U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

U.S. Department of Agriculture

A Regional Classification of the Effectiveness of 

Depressional Wetlands at Mitigating Nitrogen 

Transport to Surface Waters in the Northern 

Atlantic Coastal Plain

Scientific Investigations Report 2012–5266
Cover. Clockwise from upper left: [1] Very Flat Poorly Drained Upland (VFPDU) in North Carolina, [2] Flat Poorly Drained Lowland 

(FPDL) in North Carolina, [3] circular fields in the Flat Sandy Uplands (FSU) near Hurlock, Maryland, and [4] Flat Sandy Lowland (FSL) in 

the northern part of the Delmarva Peninsula. Aerial photographs from the U.S. Department of Agriculture (USDA) National Agricultural 

Imagery Program (NAIP), 2007.
A Regional Classification of the 

Effectiveness of Depressional Wetlands 

at Mitigating Nitrogen Transport to 

Surface Waters in the Northern Atlantic 

Coastal Plain

By Scott W. Ator, Judith M. Denver, Andrew E. LaMotte, and Andrew J. Sekellick

Prepared in cooperation with the 

U.S. Department of Agriculture

Scientific Investigations Report 2012–5266

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Ator, S.W., Denver, J.M., LaMotte, A.E., and Sekellick, A.J., 2013, A regional classification of the effectiveness of 

depressional wetlands at mitigating nitrogen transport to surface waters in the Northern Atlantic Coastal Plain: 

U.S. Geological Survey Scientific Investigations Report 2012–5266, 23 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Study Area—The Northern Atlantic Coastal Plain ..................................................................................2

Methods...........................................................................................................................................................4

Delineation of Catchments..................................................................................................................5

Landscape Metrics...............................................................................................................................5

Statistical Analysis and Interpretation..............................................................................................6

Classification and Description of Wetland Landscapes in the Northern Atlantic Coastal Plain......7

Statistical Analyses..............................................................................................................................7

Wetland Landscape Regions ..............................................................................................................8

Applications and Limitations......................................................................................................................17

Summary and Conclusions.........................................................................................................................19

References Cited..........................................................................................................................................19

Appendix........................................................................................................................................................23

Reference Cited............................................................................................................................................23
iv

Figures

1. Map showing the distribution of palustrine wetlands within the Northern 

Atlantic Coastal Plain study area...............................................................................................3

2. Map showing elevation data from 2007 showing topographic depressions 

that are common in parts of the Northern Atlantic Coastal Plain, 

particularly in parts of the Delmarva Peninsula and North Carolina...................................4

3. Map showing fundamental landscape units defined by NHDPlus and 

12-digit hydrologic units in part of the Northern Atlantic Coastal Plain..............................5

4. Boxplots showing distribution of selected topographic metrics computed 

from 30-meter elevation data among wetland landscape regions.......................................9

5. Boxplots showing distribution of selected soil metrics among wetland 

landscape regions ......................................................................................................................10

6. Boxplots showing distribution of selected land cover among wetland 

landscape regions ......................................................................................................................11

7. Map showing wetland landscape regions within the Northern Atlantic 

Coastal Plain................................................................................................................................12

8. Aerial photograph showing parts of the Very Flat Poorly Drained Upland 

(VFPDU) in North Carolina.........................................................................................................13

9. Aerial photograph showing that Flat Poorly Drained Lowlands (FPDL), as in 

this area of North Carolina, commonly form in large depressions such 

as Carolina Bays, and are often artificially drained for cultivation....................................14

10. Aerial photograph showing that in the Flat Sandy Lowlands (FSL) in the 

northern part of the Delmarva Peninsula (A), wetlands often occur in 

isolated depressions, indicated by yellow arrows (such as Delmarva 

Bays), and minimal artificial drainage is required to support agriculture. 

Further south on the peninsula, however, (B), natural drainage is 

extremely poor and extensive ditching is common ..............................................................15

11. Aerial photograph showing that circular fields in the Flat Sandy Uplands (FSU) 

near Hurlock, Maryland are indicative of center-pivot irrigation systems. 

Because FSU watersheds are generally well-drained, irrigation rather 

than artificial drainage is often used to support crop production......................................18

Tables

1. Results of principal components analysis on 16 landscape metrics defined 

for 33,799 individual catchments in the Northern Atlantic Coastal Plain............................6

2. Summary of depressional wetland landscape regions within the Northern 

Atlantic Coastal Plain...................................................................................................................8
v

Conversion Factors and Datums

Multiply By To obtain

Length

centimeter (cm) 0.0328 foot (ft)

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2

) 247.1 acre

square kilometer (km2

) 0.3861 square mile (mi2)

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C = (°F - 32) / 1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

A Regional Classification of the Effectiveness of 

Depressional Wetlands at Mitigating Nitrogen 

Transport to Surface Waters in the Northern 

Atlantic Coastal Plain

By Scott W. Ator, Judith M. Denver, Andrew E. LaMotte, and Andrew J. Sekellick

Abstract

Nitrogen from nonpoint sources contributes to eutrophication, hypoxia, and related ecological degradation in Atlantic 

Coastal Plain streams and adjacent coastal estuaries such as 

Chesapeake Bay and Pamlico Sound. Although denitrification 

in depressional (non-riparian) wetlands common to the Coastal 

Plain can be a significant landscape sink for nitrogen, the 

effectiveness of individual wetlands at removing nitrogen 

varies substantially due to varying hydrogeologic, geochemical, and other landscape conditions, which are often poorly or 

inconsistently mapped over large areas. A geographic model 

describing the spatial variability in the likely effectiveness 

of depressional wetlands in watershed uplands at mitigating 

nitrogen transport from nonpoint sources to surface waters was 

constructed for the Northern Atlantic Coastal Plain (NACP), 

from North Carolina through New Jersey. 

Geographic and statistical techniques were used to 

develop the model. Available medium-resolution (1:100,000-

scale) stream hydrography was used to define 33,799 individual watershed catchments in the study area. Sixteen landscape 

metrics relevant to the occurrence of depressional wetlands 

and their effectiveness as nitrogen sinks were defined for each 

catchment, based primarily on available topographic and soils 

data. Cluster analysis was used to aggregate the 33,799 catchments into eight wetland landscape regions (WLRs) based on 

the value of three principal components computed for the 16 

original landscape metrics. Significant differences in topography, soil, and land cover among the eight WLRs demonstrate 

the effectiveness of the clustering technique. Results were 

used to interpret the relative likelihood of depressional wetlands in each WLR and their likely effectiveness at mitigating 

nitrogen transport from upland source areas to surface waters.

The potential effectiveness of depressional wetlands 

at mitigating nitrogen transport varies substantially over 

different parts of the NACP. Depressional wetlands are common in three WLRs covering 32 percent of the area, and have 

a relatively high potential to mitigate nitrogen transport from 

nonpoint sources. Conversely, 37 percent of the study area 

includes rolling hills with relatively high slope and relief, and 

little likelihood of depressional wetlands. The remainder of the 

Coastal Plain includes relatively flat watersheds with moderate 

to low relative likelihood of nitrogen mitigation. The delineation of WLRs in this model should be useful for targeting 

wetland conservation or restoration efforts, and for estimating 

the effects of depressional wetlands on the regional nitrogen 

budget, but should be considered in light of limitations and 

assumptions inherent in the model.

