Download to read offline
UnderGround water table in Hydrology and irrigation engineering
- 1.
- 3. 3 Global Hydrologic Cycle Domenicoand Schwartz, 1990.
- 4.
- 5. 5 Basin Hydrologic Cycle– GW/SW Interaction
- 6. 6 Global Hydrologic Cycle– Schematic Domenico and Schwartz, 1990.
- 7. 7 Basin Hydrologic Cycle– Schematic Domenico and Schwartz, 1990.
- 8. 8 Basin Hydrologic Cycle– Quantitative Representation I Basin Aquifer Surface/ Soil Domenico and Schwartz, 1990.
- 9.
- 12.
- 13.
- 19. How Does WaterMove Between the Surface and Subsurface? Water table generally below surface, so water can seep in Water can soak into subsurface and become groundwater Where water table intersects surface, water can flow out 17.05.a1
- 20. How Are Lakesand Wetlands Related to Groundwater? Observe the settings of lakes compared to the water table Water table intersects surface Perched above water table Wetlands at water table or perched On topographic bench Valley bottom 17.05.c1
- 21. How Do StreamsInteract with Water Table? Stream higher than water table: loses water Stream lower than water table: gains water Mound of groundwater below stream from water flowing into subsurface 17.05.d
- 22. How Do CavesForm? Groundwater dissolves material Most in limestone (soluble) Features widen to cavities and caves Above water table, cave may be dry Below water table water further dissolves material 17.06.a1
- 23. What Features AccompanyCaves? Dripping water evaporates, precipitates calcite Roof collapse can form sinkhole on surface Cave formations on roof, floor, and walls Surface with sinkholes, limestone pillars, disappearing streams = karst topography 17.06.c1
- 24. How Do WeStudy Groundwater? Geophysical surveys: depth to bedrock and water Field studies: sequence and character of rocks Drilling: verify geology, depth to water table, provide samples, pump tests Measure discharge, losses/gains, and water quality 17.07.a1
- 25. How Do WeDepict the Water Table? Contour: water table at same elevation; blue arrows show flow Numbers show elevations of the water table: what is the pattern? Compare water table to other features 17.07.b
- 26. Effects of OverpumpingGroundwater After pumping: cone of depression Before overpumping 17.08.a
- 27. Other Problems ofOverpumping Along coast: freshwater floats on saltwater, so draw in saltwater Water table drops, aquifer compacts, land subsidence Earth fissures 17.08.b,c
- 28. How Can Water BecomeContaminated? Identify possible sources of surface water and groundwater contamination Landfill Mining and naturally mineralized rock Septic tank Farm Leaking tank in gas station Truck fuel spill Dry cleaner Brought by river Factory wells, spills, emissions Fuel storage tanks Brought from outside area by groundwater Households 17.09.a1
- 29. How Does Groundwater ContaminationMove? Can be filtered if flows slowly, like through sands With groundwater down water table Not filtered if flows rapidly, like in limestone caves 17.10.a
- 30. How Do WeTrack and Remediate Groundwater Contamination? Contour water table and contamination Drill wells to intercept plume, pump and treat water Spreads out due to diffusion and mixing, forming contamination plume 17.10.c
- 32. 32 Aquifers Definition: A geologicalunit which can store and supply significant quantities of water. Principal aquifers by rock type: Unconsolidated Sandstone Sandstone and Carbonate Semiconsolidated Carbonate-rock Volcanic Other rocks
- 33. 33 Occurrence of GroundWater • Ground water occurs when water recharges the subsurface through cracks and pores in soil and rock • Shallow water level is called the water table
- 34. 34 Recharge Natural • Precipitation • Meltingsnow • Infiltration by streams and lakes Artificial • Recharge wells • Water spread over land in pits, furrows, ditches • Small dams in stream channels to detain and deflect water
- 35. 35 Example Layered AquiferSystem Bedient et al., 1999.
- 36.
