Chapter

10

By Mark Stolt, University of Rhode Island, and James Turenne and 

Maggie Payne, USDA-NRCS.

Subaqueous Soil Survey

Introduction

Subaqueous soils differ from subaerial, or terrestrial, soils by 

having perennial water on the soil surface. These soils occur in 

shallow freshwater and marine environments, such as ponds, 

lakes, and the subtidal areas of estuaries and tidal embayments. The 

Soil Survey Staff (2014) defines “shallow” as approximately 2.5 m. 

At depths greater than 2.5 m, sunlight is typically attenuated and 

submerged aquatic vegetation (SAV) is typically absent. In especially 

clear waters, however, this depth may be much greater. Thus, for 

interpretive purposes, mapping is typically done to depths of 5 meters of 

water. Areas with extreme tidal ranges are also included as subaqueous 

soils even though they may be exposed for an hour or two during a neap 

tide or similar event. Subaqueous soils occur on a range of subtidal and 

limnic landforms, such as flood-tidal deltas, washover fans, and lake 

beds (Schoeneberger and Wysocki, 2012; USDA-NRCS, 2016). These 

soils are currently classified in the Histosol and Entisol orders of Soil 

Taxonomy (Soil Survey Staff, 2014).

Occurrence

Subaqueous soils occur worldwide, except in the driest inland areas 

where water does not pond permanently to form lakes or ponds. In 

coastal areas, these may be the most extensive soils on the landscape. 

For example, Rhode Island has approximately 700,000 acres of 

subaerial soils. If the area of subaqueous environments having water 

depths of less than 5 m are included, the total area of soils would almost 

double. 
506 Chapter 10

Importance

The world’s population centers are in coastal areas. In the United 

States, coastal watershed counties make up less than 10 percent of the 

land area, but more than 50 percent of the population lives in these 

counties. If the cities and towns within a short distance of the Great 

Lakes were included, that number would be significantly larger. Coastal 

and inland waters are used for transportation, recreation, farming, and 

other livelihoods. From an ecological perspective, shallow and inland 

waters are the nursery grounds for most of the animals that inhabit these 

ecosystems. The subaqueous soils in these shallow habitats provide the 

foundation and structure of the ecosystem. The many anthropogenic 

activities in the shallow water habitats disrupt, disturb, and may even 

destroy these habitats. Therefore, understanding the distribution and 

characteristics of the subaqueous soils is critical to properly managing 

and using these habitats so that these ecosystems properly function 

and continue to be healthy and vibrant. The use and management of 

subaqueous soils in shallow water ecosystems may include dredging, 

dredge placement, restoration of submerged aquatic vegetation, 

identification of areas for shellfish aquaculture, restoration of wild stocks 

of shellfish, and identification of areas for docks and moorings (e.g., Stolt 

and Rabenhorst, 2011; Erich et al., 2010). 

Sampling, Description, Characterization, and 

Classification

Chapters 2 and 3 provide standards for describing soil profiles 

and their site characteristics in the subaerial settings common to soil 

survey. The subaqueous soil environment presents unique challenges 

for observing soil profiles, collecting samples, and describing soil 

properties. This section provides information specific to subaqueous 

soils. The Field Book for Describing and Sampling Soils has a section 

that provides important information specific to mapping, describing, 

and sampling subaqueous soils (McVey et al., 2012).

Sampling

Subaqueous soils can be sampled by several traditional soil 

approaches, but marine science approaches are best. For cursory 

descriptions and sampling, a standard bucket auger can be used. In order 

to sample from the exact location with depth, some soil mappers use a 
Soil Survey Manual 507

piece of PVC pipe with an inside diameter a little larger than the teeth 

on the bucket auger. The auger is placed into the pipe, and the sample 

is collected in the typical fashion. While the bucket is being removed, 

the PVC pipe is pushed deeper into the soil. The sample is retrieved 

and placed in a tray (typically a meter-long piece of vinyl gutter). The 

auger bucket is pushed down the pipe again, the spoil from pushing the 

PVC pipe down is removed, and then the next depth is sampled. This 

procedure is effective for sampling the upper 75 cm of the soil. Below 

this depth, however, collecting samples with a bucket auger becomes 

very difficult. 

