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Zeros of Functions: Inputs to a given function that result in an output of zero. Zeros of a function are the x-intercepts of the function’s graph.
Zeros of Functions: Inputs to a given function that result in an output of zero. Zeros of a function are the x-intercepts of the function’s graph.
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3. Any use of this site or any of the content of this site that is inconsistent with these terms is strictly prohibited.
Fundamental Fluid Concepts
Fluid Mechanics: The study of the behavior of a gas or liquid that is in rest or in motion.
Hydrostatics: The study of fluids at rest or constant velocity.
Kinematics: The study of the geometry of fluid motion.
Fluid Dynamics: The study of the forces that cause accelerated motion.
Solid: Maintains a definite shape and volume. Solids maintain their shape because the molecules or atoms are densely packed and are held tightly together. The cohesive forces in a solid prevent movement, except for slight vibrations, resulting in the limited deformation of the solid when a load is placed on it.
Liquid: A fluid that is composed of molecules that are more mobile in comparison to a solid. Due to weaker intermolecular forces, liquids do not hold their shape. Liquids flow and take the shape of their container. Because of their molecular spacing, liquids can resist compressive forces when confined.
Gas: A fluid that fills the entire volume of its container. Gases are composed of molecules that are much farther apart than those of a liquid. The molecules of a gas are free to travel away from one another until a force of repulsion pushes them away from other gas molecules, or from the molecules on the surface of a container.
Continuum: A continuous distribution of matter, leaving no empty space. Allows for the use of average properties of the fluid at any point throughout its volume.
A fluid and its flow characteristics are described using combinations of units based on five basic quantities: length, time, mass, force, and temperature.
U.S. Customary / Foot-Pound-Second : Length (feet / ft), time (seconds / s), force (pounds / lb), mass (slug).
Slug: 1 slug is equal to the amount of matter accelerated at 1 ft/s2 when acted upon by a force of 1 lb. (slug = lb ⋅ s2/ft).
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Temperature: A measure of the average energy related to molecular motion of a substance.
Absolute Temperature: The temperature measured from a point where the molecules have “zero energy” or no motion. The unit for absolute temperature in the U.S. customary system is degree Rankine (°R).
SI Units: length (meters / m), time (seconds / s), mass (kilograms / kg), force (newton / N).
Newton: 1 newton is equal to the force required to give 1 kilogram of mass an acceleration of 1 m/s2. (N = kg ⋅ m/s2).
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Absolute Temperature: The unit for absolute temperature in the SI system is the kelvin (K).
Conversions exist to convert from US Customary Units to SI Units.
Density: The mass of the fluid that is contained in a unit volume.
Liquid Density: The density of a liquid varies little with pressure and varies slightly more with temperature.
Gas Density: The density of a gas can be greatly affected by temperature and pressure.
Specific Weight: Weight per unit volume of a fluid.
Specific Gravity: The ratio of a substance’s density or specific weight to that of another substance’s density or specific weight. The other substance is taken as a standard.
Bulk Modulus: A measure of the amount by which a fluid offers resistance to compression.
A liquid’s bulk modulus is very high because its density changes little with pressure.
A gas’s bulk modulus will be small because of its low density.
Ideal Gas: A gas that is assumed to have enough separation between molecules so that the molecules are not attracted. Also, the gas must be at a temperature or pressure so that it cannot condense into a liquid or a solid.
Fluids: Liquids and gases that continuously deform or flow when subjected to a shear or tangential force.
Viscosity: The measure of a fluid's ability to resist flow. The higher the viscosity, the more difficult it is to move through the fluid.
Newtonian Fluids: Any fluid that maintains a constant viscosity between the applied shear stress and the velocity gradient.
Non-Newtonian Fluids: Fluids whose very thin layers exhibit a nonlinear behavior between the applied shear stress and the shear strain rate.
Dilatant Fluids: Fluids that have an increase in apparent viscosity with an increase in shear stress.
