# Chapter 8 Study Guide, Version 2: Subsidence and Soils This version follows the textbook section by section. Review each section's summary, key ideas, examples, and questions before moving to the next section. ## Chapter Learning Objectives - **LO:1:** Describe what a soil is and the processes that form and maintain soils. - **LO:2:** Explain why soil erosion might threaten our society. - **LO:3:** Summarize the causes and effects of subsidence and volume changes in the soil. - **LO:4:** Locate on a map the geographic regions at risk for subsidence and volume changes in the soil. - **LO:5:** Discuss the hazards associated with karst regions. - **LO:6:** Summarize the effects of subsidence and soil volume changes. - **LO:7:** Identify linkages between subsidence, soil expansion and contraction, and other hazards, as well as natural service functions of karst. - **LO:8:** Explain how humans interact with subsidence and soil hazards. - **LO:9:** Evaluate what can be done to minimize the hazard from subsidence and volume changes in the soil. ## 8.1 Soil and Hazards (LO:1 and LO:2) ### Section summary Soil forms slowly in the upper few meters of Earth's surface as rock, water, air, and living organisms interact. Soil supports plant life, stores and transports water, and records landscape history. Soil erosion becomes a hazard when soil is removed faster than it forms, threatening agriculture, water quality, and downstream environments. ### Soil formation and profiles Soil contains mineral and rock fragments, organic matter, water, air, and organisms. Different disciplines emphasize different properties: geologists focus on altered earth material and water movement, biologists on the medium for rooted plants and habitat, and engineers on material that can be manipulated or removed without blasting. Weathering physically breaks down and chemically decomposes parent material. Climate, topography, parent material, organisms, and time influence soil development. A **soil profile** is a vertical section showing **horizons**, or layers that differ in color, texture, composition, and formation processes. | Horizon | Main characteristics | |---------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | **O** | Mostly plant litter and other organic material; often dark. | | **A** | Mixture of mineral and organic material; commonly biologically active and subject to leaching. | | **E** | Light-colored zone from which clay, iron, aluminum, and other materials have been leached. | | **B** | Zone of accumulation; may receive clay, iron oxides, silica, carbonate, and other materials from above. **Bt** indicates clay accumulation; **Bk** indicates calcium carbonate. | | **C** | Partly altered or weathered parent material. | | **R** | Unaltered, consolidated bedrock. | **Leaching** is the dissolving or downward transport of material by percolating water. **Translocation** moves clay and other materials between horizons, commonly downward. **Oxidation** of iron in well-drained soil often produces red colors; poorly drained, reduced iron may produce yellow colors. **Caliche** is an irregular accumulation of calcium carbonate, common in arid and semiarid soils. A **hardpan** is a dense, nearly impermeable soil layer that restricts infiltration and can increase runoff and flooding. ### Soil properties and development - **Color** reflects organic matter, iron chemistry, and drainage. Dark O and A horizons commonly contain abundant organic matter; red soils commonly indicate oxidized iron and good drainage. - **Texture** is the relative proportion of sand, silt, and clay. Using the engineering sizes in the textbook, sand is about 0.074-2.0 mm, silt is 0.004-0.074 mm, and clay is smaller than 0.004 mm. Texture affects drainage, water storage, strength, and shrink-swell behavior. - **Structure** describes the arrangement of soil particles into aggregates called **peds**. Common ped shapes include granular, blocky, prismatic, columnar, and platy. Soil structure generally becomes more complex as a profile develops. - **Soil fertility** is the capacity of soil to supply nutrients needed for plant growth. Relative profile development is a useful first indication of a landscape's history: - A **weakly developed** profile commonly has an A horizon directly over C material, with little or no B horizon. - A **moderately developed** profile commonly has A-Bt-C horizons, more translocated clay, and better-developed color and structure. - A **well-developed** profile has a strongly developed, commonly redder Bt horizon, more clay translocation, and stronger structure; a K horizon may also occur. A **soil chronosequence** orders soils from youngest to oldest using relative profile development. With numerical dating, it can help evaluate landscape stability and the timing or recurrence of events such as earthquakes and landslides. ### Water and soil classification Water moves vertically and laterally through pores and fractures. A soil is **saturated** when all pore spaces are filled with water and **unsaturated** when pores contain both water and air. In unsaturated flow, water moves as films around grains and more quickly through the centers of pores. Soil moisture affects strength, drainage, plant growth, subsidence, and shrink-swell behavior. Two classification systems are commonly used: - **Soil taxonomy** is used by soil scientists and emphasizes horizons, morphology, chemistry, nutrient status, organic content, and climate. Its orders include