Introduction

Excessive nitrogen from nonpoint sources has contributed 

to eutrophic and hypoxic conditions and related ecological 

degradation of surface waters worldwide (Howarth and others, 

1996; Vitousek and others, 1997; Carpenter and others, 1998). 

Rivers of the eastern United States export 3 to 14 times more 

nitrogen (commonly in the form of nitrate) than under natural 

conditions, and average nitrogen yields from the northeastern 

United States to the North Atlantic Ocean are the greatest 

in the Western Hemisphere (Howarth and others, 1996). 

Nonpoint sources of nitrogen include numerous human activities, but are often dominated by agricultural fertilizer applications (Howarth and others, 1996; Dubrovsky and others, 2010; 

Ator and others, 2011). Whereas point sources contribute the 

majority of nitrogen to certain individual streams, they account 

for less than 20 percent of the total nitrogen delivered to 

Chesapeake Bay (Ator and others, 2011) or to the wider North 

Atlantic Ocean (Howarth and others, 1996). Nitrate carried in 
2 A Classification of the Effectiveness of Depressional Wetlands at Mitigating Nitrogen in the Northern Atlantic Coastal Plain

groundwater discharging to streams is a substantial nonpoint 

source of nitrogen in surface water (Dubrovsky and others, 

2010). In the Northern Atlantic Coastal Plain (NACP) adjacent 

to Mid-Atlantic coastal estuaries, groundwater provides 40 to 

95 percent of streamflow (Sinnott and Cushing, 1978; Leahy 

and Martin, 1993) and as much as 70 percent of the nitrogen 

flux (in the form of nitrate) in streams (Domagalski and others, 

2008; Ator and Denver, 2012). Although primary production 

in streams and some tidal waters is typically limited by phosphorus concentrations, production and resulting eutrophication 

in temperate estuaries is often nitrogen-limited (Vitousek and 

others, 1997; Prasad and others, 2010).

Wetlands mitigate nitrate transport from nonpoint sources 

to surface waters in many landscapes and are often a key 

component of management and restoration strategies to reduce 

nitrogen pollution (Brinson and Eckles, 2011). Wetlands are 

commonly interspersed with agricultural land in Coastal Plain 

settings where they occur in riparian zones, depressions, and 

on flats (De Steven and Lowrance, 2011). Organic matter and 

reducing conditions typical of wetlands promote the removal 

of nitrogen as dissolved nitrate to the atmosphere through 

denitrification (Howarth and others, 1996; Whigham and 

Jordan, 2003). This process occurs where water containing 

nitrate passes through any sufficiently reducing environment 

(Puckett, 2004), and has been observed in riparian and hyporheic sediments (De Steven and Lorwrance, 2011; Kennedy 

and others, 2009; Hill, 1996; Puckett and others, 2008) and 

along groundwater flowpaths (Tesoriero and others, 2000; 

Böhlke and others, 2007; Mehnert and others, 2007; Denver 

and others, 2010). The effects of depressional wetlands on 

nitrate reduction have been less widely studied (Leibowitz and 

Nadeau, 2003). Whereas the restoration of riparian zones is the 

most widespread wetland conservation practice, the restoration of depressional wetlands is also a common practice in the 

Coastal Plain (DeSteven and Lowrance, 2011). The restoration 

of cultivated former depressional wetlands where soils may 

retain organic matter and reducing conditions are sufficient to 

promote denitrification is most likely to be successful (David 

and others, 2009; De Steven and Lowrance, 2011). Such restorations often include restoring wetland hydrology through the 

blocking of ditches installed to promote cultivation and allowing the re-establishment of wetland vegetation (De Steven and 

others, 2006). 

Coupled conceptual and geographic models can be useful 

for understanding and predicting the spatial variability in 

the effectiveness of wetlands at reducing nitrogen transport. 

A thorough understanding of nitrogen mitigation and other 

services provided by wetlands or other ecosystem compartments is critical in assessing their effects on regional water 

quality and for prioritizing management or restoration investment (Palmer and Filoso, 2009). Nitrogen losses in wetlands 

vary spatially and temporally in response to similar variability 

in hydrology and geochemistry, however, and extrapolating 

observations of nitrogen losses in local wetlands to large 

regions is therefore difficult (Howarth and others, 1996). 

Predicting nitrogen fate and transport can be particularly 

complicated where groundwater delivers nitrate to streams at 

depth beneath reducing environments associated with wetlands (Böhlke and Denver, 1995; Hill, 1996; Gold and others, 

2001; Puckett, 2004). Winter (2001) proposed the concept 

of hydrologic landscapes composed of various combinations 

of uplands, lowlands, and intervening slopes as an objective 

framework for predicting physical hydrology and resulting 

geochemical and biological processes in different areas. 

Geographic models delineating selected relevant landscape 

features over large areas coupled with conceptual models of 

hydrology and nitrate transport through individual watersheds, 

for example, can be useful for understanding and (or) predicting the spatial variability in the effectiveness of wetlands at 

mitigating nitrogen transport from nonpoint sources to surface 

waters.

A regional geographic model predicting the mitigation 

of nitrogen transport from nonpoint sources to surface waters 

by depressional (non-riparian) wetlands located in watershed 

uplands is presented and discussed in this report. The model 

was developed for the NACP (New Jersey through North 

Carolina) (fig. 1), where depressional wetlands, including 

Carolina Bays and smaller Delmarva Bays, are common. 

Multivariate statistical techniques, such as principal components analysis (PCA) and cluster analysis, have been frequently used to classify landscapes on the basis of physical 

hydrology or watershed characteristics (for example, Mosely, 

1981; Lipscomb, 1998; Preston, 2000; Chiang and others, 

2002; Caratti and others, 2004; Wolock and others, 2004; 

Rao and Srinivas, 2006; Isik and Singh, 2008). In this study, 

however, such techniques were used with available geographic 

data particularly relevant to nitrogen fate and transport 

around depressional wetlands to classify 33,799 individual 

watershed catchments in the NACP into wetland landscape 

regions (WLRs) (see Appendix). The predicted likelihood of 

depressional wetlands to occur and to mitigate the transport 

of nonpoint-source nitrogen to surface waters in each WLR is 

described. Implications for regional watershed management 

and restoration also are discussed.

Study Area—The Northern Atlantic 

Coastal Plain

The NACP study area covers approximately 114,000 

square kilometers (km2

) in the eastern United States and 

includes areas of the Atlantic Coastal Plain Physiographic 

Province (Fenneman and Johnson, 1946) in five states (North 

Carolina, Virginia, Maryland, Delaware, and New Jersey) and 

the District of Columbia (fig. 1). The NACP is underlain by 

unconsolidated and (in parts of North Carolina) semi-consolidated sediments that thicken from 0 meters (m) in the west at 

the Fall Line to more than 3,000 m along the North Carolina 

coast (Winner and Coble, 1996; Ator and others, 2005). The 

texture of near-surface sediments varies laterally and vertically from relatively impermeable silt and clay to extremely 
Study Area—The Northern Atlantic Coastal Plain 3

PENNSYLVANIA

NEW

YORK CONNECTICUT

NEW

JERSEY

MARYLAND

DE

Delmarva

Peninsula

Long Island

Washington, 

D.C.

