National Wetland Condition Assessment:

KEY FINDINGS ON 2021 CONDITION

The survey characterized wetlands using indicators of ecological condition and stress. Key results for 2021 are summarized below. Following standard practices (described in the background section), EPA analysts classified results for most indicators as good, fair or poor. The nonnative plant indicator had an additional condition category of very poor. For microcystins, a human health indicator, the EPA classified wetlands based on whether they exceeded the agency’s recommended recreational water quality criterion. Note that analyses of the soils data are still underway; this report will be updated when the results are available.

Less than half of wetland area was rated good, based on an analysis of plant communities.

• The EPA calculated a vegetation indicator score by combining several metrics based on abundance and types of plant species into one value.
• The EPA found 45% of wetland area was in good condition.

Nonnative plants were a widespread concern.

• Using an indicator based on the occurrence and abundance of nonnative plants, the EPA found less than half of wetland area, 48%, was rated as being in good condition, and 24% was rated poor or very poor.

Physical alterations to wetlands were the most widespread stressors measured. The NWCA reports on six indicators of physical alteration, based on evidence of certain human activities at each site. These alterations directly affect vegetation, hydrology (water levels and the flow of water), or soil. The NWCA also measures the presence of multiple types of alterations at each site by combining the results of the six indicators.

• The combined indicator showed that 82% of wetland area was in fair or poor condition.
• Indicators for both vegetation replacement (for example, replacement of native vegetation with pasture or croplands) and vegetation removal (such as clearing or excessive grazing by livestock) showed 56% of wetland area was in fair or poor condition.
• The indicator for soil hardening (for example, soil compaction or hardened surfaces such as roads) showed fair or poor condition for 49% of wetland area.

Nutrient levels were elevated for some wetlands.

• Nitrogen and phosphorus levels were measured at wetland sites where enough surface water was present to collect a water sample. Nitrogen and phosphorus conditions were found to be poor at 17% and 24% of wetland area, respectively. However, because many wetlands did not have surface water, 40% of wetland area could not be assessed for water chemistry.
• In wetlands where phosphorus and nitrogen were elevated, biological condition expressed by the nonnative plant indicator was 1.6 and 1.7 times more likely, respectively, to be rated poor or very poor.

Microcystins, a type of cyanobacterial toxin, were present — but at very low levels — posing minimal recreational human health concerns.

• Microcystins were detected in 30% but exceeded the EPA’s recommended water quality criterion in less than 1% of wetland area. However, because many wetlands did not have surface water, 40% of wetland area could not be assessed for microcystins.

KEY FINDINGS ON CHANGE FROM 2016 TO 2021

For biological indicators (vegetation and nonnative plants), there was little change between surveys at the national level.

There were significant changes in some chemical and human health measures in 2021.

• Wetland area in good condition for nitrogen levels increased 8.4 percentage points, to 29%.
• Detection of microcystins increased by almost 22 percentage points, to 30%, but the small percentage of wetland area with concentrations that exceeded human health risk benchmarks (<1%) did not change significantly.

NWCA DASHBOARD

The EPA developed an interactive dashboard to accompany this report. It contains regional results and allows comparisons between NWCA wetland groups. The NWCA dashboard also provides wetland acreage for condition categories.

In addition to the link above, readers can access the dashboard by following the link at the bottom of each graph in this report, which will bring them to a customized page with regional data for each indicator. Dashboard users can then navigate to other views using the "Condition Estimates" dropdown and other dashboard controls.

HOW CAN I FIND OUT MORE?

Read the other sections of this report for more detail on the national results for each indicator, including data from 2011 and 2016 where available. Explore the interactive dashboard to compare conditions in different types of wetlands. See the NWCA 2021 Technical Support Document (U.S. EPA 2024) for technical details on the survey design and data analyses that underpin the findings in this report. Additional information about the NWCA and previous reports is available at the NWCA home page. The page is updated as new resources come out, so readers may want to visit periodically. Other NWCA information, such as published scientific research, is available at the NARS home page, along with NARS results for other surveys.