Entisols, Vertisols, Inceptisols, Aridisols, Mollisols, Andisols, Spodosols, Alfisols, Ultisols, Oxisols, and Histosols. - The **Unified Soil Classification System** is used by engineers and groups coarse-grained, fine-grained, and organic soils according largely to particle size and engineering behavior. Symbols include GW, GP, GM, GC, SW, SP, SM, SC, ML, CL, CH, OL, OH, and PT. ### Soil erosion as a hazard Soil is produced slowly but can be removed quickly. The textbook estimates that forming 50 mm of soil may take 500-1,000 years, while poor conservation can remove centuries of accumulation in less than a decade. Erosion can: - remove organically rich topsoil and reduce fertility and agricultural productivity; - increase sedimentation in streams, reservoirs, and wetlands; - degrade water quality; - increase runoff and downstream flooding; and - destabilize slopes and contribute to landslides. The chapter gives a natural erosion rate of about 0.5-1.0 ton per hectare per year, compared with typical agricultural-region averages of about 10-20 tons per hectare per year in North, Central, and South America and about 30 tons per hectare per year in Asia. Rates vary with soil, rainfall, runoff, slope, vegetation, and land use. Erosion rates can be estimated by measuring soil loss on slopes over several years, resurveying reservoirs and measuring sediment accumulation, or applying an equation such as the **Universal Soil Loss Equation**, which uses factors including rainfall, runoff, soil erodibility, slope, and land use. ### Section 8.1 case study: Haiti and the Dominican Republic The two countries share Hispaniola but developed very different land-use histories and forest cover. Extensive deforestation in Haiti, combined with population pressure, limited regeneration, and soil erosion, has degraded land and water resources. The contrast with the Dominican Republic illustrates how historical and human decisions can change erosion and environmental conditions even in neighboring areas with broadly similar regional settings. ### Key terms - **Soil profile / horizon:** A vertical section of soil / a layer within that section. - **Parent material:** Rock or sediment from which soil develops. - **Weathering:** Physical breakdown and chemical decomposition of rock and minerals. - **Leaching:** Removal or downward movement of materials by percolating water. - **Translocation:** Movement of material from one soil horizon to another. - **Soil texture / structure:** Relative particle-size proportions / arrangement of particles into peds. - **Soil chronosequence:** A series of soils ordered by relative profile development. - **Soil erosion:** Removal of soil by wind or water, especially faster than soil forms. - **Universal Soil Loss Equation:** Equation for estimating soil erosion from environmental and land-use factors. ### Section 8.1 review questions 1. How do geologists, biologists, and engineers define or use the concept of soil? 2. Describe the O, A, E, B, C, and R horizons. How do leaching, translocation, and accumulation form them? 3. How do soil color, texture, and structure provide information about soil properties? 4. What is relative soil profile development, and how can a chronosequence help evaluate hazards? 5. How do saturated and unsaturated water flow differ? 6. How do soil taxonomy and the Unified Soil Classification System differ? 7. Why is soil erosion a hazard, and how are erosion rates measured or estimated? 8. What does the Haiti-Dominican Republic comparison show about human influences on erosion? ## 8.2 Introduction to Subsidence and Soil Volume Change (LO:3) ### Section summary **Subsidence** is nearly vertical downward deformation or sinking of earth materials. It may occur on flat ground as well as slopes. **Soil volume change** includes shrinking, swelling, heaving, and settlement as water content or temperature changes. The processes may be natural, human-induced, or both. ### Karst formation and sinkholes **Karst topography** forms when soluble rock dissolves underground. Soluble rocks include rock salt, rock gypsum, limestone, dolostone, and marble. Salt and gypsum dissolve readily in fresh water; limestone, dolostone, and marble dissolve more readily in acidic water. Carbon dioxide from soil respiration dissolves in water to form weak carbonic acid, which dissolves limestone along fractures and bedding planes. As dissolution enlarges openings in bedrock, surface water may be diverted underground, caves and caverns may form near the groundwater table, springs may discharge groundwater at the surface, and streams may enter underground passages. A **disappearing stream** continues flowing through the groundwater system rather than truly vanishing. Two main sinkhole types are: - **Solutional sinkholes:** Develop gradually as acidic groundwater dissolves bedrock, often where water is concentrated along joints and fractures. - **Collapse sinkholes:** Form when surface or near-surface material collapses into an underground cavern. A falling groundwater table can remove buoyant support from a cavern roof and trigger collapse. Caves form as groundwater enlarges fractures and bedding planes. Mineral deposits left by dripping or flowing water produce formations such as **stalactites** (hang from ceilings), **stalagmites** (grow from cave floors), and flowstone. **Tower karst** consists of steep, isolated limestone towers, common in humid tropical regions. ### Permafrost thaw and thermokarst **Permafrost** is soil or sediment