WEST

VIRGINIA

VIRGINIA

NORTH

CAROLINA

SOUTH

CAROLINA

ATLANTIC OCEAN

Chesapeake Bay

Delaware

Bay

Potomac River

Pamlico Sound

EXPLANATION

Northern Atlantic Coastal Plain

Palustrine wetlands 

U.S. Fish and Wildlife Service, 2011)

(National Wetlands Inventory, 

78

72

41

35

38

O

O

O

O

O

0 30 60 MILES

0 30 60 KILOMETERS

Northern Atlantic

Coastal Plain

N

Base from U.S. Geological Survey,1:2,000,000 DLG, 

Albers Equal-Area Conic Projection, NAD 1983

Figure 1. The distribution of palustrine wetlands within the 

Northern Atlantic Coastal Plain study area.

permeable sand and gravel. Soils are generally permeable, 

but can be quite variable in texture reflecting the variability 

in geologic parent material. Much of the NACP is relatively 

flat and low-lying (particularly along the coast of the Atlantic 

Ocean and major estuaries), although stream incision and 

land-surface slopes generally increase toward the west. 

Climate in the NACP is humid and temperate to subtropical. Precipitation averages approximately 120 centimeters 

per year, about half (51 percent) of which returns to the 

atmosphere through evaporation or transpiration (Leahy and 

Martin, 1993). Approximately two-thirds of the remaining 

precipitation recharges the shallow groundwater (Leahy and 

Martin, 1993). Because of the humid climate and commonly 

flat, low-lying topography in the NACP, the water table 

generally occurs within a few meters of the land surface. 

Groundwater provides 40 to 95 percent of flow in Coastal 

Plain streams (Sinott and Cushing, 1978; Leahy and Martin, 

1993); most water moves through the surficial aquifer system 

from the water table to discharge areas in local streams within 

a few decades (Dunkle and others, 1993; Böhlke and Denver, 

1995; McFarland, 1995; Speiran, 1996; Szabo and others, 

1996; Kauffman and others, 2001).

Abundant nonpoint nitrogen sources and commonly 

permeable soils and near-surface sediments in the NACP 

contribute to widespread nitrate contamination in shallow 

groundwater and surface waters. Although the NACP is 

predominantly forested, areas of intensive agriculture (particularly on the Delmarva Peninsula and in parts of North 

Carolina) and densely populated urban areas (particularly 

along the Fall Line north of the Washington, D.C. area) are 

common. Nitrate attributable to human sources has been 

reported in the aquifer since at least the early 1970s, and 

concentrations exceeding the Federal drinking-water standard 

of 10 milligrams per liter (mg/L) are not uncommon (Cushing 

and others, 1973; Bachman, 1984; Denver, 1989; Böhlke 

and Denver, 1995; Eckhardt and Stackelberg, 1995; Bolton, 

1996; Spruill and others, 1997; Debrewer and others, 2008; 

Ator, 2008; Denver and others, 2010). As the predominant 

source of water to NACP streams, groundwater also contributes as much as 70 percent to the total nitrogen load in many 

NACP streams (Bachman and Phillips, 1996; Ator, Denver, 

and Brayton, 2005; Domagalski and others, 2008; Ator and 

Denver, 2012) and downstream coastal estuaries. This substantial export of nitrogen is associated with documented waterquality problems. For example, only 12 percent of Chesapeake 

Bay, the largest estuary in North America, met ecological 

criteria for dissolved oxygen in 2007 (U.S. Environmental 

Protection Agency, 2008), and a Total Maximum Daily Load 

was established for nitrogen in the bay watershed in 2010 

(U.S. Environmental Protection Agency, 2011). Other MidAtlantic estuaries in which nitrogen has contributed to poor 

water quality include Delaware Bay, Delaware and Maryland 

Coastal Bays, Barnegat Bay, and Albemarle, Long Island, and 

Pamlico Sounds (Spruill and others, 2005; U.S. Environmental 

Protection Agency, 2007).

The NACP has one of the most wetland-rich landscapes 

in the United States (Carter, 1996) (fig. 1). Depressional 

(non-riparian) wetlands, including Carolina Bays and smaller 

Delmarva Bays, are abundant and represent a significant 

potential landscape sink for nitrate from agricultural and 

other sources. They are found in various landscape positions 

from relatively flat interstream divides, isolated topographically low depressions in otherwise well-drained landscapes, 

to coastal terraces where they are commonly embedded in 

areas with large expanses of wetland flats (Tiner, 1996; De 

Steven and Lowrance, 2011) (fig. 2). Pocosins, palustrine 

wetlands with evergreen scrub vegetation, are common in the 
4 A Classification of the Effectiveness of Depressional Wetlands at Mitigating Nitrogen in the Northern Atlantic Coastal Plain

Figure 2. Elevation data from 2007 showing topographic depressions that are common in parts of the 

Northern Atlantic Coastal Plain, particularly in parts of the Delmarva Peninsula and North Carolina. 

Lighter colors indicate higher elevations.

southern part of the NACP (Richardson, 1983; Tiner, 1996). 

Nitrate in groundwater in poorly drained areas of the NACP 

that promote the formation of wetlands generally occurs at 

lower concentrations than in well-drained areas (Ator, 2008). 

Many of the areas with agricultural land use and lower nitrate 

concentrations were previously wetlands that have been 

drained, such as in coastal North Carolina, where 50 percent 

of previous wetland areas have been converted mostly for 

agricultural use (Dahl, 1990). Whether wetlands or other 

landscape sinks will intercept and reduce nitrate in groundwater from agricultural sources depends on local hydrogeologic 

conditions including the orientation of groundwater flowpaths, 

the location of the wetlands within the groundwater-flow 

system, and the thickness of the surficial aquifer (Böhlke and 

Denver, 1995; Leibowitz and Nadeau, 2003; Whigham and 

Jordan, 2003; Puckett, 2004). These factors vary regionally 

and control how much nitrate encounters reducing conditions 

conducive to denitrification.

Methods

Geographic and statistical techniques were used to create 

a geographic model that identifies and delineates WLRs, 

areas of the NACP within which depressional wetlands 

have a similar potential to mitigate nitrogen transport from 

agricultural sources to local streams. Methods for defining 

WLRs were adapted from those used by Wolock and others 

(2004) to define hydrologic landscapes for the United States. 

Fundamental landscape units for the model were defined 

by individual watershed catchments within the study area. 