ABOUT WETLANDS

Wetlands are areas where water covers the soil or is present either at or near the surface of the soil all year or for varying periods of time during the year. For the NWCA, the EPA, states, Tribes and other partners surveyed many types of wetlands across the United States, including coastal salt marshes, mangrove swamps, inland marshes, seasonal wet prairie, bogs and forested wetlands. See the EPA’s wetland pages for a description of common wetland types.

CHOOSING INDICATORS

The EPA used 13 indicators to assess the biological, physical and chemical condition of wetlands. Indicators were chosen for their utility in reflecting ecological condition of wetlands or in indicating stress, which may influence condition. The EPA also considered indicators’ applicability across wetland types and environments and the ability of crews to collect the needed data during a one-day site visit (see the technical support document). The EPA also used one indicator to assess characteristics that pose risks to human health. Although there are others that could be used to describe wetland condition, the agency has determined these indicators align with the goals of the survey described earlier and are the most representative at a national scale. The EPA grouped indicators into four categories.

The survey included two biological indicators:

Vegetation | Nonnative plants.

The EPA measured seven physical indicators. Six individual indicators measured physical alterations to vegetation, hydrology, and soil, and one combined results from all six:

Vegetation removal | Vegetation replacement | Flow obstruction | Water addition or subtraction | Soil hardening | Surface modification | Physical alterations (sum).

There were three chemical indicators:

Soil heavy metals (coming soon) | Water chemistry (nitrogen and phosphorus).

Lastly, the survey tracked one human health indicator:

Microcystins.

SELECTING WETLANDS

The EPA used a statistical sampling approach to randomly select 933 of the wetland sites visited for this assessment. This set of sites was used to estimate the condition of the broader population of wetlands across the United States and ensure that survey results were unbiased. For more information on statistical surveys, see What Are Probability Surveys? and Selecting a Sampling Design. The target population for the NWCA was the set of wetlands in the conterminous United States meeting the definition below.

Four Wetland Groups Used to Classify Wetlands for NWCA 2021

The 933 wetland sites mentioned above were identified using a statistical method called stratified random sampling. This approach is used in environmental, social science and health fields to more accurately assess subgroups within the larger population being studied. The NWCA 2021 stratified design randomly selected wetlands within several different categories, including ecoregion and wetland group. This method improves confidence in results associated with these subpopulations while maintaining robust and representative national results. See the design documentation and technical support document for more information.

The statistical design of the survey allows the EPA to extrapolate the results from the 933 wetland sites sampled to the 81,694,381 acres of wetlands meeting the criteria for the NWCA target population. Throughout this report, percentages reported for a given indicator apply to the 81,694,381 acres of wetlands in the target population. For example, if the condition is described as poor for 10% of wetlands nationally, this means that the area estimated to be degraded for that indicator is 8,169,438 wetland acres.

To produce the national results for each indicator, the EPA used established statistical approaches to assign each of the 933 sites a weight based on the total wetland area that the site represents. This enabled the EPA to estimate the proportion of wetland area in each condition category (for instance, good, fair or poor). See the technical support document for details.

When designing the survey, the EPA considered the number of wetland sites that should be sampled; a greater number of sites provides more confidence in the results. The 933 sites sampled in NWCA 2021 allow the EPA to determine the extent of wetlands in each condition category for each indicator within a margin of error of approximately ±5%, with 95% confidence at the national scale. See Exhibit 2 for a map of the 2021 sampling sites and their distribution across five ecoregions.

Map of NWCA 2021 Sampling Sites in Each Ecoregion

FIELD SAMPLING

To ensure consistency in collection procedures and to assure the quality of resulting data, field crews participated in training, used standardized field methods and followed strict quality control protocols (U.S. EPA 2021). Field and lab results were used to calculate scores for each indicator.

Field crews visited each wetland site once. They spent a full day collecting ecological data in a 0.5-hectare (1.24 acres) assessment area whose location was specified by the survey design documents. Crews collected additional data in a buffer area extending 100 meters from the assessment area edge (see Exhibit 3).

The crews collected data on vegetation, soils, hydrology and water chemistry from the assessment area. Crews also collected data on the presence of human-caused physical alterations observed in the assessment area and buffer.

Field crews used the standard layout shown in Exhibit 3 at most sites. Occasionally this layout was not possible because of the size and shape of the wetland at the specified location or because upland or deep water was present. In such cases, the NWCA protocols allowed alternate assessment area layouts.