that remains frozen for at least two years. Its upper **active layer** thaws in summer and refreezes in winter. Ice-rich permafrost can support overlying ground; thawing removes ice volume and weakens the material, causing subsidence. Extensive thaw can form irregular terrain called **thermokarst**. The chapter notes that climatic warming has thawed Arctic permafrost, while human structures can locally add heat and accelerate thaw. ### Compaction and other sources of subsidence - **Fine sediment:** Recently deposited sand and mud contain water-filled pore spaces. Drainage or lower fluid pressure lets grains move closer together, compacting the sediment. This is common in river deltas. - **Delta subsidence:** Deltas naturally compact. Ongoing sedimentation by rivers helps keep the delta plain above sea level. Floods or earthquakes can remobilize delta sediment. - **Collapsible soil:** Dry, loosely packed or weakly cemented deposits, including some loess and arid-region stream deposits, can collapse when water weakens clay bonds or dissolves soluble cement. Settlement can exceed a meter. - **Organic soil:** Draining peat or muck allows compaction and oxygen-driven decomposition of organic matter. Erosion and burning can further reduce the soil, often irreversibly. - **Earthquakes:** Fault-related deformation may lower coastal land. - **Volcanic processes:** A surface uplifted by magma may subside when a magma chamber drains during an eruption. A drained lava tube may leave a shallow void that later collapses. ### Expansive and frost-susceptible soils - **Expansive soils** contain clay, particularly smectite, that attracts water. They swell when wet and shrink when dry. Surface signs can include deep drying cracks, popcorn-like bare ground, small mounds and depressions, wavy pavement, and tilted posts. - **Frost-susceptible soils** are commonly silt-rich and can draw water toward a freezing zone. Water forms **ice lenses**; their growth pushes soil upward in **frost heaving**. Thawing can then cause settlement. Three conditions are required for frost heaving: soil must remain below freezing long enough for ice to form, water must be available, and enough silt-sized material must be present to draw and retain water in the freezing zone. ### Section 8.2 case studies - **Scott Lake, Florida, 2006:** After drought lowered the Floridan aquifer, more than 8 inches of rain fell. A sinkhole opened in the lake bottom, and more than 95 percent of the lake's volume drained. Wildlife died, nearby houses and pools were damaged, and the lake later began to refill as debris partly plugged the opening. - **Mississippi River Delta:** The delta's lobes are built by distributary channels and then abandoned as the river shifts to a shorter route to the sea. Levees and upstream dams reduce sediment delivery; organic-soil compaction, fluid withdrawal, canals, erosion, and possible tectonic subsidence add to land loss. Wetland loss increases exposure to storm surge. ### Key terms - **Subsidence:** Nearly vertical sinking of earth materials. - **Karst topography:** Landscape formed by dissolution of soluble bedrock. - **Carbonic acid:** Weak acid formed when carbon dioxide dissolves in water. - **Solutional / collapse sinkhole:** Depression formed by gradual dissolution / sudden collapse into a void. - **Cavern:** Large underground void or cave system. - **Disappearing stream / spring:** Stream diverted underground / natural groundwater discharge at the surface. - **Permafrost / active layer:** Ground frozen for at least two years / surface layer that thaws seasonally. - **Thermokarst:** Irregular terrain formed by thawing ice-rich permafrost. - **Collapsible soil:** Loose or weakly cemented soil that settles when wetted. - **Expansive soil:** Clay-rich soil that swells when wet and shrinks when dry. - **Frost heaving / ice lens:** Upward soil movement caused by growing ground ice / discrete body of ice in soil. - **Delta plain:** Low-lying delta surface built by sediment deposition. ### Section 8.2 review questions 1. Define subsidence and distinguish it from soil volume change. 2. How does acidic groundwater produce karst topography? 3. Compare solutional and collapse sinkholes. How can groundwater-table decline trigger collapse? 4. How do caves, springs, disappearing streams, and tower karst form? 5. How do permafrost thaw, sediment compaction, organic-soil drainage, earthquakes, and volcanic processes cause subsidence? 6. Compare expansive, collapsible, and frost-susceptible soils. 7. What three conditions are necessary for frost heaving? 8. Why does a delta need continual sedimentation? ## 8.3 Geographic Regions at Risk for Subsidence and Soil Volume Change (LO:4 and LO:5) ### Section summary Subsidence and soil-volume-change hazards occur across broad regions, but the dominant process varies with geology, climate, groundwater, and soil type. Karst, permafrost, rapidly deposited sediment, organic soils, expansive clays, collapsible soils, and frost-susceptible soils each create distinct risks. ### Karst and evaporite regions Karst may cover up to 10 percent of Earth's surface. In the contiguous United States, about 25 percent of land is underlain by limestone or evaporites with some degree of karst development. Major U.S. regions include: - Tennessee, Virginia, Maryland, and Pennsylvania; - south-central Indiana and west-central Kentucky; - the Salem-Springfield plateaus of Missouri and northern Arkansas; - the Edwards Plateau of central