Relevant landscape (geographic) metrics were compiled or 

computed for each catchment and multivariate statistical 

techniques (PCA and cluster analyses) were used to classify 

catchments into groups (WLRs) with similar values for those 

metrics. Conceptual models were developed to predict likely 

effects of depressional wetlands on regional nitrogen transport 

in each WLR.
Methods 5

Delineation of Catchments

Individual watershed catchments within the NACP study 

area for use as fundamental landscape units were developed 

from the geospatial dataset NHDPlus, a medium resolution 

(1:100,000-scale) representation of stream hydrography built 

upon the National Hydrography Dataset (NHD) (Johnston and 

others, 2009; Horizon Systems, 2010; Simley and Carswell, 

2010). The model area was defined as areas of 12-digit 

hydrologic units (HUC12s; Natural Resources Conservation 

Service, 2007a; U.S. Geological Survey and U.S Department 

of Agriculture, National Resources Conservation Service, 

2012) that intersect the NACP. The model area thus extends 

beyond the Coastal Plain, but was selected to support summarization and reporting of results for HUC12s (which are generally familiar to watershed managers) and to provide flexibility 

in the computation of landscape metrics based on watersheds 

at multiple scales (fig. 3). Catchments contributing to individual stream reaches as defined by NHDPlus in this area constitute the fundamental landscape units for modeling, and define 

the finest spatial resolution of model predictions. Catchments 

smaller than 0.5 km2

 or including more than 50 percent open 

water (LaMotte, 2008a, 2008b) were omitted. Catchments 

with more than 75 percent of land area below 2 m in elevation 

(Gesch and others, 2009) also were omitted to avoid the 

influence of tidal conditions. Catchments with missing soils 

data (Natural Resources Conservation Service, 2007b) were 

similarly dropped from consideration in the model; these areas 

generally occur in heavily developed urban areas that probably 

lack natural soils. The remaining 33,799 individual catchments 

within the model area have a median incremental area of 2.2 

km2

 (interquartile range, 2.6 km2).

Landscape Metrics

Landscape metrics (geographic variables) for inclusion 

in the model were derived mainly from soils data and topographic metrics to represent conditions in each catchment that 

are particularly relevant to nitrogen fate and transport around 

wetlands (table 1). Water is the primary vector for the transport of nitrogen in the NACP, much of which occurs through 

shallow groundwater in the form of dissolved nitrate (Böhlke 

and Denver, 1995; Bachman and Phillips, 1996; Ator and 

Denver, 2012). Understanding (and predicting) nitrogen fate 

and transport in the NACP therefore requires consideration 

of local hydrology and the occurrence of reducing conditions 

associated with wetlands that may promote nitrogen losses 

through denitrification. 

Landscape metrics were selected primarily to represent 

the likelihood of depressional wetlands being present in 

model catchments and describe their position relative to local 

hydrologic flowpaths. The likelihood of wetlands being present is represented by topographic variables indicative of the 

flatness of the landscape and by soil variables representative of 

soil texture and drainage (table 1). Topographic metrics were 

Figure 3. Fundamental landscape units defined by NHDPlus 

and 12-digit hydrologic units in part of the Northern Atlantic 

Coastal Plain.

computed from 30-m digital elevation data (Gesch and others, 2009); soil metrics were computed from the Soil Survey 

Geographic Database (SSURGO) data in conformance with 

National Resources Conservation Service (NRCS) methods 

for aggregation for area-depth weighted averaging (Natural 

Resources Conservation Service, 2007b). Topographic wetness index (TWI), a function of slope and upstream contributing area (Beven and Kirk, 1979; Wolock and McCabe, 1995) 

was computed from NHDPlus attributes (U.S. Environmental 

Protection Agency, 2007; Horizon Systems, 2010; Simley and 

Carswell, 2010). High TWI values represent low slope (flat) 

areas with a large upslope contributing area. Under dry conditions, areas with high TWI values tend to remain saturated 
6 A Classification of the Effectiveness of Depressional Wetlands at Mitigating Nitrogen in the Northern Atlantic Coastal Plain

Table 1. Results of principal components analysis on 16 landscape metrics defined for 33,799 individual catchments in the Northern 

Atlantic Coastal Plain.

[PC, principal component; loadings with absolute value greater than 0.7 are bold; loadings with absolute value less than 0.4 are omitted]

Watershed 

characteristic

PC1 PC2 PC3

Communality 

estimate (percent of variance)

(43) (19) (16)

Percent flat land1,2 0.927 0.890

Percent hydric soils3 0.890 0.807

Mean soil hydric rating3,4 0.883 0.855

Mean soil hydrologic group, undrained3,5 0.661 0.559 0.761

Mean soil drainage class3,6 0.891 0.869

Mean topographic wetness index7 0.866 0.815

Mean soil percent organic matter3 0.410 0.248

Mean watershed slope, in percent1 -0.857 0.806

Watershed relief, in meters1 -0.817 0.733

Percent lowland1,8 0.947 0.962

Percent flat lowland1,2,8 0.615 0.690 0.862

Percent upland1,8 -0.947 0.962

Percent flat upland1,2,8 0.660 -0.627 0.907

Mean soil available water capacity3 0.734 0.710

Mean soil percent sand3 -0.798 0.748

Mean soil saturated hydraulic conductivity3 -0.702 0.673

1

 Topographic metrics were computed from 30-meter digital elevation data (Gesch and others, 2009). 

2

 Flat land is the percentage of the watershed with less than 1 percent slope.

3

 From the Soil Survey Geographic Database (SSURGO) (Natural Resources Conservation Service, 2007b).

4

 Soil hydric rating ranges from 1 (excessively drained) to 7 (very poorly drained).

5

 Soil hydrologic group ranges from 1 (soil class A) to 4 (soil class D).

6

 Soil drainage class ranges from 1 (excessively drained) to 7 (very poorly drained).

7

 TWI (Beven and Kirk, 1979; Wolock and McCabe, 1995) was computed from NHDPlus attributes (Horizon Systems, 2010; Simley and Carswell, 2010).

8

 Uplands (and lowlands) within each watershed are defined by areas above (or below) the midpoint elevation (the mean of the maximum and minimum 

elevation).

(Juracek, 2000). Also important to nitrate fate and transport 

in the surficial aquifer in the NACP is the orientation of local 

groundwater flowpaths relative to wetlands and nitrogen 

sources. Metrics quantifying the percentage of each catchment 

that is flat upland or flat lowland (table 1) were included to 

represent the likelihood of the depressional wetlands in each 

catchment occurring hydrologically upgradient or downgradient, respectively, of agriculture or other potential nitrogen 

sources. Depressional wetlands in watershed uplands are generally more likely be hydrologically upgradient of agriculture 

than those occurring in lowlands, and are therefore presumably 

less likely to intercept agricultural nitrogen. Depressional 

wetlands in the NACP often occur along drainage divides in 

forested areas that are upgradient of agriculture or other potential nitrogen sources and therefore have minimal impact on 

exported nitrogen loads. These wetlands can also be embedded 

within large expanses of wetland flats that have been drained 

for crop production and have minimal hydrologic gradients 

(DeSteven and Lowrance, 2011). 

Statistical Analysis and Interpretation

Cluster analysis was used to group the 33,799 individual 

catchments in the study area into eight classes within which 

landscape metrics relevant to wetland occurrence and 
Classification and Description of Wetland Landscapes in the Northern Atlantic Coastal Plain 7

effectiveness at nitrogen mitigation are similar. Because the 

16 landscape metrics are often correlated with one another and 

may contain redundant information, the cluster analysis was 

conducted on three principal components (PCs) summarizing 

the information in the 16 landscape metrics, rather than on the 

original metrics themselves.

