Freshwater Explorer Quick Guides

Anion

A negatively charge ion, e.g., Cl^-

Assemblage, stream

A taxonomic or sampled subset of a community as may be collected from a stream.

Assessment endpoint

An explicit expression of the actual environmental value that is to be protected, operationally defined by an ecological entity and its attribute or characteristics.  An assessment endpoint may be identified at any level of organization (e.g., organism, population, community).

Background

The range of naturally occurring substances in waters that have not been substantially influenced by human activity.

Background specific conductivity

The specific conductivity (SC) in streams in a region that occurs naturally and not as the result of human activity.  Background may also be characterized as a population of minimally affected sites or low SC sites using a weight of evidence.

Background-to-Criterion (B-C) Model

A log-log linear regression model of the background conductivity values associated with the estimated 5% extirpation of stream organisms from 24 ecoregions in the United States (Cormier et al 2018).

Benchmark

A dose or concentration of a pollutant that, if exceeded, is expected to produce an adverse effect (called the benchmark response) in one or more assessment endpoints, signifying a decline in water quality or human health.

Bootstrapping

A statistical technique of repeated random sampling from a data set that is often used in environmental studies to estimate confidence and prediction limits of a parameter.

Box plot

A depiction of the 25th, 50th, and 75th quantiles of a distribution as a rectangle with a central line.  The two standard deviation range is depicted as “whiskers” extending from the box.  Data beyond two standard deviations are indicated by individual circles or dots beyond the whiskers.

Catchment area

The spatial extent of the surrounding landscape that drains into a particular river, stream, or other waterbody.

Cation

A positively charged ion, e.g., Na⁺

Community

The full complement of interacting organisms within a defined area of an ecosystem.

Conductance, specific

Conductance is the inverse of resistance for a particular sample expressed as Seimens (S) usually at 25°C.  In the literature, it is sometimes used synonymously with specific conductivity, but to avoid confusion, the term conductance is not used in this document.

Conductivity, specific (or specific electrical conductivity)

A measure of ionic concentration based on the electrical property of water and dissolved ions.  As ionic concentration increases, conductivity increases.  Standardized measurements in this document refer to specific conductivity, μS/cm (also seen as: μmho/cm) at 25°C.

Confounder

An extraneous variable that correlates with both the dependent and independent variable.  The presence of confounders can interfere with the ability to characterize a causal relationship.

Criterion continuous concentration (CCC)

An estimate of the highest concentration of a material in surface water to which an aquatic community can be exposed indefinitely without resulting in an unacceptable effect.

Criterion maximum exposure concentration (CMEC)

An estimate of the maximum concentration of a material in surface water to which an aquatic community can be exposed for a short time without resulting in an unacceptable effect.  In this document, the CMEC is estimated at the 90th centile of specific conductivity observations that contribute to the annual CCC.

Cumulative frequency distribution (CFD)

The probabilities that a random variable with a given probability distribution will be found at a value less than or equal to x.  Weighted CFDs are used to estimate extirpation concentrations of individual genera or species and unweighted CFDs to estimate a SC level that is expected to extirpate 5% of aquatic invertebrate genera. Similar to cumulative distribution function (CDF)

Ephemeral stream

A stream that flows briefly only in direct response to local precipitation, and whose channel is above the local ground water table at all times.

Extirpation

The depletion of a population of a species or genus to the point that it is no longer a viable resource or is unlikely to fulfill its function in the ecosystem.

Extirpation concentration

The level above which a genus is effectively absent from its normal habitat.  The threshold for extirpation is operationally defined by the level below which 95% of the observations of the genus occur.

Extrapolation

The process of extending the applicability of a model beyond the measured range of the original data set from which the model was derived.

Flowing waters

Inland waters with a unidirectional flow including permanent, intermittent and ephemeral streams.

Fresh water

Naturally occurring water characterized by low concentrations of dissolved salts and other total dissolved solids, not brackish or marine water.

Freshwater

Adjective indicating low salt content, typically less than 1,000 ppm or 1500uS/cm.

Generalized additive model

A nonparametric, likelihood based local regression model that replaces the linear function of a generalized linear model with a locally smoothed additive function.

Hazardous concentration

A concentration threshold that is hazardous for a proportion of taxa.  In this document, it is the concentration that is hazardous to 5% of genera calculated as the 5th centile of a taxonomic extirpation concentration distribution.

Hydroline

A feature-based database that interconnects and uniquely identifies the stream segments or reaches that make up the nation's surface water drainage system. The National Hydrography Dataset (NHD) data was originally developed at 1:100,000-scale and exists at that scale for the whole country.

Hydrology

Distribution and connectivity of water such as streams and lakes

Intermittent stream

A stream that flows continuously for only part of the time.  During low flow there may be dry reaches alternating with wetted, nonflowing reaches.  The stream bed may lie below the local ground water table for at least part of the year.

