Glossary of Terms

About Federal Drinking Water Standards and Advisories

Contaminant concentrations greater than federal health-based thresholds are considered unsafe. The criteria we use to evaluate groundwater quality are all federal, health-based guidelines for safe drinking water, but these guidelines do not all have the same name. Maximum Contaminant Levels were adopted to protect public water supplies under the Safe Drinking Water Act. Health advisories are established for some pollutants that may accumulate in groundwater at levels that are unsafe for those who rely on private wells, but which aren't commonly found in public water supplies. The standards and guidelines we use are described below.

Maximum Contaminant Level (MCLs)

Maximum Contaminant Levels (MCLs) are the highest level of a contaminant that is allowed in public drinking water systems serving more than 25 people for more than 60 days per year. MCLs are set as close to health-based Maximum Contaminant Level Goals as feasible, based on the best available treatment technology and cost considerations. MCLs are enforceable standards, and they are frequently used to evaluate groundwater quality.

Health Advisories (DWAs, LHAs, and CHAs)

The U.S. Environmental Protection Agency publishes unenforceable recommendations for drinking water quality in the form of Health Advisories. These are set at levels that are not expected to cause adverse non-cancer health effects generally (Drinking Water Advisories, or DWAs), in adults exposed over a lifetime (Lifetime Health Advisories, or LHAs), or in children exposed for 1-10 days (Child Health Advisories, or CHAs). Ashtracker relies on Health Advisories to evaluate safe levels of pollutants for which EPA has not established a Maximum Contaminant Level.

Regional Screening Levels (RSLs)

Regional Screening Levels (RSLs) are purely health-based guidelines jointly published by three EPA regions to assist in the investigation of potential superfund sites. These thresholds are updated more often than MCLs and Health Advisories. RSLs cover a range of exposure routes, and Ashtracker uses the RSLs for tapwater to evaluate safe levels of pollutants that don’t have an MCL or a Health Advisory threshold.

Drinking Water Contaminants

Name Threshold Standard Type More Information
ammonia 30.0 mg/L LHA ATSDR 1
antimony 0.006 mg/L MCL ATSDR 1
arsenic 0.01 mg/L MCL ATSDR 1
barium 2.0 mg/L MCL ATSDR 1
beryllium 0.004 mg/L MCL ATSDR 1
boron 3.0 mg/L CHA ATSDR 1
cadmium 0.005 mg/L MCL ATSDR 1
chromium 0.1 mg/L MCL ATSDR 1
cobalt 0.006 mg/L RSL ATSDR 1
copper 1.3 mg/L MCL ATSDR 1
cyanide 0.2 mg/L MCL ATSDR 1
fluoride 4.0 mg/L MCL EPA 2
gross alpha particle 15.0 pCi/L MCL EPA 2
gross beta particle 50.0 pCi/L MCL EPA 2
hexavalent chromium 0.1 mg/L MCL ATSDR 1
lead 0.015 mg/L MCL EPA 2
lithium 0.04 mg/L RSL EPA 2
manganese 0.3 mg/L LHA ATSDR 1
mercury 0.002 mg/L MCL ATSDR 1
molybdenum 0.04 mg/L LHA WHO 3
nickel 0.1 mg/L LHA ATSDR 1
nitrate 10.0 mg/L MCL ATSDR 1
nitrite 1.0 mg/L MCL ATSDR 1
radium 5.0 pCi/L MCL ATSDR 1
selenium 0.05 mg/L MCL ATSDR 1
silver 0.1 mg/L MCL ATSDR 1
strontium 4.0 mg/L LHA ATSDR 1
sulfate 500.0 mg/L DWA WHO 3
thallium 0.002 mg/L MCL ATSDR 1
tritium 20,000.0 pCi/L MCL EPA 2
uranium 0.03 mg/L MCL ATSDR 1

1 Agency for Toxic Substances and Disease Registry, 2 Environmental Protection Agency, 3 World Health Organization

About Groundwater Well Gradient Designations

The words "upgradient" and "downgradient" refer to a well's location relative to the nearest coal ash disposal area and the direction of groundwater flow. Downgradient wells are sited to detect impacts from the nearest coal ash disposal area, while upgradient wells are (typically) sited to reflect "natural" or "background" groundwater conditions.

Gradient designations on Ashtracker are based on the most recent information provided by facilities to state agencies in groundwater monitoring reports. If gradients were not available in the groundwater monitoring reports, we reviewed other documents filed by operators, such as groundwater monitoring plans, permit applications, boring or well drilling logs, and maps.

Users should be aware that up and down gradient designations are self-reported by the operators and may not be accurate. Many disposal facilities are located near large bodies of water and groundwater flow can be influenced by time, seasonality, temperature, and other environmental factors. Coal ash disposal sites themselves can also impact groundwater flow over time. When leachate infiltrates soil under facilities shaped like a bowl, it can cause the water table underneath the bottom of the bowl to rise. This can cause groundwater to flow radially from the center to the edge of the facility’s footprint, contaminating wells that were previously upgradient and therefore outside the path of contaminant flow.

Note: Contamination in upgradient, background, and sidegradient wells are not always indicative of naturally occurring concentrations of pollutants. Wells listed with these designations may be contaminated.

Designation Description
Upgradient or Background
Water samples tested from these wells theoretically show naturally occurring (background) concentrations of contaminants in groundwater. These wells are presumed to be located in groundwater that has not been contaminated by coal ash.
Downgradient or Compliance
Water samples collected from wells that are downgradient of coal ash disposal areas should show signs of coal ash contamination, if there is any.
Groundwater passing through a sidegradient well is moving parallel to the nearest coal ash disposal area. These wells are less likely to be contaminated than downgradient wells, but since groundwater flows change over time, sidegradient wells are occasionally affected.
Groundwater wells situated at a depth between the shallow and deep aquifer.
Groundwater wells located between upgradient and downgradient wells that warn of groundwater contamination moving down the hydraulic gradient of an aquifer toward a downgradient well.
Groundwater wells in which the designation (upgradient or downgradient) of the well is not defined; however, groundwater is tested from these wells determine to contaminant concentrations.
Groundwater-Surface Water Interface (GSI)
Wells that collect water samples from the area between the shallow surface water column and the shallow groundwater zone.
Groundwater is removed from an aquifer using these types of wells in order to obtain water samples that characterize local groundwater quality.