In shallow water where soils are non-fluid, sampling with a bucket 

auger can be done while wading. A small light raft (typically made from 

floating dock material of Styrofoam) works well in holding the sample 

tray. A small anchor (e.g., a brick with holes) is used to keep the raft in 

place. In deeper water, samples can be collected from the side of a boat. 

A boat with a deck at the bow can be used, but a pontoon boat with a 60 

x 60 cm sampling port cut into the deck between the two pontoons (i.e., 

a moon-pool) is preferred (fig. 10-1). The boat is anchored at two points, 

and the bow faces into the wind or the direction of oncoming waves to 

keep the boat in the place while sampling. Sampling should be avoided 

if there is significant wind or waves. In freshwater systems in northern 

climates, sampling can be done through ice. A standard ice auger is used 

to cut a hole in the ice, and the soil is sampled through the hole.

For organic soil materials or fluid mineral materials, a Macaulay 

peat sampler is very effective. Most Macaulay samplers have sampling 

chambers 50 cm long. Machine shops can construct auger chambers 

to longer lengths (such as 1 m), which work well for subaqueous soil 

sampling. These samplers can be used easily through the moon-pool, off 

the side of the boat, or through a hole in ice. Because water depths vary, 

extensions on the bucket auger or peat sampler should be easy to add on 

or remove.

For detailed sampling and description of non-fluid materials, a 

vibracore sampler is ideal. Vibracore samplers consist of a concrete 

mixer motor, a cable that delivers the vibration from the motor to the 

sampling tube, and a vibration head that clamps onto the irrigation pipe 

(core barrel) to deliver the vibration to the core barrel that collects the 

sample (fig. 10-2). The vibration loosens (or liquefies) the soil material 

around the core barrel so that the core can be easily pushed into the 

soil with a minimal amount of pressure from the top of the pipe. This 

sampling approach is typically done from a pontoon boat with a moonpool. If the body of water is too small for a pontoon boat, or gas-powered 
508 Chapter 10

Figure 10-1

A pontoon boat used for subaqueous soil sampling. The sampling is conducted through 

the moon-pool. A sealed core is strapped to the end of the chain fall. The chain fall is 

attached to the tripod, which is centered over the moon-pool.

boats cannot be used, a small barge with a moon-pool in the middle can 

be used. In this case, the concrete mixer motor is in the small boat used 

to pull the barge and the soil is collected and retrieved through the moonpool in the barge. There are “backpack” types of vibracores that can be 

used off barges or through the ice. In some cases, these types of corers 

are not powerful enough to push the tubes through dense soil materials.

Core barrels used for vibracoring are typically 10 cm in diameter 

(fig. 10-3) and should be long enough to accommodate both the soil 

sample and the overlying water. Once the barrel is pushed into the 

soil to the desired depth, a lead weight (sinker) attached to a string is 
Soil Survey Manual 509

dropped into the core barrel to measure the distance from the top of 

the core barrel to the soil surface in the tube. The same thing is done 

on the outside of the core. These measurements are made to determine 

the amount of collapse (settlement) that has occurred to the soil during 

coring (McVey et al., 2012). Significant collapse should be noted when 

recording horizon depths and reporting soil bulk density.