PCA was used to identify and eliminate redundancy in the 

16 wetland landscape metrics (McGarigal and others, 2000; 

SAS Institute, Inc., 2009). PCA generates uncorrelated PCs 

that are linear combinations of input variables, and simplifies 

subsequent analyses by removing correlation and redundancy 

among input variables (Helsel and Hirsch, 1992; McGarigal 

and others, 2000). Loadings (table 1) that illustrate how each 

PC is correlated with each input variable are computed and are 

useful in interpreting each PC (SAS Institute, Inc., 2009). For 

the wetland landscape model, PCA was run on standardized 

landscape metrics, and PC scores were computed to quantify 

how each of the 33,799 watershed catchments in the study 

area plot on each PC based on the values of the original input 

landscape metrics (SAS Institute, Inc., 2009). 

Cluster analysis was used to classify the study-area 

catchments into clusters with relatively similar PC scores and 

therefore similar combinations of landscape characteristics 

(metrics) relevant to nitrogen mitigation by wetlands. 

Agglomerative clustering methods accumulate individual 

observations (data values) into continually larger clusters. 

In a hierarchical technique, the clusters are arranged into a 

structured hierarchy with relations that can be meaningful. 

Agglomerative hierarchical clustering techniques have been 

previously used for similar landscape analyses (for example, 

Lipscomb, 1998; Preston, 2000; Wolock and others, 2004). 

Clusters of catchments relevant to wetlands in the NACP 

were identified using Ward’s minimum variance method, an 

agglomerative hierarchical clustering technique (SAS Institute, 

Inc., 2009) that joins clusters to minimize within-cluster 

variance (McGarigal and others, 2000). Although numerous 

guidelines are available, the number of clusters to retain and 

interpret is subjective, and was determined in this case through 

consideration of the clusters in light of their interpretability. 

Additional clusters may more accurately represent input data, 

but may also be more difficult to interpret and ultimately less 

useful.

Classes of watershed catchments were interpreted to 

predict the likely effects of depressional wetlands on nitrogen 

transport. Rank-transform analysis-of-variance and Tukey 

tests (Helsel and Hirsch, 1992) were used to compare values 

of landscape metrics and other geographic variables among 

clusters to support interpretation of the clusters and to evaluate 

the effectiveness of the cluster analysis. The likely effectiveness of depressional wetlands at mitigating nitrogen transport 

in each cluster was estimated on the basis of four factors: 

(1) the likelihood of the occurrence of such wetlands; (2) the 

relative position of likely wetlands along presumed hydrologic 

flowpaths; (3) selected soil properties; and (4) the likely position in the landscape of agricultural sources of nitrogen.

Classification and Description of 

Wetland Landscapes in the Northern 

Atlantic Coastal Plain

Statistical analyses (PCA and cluster analysis) were 

performed on the 16 landscape metrics and were used to classify each of the 33,799 original stream catchments into one of 

eight WLRs. Variable topography, soil conditions, and other 

landscape characteristics among the WLRs contribute to a 

similar variability in the likely effectiveness of depressional 

wetlands at mitigating nonpoint nitrogen transport in different 

parts of the NACP. The geographic model defined by the eight 

delineated WLRs can be used for watershed management and 

further research of regional nitrogen fate and transport.

Statistical Analyses

The 16 input landscape metrics are generally well correlated with one another and are therefore good candidates 

for PCA (table 1). The first three unrotated PCs explain at 

least 15 percent (individually) and 77 percent (collectively) 

of the variability in the 16 input landscape metrics (table 1), 

and were retained for cluster analysis. The first PC (PC1) 

explains nearly half (43 percent) of the variability in the input 

data and represents a measure of the likelihood of conditions 

conducive to the occurrence of depressional wetlands. Input 

landscape metrics with strong positive loading on PC1 include 

measures of overall catchment flatness (including overall flat 

land as well as flat land in uplands and lowlands) and indicators of poor drainage (including hydric soils and soil drainage 

class). PC1 is also moderately positively correlated with soil 

organic matter. Landscape metrics with strong negative loadings for PC1 include slope and relief (table 1). PC2 represents 

a distinction between catchments with flat lowlands (positive 

values) and those with flat uplands (negative values), which 

can be interpreted as an indicator of where wetlands in each 

catchment may occur relative to nitrogen sources (such as 

agriculture) along local topographic gradients and (presumably) hydrologic flowpaths from uplands towards lowlands. 

PC2 may also act as a surrogate for understanding stream incision that affects overall watershed drainage. PC3 is an indicator of soil texture. Catchments with negative values of PC3 

contain relatively sandy, conductive soils; those with positive 

values include less well-drained soils with higher available 

water capacity (table 1).

Eight groups of catchments were selected from the cluster 

analysis based on the distribution of input landscape metrics 

reflected in the PCs (table 2). These eight groups (clusters) 

were selected on the basis of their interpretability and various 

metrics (McGarigal and others, 2000), and account for 68 

percent of the variability in the input PC1, PC2, and PC3. 

The cluster results (table 2) represent a meaningful classification of the NACP with respect to the input landscape metrics; 

the distributions of input topographic (fig. 4) and soil (fig. 5) 
8 A Classification of the Effectiveness of Depressional Wetlands at Mitigating Nitrogen in the Northern Atlantic Coastal Plain

Table 2. Summary of depressional wetland landscape regions within the Northern Atlantic Coastal Plain.

[km2, square kilometer; %, percent; N/A, not applicable]

Wetlands 

landscape 

Area 

(km2

) 

[%]

Summary of setting and predicted depressional wetland landscape

Relative 

likelihood 

of nitrogen 

mitigation

Very Flat Poorly 

Drained 

Uplands 

(VFPDU)

(6,500)

[6]

Mostly flat and poorly drained upland-dominated watersheds. Soils are mostly finegrained and hydric with relatively high available water capacity. High potential 

for nitrogen losses in prior-converted cropland. Agricultural and timber harvesting 

common in drained former wetlands.

High

Flat Poorly 

Drained 

Lowlands 

(FPDL)

(11,700)

[11]

Mostly flat and poorly drained lowland-dominated watersheds, including relatively 

large Carolina Bays to the south. Soils have mixed permeability but relatively high 

available water capacity. Wetlands are abundant and may be drained, but drainage 

would be difficult due to flat lowland topography. High potential for nitrogen losses 

in prior-converted cropland. Agriculture common in drained former wetlands.

High

Flat Sandy 

Lowlands 

(FSL)

(16,700)

[15]

Typically flat lowland-dominated watersheds with mixed drainage. Soils are typically 

sandy and permeable with low available water capacity. Wetlands are common 

and often forested or drained for agriculture. Prior-converted cropland is likely to 

intercept nitrogen that is direcly applied, and remaining wetlands interspersed with 

agriculture may intercept nitrogen from adjacent uplands.

High

Flat Mixed 

(FM)

(10,400)

[9]

Typically flat watersheds with mixed drainage, geomorphology, and soils. Mixed 

potential for wetlands and mitigating agricultural nitrogen.

Moderate

Flat Mixed 

Uplands 

(FMU)

(17,700)

[16]

Typically flat upland-dominated watersheds. Mixed soils, potential for wetlands, 

and nitrogen losses. Wetlands may be drained for agriculture, although artificial 

drainage is likely less common than in the FSL, and irrigation may be required. 

Some potential for nitrogen losses in prior-converted cropland; however, adjacent 

agricultural areas contributing to wetlands may be very localized.

