Groundwater. Importance of Groundwater. The Water Table. Geol 104: Groundwater

Transcription

1 Groundwater Subsurface water contained in soil and bedrock. There is ~ 60 times as much water underground than in freshwater streams and lakes. Source of groundwater is rain and snow. Represents the infiltration of precipitation. In general, ~15% of precipitation ends up in groundwater (but varies from 1 to 20%). Importance of Groundwater Over 50% of US population rely on groundwater for drinking. Tens of millions of acres of farmland is irrigated using groundwater. Problems due to excessive withdrawl and contamination of groundwater systems clearly demonstrates the need for better management of this valuable resource. The Water Table Vadose Zone - subsurface zone above the water table that may be moist but is not saturated. Capillary fringe - transition zone with higher moisture content at base of vadose zone. Water Table - top of the saturated zone. Saturated Zone - subsurface zone in which soil and bedrock are saturated with water. See fig

2 The Water Table Springs - occur where the water table intersects the surface and water flows onto the land. Perched Water Table - top of a localized lens of groundwater that lies above the main water table, formed by a layer of impermeable rock. See fig Storage & Movement Porosity - proportion of the volume of a material that consists of open (void) space. Porosity is greatest in sediments and rocks composed of well-sorted, loosely packed, spherical grains. Permeability - measure of the ability of a fluid to travel through a porous material; related to interconnectivity of porosity. See Table 11.1 Aquifer Aquifer - body of rock or sediments that can yield economically significant quantities of water. Must be able to store (high porosity) and transmit (high permeability) water. Good aquifers: sand/gravel, sandstone, fractured carbonates (limestone). Poor aquifers: clay, shale, unfractured crystalline rock. Aquiclude - impermeable rock or sediment (often termed aquitard). 2

3 Types of Aquifers Unconfined Aquifer Aquifer in which there is no impermeable layer restricting the upper surface of the saturated zone. Confined Aquifer An impermeable layer restricts the upper surface of the saturated zone. See fig Groundwater Flow Groundwater flows from areas of high water pressure to low water pressure. The pressure is proportional to the weight of water above any given point. Recharge - replenishment of an aquifer by the addition of water. Discharge - depletion of an aquifer by removal (pumping) of water. Groundwater Flow Darcy s law - relationship to quantify the rate of groundwater movement; states that the discharge is equal to the product of the cross-sectional area times the permeability times the gradient of the aquifer. 3

4 Darcy s Law Important relations: Hydraulic head = elevation + pressure Hydraulic gradient = difference in head / distance Velocity = permeability x hydraulic gradient Fluid potential = gravity x head See Box 11.1 figs. 1 & 2 Wells Well - a hole drilled through the unsaturated zone and into the saturated zone. The water level in a well is the level of the local water table. Artesian Well - well drilled into an Artesian aquifer in which water rises to the surface without pumping due to water pressure in the confined aquifer. Effects of Pumping a Well Cone of Depression - cone-shaped depression in the water table due to the pumping of water out of the well. Drawdown - difference between initial and present water level in a well. See fig

5 Groundwater & Streams Effluent (gaining) streams - streams that are recharged from groundwater. Influent (losing) streams - streams that are recharging from the groundwater. See fig Groundwater Problems May be due to: Water table depression Excessive pumping Groundwater contamination Water Table Depression Occurs when: Pumping rate exceeds recharge rate. Recharge rate is reduced due to urbanization (decreased infiltration). Pumped water is discharged into surface water system not returned to ground near its source. Pumped water used for irrigation is lost to evaporation. 5

6 Effects of Excessive Pumping Excessive groundwater mining - water withdrawn much faster than aquifer recharge. Subsidence - sinking or settling of earth s surface in response to excess removal of groundwater. Saltwater intrusion - displacement in the saturated zone of freshwater by saltwater. See fig Effects of Excessive Groundwater Mining Major effect is loss of groundwater resource. Example: Ogallala Aquifer Originally held ~2 billion acre-feet of water (1 acreft = 325,821 gallons). Aquifer filled during the last ice age as glaciers melted in the Rocky Mountains. >14 million acres of land are irrigated from the Ogallala aquifer. Half of the water has been removed and it would take thousands of years to recharge the aquifer if pumping ceased. Current pumping rates cannot be sustain and many areas will run out of water in the next decade. Effects of Subsidence Subsidence can cause: Cracks in foundations as ground settles. Ruptures of underground pipes. Cracks in roadways. Increased flood potential Examples of Subsidence: Venice, Italy San Joaquin Valley, CA (>9 m of subsidence) 6

7 Saltwater Intrusion Fresh water is less dense than saltwater and floats on saltwater in an aquifer. Too much pumping leads to saltwater intake and water becomes too salty for drinking. Examples: Coastal areas: Long Island & Cape Cod. Inland areas: S. Wisconsin (due to deeper saline groundwater). Groundwater Contamination Aquifer becomes contaminated with toxic chemicals or biological agents. Contaminants move within the groundwater flow as a contaminant plume. See fig Note, not all pollutants move within the saturated zone ( floaters vs sinkers ). See fig Groundwater Pollution Because of its widespread use for drinking water, pollution of groundwater is an important concern. Contamination of groundwater is often unnoticed ( out of sight - out of mind ) and is difficult to assess due to complexity of subsurface geology. 7

8 Pollutants Groundwater pollutants include: Oxygen demanding waste Decomposing organic matter (sewage, agricultural waste). Pathogenic organisms Disease causing micro-organisms (fecal coliform bacteria). Inorganic plant nutrients Nitrates and phosphates (fertilizer, sewage, detergent). Organic substances Petroleum products, organic solvents (PCB, TCE, etc), herbicides and pesticides (DDT, atrazine, etc). Inorganic substances Heavy metals, radioactive waste, acid mine waste. Sources of Groundwater Pollution Leakage from: Underground storage tanks Landfills Septic systems Seepage from the surface from: Feedlots/Megafarms Irrigation Mine tailings/acid mine waste Sources of Groundwater Pollution Infiltration of urban, industrial, and agricultural runoff. Saltwater intrusion Improper operation of hazardous waste injection wells. Thermal pollution Natural sources 8

9 Effects of Groundwater Action Dissolution by groundwater in soluble rocks (especially limestones) can produce: Caverns (caves) Sinkholes Karst topography Disappearing streams Caverns Naturally formed underground chambers. Most develop when slightly acidic groundwater infiltrates and dissolves limestone along joints and bedding planes. A drop in the water table allows caverns to greatly enlarge. See fig Sinkholes Closed depressions found on land surface underlain by limestone or other readily soluble rocks. Form when the roof of a cavern collapses. See fig

10 Karst Topography Form in broad regions typically underlain by limestone or other readily soluble rocks. Caverns and sinkholes are common and surface streams often disappear into sinkholes. See fig Hydrothermal Waters Heating of downward percolating groundwater by shallow magma bodies leads to circulation that brings hot water to the surface as: Hotsprings - spring formed when heated groundwater flows to the surface. Geyers - violent eruptions of heated water and steam (fig ). Geothermal Energy Naturally occurring steam and hot water can be used to generate electrical power. 10