01 Landfills 1
What happens beneath an old landfill when rain percolates through the waste?
What you’re watching A cross-section of a landfill leaching a plume (red high concentration fading to blue) that sinks below the water table and migrates down-gradient through the aquifer toward a pond.
The mechanism Infiltration through buried waste dissolves contaminants into leachate, which enters the aquifer as a continuous source. Advection carries it along the flow lines while dispersion spreads it — a plume that grows as long as the source persists.
Why it matters Legacy landfills are among the most common contamination sources; a single leaking cell can threaten down-gradient wells and surface water for decades.
IGW-NET Sketch the landfill and the aquifer and IGW-NET releases the plume and carries it down-gradient as it solves — the invisible leachate made visible from source to receptor.
02 Landfills 2
How does the geology beneath a landfill steer its plume?
What you’re watching A second landfill case where the plume’s path bends with the aquifer’s flow field and layering rather than going straight down-gradient.
The mechanism Leachate follows the local flow direction and the high-conductivity layers. A confining layer can perch or deflect it; a permeable lens can channel it far and fast.
Why it matters Predicting which wells a landfill threatens requires the site’s actual hydrogeology — intuition about ‘downhill’ is often wrong underground.
IGW-NET Changing the layering and watching the plume re-route shows why site-specific modeling beats rules of thumb.
03 Landfills 3
What controls how far a landfill plume eventually reaches?
What you’re watching A third landfill scenario showing the long-term extent of the plume as it approaches a discharge boundary.
The mechanism Plume length is set by advection (push) against attenuation — dispersion, sorption (retardation), and decay. Strongly sorbing or decaying contaminants stay short; conservative ones travel to the discharge zone.
Why it matters Whether a plume stabilizes short of a well or reaches surface water determines the entire risk and remediation strategy.
IGW-NET Toggling sorption and decay and watching the plume lengthen or shrink connects the abstract transport terms to a real landfill’s footprint.
04 Solid Waste Dump
How is an open dump different from an engineered landfill?
What you’re watching A solid-waste dump leaching contaminants directly into the subsurface, with no liner to slow the source.
The mechanism Unengineered dumps have no liner or leachate collection, so infiltration drives a strong, continuous leachate source straight into the water table.
Why it matters Thousands of legacy dumps predate modern regulation and remain a major remediation burden worldwide.
IGW-NET Comparing a lined and an unlined source in real time shows how much containment actually buys.
05 Contamination from Waste Disposal Site
What does a hazardous-waste disposal site do to the aquifer below it?
What you’re watching A waste-disposal site releasing a plume that spreads through the aquifer toward down-gradient receptors.
The mechanism Disposal sites concentrate many contaminants; their plumes are chemically complex, with different components sorbing, decaying, or traveling at different rates — separating into a multi-component plume.
Why it matters Superfund-scale sites (the Woburn case among them) are defined by exactly this problem — characterizing what is in the plume and where it is going.
IGW-NET Releasing several species at once and watching them separate by sorption and decay turns a ‘plume’ into the distinct chemicals it really contains.
06 Gas Leaks
Where does fuel go when an underground tank leaks?
What you’re watching A fuel leak releasing a light hydrocarbon plume that spreads along the top of the water table rather than sinking.
The mechanism Gasoline and fuels are lighter than water (LNAPLs). They migrate down to the water table and then spread laterally along it as a floating layer, dissolving a soluble plume down-gradient.
Why it matters Leaking underground storage tanks are one of the most widespread contamination sources; the floating product is both the ongoing source and the cleanup target.
IGW-NET Seeing the light plume ride the water table makes LNAPL behavior — and why it differs from a sinking solvent — immediately clear.
07 Leaky Septic System
Can a single household septic system really threaten groundwater?
What you’re watching A septic system leaching nutrients and pathogens into the shallow aquifer, forming a plume beneath the drain field.
The mechanism Septic effluent adds nitrate, bacteria, and dissolved organics to the water table. Nitrate is mobile and persistent; in dense developments, many small sources sum to a regional plume.
Why it matters Septic-derived nitrate is a leading cause of rural well contamination and of nutrient loading to lakes and streams.
IGW-NET Adding multiple septic sources and watching their plumes merge shows how distributed loading becomes a basin-scale issue.
08 Industrial Spills
What happens when an industrial solvent reaches the water table?
What you’re watching An industrial spill forming a plume that migrates down-gradient — the classic source behind contaminated community-well cases.
The mechanism Industrial solvents such as chlorinated compounds are often dense, persistent, and toxic at tiny concentrations. They form long plumes that can reach water-supply wells far away.
Why it matters This is the Woburn / ‘A Civil Action’ scenario the foundational courses are built around: an industrial solvent reaching a community wellfield, and the legal and public-health fight that follows.
IGW-NET Building the spill, the aquifer, and the down-gradient wells, then watching the plume advance toward them, is exactly the live investigation the IGW classroom uses to teach site characterization.
09 Poorly Maintained Animal Lots
How do feedlots contaminate the water beneath them?
What you’re watching An animal lot leaching nutrients and pathogens into the shallow groundwater below.
The mechanism Concentrated animal waste loads the soil with nitrate, ammonia, and pathogens; infiltration carries them to the water table as a persistent nutrient plume.
Why it matters Feedlot nitrate contaminates rural wells and drives nutrient pollution of connected streams and lakes.
IGW-NET Watching the nutrient plume spread from a lot connects land-use decisions to the water beneath.
10 Contamination of Community Wells
How does contamination from across town end up in the drinking-water wells?