NWCA 2021 Standard Assessment Area Layout

ASSESSMENT BENCHMARKS

NWCA analysts reviewed the raw data for each indicator. Based on each site’s data, they assigned a condition category to the site (for example, good, fair or poor) for each indicator. Thus, a site could be in good condition based on data for one indicator and poor condition based on another. To assign the appropriate condition category, the NWCA 2021 used two types of assessment benchmarks.

The first type consisted of fixed benchmarks based on values in the peer-reviewed scientific literature, values published by the EPA, or professional judgment by the scientists analyzing the data. For example, the EPA’s recommended water quality criterion was used nationally to classify wetlands as "above the criterion" or "at or below the criterion" for microcystins. Most fixed benchmarks did not change, but the physical alteration indicators were revised for NWCA 2021 as part of an adjustment of physical alteration assessment protocols. Please see the appendix for additional information.

The second type consisted of benchmarks based on the distribution of indicator values from a set of wetland reference sites. The EPA set benchmarks nationally, regionally or by wetland group, based on the indicators used or variations in indicator values. Such variations reflect differences in climate, geology, chemistry, ecology and human disturbance. See the results chapter of this report for more on how benchmarks were applied.

The steps below describe the EPA’s process for setting benchmarks based on these reference site distributions and then determining wetland conditions. Exhibits 4‑7 provide an example of how the benchmark for the vegetation indicator for estuarine herbaceous wetlands was derived.

The process for setting condition benchmarks for the other wetland groups and for chemical indicators was similar. The technical support document provides indicator-specific details (including details on screening for reference wetland sites). The EPA developed the reference-based benchmarks used in this report during earlier assessments.

1. Screen Wetlands to Identify Reference Sites. First, NWCA teams compiled wetland information from the 2011 and 2016 surveys for both the statistically selected sites and a smaller set of targeted sites thought to have low levels of human disturbance. EPA scientists evaluated these sites by considering reference screening factors such as physical alterations, soil heavy metals and nonnative plant cover. Wetlands that passed screening were considered less disturbed than others and were used in establishing the reference distribution (that is, the range of conditions observed at reference wetlands and used to set benchmarks). Exhibit 4 shows a map of estuarine herbaceous wetland reference sites. All were located in the Coastal Plains and Eastern Mountains/Upper Midwest ecoregions.

Estuarine Herbaceous Wetland Reference Sites

Each dot in Exhibit 5 indicates the observed vegetation score at one of the reference sites. Possible index values range from zero to 100, with higher values indicating better wetland condition. Many of the dots overlap because they have similar values.

Vegetation Index Values at Estuarine Herbaceous Reference Sites

2. Calculate Condition Benchmarks Using Reference Site Data. The EPA then used the 5th and 25th percentiles of the distribution of vegetation values observed at reference sites to set the benchmarks for the condition categories (see Exhibit 6).

Benchmarks for Condition, Based on Vegetation Index Values at Estuarine Herbaceous Reference Sites

3. Assign Condition Categories to Wetlands. Using those benchmarks, the EPA assigned the condition (good/fair/poor) to each of the estuarine herbaceous wetlands that were randomly sampled as part of the NWCA. Exhibit 7 shows the vegetation value for each estuarine herbaceous wetland site sampled in 2021 and the condition category in which it falls.

Vegetation Index Values at Estuarine Herbaceous Sites Randomly Sampled During NWCA 2021

The NWCA assessment benchmarks have no legal effect and are not equivalent to individual state water quality standards, nor do the condition categories correspond to the categories states and Tribes use when they assess the quality of wetlands relative to their specific water quality standards under the Clean Water Act. For example, a rating of poor condition in this report does not necessarily mean a site is "impaired" as defined by state and Tribal water quality standards assessment protocols. For additional information on state-specific and local water quality data and assessments, visit the EPA How’s My Waterway application. To learn more about water quality standards, visit the EPA Water Quality Standards Academy.

BIOLOGICAL INDICATORS

Vegetation is a significant component of the biodiversity and structure of wetlands, and it provides habitat for microbes, insects, amphibians, reptiles, birds and mammals. The composition and abundance of plant species reflect and influence wetland hydrology, water chemistry and soil properties. Vegetation is a particularly good indicator of wetland condition because of its ability to respond to physical, chemical and biological disturbances at multiple temporal and spatial scales. The EPA used a vegetation indicator to evaluate wetland conditions as a whole and used a nonnative plant indicator to assess the impact of nonnative species on wetlands.