Low

Flat Sandy 

Uplands 

(FSU)

(7,300)

[7]

Typically flat upland-dominated watersheds. Soils are typically sandy and permeable with relatively low available water capacity. Limited wetlands interspersed 

with well-drained areas. Artifical drainage is likely minimal and irrigation is often 

required for crop production. Nitrogen losses in prior-converted cropland may be 

limited by short hydroperiod, and adjacent agricultural areas contributing to 

wetlands may be very localized.

Low

Rolling Hills with 

Mixed Soils 

(RMS)

(32,900)

[30]

Watersheds with relatively high slope and relief and good drainage. Geomorphology 

and soil conditions are mixed. Wetlands limited to riparian zones.

N/A

Rolling Hills with 

Sandy Soils 

(RSS)

(7,800)

[7]

Watersheds with mostly relatively high slope and relief and relatively good drainage. 

Geomorphology is mixed. Soils are generally sandy and permeable with low 

available water capacity. Wetlands limited to riparian zone.

N/A

metrics generally vary significantly among the eight clusters. 

The distribution of different land covers also varies significantly among the eight clusters, and the occurrence of mapped 

wetlands are generally consistent with interpretations from the 

cluster-analysis based model (fig. 6).

Wetland Landscape Regions

Significant differences in topographic, soil, and other 

landscape conditions among the eight WLRs contribute to 

similar differences in the likely effects of depressional wetlands on nitrogen fate and transport. 

The Very Flat Poorly Drained Uplands (VFPDU) cover 

6,500 km2

 of particularly flat near-coastal low-lying areas 

primarily in North Carolina and southern Virginia (fig. 7, table 

2). Catchments in the VFPDU have significantly less relief and 

a greater percentage of flat land than in any other WLR. More 

than half of the catchments in the VFPDU include greater than 

90 percent flat land; in no other WLR is the median percentage of flat land as high as 80 percent (fig. 4). Most of the flat 

land in VFPDU catchments is located within local uplands; 

VFPDU catchments are typically broad, flat plains with 

limited minor lowlands, likely including incised streams or 

artificial ditches (fig. 8). Much of the area of the VFPDU that 
Classification and Description of Wetland Landscapes in the Northern Atlantic Coastal Plain 9

Figure 4. Distribution of selected topographic metrics computed from 30-meter elevation data among wetland 

landscape regions (see table 2).

is currently cultivated was previously wetland flats. Soils in 

the VFPDU are significantly finer in texture with greater available water capacity (AWC) and more commonly hydric than 

in any other WLR (fig. 5). Soils in the VFPDU also contain 

significantly more organic matter than in most other WLRs. 

In spite of the soil conditions and extremely flat topography in 

the VFPDU, overall wetlands are less common than in other 

WLRs (fig. 6), likely due to artificial drainage (ditching) to 

support agriculture. 

Natural or restored depressional wetlands in the VFPDU 

would likely have a high potential to mitigate nitrogen 

transport from nonpoint sources to local streams. The area is 

extremely flat and is underlain by organic soils with relatively high AWC and likely reducing geochemical conditions; 

water would move slowly through the low-gradient landscape 

providing ample opportunity for denitrification. The distinction between relative uplands and lowlands is probably less 

important in the VFPDU than in other WLRs with greater 

relief. Ditching in depressional wetlands and wetland flats in 

the VFPDU would lower the water table locally and induce 

greater flow from surrounding areas to the wetland, regardless of whether they are in natural relative “uplands” or 

“lowlands.” Ditching of VFPDU wetlands may thus increase 

their effectiveness as nitrogen sinks, although ditching may 
10 A Classification of the Effectiveness of Depressional Wetlands at Mitigating Nitrogen in the Northern Atlantic Coastal Plain

Figure 5. Distribution of selected soil metrics among wetland landscape regions (see table 2).

increase nitrogen transport from wetlands by reducing the 

length of groundwater flowpaths and therefore, the time available for denitrification to occur (Phillips and Donnelly, 2003). 

Successful drainage through artificial ditching may also oxidize upper soils and decrease the area of likely denitrification.

Catchments in the Flat Poorly Drained Lowlands 

(FPDL) cover 11,700 km2

 of the NACP (fig. 7, table 2). 

FPDL catchments, many of which are likely former Carolina 

Bays, are interspersed among VFPDU catchments in much 

of North Carolina and southern Virginia, but also occur more 

widely throughout the Coastal Plain, particularly south of the 

Potomac River. As in the VFPDU, FPDL catchments are typically extremely flat with a median relief of less than 10 m (fig. 

4) and likely formed in Carolina Bays and other landscape 

depressions (fig. 9). Unlike in the VFPDU, however, flat land 

in the FPDL is typically located within relative lowlands. Soils 

in the FPDL have greater mean AWC and are more commonly 

hydric than in any other WLRs except for the VFPDU, and are 

also relatively fine-grained and organic rich (fig. 5). Wetlands 

are more common in the FPDL than in any other WLR (fig. 6). 

Among the eight WLRs, depressional wetlands in the 

FPDL may have the greatest potential to mitigate agricultural 

nitrogen. Soil and topographic conditions would promote slow 

movement of water and conditions favorable for denitrification 

in the FPDL, as in the VFPDU. Unlike in the VFPDU, however, the dominance of flat lowlands in the FPDL indicates 

that depressional wetlands, which are likely located in areas 

with drained wetland flats that are currently cultivated (fig. 9), 

may occur downgradient of nitrogen sources and be more 

likely to intercept nitrogen. The distinction between uplands 
Classification and Description of Wetland Landscapes in the Northern Atlantic Coastal Plain 11

Figure 6. Distribution of selected land cover among wetland landscape regions (see table 2).

and lowlands may be relatively insignificant in the low-relief 

FPDL as in the VFPDU, however, and artificial drainage may 

draw groundwater toward wetlands regardless of apparent 

topographic position. 

The Flat Sandy Lowlands (FSL) cover 16,700 km2

 distributed over most of the Coastal Plain, particularly over broad 

areas of the Delmarva Peninsula and southern New Jersey, 

and also are common in southern North Carolina (fig. 7, table 

2). FSL catchments are moderately flat, particularly in local 

watershed lowlands (fig. 4). Soils in FSL catchments are generally sandy with low mean AWC but relatively high organic 

matter content (fig. 5). The median coverage of hydric soils 

among FSL catchments is around 40 percent, and wetlands are 

more common in the FSL than in any other WLR except the 

FPDL (fig. 6). 

