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Environmental Geology

Understanding the rocks and soils which form the geologic environment is necessary to properly assess the condition and remediate a contaminated site and make sound decisions regarding storage or isolation of wastes. Just as the physical, chemical and hydrological properties of soil and rock vary across the landscape and into the ground, so also will the location and "fate and transport" of contaminants vary, as will as the conditions for site remediation or storage.

This information sheet first lists those categories and words that provide a general framework and context for understanding geology:

• General Terms,
• General Sediment Types and
• General Mineral Types.

It then lists terms within more specific categories:

• Rock Types,
• Rock Features,
• Sediment Types,
• Sediment and Soil Features and
• Hydrogeologic Features.

Bedrock:

Rock which is still in place in the ground. That is, rock which has not been eroded or broken apart from its original state.

Geophysics:

The study of the earth and underground phenomena using the methods of physics that yield measurable results. Methods may include seismic, gravity, magnetic, electrical, electromagnetic, radiometric and geothermal.

Geotechnical properties:

The properties of the soil or rock affecting engineering-related characteristics such as its ability to bear a load or suitability for construction of a landfill. Geotechnical properties are measured with remote sensing geophysical techniques such as nuclear density tests and by laboratory techniques that measure the physical parameters of the material, such as plasticity.

Hydrogeochemical:

Referring to the chemical properties of groundwater. Particulate, colloidal and dissolved components all affect groundwater chemistry. These are in turn affected by the surrounding geology and the local hydrologic conditions.

Hydrogeology:

The study of water and its movement and properties in geologic media. Hydrogeologic characteristics include hydraulic conductivity, porosity, gradient and specific yield.

Ion:

An element or molecule which has either insufficient or excess electrons such that it has an electrical charge. Metals and minerals dissolved in water are often in the ion state.

Mineral:

A substance, in the solid state, comprised of one or more elements of the periodic chart which are arranged in a regular, repetitive structure.

Overburden:

Any material, consolidated or unconsolidated, that overlies a deposit or rock of interest, such as coal or limestone. Often overburden refers to the material moved by surface mining.

Profile:

A set of data, often represented in map form, compiled from sequential sampling and analysis at successive depths of the ground. Profiling can also refer to measurements taken across the surface of the landscape or horizontally underground. Because all rock and soil characteristics change with depth and distance, profiling often involves a three-dimensional approach.

Rock:

Solid inorganic material comprised of one or more types of mineral grains or crystals held together by either interlocking crystal growth, cementing minerals in the pore spaces, or surface interactions between minerals (similar to static electricity).

Sediment:

Mineral and organic particles that have been transported by wind, water or ice and deposited in a new location either on the earth's surface or under water. Sediment can refer to particles ranging in size from microscopic to several meters in diameter.

Soil:

Sediment that can naturally sustain plant growth.

Strata:

Layers of rock or sediment.

Stratigraphy:

The branch of geology that studies how stratified, or layered, rock is formed, its composition, sequence and correlation. Stratified sediments are deposited by wind or water and exhibit layering features which reflect the depositional conditions of the sediment. Local variations in stratigraphy will influence contaminant and groundwater movement underground.

Just as a chemist thinks of matter in its three different states - solid, liquid and gas, the geologist likewise uses the following sediment terms to define the "states" of geologic material.

Boulder:

Those rock or mineral particles that are greater than 300 mm.

Clay:

The term "clay" is often used in two ways - either to refer to a type of sediment (based on particle size) or to a type of mineral content (see definition below of "clay minerals"). As a sediment term, clay particles are defined as those less than 0.002 mm in size. Clays are usually cohesive (stick together) and plastic (can be molded into shapes).

Cobble:

Those rock or mineral particles that range in size from 75 mm to 300 mm.

Fill:

Soil, rock and/or debris that is placed by human activity and is not of geologic origin.

Gravel:

Those rock or mineral particles that range in size from 4.8 mm to 75 mm.

Sand:

Rock or mineral particles ranging in size between 0.02 mm and 4.75 mm. Sand-size particles may originate from any kind of rock and mineral and therefore will vary chemically. Frequently, sand-size particles are often comprised of quartz or feldspar which take longer to weather and are less active chemically than clay minerals.

Silt:

Those rock or mineral particles that fall in size between sand and clay-sized material and vary in mineral content, as does its parent rock. Fine silt is often a component of glacial runoff and is a result of the grinding action of the ice directly on the rock surface.

Soil:

Sediment that can naturally sustain plant growth. Soil is made up of mineral particles and of organic matter. Oxygen, water and microbial life, in addition to mineralogy and type of organic matter present in soils, greatly affect chemical activity and the fate of any contaminants spilled. Soil also refers to all unconsolidated material found above bedrock.