Texas; - most of central Florida; and - Puerto Rico. The Permian Basin of eastern New Mexico and western Texas has extensive evaporite karst. Karst hazards include sinkhole collapse, groundwater pollution, and unreliable water supplies. The chapter notes that subsidence is a hazard in more than 45 U.S. states and most Canadian provinces. ### Permafrost and organic soils Permafrost covers more than 20 percent of the world's land surface. It underlies most of Alaska and more than half of Canada and Russia, as well as Arctic and high-elevation regions worldwide. Towns at risk from thaw include Barrow, Alaska; Inuvik, Canada; and Yakutsk, Norilsk, and Vorkuta in Russia. Organic soils occur in northern wetlands, the Upper Midwest, Pacific Northwest, New England, Alaska, Canada, the Everglades, the Sacramento-San Joaquin Delta, and coastal Louisiana and North Carolina. ### Expansive, collapsible, and frost-susceptible soils - Expansive soils occur across the western United States and Canada. High swelling potential is found in the Blackland Prairies from Texas to Alabama and in parts of the High Plains from eastern Colorado north through Saskatchewan and Alberta. Swelling clays occur in all 50 U.S. states and many Canadian provinces and territories. - Frost-susceptible soils occur throughout Canada, Alaska, and the northern contiguous United States, and at high elevations in the Appalachians, Rocky Mountains, Sierra Nevada, and Cascades. - Collapsible soils are common in arid and semiarid settings, including the southwestern United States. ### Deltas and volcanic regions Subsidence from sediment compaction is important in the Mississippi, Sacramento-San Joaquin, and Nile deltas. Large-scale earthquake-related subsidence is possible in Alaska, Hawaii, and the Pacific Northwest. Magma-chamber subsidence can occur in volcanic areas, while lava-tube collapse is a particular hazard in Hawaii and some western North American lava flows. ### Section 8.3 review questions 1. Where are the major U.S. regions at risk from karst and evaporite dissolution? 2. Where are permafrost, organic soils, expansive soils, and frost-susceptible soils most common? 3. Which regions are particularly susceptible to collapsible-soil and delta subsidence? 4. Why is karst a water-supply resource and also a hazard? ## 8.4 Effects of Subsidence and Soil Volume Change (LO:6) ### Section summary Subsidence and soil volume change damage buildings, roads, utilities, and water systems; alter wetlands and groundwater; and create substantial economic losses. Effects range from gradual cracking and settlement to sudden sinkhole collapse. ### Sinkhole and groundwater effects Sinkholes can damage or swallow highways, homes, businesses, swimming pools, sewage facilities, and utilities. Their positions are difficult to predict precisely. Lowering the groundwater table, often during drought or from pumping, can remove buoyant support from cavern roofs and trigger collapse. Karst allows rapid connections between surface water and groundwater. This makes karst aquifers productive but vulnerable to pollution from waste disposal, sewage, and contaminated runoff. Groundwater changes can also reduce spring flow, threatening communities and species that depend on springs. ### Permafrost, delta, and coastal effects - Permafrost thaw can crack, tilt, or collapse buildings and buckle roads, runways, pipelines, and railroads. Alaska spends a significant share of its budget repairing permafrost damage. - Delta subsidence lowers land relative to sea level, contributes to wetland loss, and increases exposure to coastal flooding and storm surge. - New Orleans and nearby areas subside relative to sea level. The chapter reports an average rate of about 8 mm per year; much of the city is below sea level, increasing dependence on levees and flood walls. - Loss of coastal wetlands can increase storm-surge exposure and damage fisheries and other local economies. ### Soil-volume-change effects - Expansive soils crack and shift foundations, walls, sidewalks, roads, and pipelines. Water-line breaks can reduce pressure and allow contamination. - Frost heaving can crack pavement and foundations, lift fence posts and utility poles, and move buried stones upward. Freeze-thaw action also contributes to potholes. - Collapsible soils can settle suddenly when wetted. The chapter reports that expansive-soil damage exceeds $15 billion annually in the United States, in addition to billions of dollars of frost-action damage. More than 250,000 new U.S. homes are built on expansive soils each year; many experience minor damage and some severe damage. ### Section 8.4 case studies - **Winter Park, Florida, 1981:** A rapidly growing sinkhole swallowed part of a swimming pool, a street, businesses, houses, and vehicles. Damage exceeded $2 million. It developed during drought when groundwater levels were very low. - **Seffner, Florida, 2013:** A sinkhole collapsed beneath a home and killed a resident. The event illustrates the danger and unpredictability of collapse sinkholes, even though fatalities are rare. - **Mississippi Delta and New Orleans:** Reduced sediment supply, subsidence, wetland loss, and sea-level rise combine to increase flood and storm-surge exposure. ### Section 8.4 review questions 1. What are the main effects of subsidence and soil volume change on structures and infrastructure? 2. Why can groundwater-table decline trigger a collapse sinkhole? 3. How can sinkholes and other karst features affect groundwater quality and supply? 