- 37. 37 • Leaky confinedaquifer: represents a stratum that allows water to flow from above through a leaky confining zone into the underlying aquifer • Perched aquifer: occurs when an unconfined water zone sits on top of a clay lens, separated from the main aquifer below Leaky and Perched Aquifers
Editor's Notes
- #19 EXPLANATION Precipitation, snowmelt, and surface water can soak into subsurface and become groundwater Water table is generally below surface, providing unsaturated zone into which water can seep Where water table intersects surface, water can flow out into spring, lake, or river
- #20 INSTRUCTIONS TO STUDENTS Observe how the water table relates to these lakes and wetlands (After explanation) How do you think you could determine which of these settings a lake would be in? Possible answers determine elevation of water table by drilling see whether a lake was losing or gaining water by modeling inflow, outflow, and evaporation compare the chemistry of the lake water with runoff and with groundwater EXPLANATION Perched above water table with outflow to subsurface Where water table intersects the surface (normal setting for lakes) Lake on bench (flat area) in topography Lake on bottom of valley, where water table is flat Wetlands can be at water table or perched above an impermeable zone
- #21 INSTRUCTIONS TO STUDENTS Observe how each river relates to the water table and whether water flows from the river to groundwater or from groundwater to the river How do you think you could determine which situation was occurring? Possible answers determine elevation of water table by drilling see whether a lake was losing or gaining water by modeling inflow, outflow, and evaporation compare the chemistry of the lake water with runoff and with groundwater EXPLANATION River lower than water table can gain water from inflow of groundwater River higher than water table can lose water to groundwater Mound of groundwater can form below river due to outflow of river water into groundwater
- #22 INSTRUCTIONS TO STUDENTS (after explanation) How might we determine whether a cave is present at depth but not expressed at the surface? Possible answers see if the rock limestone at the surface drilling geophysical surveys, such as ground-penetrating radar, if shallow EXPLANATION Most caves form in soluble rocks, such as limestone Groundwater in fractures and bedding planes dissolves material Dissolution widens bedding planes and fractures until they become cavities and caves Part of cave above water table may be mostly dry Cave below water table will contain water that further dissolves material, enlarging cave A sinkhole develops over the collapsed part of the cave on the right side A cave may not have any expression at the surface, as on the left side
- #23 MEDIA 1706a2_Plateau_Sinkholes.kmz 1706a2_Florida_Sinkhole_Lakes.kmz 1706b1_Karst_WV.kmz EXPLANATION Roof of a cave can collapse, forming sinkhole on surface (especially a cave above water table) Water dripping off roof evaporates, precipitating calcite and other minerals Minerals build up cave formations from roof, floor, and walls Surface with sinkholes, limestone pillars, disappearing streams = karst topography If the cave is filled with water, it is less likely to collapse because the water is helping hold up the roof If the cave is filled with water, typical cave formations are not forming because there are no drips or evaporation Some caves may have been partially filled with water or were once dry, and have since been refilled when the water table rose Water in caves can flow or be stationary
- #24 EXPLANATION Explain a pumping test Field studies Types of rock and sediment Estimate of overall permeability Abundance and orientation of fractures Orientation of layers Presence or absence of cavities Characteristics of the material, such as clast size and degree of sorting Geophysical surveys Gravity surveys to determine depth to bedrock Seismic surveys to determine geometry of layers, units, and faults Electrical surveys to determine location of water table and geometry of units Drilling Describe the sequence of materials versus depth Characteristics of the materials (clast size) Abundance and dip of fractures Lowering special video cameras down the drill hole Geophysical surveys done within a drill hole, such as how much radioactivity each layer gives off
- #25 MEDIA 1707b3_groundwater_map.mov Relates the sloping groundwater surface in the aquifer to the land surface and a groundwater contour map INSTRUCTIONS TO STUDENTS Examine the map on the upper left, which shows elevations of the water table What is the pattern of elevations, and which way do you think groundwater would flow? EXPLANATION Contours mark where water table is at same elevation; blue arrows show flow We visualize system to compare water table to other features, such as farms with wells that pump high volumes (this is a different area) EXERCISE Have students contour a simple map showing only the elevation of points, and then draw arrows perpendicular to contours and in a downslope direction
- #26 EXPLANATION Before over-pumping, water table is fairly flat and water flows to left, past small well Larger well causes cone of depression in water table, changing direction of flow A cone of depression forms because water cannot flow fast enough to replenish the amount of water that is withdrawn from the well In many cones, the water table becomes steeper near the well because water is withdrawn too fast
- #27 MEDIA 1708b4_Picacho_Peak_Fissures.kmz Google Earth file of Picacho Peak area, Arizona, shows earth fissures crossing desert, canal, and I-10 Central Arizona Canal (up against the mountain) was built close to bedrock to try to avoid the fissures EXPLANATION As water table drops, the aquifer compacts, causing subsidence of land surface Fissures form along boundaries between more and less subsidence Along coast, freshwater floats on saltwater, so overpumping can draw saltwater into bottom of coastal wells Areas can drop meters to tens of meters in elevation Detect subsidence with repeated surveying with conventional surveying equipment, GPS, and other satellite measurements Once compaction occurs, the pore space cannot be regained if the water table is allowed to rise (lost permeability, porosity, and storage capacity of the reservoir) Fissures related to subsidence can be tens of meters wide, kilometers long, and tens to over a hundred meters deep Fissures provide easy access for contaminant into the groundwater without being filtered
- #28 INSTRUCTIONS TO STUDENTS Observe this scene and identify all possible sources of water contamination
- #29 EXPLANATION Contamination largely moves with groundwater down the slope of the water table Can be naturally filtered out with enough time: flows slowly and in contact with material like sand Not filtered if flows rapidly through a rock, such as a limestone with open cavities Importance for considering the direction of groundwater flow when sighting the relative positions of a water well and contamination sources, such as a septic tank In general, put the well uphill of a septic tank, but be aware of what is farther up the hill
- #30 EXPLANATION Contamination spreads out due to diffusion and mixing, forming a contamination plume Contour water table and concentration of contaminant to track plume Drill wells to intercept plume, pump out and treat water Dispose of contaminated water or treat it with activated charcoal or certain geologic materials