8_Groundwater - LECTURE 8 GROUNDWATER “a hidden...

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Unformatted text preview: LECTURE 8 GROUNDWATER “a hidden reserve” drinking water for more 50% of all people  40% of irrigation water irrigation  important for livestock & industry livestock industry -an overused resource resulting in: overused - water shortages - land subsidence - Contamination -Underground “lakes” and “rivers” are rare Most underground water exists in spaces between grains (in “pore spaces”) -geological important erosional agent Groundwater storage & movement Important factors : •Porosity –Measures amount of water that can be held by rocks/sediments –Volume of voids / total volume of material sediment gravel sand silt clay porosity (%) 25-40 30-50 35-50 35-80 10-30 20-30 0-10 0-30 0-20 up to 50 0-5 5-10 0-50 permeability excellent good to excellent moderate poor moderate to excellent good to very good poor to moderate very poor to poor poor to good excellent very poor poor poor to excellent rock conglomerate sandstone well-sorted, no cement poorly sorted, well-cemented shale limestone cavernous limestone crystalline rock unfractured fractured volcanic rocks Origin of Groundwater Glaciers and ice caps 2.14 Groundwater 0.61 Surface water 0.009 Soil moisture 0.005 Atmosphere 0.001 Groundwater is the largest source of readily available freshwater. It has: (1) less bacteria due o the natural filtering effect of rocks (2) widespread (3) constant temperature Groundwater is all water contained in the spaces within bedrock and regolith. Hydrologic Cycle Inflow =Outflow Precipitation =Evapotranspiration + Runoff + GW GW =Precipitation – Evapotranspiration – Runoff From Physical Geology by Charles C. Plummer et al. 9th edition, 2003. –Affected by grainsize, sorting and grain packing •Poorly sorted less porous •Cubic vs rhombohedral packing •Permeability –Ability to transmit fluids –degree of interconnection of voids in the material Porosity and Permeability of different materials Groundwater System -Layer of Soil Moisture Zone of Aeration – open spaces in regolith or bedrock which are mainly filled with air (also called the vadose zone) Capillary Fringe – narrow fringe that is kept wet by capillary attraction that temporarily holds water. Water Table - boundary between the zone of aeration and zone of saturation Zone of Saturation – all openings are filled with water. Zone all Groundwater Transport •Aquifer –Stores and transmits sufficient amount of water •Confining units –Aquitard – stores, but slowly transmits water –Aquiclude – stores, but does not transmit water 1 –Aquifuge – does not store nor transmit water Types of Aquifer •Unconfined aquifer –Bounded at the bottom by a confining unit –Water rises up to the water table •Perched aquifer –Unconfined aquifer defined by a discontinuous confining unit –Local water table (usually above the main/regional water table) •Confined aquifer –Bounded at top and bottom by confining units –Water rises up to the piezometric water level (also called potentiometric line/surface) •Darcy’s Law – describes the rate of •Wells can supply water ­ if they intersect the water table. –Pumping a well at a rate faster than water can flow in the aquifer creates a cone of depression. Groundwater recharge and discharge takes time! •The rate of movement of groundwater depends on many factors including the flowpath. Manifestations of groundwater on the surface: Springs, hot springs, geysers groundwater flow down a slope between two points Q = ­ KA (ΔH/L) Spring ­ Outflow of ground water from water table intersecting Earth’s surface ( also oases) hot springs- Spring w/ water 6-9oC (10-15o F) warmer than mean annual air temperature Geysers - Intermittent hot fountains/columns of water Artesian wells •Wells tapping a confined aquifer. •Analogy: water supply from elevated water tanks •Water in the well rises above the top of the aquifer under artesian pressure, but does not necessarily reach the land surface; a flowing artesian well is a well in which the water level is above the land surface. Q= discharge (volume of water flowing in a given time) K= hydraulic conductivity (a measure of permeability) A= area (through which the water flows) H= hydraulic head L= length of path (between two wells) (­) sign means direction of Q is from high to low hydraulic head Geologic Work of Groundwater ­ Caverns­ Limestone, usually formed just below water table ­ Karst topography ­ Bedrock shaped (dissolved) by groundwater (e.g. Chocolate hills in Bohol, Hundred Islands) - sink holes - Either gradual or abrupt depression in surface due to dissolved limestone bedrock. Groundwater problems and Contamination - Saltwater contamination or intrusion (coastal areas) - Groundwater contamination (due to human causes) Wells ­ Land subsidence •The removal of water allows the aquifer sediments to compact. 2 •Once compacted, the overlying unsaturated zone •Once compacted, the overlying unsaturated zone sediments will also drop in elevation - CAMANAVA “enhanced” floooding Excessive groundwater extraction Compaction of aquifers Land subsidence increased susceptibility to floods 3 ...
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