Pseudoplastic Fluids: Fluids that flow slowly at low applications of shear stress but rapidly under a higher shear stress.
Inviscid Fluid: A fluid that has zero viscosity.
Ideal Fluid: A fluid that is inviscid and incompressible.
Liquid Viscosity: When pressure increases, viscosity increases. When temperature is increased, viscosity decreases.
Gas Viscosity: When temperature is increased, viscosity increases.
Saturated: A state of equilibrium where the number of molecules that evaporate from the liquid equals the number of molecules that condense back to it.
Vapor Pressure: Pressure created by evaporated molecules that bounce off the walls of the tank and the liquid surface. A higher temperature will cause a higher vapor pressure. The liquid will begin to boil when the pressure at its surface is at or below its vapor pressure.
Cavitation: Vapor bubbles from a boiling liquid that migrate to regions of higher pressure, along the surface of the container, and suddenly collapse. A high-speed jet of the liquid enters the collapsing bubble and produces an extremely large localized pressure, which results in a shock wave that strikes the surface.
Cohesion: A force where molecules are attracted to one another.
Adhesion: A force where molecules are attracted to those of a different substance.
Surface Tension (σ): Molecules located at the surface of a liquid have cohesive forces towards the molecules next to it on the surface and also from towards the molecules below it. The net result is a cohesive downward force that tends to produce a contraction of the surface. The downward pull on the surface is similar to the same effect that occurs when molecules of a thin elastic film tend to restore it when it is stretched.
Liquid Drops: The formation of droplets is the result of surface tension. Surface tension minimizes the surface of any droplet, and so it forms a sphere.
Capillary: If the force of a liquid’s adhesion to the molecules on the surface is greater than the force of cohesion between the liquid’s molecules, then the meniscus will curve upwards. If the adhesive force is less than the cohesive force, the meniscus will curve downwards.
Fluid Statics
Pressure: The intensity of force per unit area exerted on a fluid. Pressure is the result of the impulses exerted by the fluid molecules as they collide with and then bounce off the surface.
Pascal’s Law: The pressure at a point in a fluid is the same in all directions, provided the fluid has no relative motion.
Zero Absolute Pressure: If a fluid is removed from its container, a vacuum forms, and the pressure within the container is zero.
Absolute Pressure: Any pressure that is measured above zero absolute pressure.
Gage Pressure: Any pressure measured above or below atmospheric pressure.
When the weight of a static fluid is considered, the pressure in the horizontal direction is constant; however, in the vertical direction, it increases with depth.
If a fluid is incompressible, then its specific weight is constant.
Pressure Head: The height of a column of liquid that produces the gauge pressure.
If a fluid is compressible, then the variation of the fluid’s specific weight (or density) with pressure must be taken into account to obtain an accurate measurement of pressure.
A body can float in a liquid or gas in stable, unstable, or neutral equilibrium. If its metacenter is above the center of gravity of the body, then the body will float in stable equilibrium. If its metacenter is below the center of gravity, then the body will be unstable.
When a cylindrical container of liquid has a constant rotation about a fixed axis, the liquid surface forms a forced vortex having the shape of a paraboloid.
Kinematics of Fluid Motion
System: A specific quantity of fluid that is enclosed within a region of space, apart from the fluid particles outside the region.
Surroundings: The region outside the fluid particles within the system.
Lagrangian Description (Motion of Fluid Particles): Tracking the location of each particle in the flow and reporting its motion.
Eulerian Description (Motion of Fluid Particles): Considers a specific region or point in the flow, and measures the motion of any fluid particles passing through this region or point.
Flow
Volumetric Flow / Flow / Discharge: The rate at which a volume of fluid flows through a cross-sectional area.
Laminar Flow (Frictional Classification): Fluid particles follow straight-line paths since fluid flows in thin layers.
Turbulent Flow (Frictional Classification): Fluid particles follow erratic paths that change direction in space and time.