Vegetation

As mentioned above, the plant species at a site reflect climate and environmental conditions such as hydrology, soil properties and water chemistry. For example, plants such as cattails have special adaptations for getting oxygen to their roots, allowing them to grow in flooded soils, and salt marsh grasses have adaptations that limit salt uptake. Human-caused disturbances that alter ecosystem properties can result in degraded wetlands. Plant communities may respond to this degradation with changes in the abundance of individual species or plant groups (such as shrubs, grasses, or plants favoring wetter or drier conditions) or with the disappearance or addition of species to a site. This responsiveness makes vegetation a particularly good indicator of wetland condition.

Because wetlands are complex and vary so much nationally and even locally, a good way to compare them is to create an index. Rather than comparing sites for the presence of specific species, scientists use indexes that can take into account the number and abundance of species, the kinds of species and their sensitivity to disturbance. Such indexes can also be customized by wetland type and region.

A multimetric index combines multiple individual metrics into one index score to provide a holistic description of condition. The EPA developed four such indexes for vegetation, one for each NWCA wetland group (estuarine herbaceous, estuarine woody, inland herbaceous and inland woody). Each index evaluates four to six vegetation characteristics.

The characteristics measured for each index varied but had common elements. Every index, for instance, included metrics describing species’ tolerance to human disturbance.

Botanists measure the ability of plants to tolerate disturbance by assigning a value from 0 to 10 for each species, where 10 is most sensitive to disturbance. This value may vary from state to state since species sensitivity can vary due to genetics, climate and ecological interactions.

Every index measured key aspects of the health of native species, such as the area covered by native species, the number of native species, or the percentage of all species that were native. Several indexes also considered the prominence of certain types of plants, such as graminoids (grasses and grass-like plants) or monocots (which include graminoids, orchids, lilies, irises and others). The measures chosen for each of the wetland groups are described in the technical support document.

The EPA calculated benchmarks for good, fair and poor conditions for each index (see the assessment benchmarks section for a description of the benchmark development process). For three wetlands groups, benchmarks were set for the whole group, irrespective of location. For the inland woody wetlands group, the EPA developed two sets of benchmarks, one for sites in the Xeric and Interior Plains ecoregions and one for sites in the Eastern Mountains and Upper Midwest, Western Mountains, and Coastal Plains ecoregions. See the technical support document for details.

Almost one half of U.S. wetland area, 45%, was rated good based on vegetation, as shown below. The assessment also showed that 20% of wetland area was in fair condition, and 34% was in poor condition. Regionally, wetland conditions based on vegetation varied, but the Interior Plains, Western Mountains and Xeric ecoregions had the greatest percentages of area in poor condition, with 70%, 71% and 84%, respectively.

No statistically significant changes occurred nationally. However, a statistically significant decrease in area in good condition of 15 percentage points was observed for the Western Mountains subpopulation.

Nonnative Plants

Nonnative plants can directly and indirectly affect native vegetation and ecosystem components. Some nonnative plant species become invasive outside their regions of origin because they outcompete native species or change ecosystem properties. Even species native to one part of the United States can become invasive elsewhere. Saltmarsh cordgrass, for instance, is native to the East Coast but is invasive on the West Coast. Nonnative species that are not currently considered invasive may become so over time, and the presence of nonnative species indicates disturbed conditions and change from the expected plant community.

When native plants are outcompeted by nonnative plants, the insects, amphibians, birds and mammals that rely on native plants also face pressure, because nonnative vegetation often provides inferior habitat and food. In addition, changes driven by nonnative plants to vegetation structure or to the kinds of plants present can alter ecosystem services like flood reduction, carbon storage and water-quality enhancement.

The nonnative plant indicator includes three complementary metrics readily calculated from field observations. These metrics consider the percentage of area covered by nonnative species, the number of unique nonnative species, and the number of plots containing nonnatives (the frequency of nonnative species occurrence).