Although wetlands, including isolated depressions 

(fig. 10a) and broader flat areas (fig. 10b), are relatively 

common in FSL catchments, the success of ditching to promote artificial drainage is likely facilitated by the moderate 

relief and permeable soils. Artificial drainage is common in 

agricultural areas developed on FSL areas; irrigation is generally not required (fig. 10). Because most wetlands likely occur 

in lowlands downgradient from agricultural nitrogen sources 

or are drained and cultivated, however, both natural and former (drained) wetlands likely have a high potential to mitigate 

nitrogen transport. By decreasing the residence time of water 

in organic-rich anoxic sediments that promote denitrification, 

however, ditching may alternatively increase nutrient transport 

from wetlands in the FSL, as in other landscape regions 

(Phillips and Donnelly, 2003). 
12 A Classification of the Effectiveness of Depressional Wetlands at Mitigating Nitrogen in the Northern Atlantic Coastal Plain

Figure 7. Wetland landscape regions within the Northern Atlantic Coastal Plain.
Classification and Description of Wetland Landscapes in the Northern Atlantic Coastal Plain 13

Figure 8. Parts of the Very Flat Poorly Drained Upland (VFPDU) in North Carolina. Much of the VFPDU is 

artificially drained for timber or row crop production.
14 A Classification of the Effectiveness of Depressional Wetlands at Mitigating Nitrogen in the Northern Atlantic Coastal Plain

Figure 9. Flat Poorly Drained Lowlands (FPDL), as in this area of North Carolina, commonly form in large 

depressions such as Carolina Bays, and are often artificially drained for cultivation.
Classification and Description of Wetland Landscapes in the Northern Atlantic Coastal Plain 15

Figure 10. In the Flat Sandy Lowlands (FSL) in the northern part of the Delmarva Peninsula (A), wetlands 

often occur in isolated depressions, indicated by yellow arrows (such as Delmarva Bays), and minimal artificial 

drainage is required to support agriculture. Further south on the peninsula, however, (B), natural drainage is 

extremely poor and extensive ditching is common. In neither area is irrigation often required.
16 A Classification of the Effectiveness of Depressional Wetlands at Mitigating Nitrogen in the Northern Atlantic Coastal Plain

Figure 10. In the Flat Sandy Lowlands (FSL) in the northern part of the Delmarva Peninsula (A), wetlands 

often occur in isolated depressions, indicated by yellow arrows (such as Delmarva Bays), and minimal artificial 

drainage is required to support agriculture. Further south on the peninsula, however, (B), natural drainage is 

extremely poor and extensive ditching is common. In neither area is irrigation often required.—Continued
Applications and Limitations 17

The Flat Mixed (FM) WLR covers 10,400 km2, dispersed widely throughout the Coastal Plain of North Carolina, 

southern Virginia, the Delmarva Peninsula, and southern New 

Jersey (fig. 7, table 2). FM catchments are generally flat areas 

that are relatively evenly distributed among local uplands and 

lowlands (fig. 4). Soil properties are moderate compared to the 

other seven WLRs, although the median percentage of hydric 

soils in FM catchments is nearly 40 percent (fig. 5). FM catchments generally contain more agriculture than VFPDU, FPDL, 

or FSL catchments, and less than 25 percent wetlands (fig. 6).

Generally mixed and moderate landscape and soil conditions contribute to a variable potential for nitrogen losses 

in depressional wetlands in the FM catchments. As in other 

WLRs, prior-converted wetlands will likely be effective at 

mitigating directly applied nitrogen, but the movement of 

nitrogen from adjacent areas will depend on local hydrologic 

gradients and flowpaths.

The Flat Mixed Uplands (FMU) and Flat Sandy 

Uplands (FSU) WLRs cover 25,000 km2

 (collectively) over 

much of the Coastal Plain, particularly in North Carolina 

and the Delmarva Peninsula (fig. 7; table 2). The two WLRs 

represent very similar topographic settings, including low 

relief, incised streams, and a relatively high amount of flat 

land (greater than 60 percent), which is located mostly in local 

watershed uplands (fig. 4). The major difference between the 

FMU and FSU is in soil composition; soils in FSU catchments 

are generally sandier with lower mean AWC and are less commonly hydric, but include slightly greater organic content 

(fig. 5). As might be expected in broad, flat predominantly 

well-drained uplands, FSU and FMU catchments contain the 

greatest concentration of agriculture (more than 40 percent) of 

any WLR. Because of the efficient natural drainage, irrigation 

is often required for crop production (fig. 11). Wetlands cover 

10 to 15 percent of each WLR and are mostly confined to 

riparian zones (fig. 6).

The likelihood of nitrogen mitigation in depressional 

wetlands is relatively low in catchments of the FMU and FSU 

WLRs. Wetlands are relatively rare in the flat uplands of these 

catchments, and those that do occur would be relatively easy 

to drain for cultivation, particularly in the permeable soils of 

the FSU. The relative upland location of large flat areas also 

suggests that wetlands that do occur in uplands may receive 

little drainage from agricultural areas. 

The Rolling Hills with Mixed Soils (RMS) and Rolling 

Hills with Sandy Soils (RSS) WLRs are largely confined 

within the Coastal Plain to areas of Maryland and Virginia 

west of Chesapeake Bay (fig. 7). The RMS also covers most 

of the parts of the study area in the Piedmont Physiographic 

Province west of the Fall Line. RMS and RSS catchments 

have greater relief and less flat land, hydric soils, or soil 

organic matter than any other WLRs (fig. 4, fig. 5). Whereas 

RSS soils are relatively sandy with the lowest mean AWC of 

any WLR, RMS soils have a finer texture and greater mean 

AWC (fig. 5). Wetlands and agriculture are relatively uncommon in RMS and RSS catchments, as might be expected in 

light of the rolling topography; forest is the predominant land 

cover in both areas (fig. 6).

Applications and Limitations

The geographic model including the eight WLRs can 

be used for any application requiring an understanding of the 

spatial variability in the potential effectiveness of wetlands at 

mitigating nitrate transport from watershed uplands to streams. 

The WLRs represent areas of the Coastal Plain where nitrate 

losses to denitrification in depressional wetlands are more 

or less likely, and can therefore be used to target wetland 

restoration or conservation to areas where they may provide 

the greatest improvement in water quality. For example, 

restoration of wetlands along watershed divides may yield 

little improvement in the nitrogen loads in receiving streams, 

but such improvements may be substantial following restoration of lowland wetlands that have been artificially drained. 

The geographic model may also be useful for improving 

regional models of nitrogen fate and transport. Groundwater 

is a significant vector for nitrogen transport from the land 

surface to streams in the Coastal Plain (Bachman and Phillips, 

1996; Ator and others, 2011; Ator and Denver, 2012), and 

denitrification is the only major sink for nitrate in groundwater. Regional models used to understand and manage nutrients 

often require spatially explicit delineations of factors affecting 

nitrogen fate and transport; such delineations of the effectiveness of depressional wetlands at nitrate mitigation have been 

previously lacking. 

The effects of several limitations must be considered in 

using the geographic model of WLRs. Fundamental landscape 

units used in the model were derived from NHDPlus hydrography at the 1:100,000 scale. Although model predictions may 

be useful for broad regional areas, similarly reliable predictions for individual catchments or other areas smaller than a 

few square kilometers in size would likely require modeling 

at a finer resolution. The model results also are limited by the 

availability of input data representing landscape characteristics particularly relevant to nitrate fate and transport around 

wetlands. The thickness of the surficial aquifer, in particular, 

affects the formation of groundwater flowpaths and the likelihood that nitrate may pass beneath wetlands unaffected by 

reducing conditions (Gold and others, 2001; Puckett, 2004). 

The thickness of the surficial aquifer has not been mapped 

at sufficient resolution or consistency across the study area, 

and the lack of this information may limit the reliability of 

the WLRs. Similarly lacking is a consistent, comprehensive 

water-quality database against which to test the effectiveness 

of the WLRs at predicting nitrate concentrations in streams or 

groundwater. Calibration of the model with such data would 

greatly improve its usefulness for a variety of applications.