Transitional Flow (Frictional Classification): A region in which both laminar and turbulent flow coexist.
Steady Flow: When the flow does not change with time.
Uniform Flow: The flow is the same at each location.
Fluid behavior must satisfy the conservation of mass, the principle of work and energy, and the principle of impulse and momentum.
Extensive Property: Any fluid property that depends on the amount of mass.
Intensive Property: Fluid properties that are independent of the system’s mass.
Conduit: A channel or passage through which fluid flows.
Closed Conduit: Fully enclosed channels such as pipes or ducts.
Fluid Work and Energy
The total energy E of a fluid system consists of three parts: kinetic energy, gravitational potential energy, and internal energy.
Kinetic Energy: The energy of motion that depends upon the macroscopic speed of the particles.
Gravitational Potential Energy: Energy due to the elevation of the particles.
Internal Energy: Vibrational or microscopic motion of the atoms and molecules that compose the fluid system. Internal energy also includes any stored potential energy within the atoms and molecules that cause the binding of the particles due to nuclear or electrical forces.
Heat Energy: The result of random motion caused by the jiggling of the fluid particles. Heat energy increases the total energy if it flows in, and decreases the total energy if it flows out.
Work: Organized energy that provides a useful purpose. Work decreases the total energy of the system when it is done by the system, and it increases the total energy of the system when it is done on the system.
Flow Work: Work done on or by the system due to pressure.
Shaft Work: Work done on or by the system due to a pump or turbine.
Shear Work: Work done on or by the system caused by viscous friction.
Open Channels
Open Channel: A conduit that has an open or free surface. The main driving force for the flow is gravity, as opposed to closed conduits, where the flow is driven by pressure.
Canals: Very long and straight, and are used for drainage, irrigation, or navigation. Canals are typically constructed with rectangular or trapezoidal cross sections.
Culverts: Do not flow full, and are usually made of concrete or masonry. They are often used to carry drainage under roadways. Culverts often have circular or elliptical shapes.
Flume: A conduit that is supported above ground and is generally designed to carry drainage over a depression. Flumes are typically constructed with rectangular or trapezoidal cross sections.
Prismatic Channel: A channel that has a constant cross-section.
Rivers and streams have nonprismatic cross sections.
Uniform Flow (Classification): When the depth of the liquid remains the same along the length of the channel, the velocity of the liquid will not change from one location to the next.
Nonuniform Flow (Classification): The depth varies along the length. This can happen if there is a change in slope or where there is a change in the channel’s cross-sectional area.
Accelerated Nonuniform Flow (Classification): When the depth of flow decreases downstream. An example would be water flowing down a chute or spillway.
Retarded Nonuniform Flow (Classification): Occurs if the depth is increasing, as when water in a downward-sloping channel backs up to meet the face of a dam.
Steady Flow (Classification): Occurs when the flow remains constant over time, and so the depth at a specific location remains constant. However, if a wave passes by a specific location, the depth and hence the flow will change with time, and so unsteady flow occurs.
Hydraulic Jump: A localized turbulence that rapidly dissipates kinetic energy, changing the flow from rapid to tranquil. Hydraulic jumps generally occur at the bottom of a chute or spillway.
Weirs: Devices placed within a channel to obstruct the flow, causing the water to back up and then flow over it. If the depth of water passing over the weir is measured, it then provides a way to measure the flow.
Turbomachines
Turbomachines: Pumps and turbines, which transfer energy between the fluid and the rotating blades of the machine.
Pumps: Fans, compressors, and blowers. Pumps add energy to fluids.
Turbines: Remove energy from fluids.
Instruments
Rotational Viscometer: Used to measure the viscosity of a liquid.
Ostwald Viscometer: Used to measure the viscosity of a liquid.
Barometer: Used to measure atmospheric pressure.
Manometer: Used to determine the gage pressure in a liquid.