Wetlands were assessed as having good, fair, poor or very poor condition for this indicator based on whether the calculated value for each metric exceeded benchmark values established using best professional judgment. See the technical support document for details.

Condition based on the nonnative plant indicator was good in 48%, fair in 27%, poor in 13% and very poor in 11% of wetland area across the United States. Regionally, good condition ranged from 13% in the Interior Plains ecoregion to 67% in the Coastal Plains ecoregion.

No statistically significant changes occurred at the national level. Significant changes in condition did occur in three ecoregions. The Eastern Mountains and Upper Midwest ecoregion had a decrease in good condition of 17 percentage points. The Interior Plains had an increase in poor condition of 9 percentage points. The Xeric ecoregion had a decrease in very poor condition of 21 percentage points.

PHYSICAL INDICATORS

Modifying physical features of wetlands or the surrounding area may impair wetland function and diminish ecological conditions. Observations of these physical alterations can indicate the degree of disturbance to a wetland and, ultimately, its condition.

The physical alteration protocol and its associated indicators were revised for 2021 (see the appendix and technical support document for description of the differences). New fixed benchmarks were developed based on professional judgment by the scientists analyzing the data. The revision was substantial enough that previous survey data from 2011 and 2016 could not be used to analyze change. Therefore, no change data are shown below.

Field crews used checklists to document physical alterations. Alterations were observed across the assessment area and in 12 plots (100 square meters each) in the buffer area surrounding the assessment area.

Some physical alterations can occur naturally. For instance, beavers build dams that impede the flow of water into and out of wetlands, and native animals browse vegetation. In calculating physical alteration indicator scores, the EPA attempted to exclude naturally occurring alterations.

The NWCA physical indicators of stress included the presence of two types of vegetation alteration (removal or replacement), two types of hydrologic alterations (flow obstruction or water addition/subtraction), and two types of alterations affecting soil (surface modification or soil hardening). The EPA also developed a summary indicator, which combined scores for the other six indicators.

For each observed alteration, the EPA assigned points based on the extent or severity of the disturbance within the assessment area and the plot’s distance from the assessment area. A score of zero meant no alterations were observed, indicating good condition. A score at or above 25 indicated poor condition for the six individual indicators. A score at or above 50 indicated poor condition for the summary indicator.

Vegetation Removal

The vegetation removal indicator evaluates the loss, removal or damage of vegetation due to human activity. Examples of removal activities include logging, mowing, excessive grazing by livestock or feral animals (such as hogs or nutria), herbicide use, and fire. Removal of vegetation may increase sediment, nutrient and pollutant loads entering or collecting in a wetland.

Nationally, the vegetation removal stressor rating was good for 42%, fair for 31% and poor for 26% of wetland area. Another 2% of wetland area was not assessed because of missing data. The Xeric ecoregion had the largest proportion of wetland area rated poor, at 59%.

Vegetation Replacement

The vegetation replacement indicator reflects conversion of natural vegetation structure and composition due to human activity. Alterations could include conversion of natural plant communities to pasture, row crops, golf courses, lawns, parkland or nursery. Native plant species in rangeland could also be replaced by nonnatives, leading to changes in plant communities and native species richness, even if land use remains unchanged. Vegetation replacement can decrease biodiversity, simplify the vertical structure and reduce habitat quality on the site.

Nationally, the vegetation replacement stressor rating was good for 42%, fair for 23% and poor for 33% of wetland area. Another 2% of wetland area was not assessed due to missing data. The Xeric ecoregion had the largest proportion of wetland area rated poor, at 72%.

Flow Obstruction

The flow obstruction indicator reflects human activities that can impound water or impede its flow into, out of, or within the wetland (for instance, construction of dikes, dams, berms, or railroad beds). Such hydrologic alterations can affect vegetation, nutrients and habitat, thereby impacting the overall ecological condition of the wetland. For example, roadbed construction near coastal wetlands can cut off or reduce tidal flow, changing a marsh from saltwater to freshwater and eliminating daily fluctuation in water levels needed to support certain types of marsh vegetation.

Stress from flow obstructions was low overall. Condition was rated good in 74%, fair in 17% and poor in 7% of wetland area in the United States. The remaining 2% of wetland area was not assessed. The Xeric ecoregion had the largest proportion of wetland area rated poor, at 25%.