Clay:

A term not only used to refer to a type of sediment (see definition of clay in General Sediment Terms) but also to a mineral type comprised of silicon and oxygen, whose crystal structures contain water molecules and grow in sheet-like structures. The molecules carry an electric charge on their surface. Thus, clay minerals can attract other charged ions such as dissolved metals in groundwater.

Feldspar:

The most abundant family of minerals on earth, occurring in all types of rocks and containing silicon, oxygen, aluminum and sometimes potassium, sodium, calcium, barium, rubidium, strontium or iron.

Quartz:

Common and found abundantly in nature, quartz crystals are comprised of silicon and oxygen.

Basalt:

Rock of volcanic origin, characterized by fine-grained dark gray or black crystals. Basalt is the rock formed from volcanic eruptions in Hawaii and other places. Upon cooling, basalt often forms joints (cracks) which result in six-sided columns of basalt rock. The Columbia Plateau in Oregon and Washington is a basalt plateau and Devil's Tower in Wyoming is a basaltic remnant.

Granite:

Rock crystallized under the earth's surface from magma. Granite is primarily made up of quartz and feldspar. Stone Mountain in Georgia is a granite remnant.

Limestone:

Rock comprised primarily of calcium carbonate derived from ocean floor sediments and carbonaceous "ooze" or mud. Limestone can be dissolved by water and therefore may develop karst features, such as enlarged rock joints or caves.

Salt Beds:

Sedimentary deposits made up of common table salt and impurities, formed after water evaporated from ancient seas. Salt is plastic-like, flexing in all three dimensions under enough pressure. It therefore does not fracture.

Sedimentary rock:

Rock made up of sediments (clay-size to boulder-size) which have been "cemented" together by mineral growth in the pore spaces or by interactions between the grains.

Shale:

A laminated (layers less than 1/4-inch thick) sedimentary rock characterized by fine-grained particles, with a tendency to come apart along a parallel break. Shale usually acts as a barrier to groundwater movement, but in some situations may serve as an aquifer whose water moves primarily through the horizontal layers.

Siltstone:

A sedimentary consolidated rock whose grains are predominantly silt size. Siltstones vary in chemical composition. Siltstones vary in their degree of hardness and may act as aquifers.

Tuff:

Rock produced from ash and rock fragments that have been ejected by violent volcanic eruptions. Some particles may partially re-melt by the residual volcanic heat, thus forming a welded tuff.

Faulting:

Fracturing of rock resulting in parallel displacement relative to each side along the fault zone. Faults easily act as conduits for groundwater.

Interbedding:

Sedimentary deposits of different particle size or mineral content (organic, limestone or sandstone layers) lying parallel to each other. Interbedding is also known as interstratification.

Jointing:

A series of regular breaks in a rock formation. Jointing may be caused by stress or the release of pressure or chemical and temperature changes. Joints provide an easy pathway for groundwater movement.

Karst:

A structure formed inside rock that has been dissolved by groundwater. The types of rock that karst generally occurs in are limestone, dolomite and gypsum. A common example of a karst feature is an underground cave. Karst topography is the visible expression on top of the ground, of karst features below the ground. Karst topography is visible around the Mammoth Cave area of Kentucky. Karst features, particularly small caves or channels, are difficult to detect. Their presence, however, may affect groundwater movement and contaminant "fate and transport."

Laminations:

Thin layers of sediment or sedimentary rock, just less than 1/4 inch thick.

Varve:

Alternating dark and light bands of very fine sediment layers.

Alluvial sediment:

Sediment deposited by flowing water (e.g., streams or rivers).

Glacial alluvial sediment:

Alluvial sediments deposited by flowing water associated with melting glaciers.

Glacial sediments:

Soil and rock material, deposited either by a glacier or by meltwater from a glacier. Particles may be as large as a boulder or as small as bacteria.

Glaciolacustrine sediment:

Fine-grain sediment deposited in stagnant water associated with glacial melting or runoff.

Granular glacial sediment:

Loose, non-layered material deposited by a glacier, probably laid down irregularly by meltwater, resulting in less clay and fine silt-sized particles and greater amounts of sand.

Lacustrine sediment:

Refers to material deposited in stagnant water, such as in lakes and ponds. These sediments are often made up of silt and clay particles less than 0.02 mm in diameter.

Loess:

A silty sediment deposited by wind and often formed near glaciers. Loess soils are easily eroded, carrying away valuable nutrients and organic matter. Loess soils are found in Western Iowa along the Missouri River.

Bulk Density:

The percentage of the rock or soil mass within the volume occupied by both the solid mass and the pore space. Bulk density is affected by the amount of organic matter present, the degree of compaction, and the total amount of water present. Bulk density affects the ease with which contaminants move through the rock or soil. Bulk density is also important in converting contaminant-to-soil weight ratios, for example converting weight of soil to volume of soil.

