13.2 Groundwater quality and contamination

Groundwater Contamination

Groundwater contamination happens when harmful substances enter aquifers, making the water unsafe to drink or damaging ecosystems that depend on it. Since roughly half the U.S. population relies on groundwater for drinking water, understanding where contamination comes from, how it spreads, and how we clean it up is a core part of hydrogeology.

Sources of Groundwater Contamination

Contamination sources fall into two broad categories: natural and anthropogenic (human-caused).

Natural sources introduce dissolved minerals from rocks and soils into groundwater as it flows through the subsurface:

• Arsenic leaches from certain rock types (especially volcanic and metamorphic rocks) and can reach dangerous concentrations in aquifers.
• Fluoride dissolves from fluoride-bearing minerals like fluorite and apatite. Low levels strengthen teeth, but high concentrations cause skeletal fluorosis.
• Radionuclides such as radon and uranium are released from granitic and phosphatic rocks and accumulate in groundwater over time.

Saltwater intrusion is a special natural-anthropogenic hybrid. In coastal aquifers, excessive pumping lowers the freshwater table enough that denser seawater migrates inland into the aquifer, contaminating wells with salt.

Anthropogenic sources cover a wide range of human activities:

• Agricultural activities are among the most widespread sources. Pesticides leach through soil into aquifers. Excess fertilizers introduce nitrates and phosphates. Animal waste from livestock operations adds pathogens and nutrients.
• Industrial activities include chemical spills and leaks from storage tanks or pipelines, improper disposal of hazardous waste (toxic chemicals, heavy metals), and mining operations that expose groundwater to acid mine drainage.
• Urban development creates multiple contamination pathways. Poorly maintained septic systems leak bacteria, viruses, and nutrients. Underground storage tanks at gas stations can release petroleum products. Landfills generate leachate, a contaminated liquid that forms when water percolates through waste and picks up dissolved pollutants. Stormwater runoff from roads and parking lots carries oil, grease, and heavy metals into the subsurface.

Contaminant Transport in Groundwater

Once a contaminant enters an aquifer, three processes control how it spreads. Understanding all three explains why a contamination plume is always larger and more diffuse than you might expect from the source alone.

Advection is the simplest: contaminants travel along with the bulk flow of groundwater, moving in the same direction and at the same average velocity as the water itself. The speed of advective transport depends on the hydraulic gradient (the slope of the water table) and the hydraulic conductivity of the aquifer material.

Dispersion spreads contaminants beyond what advection alone would predict. It happens because water doesn't all move at one uniform speed through pore spaces. Some water takes shorter, faster paths; some squeezes through narrower, slower ones. This variation in flow paths is called mechanical dispersion. When you combine mechanical dispersion with molecular diffusion, the result is hydrodynamic dispersion, which produces a broader, more spread-out contaminant plume.

Diffusion moves contaminants from areas of high concentration to areas of low concentration, driven by concentration gradients. This occurs through the random motion of molecules (Brownian motion) and happens even when groundwater isn't flowing at all. Diffusion is much slower than advection or dispersion, but over long time periods it can move contaminants into low-permeability zones like clay layers, where they become very difficult to remove.

Impacts of Groundwater Contaminants

Different contaminants cause different problems. Here are the major categories you should know:

• Nitrates cause methemoglobinemia (blue baby syndrome) in infants by reducing the blood's ability to carry oxygen. When nitrate-rich groundwater discharges into lakes or streams, it also triggers eutrophication, leading to algal blooms and oxygen-depleted water.
• Pesticides can disrupt endocrine (hormone) systems in humans and wildlife. Some are carcinogenic, and many bioaccumulate in the food chain, reaching higher concentrations in top predators.
• Heavy metals like lead and mercury are neurotoxic, meaning they damage the nervous system. Children are especially vulnerable to developmental disorders from lead exposure. Chronic exposure also damages the kidneys and liver.
• Volatile organic compounds (VOCs) are chemicals that evaporate easily, such as industrial solvents and gasoline components. Many are carcinogenic and can cause respiratory and neurological problems through ingestion or inhalation.
• Pathogens such as E. coli, Salmonella, and hepatitis A virus cause waterborne diseases ranging from gastrointestinal illness to liver inflammation.

Management of Groundwater Contamination

Managing contamination involves three strategies: monitoring, prevention, and remediation.

Monitoring tracks groundwater quality over time and space:

• Monitoring wells are drilled to collect water samples at specific depths. Piezometers measure groundwater pressure and help determine flow directions, which tells you where contaminants are heading.
• Geophysical techniques characterize subsurface conditions without drilling. Electrical resistivity surveys map variations in subsurface conductivity that can indicate contamination. Ground-penetrating radar identifies changes in subsurface materials and can help outline plume boundaries.

Prevention focuses on stopping contamination before it starts:

• Source control includes proper hazardous waste disposal, spill prevention and secondary containment systems, and improved agricultural practices like precision farming and integrated pest management.
• Wellhead protection areas restrict potentially polluting activities within a defined zone around public water supply wells.
• Recharge area protection uses land-use restrictions to preserve areas where water infiltrates into aquifers, keeping contaminants from entering at these vulnerable points.

Remediation cleans up contamination that has already occurred. The main techniques are:

1. Pump-and-treat extracts contaminated groundwater through wells and treats it at the surface using methods like air stripping or activated carbon adsorption. Treated water can then be discharged or reinjected into the aquifer. This is the most common approach, though it can take years to decades.
2. In-situ bioremediation stimulates naturally occurring microorganisms to break down contaminants underground. Nutrients and oxygen are injected into the aquifer to boost microbial activity. This works well for petroleum hydrocarbons and some organic compounds.
3. Permeable reactive barriers are subsurface walls filled with reactive materials (like zero-valent iron or activated carbon) installed across the path of a contaminant plume. As groundwater flows through the barrier, contaminants are either trapped or chemically transformed into less harmful substances.
4. Phytoremediation uses plants, often deep-rooted species like poplars and willows, to absorb, accumulate, or degrade contaminants from shallow soil and groundwater. It's a slower, lower-cost option best suited for sites with shallow, moderate contamination.