Assessing Water Quality (Questions and Answers)This fact sheet answers basic questions about how states assess and report on water quality conditions, as summarized in the National Assessment Database.
Water quality assessment begins with water quality standards. States and other jurisdictions adopt water quality standards for their waters. EPA must then approve these standards before they become effective under the Clean Water Act.
Water quality standards have three elements: the designated uses assigned to waters (e.g., swimming, the protection and propagation of aquatic life, drinking), the criteria or thresholds that protect fish and humans from exposure to levels of pollution that may cause adverse effects, and the anti-degradation policy intended to prevent waters from deteriorating from their current condition.
After setting standards, states assess their waters to determine the degree to which these standards are being met. To do so, states may take biological, chemical, and physical measures of their waters; sample fish tissue and sediments; and evaluate land use data, predictive models, and surveys.
Waters rated by the states as "good" fully support all of their designated uses.
Waters rated by the states as "threatened" currently support all of their designated uses, but one or more of those uses may become impaired in the future (i.e., water quality may be exhibiting a deteriorating trend) if pollution control actions are not taken.
Waters rated as "impaired" by the states cannot support one or more of their designated uses. top
Where possible, states, tribes and other jurisdictions identify the pollutants or stressors causing water quality impairment. These causes of impairment keep waters from meeting the criteria adopted by the states to protect designated uses. Causes of impairment include chemical contaminants (such as PCBs, metals, and oxygen-depleting substances), physical conditions (such as elevated temperature, excessive siltation, or alterations of habitat), and biological contaminants (such as bacteria and noxious aquatic weeds). top
Where possible, states, tribes and other jurisdictions identify where pollutants or stressors (causes of impairment) are coming from. These sources of impairment are the activities, facilities, or conditions that generate the pollutants that keep waters from meeting the criteria adopted by the states to protect designated uses. Sources of impairment include, for example, municipal sewage treatment plants, factories, storm sewers, modification of hydrology, agricultural runoff, and runoff from city streets. top
It is not appropriate to use the information in this database to make statements about national trends in water quality. The methods states use to monitor and assess their waters and report their findings vary from state to state and even over time. Many states target their limited monitoring resources to waters they suspect are impaired and, therefore, assess only a small percentage of their waters. These may not reflect conditions in state waters as a whole. States often monitor a different set of waters from cycle to cycle. Even weather conditions - such as prolonged drought - can have an impact on whether waters meet their standards from one year to the next.
The science of monitoring and assessment itself changes. We know that a number of states have increased the amount of fish tissue sampling they conduct and, as a result, are issuing more protective fish consumption advisories. We don't think this means necessarily that there are new pollution problems; it's likely that states are able to identify them better as monitoring and analytical methods progress. States may also, over time, change how they issue or count fish consumption advisories.
Another fact to consider is that, for this reporting cycle and beyond, EPA has dramatically changed its monitoring and assessment guidance to the states. For the first time in November 2001, we encouraged states to combine water quality reporting under Section 305(b) of the Clean Water Act with Section 303(d) (which requires identification of impaired waters and implementation of pollution controls for those waters). States were also to submit their reports electronically using either the Assessment Database developed by EPA or a compatible database. Some states followed this guidance, others did so only partially, and others could not change their reporting systems in time for the 2002 submissions. top
National water quality trends are best determined using scientifically-based studies designed to sample water quality conditions at randomly-selected sites that are statistically representative of the Nation's many distinct ecological regions. EPA and the states have embarked on such probability-based studies of near coastal conditions (see the National Coastal Condition Report II or NCCR; http://www.epa.gov/owow/oceans/nccr/index.html) and the biological health of wadeable streams. The NCCR uses nationally consistent methods and probability-based design to assess five key indicators of coastal water health. The Wadeable Streams Assessment (http://www.epa.gov/owow/monitoring/wsa/index.html) will provide a scientific baseline of stream water quality based on 1,100 randomly-selected sites across the country. top
State water quality assessments are normally based upon five broad types of monitoring data: biological integrity, chemical, physical, habitat, and toxicity. Each type of data yields an assessment that must then be integrated with other data types for an overall assessment. Depending on the designated use, one data type may be more informative than others for making the assessment. For example:
Biological integrity data are objective measurements of aquatic biological communities (usually aquatic insects, fish, or algae) used to evaluate the condition of an aquatic ecosystem. Biological data are best used when deciding whether waters support aquatic life uses.
Chemical data include measurements of key chemical constituents in water, sediments, and fish tissue. Examples of these measurements include metals, oils, pesticides, and nutrients such as nitrogen and phosphorus. Monitoring for specific chemicals helps states identify the causes for impairment and helps trace the source of the impairment.
Physical data include characteristics of water such as temperature, flow, dissolved oxygen, and pH. Physical attributes are useful screening indicators of potential problems, often because they can have an impact on the effects of chemicals.
Habitat assessments include descriptions of sites and surrounding land uses; condition of streamside vegetation; and measurement of features such as stream width, depth, flow and substrate. They are used to supplement and interpret other kinds of data.
Toxicity testing is used to determine whether an aquatic life use is being attained. Toxicity data are generated by exposing selected organisms such as fathead minnows or daphnia ("water fleas") to known dilutions of water taken from the sampling location. These tests can help determine whether poor water quality results from toxins or degraded habitat. top
Hundreds of organizations around the country conduct some type of water quality monitoring. These include federal agencies such as the EPA and the U.S. Geological Survey. They also include state, interstate, tribal and local water quality agencies; research organizations such as universities; industries and sewage and water treatment plants; and citizen volunteer programs. They may collect water quality data for their own purposes or to share with government decision makers. States evaluate and use much of these data when preparing their water quality reports.
The states, territories, and tribes maintain monitoring programs to support several objectives, including assessing whether water is safe for drinking, swimming, and fishing. States also use monitoring data to