To retrieve the soil, the core is first slowly filled with water. A cap 

(test plug) is placed on the top that, when tightened, seals the top of the 

core. This creates a suction and, for most materials, keeps all of the soil 

material in the core barrel when it is removed. In some soil materials, 

particularly those that are coarse textured with many rock fragments, 

some of the soil material may fall out of the bottom of the core barrel 

when it is retrieved. In these cases, a core catcher can be added to the 

base of the core before it is vibracored into the soil (fig. 10-3). Once the 

core is sealed, it is attached with a heavy strap to a winch or chain fall 

and pulled out of the soil. Using a chain fall attached to a tripod mounted 

Figure 10-2

Equipment for vibracore sampling. A vibracore sampler consists of an engine, cable, 

head, and core barrel. The engines are typically gasoline driven (sold as a device to 

keep concrete mixed so it does not set). The cable carries the vibrations to the head, 

which is attached to the core barrel with four bolts.

Vibracore 

engine

Vibracore 

head

Moonpool

3” 

Aluminum 

tubing

Chain 

fall
510 Chapter 10

Figure 10-3

A core barrel and core catcher. Left image—Core barrels used for vibracoring are 

typically 10 cm in diameter. The view of the sand catcher is from the perspective of the 

inside of the barrel. Right image—For sandy or loose materials, a core catcher may be 

attached to the bottom of the core barrel.

over the moon-pool is the safest way to remove the core (fig. 10-1). For 

sampling off a barge or through the ice, an aluminum ladder is commonly 

used instead of the tripod because of its lighter weight. 

Once the core is pulled from the soil and sealed at the bottom (to 

prevent loss of the sample), a small hole is created in the irrigation pipe 

just above the top of the soil in the core barrel to allow the water on top of 

the core to drain slowly (see above information on measuring the depth 

from the top of the core barrel to the top of the soil). After the water 

is drained, the tube is cut just above the soil surface with a tube cutter 

and the cap is screwed back in place to preserve the core. The bottom is 

sealed with a cap first to prevent loss of suction. The core barrel is dried 

and clearly labeled with a pedon number and the correct orientation. The 

core can be opened on the boat, or it can be stored in a rack and later 

described and sampled on land or in a lab. Vibracores in the lab should 

be maintained at 4 0

C to minimize drying and oxidation of sulfides prior 

to sampling and analysis. 

Core barrels are best opened by laying the core down on a table or 

lab bench and cutting lengthwise on opposite sides with electric metal 

shears (fig. 10-4, top image). A circular saw works but creates safety 

issues and produces aluminum shavings, which need to be collected. A 

piano wire or steel guitar string is slid between the cuts in the aluminum 

tube to split the core in two. The two sides are lifted apart. One side is 

described and sampled, and the other is archived if needed (fig. 10-4, 

bottom image). 
Soil Survey Manual 511

Figure 10-4

A core from subaqueous sampling. Top image—Cutting open the core with electric 

shears. Bottom image—A split core exposing the soil. One side is typically sampled, 

and the other may be stored and archived.

Description

In general, subaqueous soils are described the same way as subaerial 

soils (Schoeneberger et al., 2012) but using the setting terminology, 

sampling method, and various chemical and morphological properties of 

subaqueous soils described by McVey et al. (2012). It is worth reiterating 

that caution is needed when estuarine soils are sampled and described 

since these soils typically have significant amounts of sulfides. Once 

soil materials containing sulfides are removed from their natural state 

(underwater), the sulfides begin to oxidize (even when stored in a sealed 

core barrel at 4 0

C). Mono-sulfides are more reactive to oxidation than 

di-sulfides, such as pyrite. The oxidation of sulfides may cause a change 

in soil color and a rapid decrease in pH as the sulfides are converted to 

sulfuric acid (in some cases pH decreases from more than 7 to less than 
512 Chapter 10

3; see fig. 10-5) (Fanning et al., 2010; Rabenhorst and Stolt, 2012). If the 

materials have a very low pH, the aluminum pipe housing the soil core 

may start to corrode and create Al salts. If soil color, pH, salinity, and 

particle-size distribution are important to the soil characterization, the 

soils should be sampled and described as soon as possible or samples 

should be frozen immediately after sampling to minimize the amount of 

sulfide oxidation.