Water Addition or Subtraction

The water addition or subtraction indicator reflects modifications that drain or add water to the site (for example, ditches, culverts, or stormwater outfalls). Some modifications, such as vehicle ruts that become stream channels, may be unintentional but are impactful nevertheless. Changes in how water moves into or out of a wetland can affect plant productivity, nutrient cycling in the soil and water, and the physical habitat, thereby impacting the overall ecological condition of the wetland. For example, ditches allow water to drain from the soil quickly and can lower the water table enough to dry out the soil surface.

The water addition or subtraction indicator showed good condition in 79%, fair in 15% and poor in 4% of wetland area across the United States. The remaining 2% of wetland area was not assessed. The Xeric ecoregion had the largest proportion of wetland area rated poor, at 22%.

Soil Hardening

The soil hardening indicator reflects activities and infrastructure that result in compaction of soil and the addition of impervious surfaces (for instance, parking lots, roads, buildings, or unpaved trails). Agriculture and vehicle use are examples of activities that can cause compaction. Soil compaction and impervious surfaces can directly impact wetland habitat, plants and soil integrity by hindering root growth and diminishing a wetland’s natural ability to filter and retain floodwaters.

The soil hardening indicator showed good condition in 49%, fair in 38% and poor in 12% of wetland area across the United States. The remaining 2% of wetland area was not assessed. The Xeric and Western Mountain ecoregions had the largest proportion of wetland area in poor condition, with 46% and 27%, respectively.

Surface Modification

The surface modification indicator reflects evidence of soil erosion or deposition. Examples include the following:

• Soil loss or exposure of plant roots.
• Excavation or dredging (removal of soil or sediment).
• Deposition of fill or spoil from elsewhere.
• Freshly deposited sediment carried by runoff.

Soil erosion within a wetland can result in wetland loss, while erosion outside a wetland may indicate that excess sediment is being deposited in the wetland. Filling can raise the wetland surface far enough above the water table that it is no longer a wetland. The soil or sediment deposited may also have a different physical and chemical composition than the native wetland soil, affecting the health and growth of native species.

The surface modification indicator showed good condition in 74%, fair in 18% and poor in 6% of wetland area across the United States. The remaining 2% of wetland area was not assessed. The Xeric ecoregion had the largest proportion of wetland area rated poor, at 12%.

Physical Alterations (Sum)

In addition to looking at the six physical alterations separately, analysts also examined the combined impact by adding up the numeric scores across all indicators. This enabled the EPA to determine the extent to which wetlands were subject to multiple types of alterations and to estimate the combined effect on wetland conditions. The benchmarks for good condition remained the same as for individual physical alteration indicators, with zero (no observed alterations) indicating good condition. However, the benchmark for poor condition was raised to a score of at or above 50.

The combined physical alteration indicator showed good condition (low levels of physical stressors) in 17%, fair in 40% and poor in 42% of wetland area across the United States. The remaining 2% of wetland area was unassessed. Each of the physical alteration indicators included in the combined indicator showed good condition in 42% or more of wetland area. The finding that only 17% of area was in good condition for the combined indicator illustrates that many sites are subject to multiple physical alterations.

The Eastern Mountains and Upper Midwest had the largest area in good condition, at 24%. The Xeric ecoregion had the largest percentage of area in poor, at 84%.

CHEMICAL INDICATORS

Chemical stressors affecting wetland conditions include excess nutrients, metals, organic toxins and others. These stressors can disrupt nutrient cycles, affect plant and animal growth, and be detrimental to human health. Three chemical indicators of stress were developed for the NWCA using data from samples collected at each site: a soil heavy metal indicator and two water chemistry parameters (total nitrogen and total phosphorus). At some sites, collecting a soil sample was impossible because of ponded water or other site conditions on the sampling day. Water samples were only collected at sites where surface water was available.

