some delegated authority to states.

Non-Aqueous Phase Liquids (NAPLs)

NAPLs are a critical category of groundwater contaminants due to their unique physical properties.

  • Definition: Organic liquids (e.g., gasoline, diesel fuel, dry cleaner solvents) that are not miscible with water. While immiscible, they are sparingly soluble, meaning they can dissolve into groundwater at low, but often toxic, concentrations.

  • Types:

    • Light Non-Aqueous Phase Liquids (LNAPLs): Less dense than water, they float on the water table. Examples include gasoline and light petroleum products. LNAPLs form a floating