Bourdon Gage: Used if the gage pressures are very high.
Pressure Transducer: Used to measure pressure.
Hydrometer: Used to measure the specific gravity of a liquid.
Pitot Tube: Used to measure a fluid’s velocity at a point in an open channel.
Pitot Tube + Piezometer: Used to measure a fluid’s velocity in a closed conduit.
Venturi Meter: Used to measure the average velocity or volumetric flow of a fluid through a closed duct or pipe.
The flow of a fluid can also be measured using a nozzle meter, orifice meter, rotameter, turbine flow meter, vortex flow meter, thermal mass flow meter, positive displacement flow meter, nutating disk flow meter, magnetic flow meter, laser doppler flow meter, or ultrasonic flow meter.
Hydrological Cycle
Hydrology: The science of water. Hydrology deals with the occurrence, circulation, and distribution of water on the Earth and the Earth's atmosphere. Hydrology is concerned with streams, lakes, rainfall, snowfall, snow, ice, and water occurring below the Earth's surface in the pores of the soil and rocks.
Water occurs on Earth in all three states: liquid, solid, and gaseous.
Water can vary in its degree of motion.
Transportation Components of the Hydrologic Cycle: Precipitation, evaporation, transpiration, infiltration, and runoff.
Storage Components of the Hydrologic Cycle: Storage on the land surface (depression storage, ponds, lakes, reservoirs), soil moisture storage, and groundwater storage.
The total water resources of the Earth are constant, and the sun is the source of energy for the hydrologic cycle.
Watershed / Catchment Area / Drainage Area / Drainage Basin: The area of land draining into a stream or a water course at a given location. A watershed is separated from its neighboring areas by a ridge.
Glaciers and Permanent Ice Caps: Account for 69% of the total freshwater resource.
Groundwater: Accounts for 30% of the total freshwater resource.
Freshwater Lakes: Account for 0.26% of the total freshwater resource.
Reservoirs: Artificial lakes created by humans through the construction of dams across rivers. Reservoirs are used for irrigation, drinking water, hydro-power generation, and industrial use.
Wetlands / Marshes / Lagoons / Swamps / Bogs/ Mires: Account for 0.04% of the total freshwater resource.
Rivers: Account for 0.006% of freshwater resource.
Typical hydrological data includes weather records, precipitation data, stream flow records, evaporation and evapotranspiration data, infiltration characteristics, soils data, land use and land cover data, groundwater characteristics, physical and geological data, and water quality data.
Precipitation
Precipitation: All forms of water that reach the earth from the atmosphere. Precipitation includes rainfall, snowfall, hail, frost, and dew. The magnitude of any form of precipitation varies with time and space.
Variation is responsible for many hydrological problems, such as floods and droughts.
Rainfall: The predominant form of precipitation. Rainfall is classified by its intensity.
Snow: Consists of ice crystals, which usually combine to form flakes.
Drizzle: A fine sprinkle of numerous water droplets of small size and weak intensity.
Glaze: When rain or drizzle comes in contact with cold ground, the water drops freeze to form an ice coating called glaze.
Sleet: Frozen raindrops that form when rain falls through air at sub-freezing temperatures. Sleet may also mean precipitation of snow and rain simultaneously.
Hail: A showery precipitation in the form of irregular pellets or lumps of ice of size more than 8mm. Hail can occur in violent thunderstorms in which vertical currents are very strong.
For precipitation to form, the atmosphere must have moisture, there must be sufficient nuclei present to aid condensation, weather conditions must be good for condensation of water vapor to take place, and the products of condensation must reach the earth. The water vapor condenses over nuclei to form tiny water droplets. The nuclei are usually salt particles or products of combustion. Precipitation results when water droplets come together and coalesce to form larger drops that can fall.
Weather Systems Associated with Precipitation: Front, cyclone, tropical cyclone, extratropical cyclone, anticyclone, convective precipitation, and orographic precipitation.