4. How does permafrost thaw damage infrastructure? 5. Why does delta subsidence increase coastal flooding and storm-surge risk? 6. How do expansive, collapsible, and frost-susceptible soils damage property? ## 8.5 Linkages between Subsidence, Soil Volume Change, and Other Natural Hazards (LO:7) ### Section summary Subsidence can result from earthquakes, volcanoes, severe weather, climate change, or human actions, and it can also intensify other hazards. Soil expansion, contraction, and frost heaving can interact with landslides and creep. These linkages often involve changes in water, elevation, or ground stability. ### Important hazard linkages - Subsidence lowers land relative to rivers, lakes, or the ocean, increasing flood frequency and severity. Groundwater mining has contributed to flooding in low-lying cities such as Bangkok and Mexico City. - In coastal settings, subsidence adds to relative sea-level rise and land loss in Venice, New Orleans, Galveston Bay, and the Nile Delta. - Shrink-swell cycles and frost heaving can destabilize slopes, increase gravity-driven creep, and create uneven ground. - Earthquakes can cause subsidence; subsiding areas may then become more exposed to flooding. - Volcanic uplift followed by subsidence records the movement and drainage of magma and can help identify eruption cycles. - Drought lowers groundwater levels, which can promote sinkhole collapse, aquifer compaction, subsidence, and desiccation cracks. - Permafrost thaw linked to climate warming damages infrastructure and changes drainage and slope stability. - Soil erosion removes agricultural resources, adds sediment to water, and can increase flooding and landsliding. ### Section 8.5 review questions 1. How can subsidence increase river, urban, and coastal flood risk? 2. How are subsidence and sea-level rise linked in coastal areas? 3. How can shrink-swell and freeze-thaw cycles contribute to mass wasting? 4. How can earthquakes, volcanoes, drought, and climate change cause or intensify subsidence? 5. How are soil erosion and subsidence linked to flooding and landslides? ## 8.6 Natural Service Functions of Subsidence and Soil Volume Change (LO:7) ### Section summary Subsidence and soil volume change can provide services as well as hazards. Karst systems supply water and unique landscapes, caves preserve scientific records, and soil shrink-swell processes can form productive soil structure. ### Natural services - **Water supply:** About 25 percent of the world's population lives on or gets drinking water from karst formations. The chapter estimates that about 40 percent of U.S. groundwater used for drinking comes from karst terrains. - **Soil productivity:** Shrink-swell processes can form peds that help root growth and soil drainage, especially in clay-rich soils. - **Aesthetic and recreational value:** Rolling karst hills, caverns, cave formations, and tower karst attract visitors and support tourism. - **Scientific resources:** Cave formations preserve records of climate change; caves also preserve animal remains important to paleontology and archaeology. - **Unique ecosystems:** Caves support troglobites and other specially adapted organisms, including fish, shrimp, salamanders, and beetles. Bats use caves for shelter. Some karst species are found nowhere else. These benefits do not remove the hazards: karst aquifers can transmit pollution rapidly, and soil processes that create useful peds can also damage structures. ### Section 8.6 review questions 1. What water-supply, aesthetic, scientific, and ecological services do karst regions provide? 2. How can soil shrink-swell processes support soil productivity? 3. Why can a natural service function coexist with a serious hazard? ## 8.7 Human Interaction with Subsidence and Soil Volume Change (LO:8) ### Section summary Human activities can increase subsidence and soil-volume-change hazards by removing underground support, changing water levels, disturbing frozen ground, blocking sediment delivery, draining wetlands, and altering soil moisture near structures. ### Fluid withdrawal and mining - Pumping groundwater, oil, natural gas, or geothermal fluids reduces pore-fluid pressure that supports overlying earth materials. Sediment compacts and the surface subsides. - **Groundwater mining** is extraction faster than replenishment. It has caused major subsidence in California's Central Valley and near Phoenix, Las Vegas, Houston-Galveston, San Jose, and Mexico City. It can reactivate faults and form long earth fissures. - Coal-mine pillars can weaken and collapse long after mining ends. More than 8,000 km2 of U.S. land has subsided due to underground coal mining, according to the chapter. - Salt extraction by solution mining or open-shaft mining can leave cavities. Mine flooding and roof collapse can produce sinkholes and subsidence. ### Permafrost, deltas, and drainage - Buildings, buried warm utility lines, and poor construction practices can add heat to permafrost and accelerate thaw. - Dams trap sediment; levees prevent overbank deposition on delta plains; canals divert water and sediment. These practices can protect communities from river flooding while starving deltas of material needed to offset compaction. - Draining organic soils allows compaction, decomposition, erosion, and burning. Irrigation can wet collapsible soil or increase swelling in expansive soil. - Drainage changes can lower groundwater and affect karst systems. ### Landscaping