Module 2.3: Soil and Groundwater

Soil

• Any solid earth material that can be removed without blasting (definition by engineers)

• Solid earth material that has been altered by physical, chemical, and organic processes such that it can support rooted plant life (definition by soil scientists)

Rock, air, water, and organic material

Soil involves the combination of these elements.

Mineral matter (~45%)

A soil component that is disintegrated and decomposed rock (also state the percent composition)

Water (~25%)

A soil component which is a complex solution containing many soluble nutrients (also state the percent composition)

Air (~25%)

A soil component which is the source of oxygen and carbon dioxide (also state the percent composition)

Humus or organic matter (~5%)

A soil component consisting of decayed remains of plants and animals (also state the percent composition)

Ecosystem services

Soil preservation is important in this aspect as it provides essential nutrients to forests and crops, filters water, and enhances water retention.

Land use planning

Soil preservation is important in this aspect as it is the foundation of urbanization, timber management, and agriculture.

Waste management

Soil preservation is important in this aspect as it maintains the sustainability of a site.

Climate change

Soil preservation is important in this aspect as healthy soil regulates Earth’s temperature, mitigates impacts of extreme weather events, and reduces erosion.

Weathering

• The first step in soil development involving physical and chemical breakdown of rocks

• Rocks are then further modified by the activity of soil organisms.

Parent material

An important factor in soil formation as it is the source of the mineral matter.

Residual soil

Type of soil formed directly from the weathering of underlying bedrock and remains at its place of origin

Transported soil

Type of soil formed from unconsolidated sediments carried away from its source by agents such as wind, water, or ice and deposited elsewhere

Slope

An important factor in soil formation as it influences erosion, water movement, and vegetation patterns

Steep slopes

• Slopes that lead to poorly developed soils

• Have low moisture content and accelerated erosion

Gentle slopes

• Slopes that lead to average soils

• Have some drainage and less erosion

Flat to undulating upland

• Slopes that create great soils for vegetation

• Have good drainage, minimum erosion, sufficient infiltration of water, and is the optimum terrain for soil development

Marshes and swamps

These are types of wetlands that have thick, dark soils which are poorly drained and water-logged, even at an optimum slope.

Time

An important factor in soil formation since the longer the soil has been forming, the thicker it becomes, and the less it resembles the parent material

Climate

• An important factor in soil formation and is the most influential

• It affects soil formation through temperature, precipitation levels, and the type of plant and animal life present in the area

Plants and animals

• An important factor in soil formation as they have a strong influence on the chemical and physical properties of the soil

• Their decay produces organic acids which hastens weathering.

• They provide organic matter to the soil which enhances its fertility.

Soil profile

Vertical cross-section from the surface down to the parent material

Soil horizons

Distinct layers formed in well-developed soils

Mature soil

Type of soil characterized by a well-developed profile - indicating that the environmental conditions have been relatively stable over an extended period

Immature soil, steep

Type of soil characterized by the lack of horizons as development has been going on only for a short time (e.g. _ slopes)

O horizon

Soil horizon characterized by loose and partly decayed organic matter

A horizon

Soil horizon characterized by mineral matter mixed with some humus

E horizon

• Soil horizon also known as zone of eluviation

• Eluviation is the leaching of dissolved or suspended material removed by water from an upper horizon

B horizon

• Soil horizon also known as zone of illuviation

• Illuviation is the deposition of materials from eluviation.

• It is characterized by the accumulation of clay, iron, and aluminum.

O, A, E, and B horizons

Soil horizons which constitute the soil that can support plant life

C horizon

Soil horizon made up of the partially altered parent material

Regolith

• It is the layer of loose, unconsolidated material that covers the solid bedrock.

• Composed of horizons O to C

R horizon

• Soil horizon composed of unweathered parent material

• Also known as the bedrock

Through visual or tactile inspection

Two ways to estimate the size of soil particles

Sand

• Visual: Individual grains can be seen

• Tactile: Gritty

Silt

• Visual: Grains can be seen with a 10x hand lens

• Tactile: Like baking flour

Clay

• Visual: Cannot see the grains with 10x hand lens

• Tactile: Cohesive

Soil texture triangle

• It is a diagram developed by the US Dept. of Agriculture to show soil texture classes.