Front: The interface between two distinct air masses. A front is when a warm air mass meets a cold air mass. The warmer air mass is lifted over the colder one with the formation of a front.
Cyclone: A large low-pressure region with circular wind motion.
Anticyclones: Regions of high pressure.
Convective Precipitation: A packet of air that is warmer than the surrounding air due to localized heating rises because of its lesser density. Air from cooler surroundings flows to take up its place, thus creating a convective cell. The warm air continues to rise, undergoes cooling, and results in precipitation.
Orographic Precipitation: Moist air masses get lifted up to higher altitudes due to the presence of mountain barriers. The lifted moist air undergoes cooling, condensation, and precipitation.
Evaporation
Evapotranspiration: Evaporation from water bodies and soil masses together with transpiration from vegetation.
Evaporation: The process in which a liquid changes to the gaseous state at the free surface, below the boiling point, through the transfer of heat energy.
The rate of evaporation is dependent on the vapor pressures at the water surface and air above, air and water temperatures, wind speed, atmospheric pressure, quality of water, and size of the body of water. The rate of evaporation increases with higher temperatures, higher wind speeds, and a decrease in barometric pressure. An increase in soluble salts causes a reduction in the rate of evaporation.
Methods to reduce evaporation include reducing the surface area of the body of water, using mechanical covers, and utilizing chemical films such as cetyl alcohol.
Transpiration: The process by which water leaves the body of a living plant and reaches the atmosphere as water vapor. Transpiration is affected by atmospheric vapor pressure, temperature, wind, light intensity, and plant characteristics. Transpiration is confined to daylight hours, and the rate of transpiration depends upon the growth periods of the plant.
Water Storage
Field Capacity: The maximum quantity of water that the soil can retain against the force of gravity. Any higher moisture input to a soil at field capacity simply drains away.
Permanent Wilting Point: The moisture content of a soil at which the moisture is no longer available in sufficient quantity to sustain plants.
Available Water: The moisture available for plant growth.
Interception: The volume of water caught by vegetation. Interception may result (1) in the vegetation retaining the water, and it eventually being returned to the atmosphere via evaporation. (2) in the water dripping off the plant leaves to join the ground surface. (3) in the rainwater running along the leaves and branches and down the stem to reach the ground surface.
Depression Storage: The volume of water trapped in depressions. When precipitation reaches the ground, it must first fill up all depressions before it can flow over the surface. Depression storage depends on the type of soil, the condition of the surface reflecting the amount and nature of depression, the slope of the catchment, and the antecedent precipitation, as a measure of the soil moisture.
Infiltration: The flow of water into the ground through the soil surface.
Infiltration Capacity: The maximum rate at which a given soil at a given time can absorb water.
Soil Characteristics: The percentage of sand, silt, and clay, texture, structure, permeability, and underdrainage.
Water infiltrating into the soil will have many impurities, both in solution and in suspension. The turbidity of water, especially its clay and colloid content, is an important factor, as these suspended particles block the fine pores in the soil and reduce infiltration capacity.
Group A Soils (Sand, Loamy Sand, or Sandy Loam): Soils have Low runoff potentials and high infiltration rates.
Group B Soils (Silty Loam or Loam): Soils have moderate infiltration rates when thoroughly wetted.
Group C Soils (Sandy Clay Loam): Soils have low infiltration rates when thoroughly wetted and consist chiefly of soils that impede downward movement of water and soils with moderately fine to fine texture.
Group D Soils (Clay Loam, Silty Clay Loam, Sandy Clay, Silty Clay, or Clay): Soils have high runoff potential and low infiltration rates when thoroughly wetted.
Flow
Runoff: The draining or flowing off of precipitation from a catchment area through a surface channel. Runoff represents the output from the catchment in a given unit of time. For a given precipitation, the evapotranspiration, initial loss, infiltration, and detention storage requirements must be satisfied before runoff commences.
