ES 106 2006 April 21 Groundwater Exploitation and Pollution I. Wells A. Draw water from aquifer B. Zone of saturation may be depressed due to withdrawal 1. drawdown of water table a. cone of depression b. may refill seasonally with recharge c. may be permanent if aquifer compresses C. artesian flow 1. hydraulic pressure is above water table a. flowing artesian—above well head b. non-flowing artesian—rises upon intersection of water table II. Groundwater concerns A. Withdrawal exceeds recharge in many areas 1. ‘mining’ a non-renewable resource 2. water tables in some areas drop 1 meter per year 3. may not ever recharge, if aquifer has compressed a. overlying land packs grains after water leaves void space b. land subsidence several meters in some areas B. Contamination 1. organic leachate a. sources 1) septic tanks 2) farm wastes 3) sewage system failure b. removal 1) natural filtering by sand and gravel 2) oxidation by chemicals and assimilation by organisms 3) if aquifer has proper characteristics: mostly optimal porosity 2. other pollution sources: road salt, fertilizer, pesticide, leaking underground tanks/pipelines, landfills, impoundment ponds a. Denver: pesticide production b. Minneapolis: wood preservatives c. Potato production: aldecarb antifungal compound 3. Nitrates: from fertilizer—blue baby syndrome: impaired hemoglobin 4. VOC: solvents spilled, discarded, buried storage leaking—persistent C. Cleanup of groundwater costly, difficult, mixed rate of effectiveness III. Drinking water standards A. Established by EPA 1. milligrams per liter is parts per million 2. parts per billion is milligrams per 1000 liters B. dilution is a poor solution to pollutionIV. Sewage treatment methods A. primary treatment: settling ponds removes solids B. secondary treatment: aerobic bacteria metabolize organic matter 1. sand and gravel filter bed 2. aeration encourages aerobic bacteria activity C. advanced treatment: filtration 1. activated carbon/charcoal: removes solvents, pesticides, metals 2. reverse osmosis: removes most impurities D. chlorine kills bacteria, not viruses V. Geologic work of groundwater A. Dissolves rock: limestone more common than salt, gypsum B. Caverns 1. groundwater dissolved limestone along cracks, bedding planes 2. calcium ions, bicarbonate ions, carbonate ions flushed away 3. depositional features in caverns from these ions re-precipitating a. dripstone features 1) stalactites—on ceiling 2) stalagmites—on ground 3) columns—joined stalactites and stalagmites 4) soda straws, angel wings, cave pearls b. formation due to loss of dissolved CO2 changing pH of water C. Karst terrain: usually limestone land area with solution collapse of underground openings, can be on salts also 1. sinkholes—collapsed caverns a. Florida, Kentucky, Indiana, New Mexico b. Gradual dissolution, sudden collapse c. Enhanced by removal of groundwater 1) Exploitation for municipal/industrial/agricultural purposes 2) Mitigation of swampland for development 2. typical features of karst terrain a. sinkholes b. disappearing streams, lack of through-flowing streams c. steep-sided solution valleys d. haystack buttes: common in southeastern
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