Grain Density:

The average particle density of all the particles in a representative sample of soil or rock. Used with bulk density, grain density aids in estimating porosity.

Hydraulic conductivity:

(loosely referred to as permeability) A measure of the liquid flow rate through a porous rock or soil. As the hydraulic conductivity of a rock or soil increases, so does the rate at which contaminants move.

Moisture content:

(also known as the water content) The ratio between the amount of water contained in pore spaces and the total amount of solid particles. Moisture content affects a soil or rock's ability to retain contaminants dissolved or carried in the groundwater.

Porosity:

The percentage of empty space (or voids) within the overall total space (or volume) taken up by rock and soil. As porosity increases, the ease with which a contaminant or groundwater moves through the rock or soil also increases.

Permeability:

The ability of a soil or a geologic formation to allow fluid, liquid or gas to move through it. It is dependent on the porosity, jointing, fracturing or faulting of the material.

Plasticity:

The ability of soil or rock to be deformed into a new shape without breaking or cracking or changing perceptibly in volume. Certain clays are high in plasticity, which increases their ability to retard water flow underground.

Spatial variability:

The three-dimensional variation of rock and soil composition. This variation of soils and rock across the landscape, both horizontally and vertically, requires smart sampling to generate accurate information regarding chemical, physical and hydrologic properties.

Aquifer:

A rock formation below the earth's surface that acts as a groundwater reservoir. Aquifers may store their water in joints or fractures, in such rocks as basalt or granite, or between and around rock particles, such as sand in a sandstone. Technically, aquifers must contain an economically viable amount of water of acceptable quality, otherwise they are referred to as "water-bearing units." Aquifers currently provide water to 20% of America's population, both rural and urban.

Aquitard:

A geological formation that acts to restrict or slow groundwater movement. For example, a lens of tightly packed sediment such as clay would act as an aquitard and slow the migration of groundwater contaminants through an area.

Confined Aquifer:

An aquifer that is under pressure by being squeezed between overlying and underlying rocks or sediment (aquitards). In this situation, part of the weight of the overlying rock or sediment is being supported by the water.

Groundwater:

Water found below the ground surface. Groundwater may be geologic in origin and as pristine as it was when it was entrapped by the surrounding rock or it may be subject to daily or seasonal effects depending on the local hydrologic cycle. Groundwater may be pumped from wells and used for drinking water, irrigation and other purposes. It is recharged by rainfall or snowmelt or irrigation water soaking into the ground. Thus, any contaminant in rainfall or snowmelt or irrigation water may be carried into groundwater.

Lateral transport:

The horizontal movement of liquids through geologic material.

Leach:

The action whereby material is dissolved or physically transported by groundwater movement through soil or geologic material.

Plume (contaminant):

The moving body of contaminated water or pure contaminant whose shape and size depend on the geology, groundwater flow, properties of the contaminant source, and the physical and chemical characteristics of the contaminants. Smokestack emissions also can be termed contaminant plumes.

Potentiometric surface:

The surface or level to which the water in an aquifer will rise by hydrostatic (water) pressure. In an unconfined groundwater situation, the potentiometric surface is equal to the the top of the layer of saturation, the top of the aquifer. In a confined groundwater system, part of the weight of the overlying rock or sediment is being supported by the water, thus the potentiometric surface represents the level to which the water would like to rise (as a result of being under pressure) but can't because overlying impermeable rock or sediment is in the way.

Recharge:

The movement of water into an aquifer from a surface or other underground water body.

Saturated sediment:

A sediment or rock formation whose pores are completely filled with water. If a volatile contaminant is dissolved in the water of a saturated sediment, it must diffuse through the water before it can escape into the atmosphere.

Soil gas:

Any gas (vapor), including air, found within soil pores. Sampling a soil gas helps to locate and identify volatile contaminants.

Specific yield:

An aquifer's ability to produce water within a given amount of time.

Unconfined Aquifer:

An aquifer that is not subjected to pressure from overlying rocks and deposits and is often the topmost saturated zone in the subsurface. Water from unconfined aquifers must usually be pumped for retrieval as opposed to being available through an artesian source.

Vadose zone:

The unsaturated and aerated subsurface zone located above the saturated zone where the pore spaces between the sediment or rock particles are filled primarily with air. Vadose water is water found in the zone of aeration and has, therefore, more oxygen than water in the saturated zone. This oxygen can support aerobic microbial life capable of degrading contaminants or the oxygen may also be directly involved in oxidizing contaminants and rendering them less harmful.

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Last Modified: 1 January 2002 by dave eckels
Projects: etd/library/infopages/envgeology.html