Groundwater Chapter 10 I. Importance of groundwater A. Groundwater is water found in the pores of soil and sediment, plus narrow fractures in bedrock B. Where is fresh water - 2 % of all water is fresh water. Of this Fresh Water: Glaciers 85% (not really accessible) Groundwater 14% Surface water - < 1 % I. Importance of groundwater C. Importance of groundwater Important source of drinking and irrigation water In CA 80% of used fresh water is for irrigation. In CA almost half comes from groundwater. Easily contaminated Ex. Gas stations, dry cleaners, Navy bases. Contributes water to streams and lakes Over pumping causes subsidence (lowering of land) Erosion makes: caves and sinkholes (collapsed roofs of caves). 1
A. Belt of soil moisture water held by soil particles at the near-surface. (plants) B. Zone of Aeration (Unsaturated Zone) Area under Belt of Soil Moisture but above the water table (PLINKO!!!!!!) Air and water in pore spaces (voids between the grains) Water cannot be pumped by wells C. Zone of Saturation Formation - Water not held as soil moisture percolates downward Water reaches a zone (place) where all open pore spaces are completely filled with water (no air) Water within the pores is called groundwater Water table the top of Zone of Saturation Figure 10.3 fig_11_02a D. Water Table Level of water table changes. Varies season to season Year to year depending on amount of precipitation received. Water table has a slope. Groundwater flows down slope. (just like surface water) This slope usually loosely mimics surface slope. (huge implications for groundwater contamination) E. Groundwater Flows into Surface Water In wetter climates, water table is higher than streams so groundwater flows into the streams and streams get bigger downstream. These are called Gaining Streams. Most streams in CA are Gaining Streams except in dry southeastern deserts. 2
In dry climates, like CA s deserts, water table is lower than stream, and stream gets smaller down stream, eventually drying out. Called Losing Streams Gaining and losing streams Figure 10.5 III. Movement of groundwater A. Groundwater Flow - Darcy s Law Groundwater flows very slowly about 6 feet per day in CA s Central Valley. Hydraulic gradient the water table slope Energy for the movement is provided by the force of gravity Darcy s Law if permeability remains uniform, the velocity of groundwater will increase as the slope of the water table increases III. Movement of groundwater B. Factors Affecting Flow Aquifer rock or sediment that holds groundwater and can provide groundwater if pumped from a well Porosity percentage of aquifer that are pore spaces. High = 50%; Low = 15% Determines how much water can be held there. Permeability the ability of groundwater to flow through aquifer. Aquitard an impermeable layer that hinders or prevents water movement (such as clay) 3
III. Movement of groundwater C. Springs Occur where the water table intersects Earth s surface Natural outflow of groundwater A. Wells Drill hole, place pipe with slots, pour sand A well must penetrate below the water table Pumping of wells can cause Drawdown (lowering) of the water table Cone of depression in the water table localized drawdown near a well Formation of a cone of depression B. Withdrawal and Recharge Recharge water flowing into the aquifer. Withdrawal water pumped out When withdrawal exceeds recharge = drawdown. To0 much drawdown, can: make well go dry Cause subsidence 1) Subsidence Ground sinks when water is pumped from withdrawal exceeds recharge. 4
Subsidence in the San Joaquin Valley of California Figure 10.15 Subsidence in Central Valley C. Groundwater 1. From Gas Stations and Dry Cleaners Leaking underground storage tanks of fuel and chemicals into soil and groundwater 2. From Industry (ex. Chevron Refinery) Leaking chemical tanks 3. From Agriculture fertilizers and pesticides Groundwater contamination V. Hot springs and geysers A. Hot springs The water for most hot springs is heated by cooling of igneous rock 5
Mammoth Hot Springs, Yellowstone National Park V. Hot springs and geysers B. Geysers Hot water erupts with great force Found over hot igneous rocks Figure 10.20 V. Hot springs and geysers C. Geothermal Energy Use the hot water to generate electricity Renewable clean energy resource. But not much of it and hard to find. Ex. The Geysers Power Plant north of the Bay. Link to QUEST video VI. Caves and Karst Topography A. Caves Rain water and groundwater are slightly acidic so they dissolve limestone rock making a hole in rock cave or cavern. Often form in the Zone of Aeration Limestone is precipitated in caves as stalactites (hanging from the ceiling) and stalagmites (form on the floor of a cavern) Geologic work of groundwater B. Karst topography Landscapes on top of limestone dissolved by groundwater features include Sinkhole or sinks - collapsed cave roofs These can makes depressions or small lakes 6