and development - Trees and large shrubs near foundations can dry expansive soil as roots remove moisture. - Frequent irrigation near foundations can increase swelling; excess water is a major cause of expansive-soil damage. - Ponding water near collapsible soils can trigger settlement. - Building without geologic and soil investigations increases exposure to caves, abandoned mines, expansive clay, permafrost, and weak sediment. ### Section 8.7 case study: Helen Delano, environmental geologist Pennsylvania geologist Helen Delano evaluates hazards such as sinkholes and subsidence above abandoned coal mines and limestone. Mine maps can identify potential problems; borehole cameras and geophysical methods can investigate cavities where records are incomplete, and ground-penetrating radar can help detect shallow caves. Her central lesson is that avoidance is often cheapest, and where construction proceeds, investigating and designing for the hazard in advance costs less than addressing damage later. ### Section 8.7 review questions 1. How does removing groundwater, oil, or gas lead to subsidence? 2. Why can abandoned coal and salt mines cause surface collapse years after mining? 3. How can construction and utility heat affect permafrost? 4. How do dams, levees, canals, and wetland drainage affect delta subsidence? 5. How can landscaping and irrigation increase damage from expansive or collapsible soils? 6. What methods can geologists use to investigate abandoned mines and shallow caves? ## 8.8 Minimizing Subsidence and Soil Volume Change (LO:9) ### Section summary Natural subsidence and soil volume change are difficult to prevent, but planning, regulation, mapping, monitoring, water management, and appropriate engineering can limit damage. Avoiding development in hazardous areas is generally the safest and least expensive adjustment. ### Reducing fluid-withdrawal subsidence and mining hazards - Regulate groundwater wells and reduce pumping where withdrawal is causing subsidence. - Monitor groundwater levels, fluid production, and ground-surface elevation. - Injection wells may help in some settings, such as when water is injected as oil is pumped. They generally cannot reverse compaction of fine sediment once particles have collapsed together. - Prevent mining beneath urban areas where possible. Review mine maps and investigate incomplete records using boreholes, borehole cameras, ground-penetrating radar, and other geophysical methods. Houston-Galveston shows that regulation and reduced groundwater pumping can essentially stop additional subsidence in some areas. ### Protecting permafrost Elevate buildings to reduce heat transfer into frozen ground. Use foundations or pilings designed for changing frozen-ground conditions, manage heat from buildings and utilities, and consider adjustable screw jacks or lattice-like foundations where appropriate. ### Managing deltas and wetlands - In developed areas, maintain and raise levees as needed and maintain pumping systems to remove water from protected enclosures. - In undeveloped areas, controlled diversions or levee breaches can restore freshwater and sediment supplies and help rebuild marshes. - Protecting and restoring marshes and barrier islands can reduce storm-surge exposure. Fully returning an urbanized delta to natural conditions is unrealistic. ### Managing organic, collapsible, expansive, and frost-susceptible soils - Maintain water levels in marshes and swamps to limit organic-soil drying, decomposition, and subsidence. Drained or decomposed organic soils generally cannot be restored to their former volume. - On collapsible soil, prevent ponding near buildings and limit irrigation or other infiltration. - On expansive soil, control drainage with subsurface drains, gutters, and downspouts; keep moisture near foundations as uniform as possible; and use reinforced foundations designed to accommodate movement. - Where appropriate, build on compacted fill, excavate and replace expansive soil, or mix in calcium-based stabilizers such as quicklime. - Design roads, pipelines, and foundations to accommodate frost heaving and thaw settlement. ### Section 8.8 review questions 1. How can regulation and monitoring reduce subsidence caused by fluid withdrawal? 2. Why can injection wells fail to reverse subsidence from compacted fine sediment? 3. How can mining-related subsidence be minimized? 4. How can engineering reduce damage from permafrost thaw? 5. How can delta restoration help undeveloped areas while levees remain necessary in developed areas? 6. What construction, drainage, and landscaping methods reduce damage from organic, collapsible, expansive, and frost-susceptible soils? ## 8.9 Perception of and Adjustment to Subsidence and Soil Volume Change (LO:9) ### Section summary Subsidence and soil-volume-change hazards receive less media attention than earthquakes, floods, or hurricanes, so people may underestimate them. Residents of areas with frequent sinkholes, expansive soils, permafrost, or groundwater withdrawal are more likely to recognize the risk. Avoiding hazardous sites is the preferred adjustment, supported by mapping, site investigation, disclosure, monitoring, and insurance. ### Identifying risk and choosing a site Before buying or building: - Review detailed geologic, soil, groundwater, karst, permafrost, and mine maps and local hazard records. - Determine whether the property is underlain by limestone, evaporite, abandoned mines, expansive clay, collapsible