What you’re watching A town with multiple sources — herbicides, wash water, oil and gas spillage, calcium chloride — sends a plume down-gradient, and the town’s water wells pull it toward them.
The mechanism Up-gradient sources release contaminants that advect toward the wellfield, and the wells’ own capture zones actively draw the plume in — the area of contribution and the contamination source overlap.
Why it matters This is the core public-health question — is our drinking water threatened? — and the reason wellhead protection maps source areas in the first place.
IGW-NET Overlaying the plume and the wells’ capture zones in real time shows, before it happens, which sources will reach the supply.
11 Improper Spill
What does a careless surface spill do once it soaks in?
What you’re watching An improperly handled spill infiltrating to the water table and forming a down-gradient plume.
The mechanism Surface spills percolate through the unsaturated zone to the water table, then advect and disperse — a delayed but persistent source if not cleaned up promptly.
Why it matters Prompt surface cleanup is far cheaper than chasing a plume once it reaches the aquifer.
IGW-NET Watching the lag between spill and plume arrival underscores why early response matters.
12 Chemical Spill
How fast can a chemical spill reach a well?
What you’re watching A chemical spill forming a plume that advances down-gradient at the groundwater velocity, modulated by sorption.
The mechanism Once dissolved, the chemical travels at the water velocity divided by its retardation factor. Weakly sorbing chemicals move nearly as fast as the water; strongly sorbing ones lag far behind.
Why it matters Emergency response hinges on arrival time at the nearest receptor — which depends on the chemical as much as the aquifer.
IGW-NET Running spills of different sorption strength side by side shows retardation deciding who is at risk, and when.
13 Contaminants with Different Densities
Why do some contaminants float while others sink to the bottom of the aquifer?
What you’re watching Two releases at once: a ‘floater’ (lighter than water) spreading along the top of the water table from a leaking tank, and a ‘sinker’ (denser than water) plunging from a leaky lagoon to pool at the aquifer base.
The mechanism Light non-aqueous liquids (LNAPLs — fuels) float and spread on the water table; dense ones (DNAPLs — chlorinated solvents) sink through the aquifer under their own weight and pool on low-permeability layers far below.
Why it matters DNAPLs are the nightmare of remediation: they sink out of reach, pool in unexpected places, and slowly redissolve for decades — you cannot clean what you cannot find.
IGW-NET Releasing a floater and a sinker together and watching them separate by depth makes density-driven behavior — invisible and counterintuitive — obvious at a glance.
14 Accidental Contamination1
What does an unplanned release look like as it enters the aquifer?
What you’re watching An accidental release forming a plume from a point source and beginning its down-gradient migration.
The mechanism A finite (one-time) release forms a slug that detaches from the source and travels as a discrete plume, spreading and attenuating as it goes — unlike a continuous source that keeps feeding.
Why it matters Distinguishing a finite spill from an ongoing leak changes the monitoring strategy and the cleanup timeline.
IGW-NET Watching a slug detach and travel, versus a continuous plume grow, distinguishes the two source types instantly.
15 Accidental Contamination2
How does a slug of contamination change as it travels?
What you’re watching A second accidental-release case following the slug downstream as it spreads and its peak concentration drops.
The mechanism As the slug advects, dispersion stretches it and lowers its peak, sorption delays it, and decay erodes its mass. By the time it reaches a receptor it is longer, weaker, and later than at release.
Why it matters Predicting the peak concentration and arrival time at a downstream well is the whole question for a one-time spill.
IGW-NET Tracking the slug’s shrinking peak in real time turns breakthrough-curve theory into something you watch happen.
16 Salt Contamination
Where does the salt in groundwater come from — and why is it so stubborn?
What you’re watching A salt plume — from road salt, brine, or seawater — spreading through the aquifer as a conservative, non-decaying tracer.
The mechanism Chloride salts don’t sorb or decay — they travel at the full groundwater velocity and persist indefinitely. Sources include road de-icing, oilfield brine, and seawater intrusion in coastal aquifers.
Why it matters Once an aquifer is salinized it is effectively ruined for supply; coastal saltwater intrusion driven by over-pumping is a global threat.
IGW-NET Because salt neither sorbs nor decays, its plume is the clearest demonstration of pure advection–dispersion — and of how permanent contamination can be.
17 Fertilizers and Pesticides
How is farm contamination different from a single spill?
What you’re watching Fertilizers and pesticides applied across a field leaching a broad, diffuse plume into the aquifer rather than a single point plume.
The mechanism Agricultural chemicals are applied over large areas, loading the aquifer as nonpoint-source contamination — a wide, distributed plume of nitrate and pesticide residues rather than a localized one.
Why it matters Nonpoint nitrate and pesticides are the hardest contamination to manage precisely because there is no single source to remediate — only land-use change helps.
IGW-NET Applying a contaminant across the whole surface and watching a broad plume form distinguishes nonpoint from point pollution at a glance.
18 Contamination from Different Sources
What does a real aquifer with many contamination sources look like?
What you’re watching Multiple sources — point and nonpoint — releasing simultaneously, their plumes overlapping into a complex contamination picture.
The mechanism Real basins host many coexisting sources whose plumes overlap and interact. Attributing contamination at a well to a particular source requires untangling the combined plume — the central challenge of forensic hydrogeology.
Why it matters Liability, cleanup responsibility, and protection strategy all hinge on figuring out which source contributed what — exactly the kind of dispute the Woburn case turned on.
IGW-NET Running several sources at once and watching the plumes combine shows why source attribution is so difficult — and why a model is needed to do it.