Direct Runoff: The part of the runoff that enters the stream immediately after the rainfall.
Base Flow: The delayed flow that reaches a stream, essentially as groundwater.
Natural Flow: Stream flow in its natural condition, without human intervention.
Hydrological Drought: A hydrological drought has four components: magnitude, duration, severity, and frequency of occurrence. Droughts are due to temporal and spatial aberrations in the rainfall, improper management of available water, and lack of soil and water conservation.
Drought Strategies: Cloud seeding, evaporation control, water harvesting, and change of land use.
Floods: An unusually high stage in a river, normally the level at which the river overflows its banks and inundates the adjoining area.
Flood Control Structural Measures: Storage and detention reservoirs, new floodways, watershed management, levees, channel improvements.
Flood Control Non-Structural Methods: Flood plain zoning, evacuation and relocation, flood forecast/warning, and flood insurance.
Levees: Earthen banks constructed parallel to the course of the river to confine it to a fixed course and limited cross-sectional width.
Floodways: Natural channels into which a part of the flood will be diverted during high stages.
Groundwater
Groundwater: Derived from precipitation and recharge from surface water. Infiltrated water moves downward due to gravity and fills the pore spaces of the soil medium from the bottom upward.
Recharge: Addition of surface water to the zone of saturation.
Zone of Aeration (Vadose Zone): The zone adjacent to the ground surface.
Hygroscopic Water: Water that sticks to soil and is unavailable to plants.
Capillary Water: Held by surface tension and is available to plants.
Gravitational Water: Water held loosely in soil pores and drained freely by gravity.
Soil Water Belt: A band extending from the land surface to a depth from which vegetation on the surface would draw water through the root system and discharge it into the atmosphere by transpiration.
Intermediate Vadose Belt: The subzone between the soil water belt and capillary fringe.
Capillary Fringe: The zone that lies directly above the saturated water zone. Water is held by capillary action.
Zone of saturation (Ground Water Zone): The zone where all the pore space is fully saturated.
All earth materials, from soils to rocks, have pores.
Aquifer: A saturated formation of earth material that not only stores water but yields it in sufficient quantity. Aquifers have a high permeability.
Aquitard: Only seepage is possible, and thus the yield is insignificant compared to an aquifer. Aquitards are partly permeable.
Aquiclude: Essentially impermeable to the flow of water. It is closed to water movement even though it may contain large amounts of water due to its high porosity.
Aquifuge: Neither porous nor permeable. A massive compact rock without any fractures is an aquifuge.
Water Table: The free water surface in an unconfined aquifer.
Effluent Streams (Gaining Streams): Streams that receive groundwater flow.
Influent Streams (Losing Streams): Streams that contribute to the groundwater.
Open Wells (Dug Wells): Used for drinking water supply in rural communities and in small farming operations. Wells are best suited for shallow or low-yielding aquifers.
Erosion
Erosion: The wearing away of land.
Factors Affecting Erosion: Rainfall intensity, duration of rainfall, temperature, soil mass characteristics, grain size and shape, slope of the topography, vegetation cover, agricultural practice, mining, roads, building consturtion, reservoirs.
Reservoir Sedimentation Survey: Used to obtain data on the rate of sedimentation, sediment densities, depositional patterns, and loss of storage capacity of the reservoir at various elevations.
Federal Water Pollution Control Act (FWPCA) & Safe Drinking Water Act (SDWA): Address the quality of surface waters and domestic supplies.
Safe Drinking Water Act: Regulates pesticides and other hazardous substances.
Clean Water Act: Regulates hazardous water pollutants.
Major Water Pollutants: Pathogens, organic chemicals, toxic trace organics, nutrients, heavy metals, and ionizing radiation.
Significant Water-Quality Problems: Trace levels of toxic organics, lead, copper, radionuclides, concentrated animal feedlots, and hydraulic fracturing (fracking).
Water Rights: Traditionally, a state issue.