The geographic regionalization approach applied to the 

NACP has great potential to improve the understanding of 

nitrogen fate and transport around wetlands in other areas. 

Geographic analyses and statistical techniques have been used 

widely to delineate landscape areas with similar geographic 

features for a variety of applications (for example, Mosely, 

1981; Lipscomb, 1998; Preston, 2000; Chiang and others, 

2002; Caratti and others, 2004; Wolock and others, 2004; Rao 
18 A Classification of the Effectiveness of Depressional Wetlands at Mitigating Nitrogen in the Northern Atlantic Coastal Plain

Figure 11. Circular fields in the Flat Sandy Uplands (FSU) near Hurlock, Maryland are indicative of center-pivot 

irrigation systems. Because FSU watersheds are generally well-drained, irrigation rather than artificial drainage 

is often used to support crop production.
References Cited 19

and Srinivas, 2006; Isik and Singh, 2008), and the landscape 

metrics relevant to wetland occurrence and nitrogen fate and 

transport in the Coastal Plain should be similar in other areas. 

The technique has the greatest potential for areas where such 

metrics are available and delineated for all the relevant variables. Conversely, a lack of certain types of information contributes significantly to uncertainty in the WLRs. For example, 

the formation of groundwater flowpaths as well as the fate and 

transport of nitrogen are heavily influenced by the thickness 

of the surficial aquifer, and a reliable spatial representation of 

this aquifer thickness is not currently available for the NACP. 

Suitable field data for use in calibrating the water-quality or 

other effects of geographic regions defined by cluster analyses 

would also be very useful in other areas where such techniques 

may be applied.

Summary and Conclusions

Abundant nonpoint sources of nitrogen to the land surface (such as agricultural fertilizer and manure applications) 

in much of the Northern Atlantic Coastal Plain (NACP) 

contribute to excessive nitrogen and resulting ecological 

stresses in area streams and receiving coastal estuaries such as 

Chesapeake Bay and Pamlico Sound. Depressional wetlands 

that are common in parts of the Coastal Plain can intercept and 

remove nitrogen applied in excess of plant uptake requirements from shallow groundwater and overland runoff during 

transport to surface waters; however, the effectiveness of different wetlands as sinks for nitrogen varies significantly with 

variations in hydrologic, geochemical, and soils conditions 

that are often poorly or inconsistently defined or mapped for 

large areas. The effectiveness of individual wetlands at removing nitrogen is therefore difficult to predict without extensive 

field investigations, and the importance of depressional wetlands to regional nitrogen budgets is often poorly quantified.

A regional geographic model was developed to delineate 

areas of the NACP within which depressional wetlands would 

likely be similarly effective at mitigating the transport of 

nitrogen from nonpoint sources to surface waters. Available 

hydrography at the 1:100,000 scale was used to define 33,799 

individual watershed catchments in the Coastal Plain, and 16 

landscape metrics particularly relevant to the occurrence of 

depressional wetlands and their effectiveness at mitigating 

nitrogen transport were defined for each catchment based primarily on available soil and topographic information. Principal 

components analysis (PCA) demonstrated that 78 percent of 

the variability in the 16 landscape metrics is attributable to 

three principal components (PCs) representing the probability 

of wetland occurrence, their likely position along hydrologic 

flowpaths, and soil composition. Cluster analysis was used to 

aggregate the 33,799 individual catchments into eight wetland 

landscape regions (WLRs) on the basis of these three PCs. 

Significant differences in topographic settings, soils, and 

land cover among the eight WLRs was used to evaluate the 

effectiveness of the cluster analysis and interpret the likely 

effects of wetlands at mitigating nitrogen transport.

The likely occurrence of depressional wetlands and their 

effectiveness at mitigating nitrogen transport to surface waters 

varies substantially over different areas of the Coastal Plain. 

The Very Flat Poorly Drained Uplands (VFPDU), Flat Poorly 

Drained Lowlands (FPDL), and Flat Sandy Lowlands (FSL), 

which include 32 percent of the Coastal Plain (collectively), 

are extremely flat and (or) low-lying and therefore very poorly 

drained. Natural and prior-converted depressional wetlands 

and wetland flats are common in these areas and have a 

relatively high potential to intercept nitrogen moving from 

the land surface to surface waters. Conversely, better natural 

drainage limits the occurrence of depressional wetlands in the 

Flat Mixed Uplands (FMU) and Flat Sandy Uplands (FSU), 

and the potential for nitrogen losses is relatively low in these 

WLRs, which collectively cover 23 percent of the Coastal 

Plain. Nine percent of the Coastal Plain is within the Flat 

Mixed (FM) WLR, which includes a wide range of drainage 

efficiencies, geomorphology, and soils, all of which contribute 

to its moderate likelihood of nitrogen mitigation. The Rolling 

Hills with Mixed Soils (RMS) and Rolling Hills with Sandy 

Soils (RSS) WLRs include 37 percent of the study area, with 

relatively high relief and slope. Wetlands in these areas are 

probably limited mainly to riparian zones. 

The geographic model composed of the eight WLRs may 

be useful for numerous regional applications, but must be 

considered in light of inherent model assumptions and limitations. Delineation of landscapes most conducive to nitrogen 

mitigation by depressional wetlands should be very useful for 

targeting wetland conservation or restoration investments to 

protect water quality. The regional model may also be useful 

in water-quality models designed to estimate the regional 

effects of depressional wetlands on nitrogen transport from 

the Coastal Plain. Limitations in the model include a lack of 

regional water-quality data suitable for calibration and a lack 

of available input data characterizing certain landscape properties known to be important to nitrogen transport in groundwater, particularly the thickness of the surficial aquifer. 

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Appendix

Wetland landscape regions (WLRs) in the Northern 

Atlantic Coastal Plain are available online in a commadelimited ASCII file at http://pubs.usgs.gov/sir/2012/5266/

nacp_wlrs.csv. Variables in the file are described in the header 

(denoted by lines starting with “#”) and also include the 

principal component scores that were derived from principal 

components analysis on the 16 original landscape metrics 

and were used in cluster analysis to create the WLRs. The 

file includes predictions for local catchments contributing to 

33,799 stream reaches in the Coastal Plain as defined by the 

National Hydrography Dataset Plus (NHDPlus) medium resolution (1:100,000-scale) geospatial dataset (Horizon Systems, 

2010).

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Horizon Systems, 2010, NHDPlus documentation, version 1, 

available online at ftp://ftp.horizon-systems.com/NHDPlus/

NHDPlusV1/documentation/NHDPLUSV1_UserGuide.pdf.

Prepared by USGS West Trenton Publishing Service Center.

Edited by Valerie M. Gaine.

Graphics and layout by Timothy W. Auer.

For additional information, contact:

Director, MD-DE-DC Water Science Center

U.S. Geological Survey

5522 Research Park Drive

Baltimore, MD 21228

or visit our Web site at:

http://md.water.usgs.gov
Ator, S.W., and others—A Regional Classification of the Effectiveness of Depressional Wetlands at Mitigating Nitrogen Transport to Surface Waters in the Northern Atlantic Coastal Plain—Scientific Investigations Report 2012–5266

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