Chap 17 geo 3

Groundwater Contamination

Groundwater Quality • Potable water - water that is agreeable to taste and is not dangerous to health– usually about 150 mg/L dissolved solids • fresh water • The U.S. Environmental Protection Agency has set maximum allowable concentrations of toxic compounds allowed in drinking water.

Contamination • sewage - bacteria, nitrate– source: septic tanks, broken sewer lines– treatment: natural - sediment filters bacteria • only if low enough permeability • pumping well can reverse groundwater flow– can “capture” septic tank water • Other contaminants– highway salt, fertilizers, pesticides– landfills, chemical holding bonds– waste disposal into injection wells

Sewage Groundwater flow too fast so water is not purified Groundwater flow is slower and purified a it flows

Groundwater Contamination

Contamination • Rainwater leaches toxic compounds and transports it to the groundwater– slow process– generally found only after people become ill • Cleanup of contaminated aquifer– pump-and-treat - ineffective– organic compounds • natural or enhanced biodegradation • microbes “eat” hazardous organic compounds Cone of depression changed direction of groundwater flow

Groundwater erosion of limestone creates aquifers

Groundwater erosion of limestone • limestone composed of calcite (CaCO3)– atmospheric air adds CO2 to rain making it acidic– infiltrating acidic rain dissolves calcite down to the water table • caves form above the water table– the water table goes up and down so caves form at many levels • The network of caves form underground “rivers”– aquifers with the highest groundwater velocities

Limestone cave deposits speleothems- drip stone features in caves– stalactites– soda straws– stalagmite

Limestone cave collapse in Karst topography • the roof of limestone caves can eventually collapse forming a large depression called a sinkhole sink or • infiltrating rainwater over time enlarges the hole • an area covered with many sinkholes is called karst topography– Karst areas lack surface streams because the streams flow into these sinks • “sinking streams”

Guatemala City sink holes are the result of a sewage pipe break, not of limestone caves • Guatemala City sits on volcanic ash. • Water rushing out of a broken underground sewage pipe washed away volcanic ash creating a large underground hole in the ash • A large storm increased flow in the sewage pipes, the cavern was enlarged and the roof collapsed. • Not a true “sinkhole” which must be collapse into a hole formed by dissolution of carbonate rock (limestone) at depth. Called soil piping. • A similar sized sinkhole formed once before in 2007