• It indicates the percent clay, percent silt, and percent sand of a sample.

Loam soil

It is the type of soil best for plant life ue to better moisture characteristics and nutrient storage capacity in comparison to soil that are predominantly clay or coarse sand.

Granular

• Soil structure characterized by 1-10 mm grains usually found in the A horizon

• Can also be found in B and C horizons

Blocky

• Soil structure characteritzed by 5-50 mm grains usually found in the B horizon

• Usually designated as angular or subangular

Prismatic

• Soil structure characterized by 10-100 mm grains usually found in the B horizon

• Called columnar structure if they have rounded tops

Platy

• Soil structure characterized by 1-10 mm grains usually found in the E horizon

• May also be found in B horizons

Laterite

Type of soil that is red due being highly leached by oxides of iron (Fe) and aluminum (Al)

Pedocal

Type of soil made from pedon (soil) and calcite

Pedalfer

Type of soil derived from pedon (soil) and aluminum (Al) and iron (Fe)

Groundwater

It is freshwater located beneath the ground.

Hydrologic cycle

It is the biogeochemical cycle that recharges the supply of groundwater.

Intergranular

Type of groundwater storage that occurs when it fills spaces between sand grains

Crevice

Type of groundwater storage as it permeates igneous rocks

Solution openings

Type of groundwater storage as it permeates limestones

Porosity

• It measures the amount of water that can be held by rocks or sediments.

• Calculated by “volume of voids / total volume of material”

Grain size, sorting, and grain packing

The three main factors that affect porosity

Less porosity

• When soil is poorly sorted

• When soil has rhombohedral packing instead of cubic

• When grains are fine-grained instead of well-rounded and coarse

Permeability

• It is how easily water can flow through the rock or unconsolidated sediment.

• Describes how pores are shaped and interconnected

Gravel

It is the most permeable among all sediment types.

Clay

It is the most porous among all the sediment types.

Aquifer

Rock that stores and transmits groundwater

Unconfined aquifer

• Type of aquifer bounded at the bottom by a confining unit

• Its water rises to the water table

Confined aquifer

• Type of aquifer bounded on top and bottom by confining unites

• Water rises to the piezometric water level (potentiometric line/surface) due to natural pressure in rocks

Perched aquifer

• Unconfined aquifer defined by a discontinuous confining unit

• It has a local water table usually aove the main/regional water table

Water table

The boundary between water-saturated ground and unsaturated ground

Karst topography

It is the distinctive topography that indicates dissolution of underlying rocks by surface water or groundwater.

Caverns

An underground chamber often formed by acidic groundwater dissolving soluble rock at or just below the surface in the zone of saturation

Speleothems

Mineral formations found within caverns formed by deposition of minerals from dripping water in the zone of aeration

Stalactites

Speleothems that hang from the ceiling of a cavern

Stalagmites

Speleothems that grow upward from the cave floor

Columns

Speleothems that form when stalactites and stalagmites meet

Springs

Water bodies that form when the water table, confined aquifers or groundwater-bearing fractures intersect ground surface

Sinkholes

• Formed through slow dissolution of limestone by downward-seeping rainwater charged with CO2

• Can also form when the roof of a cavern collapses under its weight

Igneous and metamorphic rocks

Types of rocks typically not known to store significant amounts of groundwater

Land subsidence, sinkhole formation, saltwater encroachment, contamination, depletion

Five main groundwater-related problems

Land subsidence

• Sinking or settling of the Earth’s surface in a downward vertical movement

• Well pumping from aquifers lowers the water table and produces a cone of depression, resulting in subsidence

Saltwater encroachment or saltwater intrusion

• The movement of saltwater into freshwater sources, including groundwater

• Well pumping can cause an unnatural flow of saltwater towards freshwater sources

Water gap

• Human demand exceed the renewable supply based on global trends (according to the Utrecht model)