Interpolate

Process of estimating an unknown value that lies between known values.

Ion

an atom or molecule with a net electric charge due to the loss or gain of one or more electrons, e.g., Na⁺ or Cl^-

Ionic composition

The specific ions dissolved in water.  In this document, the ionic composition is used to distinguish water dominated by chloride salts from those dominated by bicarbonate and sulfate salts.

Ionic mixture

An undefined or defined blend of dissolved ions.  In this document, the example case studies refer to the most common mixture of ions contaminating U.S. streams, specifically those dominated by calcium, magnesium, sulfate, and bicarbonate ions.

Ionic regulation

The passive and active physiological processes that maintain the ionic composition, pH, and water content of tissues that is necessary for life.

Least disturbed condition

The best available physical, chemical, and biological habitat conditions given today’s state of the landscape or the least disturbed by human activities (Stoddard et al., 2006).  Contrast with “minimally affected condition.”

Major ions

The most common contributors to ionic concentration in surface waters, consisting of the following cations: Ca²⁺, Mg²⁺, Na⁺, K⁺; and anions: HCO₃⁻, CO₃²⁻, SO₄²⁻, Cl−.

Measure of effect

A measurable ecological characteristic that is related to the valued characteristic chosen as the assessment endpoint and is a measure of biological effects (e.g., survival, reproduction, growth).  In this document it is the presence/absence of macroinvertebrate genera along a specific conductivity gradient.

Measure of exposure

An observed or estimated characteristic that is used to characterize the level of exposure to the stressor.  In this document, the measure of ionic exposure is specific conductivity.

Minimally affected condition

The physical, chemical, and biological habitat found in the absence of significant human disturbance (Stoddard et al., 2006).  Contrast with “least disturbed condition.”

Osmoregulation

The physiological control of water content of an organism's tissues to maintain fluid and electrolyte balance within a cell or organism relative to the surrounding environment.

Perennial stream

A stream with continuous surface or shallow interstitial flow year round, and whose stream bed intersects the local ground water table throughout the year.  Also referred to as a permanent stream.

Predicted 5% Extirpation Concentration

The concentration that is hazardous to 5% of genera calculated as the 5th centile of a taxonomic extirpation concentration distribution.

Predictive background conductivity

Mineral content is the average conductivity predicted monthly for years 2000 – 2015 as they would occur if water quality was minimally influenced by people.

Predictive background conductivity model

A random forest regression model that predicts background conductivity based on geophysical and other data at thousands of sites that were judged to be relatively unaffected by people (Olson and Cormier 2019).

Produced water

Waters that are produced by oil and gas development, mine dewatering, and related activities (e.g., coal bed methane mining, hydraulic fracturing).

Reference site

Sampling locations that have been identified as minimally affected or least disturbed based on land use, habitat, and water quality characteristics other than specific conductivity.

Salinity

The amount of salts dissolved in water.  Traditionally expressed as parts per thousand (‰) or grams of salt per kilogram of water. Salinity (‰) is often reserved for describing marine waters where sodium and chloride are the dominant ions.  In this report, salinity is equivalent to total dissolved salts and may be composed of any ionic mixture.

Sample

A single water or biological collection, multiple samples can be taken at different times.

Sample site

Location where a water or biological collection was taken.

Sensitivity analysis

A process that involves changing input values of a model in various ways to see the effect on the output value.  The main goal of sensitivity analysis is to gain insight into which assumptions are most critical for model building.

Station

Location where a water or biological collection was taken.

Topography

Mapped forms and features of land surfaces.

Total dissolved solids (TDS)

A measure of the combined content of all inorganic and organic substances dissolved in water, conventionally expressed as mg/L and operationally defined as those solids that pass through a filter, typically 0.45 μm.

Total Nitrogen

Sum of nitrate+nitrite and total Kjeldahl nitrogen (TKN) in mg/L.

Total Phosphorus

Data reported as phosphorus were converted to standardized forms and where appropriate summed as Total Phosphorus, Dissolved Phosphorus, Total Orthophosphate, Dissolved Orthophosphate.

Univoltine

An organism having one brood or generation per year.

Validation

Confirmation of the quality of a model and its results, typically by applying an independent data set.

Valley fill

A headwater valley filled with mining overburden.  This practice usually occurs in steep terrain where there are limited disposal alternatives.

Verification

Demonstrating the accuracy of measurements or calculations.

Weight of Evidence

(1) A process of making inferences from multiple pieces of evidence, adapted from the legal metaphor of the scales of justice. (2) The relative degree of support for a conclusion provided by evidence. The result of “weighing the body of evidence.”

Wells

May refer to freshwater wells or oil and gas wells.

Widget

A tool or subroutine used in the U.S. EPA Freshwater Explorer.