Soil Heavy Metals

For the soil heavy metals indicator, the EPA assessed concentrations of 12 different heavy metals closely associated with human activities: antimony, cadmium, chromium, cobalt, copper, lead, nickel, silver, tin, tungsten, vanadium and zinc. Benchmarks were established based on the extent to which regional background concentrations for each heavy metal were exceeded. Concentrations above these background levels are not very likely to occur naturally. The benchmarks established for this indicator do not reflect toxicity; rather, they are indicators of human disturbance. Sites were assigned ratings as follows:

• Good: No more than one of the 12 heavy metals had concentrations at least three times greater than the regional background level, and no single concentration exceeded five times the regional background level.
• Fair: Two heavy metals had concentrations at least three times the background level, or a single concentration was greater than five times but no greater than 10 times the background level.
• Poor: Three or more heavy metals had concentrations at least three times the background level, or any single concentration exceeded 10 times the background level.

Wetland heavy metal soil contamination can be due to runoff from roads, industrial discharges to air or water, mining operations, agriculture or illegal dumping.

Soil heavy metal results are not yet available from the laboratory. This section will be updated when results are ready.

Water Chemistry

Characterizing water chemistry is an integral part of aquatic resources assessment, because the physical and chemical properties of water directly reflect the surrounding environment, including human influences. However, wetlands differ from lakes, streams and coastal waters because surface water is not necessarily present. Furthermore, some wetland types, such as tidal wetlands and riverine wetlands, are more likely to have surface water at the time of sampling than other types, such as slopes and flats. Thus, the population of wetlands with surface water is not a random subset of the total wetland population surveyed in the NWCA (that is, the population of wetlands with surface water is biased towards wetlands that are characterized by continuous surface water). At sites with surface water, water samples were collected and analyzed for several water quality parameters. This section describes results for two indicators measured in the water column: total nitrogen and total phosphorus.

Some wetlands, such as marshes, have naturally high levels of nitrogen and phosphorus in their water due to contributions from surface water sources or decaying vegetation. Fens and bogs, on the other hand, have quite low levels since they get their water from direct rainfall and groundwater, which are naturally low in nutrients. Urban stormwater runoff, agricultural runoff, atmospheric deposition and septic systems are common sources of nutrients. Even wetlands with naturally high nutrient levels can be damaged by nutrient pollution, which can cause excessive plant and algae growth and may lead to changes in the abundance or occurrence of plant species at a site.

Since total nitrogen and total phosphorus are highly variable in wetlands, there are no national or regional literature-based benchmarks for these indicators. Thus, the EPA developed benchmarks for good, fair and poor conditions using an approach similar to the one described in the assessment benchmarks section. Separate benchmarks were developed for inland and estuarine wetland subpopulations. See the technical support document for specific details.

In 2021, 29% of wetland area was in good condition, 7% was in fair condition and 24% was rated poor for total phosphorus in water. Note that 40% of wetland area was not assessed due to insufficient surface water for sample collection. Regionally, the Interior Plains had the largest percentage in poor condition, 31%, and the Western Mountains had the largest percentage in good, 58%.

For more details, download the data for this chart, or visit the NLA dashboards for ecoregional data on water quality phosphorus.

In 2021, 29% of wetland area was in good condition, 14% was in fair condition and 17% was rated poor for total nitrogen in water. Note that 40% of wetland area was not assessed due to insufficient surface water. As with phosphorus, the Interior Plains had the largest percentage in poor condition, 25%, and the Western Mountains had the largest percentage in good, 61%.

For more details, download the data for this chart, or visit the NLA dashboards for ecoregional data on water quality nitrogen.

No statistically significant changes occurred at the national level for total phosphorus. Significant changes in condition did occur in three ecoregions. The Eastern Mountains and Upper Midwest ecoregion had an increase in good condition of 14 percentage points. The Western Mountains had a decrease in fair condition of 6 percentage points.

The percentage of wetland area that was not assessed was much lower in 2016 and 2021 than it was in 2011. Such large changes can affect change results for the other condition categories. As such, users are cautioned to consider the effect of the change in percentage of area not assessed when comparing 2011 results to those of later years.

A statistically significant increase in area in good condition, 8.4 percentage points, occurred nationally for total nitrogen. A statistically significant increase in area in good condition of 22 percentage points, along with a decrease in area in poor condition of 19 percentage points, was observed for the Eastern Mountains and Upper Midwest.

The percentage of wetland area not assessed was much lower in 2016 and 2021 than it was in 2011. Such large changes can affect change results for the other condition categories. As such, users are cautioned to consider the effect of the change in percentage of area not assessed when comparing 2011 results to those of later years.