soil, organic soil, or permafrost. - Look for cracks in ground or pavement, sinkholes, depressions, uneven floors, tilted structures, damaged foundations, and unusually wet areas. - Ask about prior subsidence, flooding, soil movement, groundwater changes, mining, and repairs. - Obtain professional geologic and geotechnical evaluations and soil tests before construction. - Check whether insurance covers sinkhole, mine-subsidence, and soil-volume-change damage; standard homeowners' policies may exclude these hazards. ### Mapping and subsurface investigation - Geologic and soil maps identify surface materials and likely hazards. - Surface observations can reveal cracks, sinkholes, and changing elevations. - Satellite radar such as **InSAR** can measure millimeter-scale vertical motion across large areas over days to years. - **Ground-penetrating radar (GPR)** and boreholes can investigate shallow caves and other subsurface conditions. - Geotechnical borings and soil tests help design foundations for expansive, collapsible, frost-susceptible, or permafrost ground. The best adjustment is to avoid building in hazardous areas when possible. Where avoidance is impractical, identify the hazard early and use appropriate land-use controls, design, drainage, monitoring, and insurance. ### Applying the fundamental concepts: Venice is sinking Venice illustrates all five fundamental concepts: 1. **Science helps predict hazards:** Historical flood records, geologic studies, hydrologic stations, and satellite measurements track subsidence and high water. 2. **Risk information can support decisions:** Venice has long recognized flood and subsidence risks, yet its historic, cultural, and economic value leads residents and officials to accept some continuing risk. 3. **Hazards are linked:** Natural subsidence, past groundwater pumping, high tides, storms, and sea-level rise combine to increase flooding. 4. **Humans can turn disasters into catastrophes:** Groundwater pumping increased subsidence; settlement and infrastructure remain exposed on low-lying lagoon islands. 5. **Consequences can be minimized:** Warning systems and the MOSE movable flood barriers aim to reduce flooding. Barriers do not stop subsidence or all flooding, are costly, and may affect the lagoon ecosystem. The textbook reports that Venice subsided about 25 cm since the late 1800s, with about 15 cm attributed to groundwater pumping. Pumping was banned after the 1966 flood, but the city and lagoon continue to subside. The case shows that engineering can reduce some consequences without removing the underlying processes or all residual risk. ### Section 8.9 review questions 1. Why do people often underestimate subsidence and soil-volume-change hazards? 2. What is the most appropriate adjustment to these hazards? 3. How do geologic and soil maps, surface observations, satellite radar, GPR, and boreholes help identify risk? 4. What should a home buyer investigate in karst, mine, expansive-soil, wetland, or permafrost settings? 5. What insurance limitations should property owners consider? 6. How does the Venice example demonstrate both the benefits and limits of hazard mitigation? ## Connections among the Sections - **8.1 to 8.2:** Soil properties and water movement influence dissolution, compaction, collapse, and volume change. - **8.2 to 8.3:** The processes that cause subsidence and soil movement determine where the hazards are concentrated. - **8.3 to 8.4:** Geographic exposure to karst, permafrost, deltas, and unstable soils shapes damage to people and infrastructure. - **8.4 to 8.5:** Subsidence can intensify flooding, while earthquakes, drought, and climate change can contribute to subsidence. - **8.5 to 8.6:** Some processes that create hazards also provide water, productive soil structure, scientific records, and habitat. - **8.6 to 8.7:** Human use of karst water, delta land, groundwater, mines, and soils can increase or alter natural processes. - **8.7 to 8.8:** Understanding human causes helps target regulation, engineering, water management, and restoration. - **8.8 to 8.9:** Mitigation works best when people recognize risk and use maps, investigations, disclosure, and insurance in decisions. ## Chapter-Wide Comparison Questions - How do geologists, biologists, and engineers define or use the concept of soil? - How do weathering, leaching, translocation, oxidation, and accumulation form soil horizons? - How do the O, A, E, B, C, and R horizons differ? - How do color, texture, structure, and soil moisture affect soil properties and hazard behavior? - How do soil taxonomy and the Unified Soil Classification System differ? - How can a chronosequence help evaluate landscape age and hazard recurrence? - Why can erosion threaten food production, water quality, slopes, and downstream communities? - How do solutional and collapse sinkholes differ? - How do dissolution, permafrost thaw, sediment compaction, organic-soil drainage, earthquakes, and volcanic processes cause subsidence? - How do expansive, collapsible, and frost-susceptible soils differ? - What conditions are needed for frost heaving? - Where are the major regions at risk from karst, permafrost, deltas, expansive soils, and frost-susceptible soils? - How can groundwater withdrawal, mine collapse, or delta subsidence cause flooding and infrastructure damage? - How do subsidence and soil volume change link with floods, landslides, earthquakes, volcanoes, drought, and climate change? - What water, aesthetic, scientific, and ecological services do karst regions provide? - How do dams, levees, canals, drainage, irrigation, vegetation, and construction alter subsidence or soil movement? - Why can levees protect communities from river floods while contributing to delta subsidence? - How do mapping, InSAR, GPR, boreholes, soil tests, regulation, engineering, and insurance reduce risk? - Why is avoiding hazardous ground usually preferable to correcting damage afterward? - How do the Haiti-Dominican Republic, Scott Lake, Mississippi Delta, Helen Delano, and Venice examples illustrate the interaction of natural processes and human decisions? ## Practice Questions 1. Define soil, soil profile, soil horizon, parent material, weathering, leaching, translocation, and oxidation. 