Invisible Pollution
Soil and groundwater contamination is among the most persistent environmental problems — pollutants can take decades or centuries to dissipate underground. Unlike surface water pollution that is visible and fast-moving, subsurface contaminants migrate slowly through porous media, often going undetected until they reach drinking water wells. This simulator models the transport of a dissolved contaminant plume through a saturated soil column, showing how advection, dispersion, and adsorption shape the plume's evolution.
Advection and Dispersion
Dissolved contaminants are carried by groundwater flow (advection) and spread by mechanical dispersion — the variation in flow velocity through different pore paths. The combined advection-dispersion equation describes how concentration changes in space and time. Dispersivity, the key parameter, typically increases with plume scale (scale-dependent dispersivity), reflecting the increasing heterogeneity encountered by a growing plume.
Retardation and Adsorption
Many contaminants interact with soil particles through adsorption, ion exchange, or precipitation. The retardation factor R quantifies how much slower the contaminant front moves compared to the groundwater itself. Organic compounds adsorb to soil organic matter; heavy metals bind to clay minerals and iron oxides. Strongly retarded compounds (R > 10) may take decades to migrate even short distances, but they also persist in the soil matrix long after the source is removed.
Remediation Challenges
Cleaning up contaminated soil is expensive and technically demanding. Pump-and-treat systems extract contaminated groundwater for above-ground treatment, but tailing effects mean concentrations decline slowly. Bioremediation uses naturally occurring or introduced microorganisms to break down organic pollutants. Permeable reactive barriers intercept plumes with reactive materials. PFAS contamination — 'forever chemicals' — presents a particularly difficult challenge, as these compounds resist degradation and adsorb weakly, spreading rapidly through aquifer systems.