HUMAN HEALTH INDICATOR

The NWCA examined one human health indicator, focusing on exposure to a group of toxins called microcystins. Exposure could occur while boating or through other contact with wetland waters. Because people do not generally swim in wetlands and many wetlands do not have fish large enough for human consumption, recreational exposure to human health risks is expected to be less in wetlands than in other waters. As a result, the NWCA does not analyze water for bacteria or fish tissue for chemical contaminants as other surveys in the NARS program do.

Microcystins

Microcystins are toxins released by cyanobacteria, one-celled photosynthetic organisms that normally occur at low levels in surface waters. Under high-nutrient conditions, cyanobacteria can multiply rapidly, forming "blooms." Not all cyanobacterial blooms are toxic, but some may release microcystins, which could cause health effects. Recreational exposure is typically a result of inhalation, skin contact or accidental ingestion. See the EPA microcystins page for more information. Pets may also be exposed to microcystins if they swim in or drink wetland surface waters, although their risk levels may differ from those that apply to human health.

Microcystin results were compared to the EPA recreational water quality criterion and swimming advisory recommendation of 8 parts per billion (ppb) (U.S. EPA 2019).

The NWCA does not capture all risks associated with cyanobacteria. Note that some types of algae and cyanobacteria release other toxins not monitored as part of the NWCA. The NWCA assesses risk of exposure to microcystins at national and regional levels. For information about risks at specific locations, recreational water users should check with state, Tribal or local governments.

Nationally, levels exceeded the EPA’s recommended water quality criterion in less than 1% of wetland area, as shown below. A total of 40% of wetland area was not assessed due to insufficient surface water for sample collection. Microcystins were detected in 30% of wetland area in 2021, as shown in the dashboard. Regionally, the detection of microcystins ranged from 12% to 46%.

No statistically significant changes occurred at the national level.

The percentage of wetlands not assessed was much lower in 2016 and 2021 than it was in 2011. Such large changes can affect change results for the other condition categories. As such, users are cautioned to consider the effect of the change in percentage of area not assessed when comparing 2011 results to those of later years.

SURVEY DESIGN

In 2016, the sample frame (the set of wetlands from which the survey sample was drawn) used elements of mapped wetland data from both the U.S. Fish and Wildlife Survey wetland status and trends survey (Dahl 2006) and the National Wetlands Inventory (U.S. FWS 2014). In 2021, the sample frame relied entirely on mapped wetland data from the National Wetlands Inventory (U.S. FWS 2019). This change aligned the sample frame with the current and full extent of mapped wetlands across the conterminous U.S., improving the spatial representation of wetlands included in the survey. In addition, the number of sites sampled in estuarine wetlands along the coasts was adjusted to ensure there were enough sites to report statistically robust results for the Pacific, Gulf and Florida, and Atlantic Coasts.

INDICATORS

The EPA updated field protocols and indicator score calculation for the physical alteration indicators between 2016 and 2021, as described below. The changes were substantive enough that it is not possible to calculate changes between surveys; thus, the EPA did not include estimates of change for these indicators in this report.

In 2016, crews recorded the presence of physical alterations in 12 plots, each 100 square meters, in the buffer area and at one plot of the same size at the center of the assessment area under a buffer assessment protocol. They also recorded hydrologic disturbances across the entire assessment area under a different protocol. In 2021, the protocols were consolidated, and crews assessed alterations from human disturbance in 12 buffer plots and across the entire assessment area. In the assessment area, crews noted not just the presence, but also the severity and extent of the disturbance.

The EPA updated how scores for physical alterations indicators are calculated in 2021. In 2016, the EPA categorized the disturbance data into six individual indicators and one cumulative indicator and calculated scores using a proximity-weighted scheme depending on where the disturbance was observed. In 2021, the disturbance data were categorized using the same individual and cumulative indicators, but the scoring was adjusted to also account for the severity and extent of disturbances observed in the assessment area. For details on these changes, see the technical support document.

BENCHMARKS

Additionally, the EPA updated benchmarks for the physical alteration indicators in 2021. The EPA adjusted the benchmarks for the indicator based on professional judgment by the scientists analyzing the data. For details on the updates, see the technical support document.