2. Describe the O, A, E, B, C, and R horizons and explain the roles of Bt, Bk, K horizons, caliche, and hardpan. 3. Compare soil color, texture, and structure as indicators of soil conditions. 4. Explain weak, moderate, and strong soil profile development and the use of a chronosequence. 5. Compare saturated and unsaturated water flow through soil. 6. Distinguish soil taxonomy from the Unified Soil Classification System. 7. Define soil erosion and explain its impacts on agriculture, water quality, flooding, and landslides. 8. Compare natural and accelerated soil-erosion rates and describe ways to measure erosion. 9. Explain how land-use history contributed to erosion in Haiti and compare the outcome with the Dominican Republic. 10. Define subsidence and distinguish it from soil volume change. 11. Explain how acidic groundwater dissolves limestone and forms karst. 12. Compare solutional and collapse sinkholes and explain how groundwater levels affect collapse risk. 13. Describe the formation of caves, springs, disappearing streams, and tower karst. 14. Explain how permafrost thaw forms thermokarst and damages infrastructure. 15. Describe how compaction of fine sediment and drainage of organic soil cause subsidence. 16. Compare expansive, collapsible, and frost-susceptible soils and list the conditions for frost heaving. 17. Identify major geographic regions at risk from karst, permafrost, organic soil, expansive soil, and frost-susceptible soil. 18. Describe the direct, water-resource, coastal, and economic effects of subsidence and soil volume change. 19. Trace links among subsidence, flooding, landslides, earthquakes, volcanoes, drought, and climate change. 20. List the natural service functions of karst and soil-volume-change processes. 21. Explain how groundwater pumping, oil and gas withdrawal, and mining cause subsidence. 22. Explain how dams, levees, canals, wetland drainage, and landscaping alter soil or delta processes. 23. Evaluate ways to reduce risk from fluid withdrawal, mining, permafrost thaw, delta subsidence, and soil volume change. 24. Explain how geologic maps, soil maps, InSAR, GPR, boreholes, and soil tests support site decisions. 25. List what a prospective buyer should investigate before building on karst, mine land, expansive soil, collapsible soil, wetland, or permafrost. 26. Use Scott Lake, the Mississippi Delta, Houston-Galveston, or Venice to explain how science and human decisions affect subsidence losses. ## Quick Review Checklist Before the quiz or test, make sure you can: - define soil, soil profile, horizon, parent material, weathering, leaching, translocation, texture, structure, and fertility; - identify and describe the O, A, E, B, C, and R horizons; - explain the roles of Bt, Bk, K horizons, caliche, hardpan, and soil chronosequences; - compare soil color, texture, structure, soil taxonomy, and the Unified Soil Classification System; - explain saturated and unsaturated water flow and how moisture affects soil behavior; - define soil erosion, compare natural and accelerated erosion rates, and describe erosion measurement; - explain why soil erosion threatens agriculture, water quality, flooding, and slope stability; - define subsidence, karst, dissolution, sinkhole, permafrost, thermokarst, compaction, expansive soil, collapsible soil, and frost heaving; - distinguish solutional from collapse sinkholes and explain the role of groundwater-table changes; - describe caves, springs, disappearing streams, tower karst, and thermokarst; - explain subsidence caused by karst, thawing permafrost, sediment compaction, organic-soil drainage, earthquakes, magma drainage, and lava-tube collapse; - compare expansive, collapsible, and frost-susceptible soils and state the conditions required for frost heaving; - identify regions at risk from karst, permafrost, organic soil, deltas, expansive soils, and frost-susceptible soils; - describe effects on homes, roads, utilities, wetlands, groundwater, and coastal communities; - connect subsidence and soil volume change with floods, landslides, earthquakes, volcanoes, drought, and climate change; - describe the water, aesthetic, scientific, ecological, and soil-productivity services of karst and soil processes; - explain how fluid withdrawal, mining, permafrost disturbance, delta sediment restriction, wetland drainage, and landscaping affect risk; - describe regulation, mapping, monitoring, InSAR, GPR, boreholes, engineering, restoration, and insurance as risk-reduction measures; - explain why avoidance and early investigation are usually preferable to repair after damage; and - use the Haiti-Dominican Republic, Scott Lake, Mississippi Delta, Helen Delano, and Venice examples to explain hazards and adjustments.