Riparian Rights: Each owner of riparian land is entitled to receive water, undiminished in quality or quantity, from upstream users. This entitlement puts an obligation on each riparian landowner not to degrade the water source.
Appropriative Rights: The system of appropriative water rights provides a mechanism for obtaining water rights as a function of water use. The use of water gives the user the right to that water supply. The earliest water users have priority for water use if the supply is limited; subsequent users have inferior rights. Water is appropriated, and rights established, through the actual consumptive use of water. If water is not used where an appropriative right exists, that right can be lost. Typically, agricultural, industrial, and domestic uses are all accepted as rights that may be secured.
Groundwater Rights: Groundwater is regulated in most states as a public good. Groundwater regulation is primarily carried out at the state and local levels.
Risk: Likelihood x Consequence. A risk is the chance of harmful effects to human health or to ecological systems resulting from exposure to an environmental hazard.
Hazard: Any physical, chemical, or biological entity that can induce an adverse effect in humans or ecosystems.
Human Health Risk Assessment (Planning Stage): Identifying the hazards, their sources, how exposure occurs, who or what is being exposed, and the team members assigned. The planning stage includes determining who/what/where is at risk, what environmental hazard is of concern (radiation, dust, heat, microbiological, biological, nutritional, socio-economic), and where these environmental hazards originate.
Point Source: A single identifiable source of air or water contaminants such as factories, sewage treatment plants, industrial pipes, smokestacks, concentrated animal feeding operations, and mining operations.
Non-point Sources: Come from many diffuse sources: automobile exhaust, pesticides, herbicides, land precipitation, atmospheric deposition, drainage, and seepage.
Three Basic Exposure Pathways: Inhalation, ingestion, or direct contact absorption.
Hazards can bioaccumulate in the food chain.
Contaminants: Inputs of alien and potentially toxic substances into the environment; not all contaminants cause pollution, as their concentrations may be too low.
Contaminant (EPA): Any physical, chemical, biological, or radiological substance or matter in water.
Contaminant (Law): Anything other than water molecules.
Contamination: The presence of a substance where it should not be or at concentrations above background.
Pollutants: Anthropogenically introduced substances that have harmful effects on the environment.
Pollution: Contamination that results in, or can result in, adverse biological effects on communities. All pollutants are contaminants, but not all contaminants are pollutants.
Acute Toxic Effect: Right away or within a few hours to a day.
Subchronic Toxic Effect: Weeks or months.
Chronic: A significant part of a lifetime (at least seven years).
The toxicity of a chemical can depend on when the chemical is absorbed.
Exposure Assessment: Focused on the magnitude, frequency, and duration of a contaminant.
Direct Exposure Assessment: Point-of-Contact approaches, which more directly measure exposures or doses.
Indirect Exposure Assessment: A method that relies on an exposure scenario to estimate exposures or doses. An exposure scenario is a set of facts, assumptions, and inferences about how exposure takes place.
Exposure Reconstruction Exposure Assessment: Estimates exposure using information on an effect or outcome.
Threshold Model (Non-Linear): Dose-Response assessment for chronic effects of exposure.
Chronic Response: One can be exposed to a limited amount of a contaminant until adverse health effects are observed.
Non-Threshold Model (Linear): EPA classifies this model as an acute effects model. This response is short-term (1-14 days) and may result in adverse health effects.
Ecological Risk Assessment: The process for evaluating how likely it is that the environment might be impacted as a result of exposure to one or more environmental stressors, such as chemicals, land-use change, disease, and invasive species. The planning of an ecological risk assessment includes problem formulation, analysis, and risk characterization.
Maximum Contaminant Levels (MCL): The highest level of a contaminant that is allowed in drinking water.
Exposure Reconstruction: Uses post-exposure information to rebuild exposure effects.
Biomarker: An objective measure that captures what is happening in a cell or an organism at a given moment.
Practice Sheets
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