Figure 10-5

Incubation pH for three horizons (A, Cse1, and Cse2) from a Fluventic Sulfiwassent, 

collected from the Thimbles Island estuary in Connecticut. Samples that have a pH 

of 4 or less after 16 or more weeks of moist incubation and that have a drop in pH of 

at least 0.5 unit meet the requirements for sulfidic materials. Both the Cse1 and Cse2 

horizons meet these requirements. The Cse2 horizon attained the critical pH in less 

than 4 weeks while the Cse1 horizon took almost 12 weeks to reach a pH of 4.

Important Properties for Classification and 

Interpretation

Soil properties important for classification and interpretations of 

subaqueous soils include type of organic soil materials, content of soil 

organic carbon, electrical conductivity (EC), fluidity, incubation pH, 

content of pore-water sulfides, particle-size distribution, mineralogy, 

and soil color. The presence of odors (such as sulfur or petroleum), the 
Soil Survey Manual 513

presence of shell fragments or former vegetation, and the nature of the 

depositional environment are also important. Subaqueous soils that are 

dominated by organic soil materials or are fluid cannot support dock 

pilings and typically require special moorings (Surabian, 2007). Electrical 

conductivity is used to discern between freshwater and estuarine soils 

and to identify soils that may have halinity issues when dredged and 

placed on the soil surface. Incubation pH is a measure of the potential 

acidity of the soil materials. Subaqueous soils dominated by mono- and 

di-sulfides may have dramatic drops in pH when dredged and placed on 

the land surface. In some cases, pH values decrease to less than 4 and 

acid sulfate soils are created (Clark and McConchie, 2004; Fanning et 

al., 2010). High sulfide contents, especially in the pore waters, can be 

toxic to submerged aquatic vegetation and may indicate a highly anoxic 

environment uninhabitable for many benthic organisms. Subaqueous 

soils typically have low chroma (< 2) and a neutral (N), yellow (5Y), or 

blue-green (BG) hue. Brighter chromas (> 2) or redder hues commonly 

indicate aerobic inputs (from ground water or the water column) or relict 

subaerial soils that are now submerged (see lower part of core profile in 

fig. 10-4, bottom image). 

Soil-Landscape Relationships

Like subaerial soils, subaqueous soils form as a result of various 

additions, losses, transfers, and transformations occurring within the soil 

(Simonson, 1959). The dominant processes that form a particular kind 

of soil depend on where the soil occurs on the subaqueous landscape. 

Figures 10-6 and 10-7 provide examples of coastal lagoons, which are 

very common in estuarine systems along the U.S. Eastern Seaboard. 

Large areas of the basins (bottoms) of coastal lagoons are dominated 

by fine textured (silt loam and silty clay loam) soils that are rich in 

sulfides, are very fluid, and have a high content of soil organic carbon 

to a depth of 2 m or more. Because the lagoon bottom is in a low-energy 

position, only fine particles (very fine sand or finer) and organic matter 

are deposited. Little oxygen is available for microbial respiration, and 

redox potentials are very low. As a result, the transformation of sulfate 

to sulfides dominates the soil processes. With continued additions of 

organic matter and little decomposition at the low redox potential, levels 

of soil organic carbon remain high. Because the silt- and clay-rich parent 

materials have such fine particles sizes, which are not readily compacted 

in the permanently inundated low-energy environment, very fluid soils 

form.
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Figure 10-6

Washover fan

Lagoon bottom

Shoal

Submerged

mainland beach

Washover fan slope

Back-barrier

dune

Mid-lagoon channel

N

A

A’

Salt marsh

developing on

barrier island

(intertidal)

Barrier island

washover fan flat

Washover fan slope

Low-energy

depositional basin

(lagoon bottom) Submerged

mainland beach

Mean sea level

Mainland beach

(intertidal)

A Shoal A ́

Examples of subaqueous landscape units across a coastal lagoon.

Higher on the coastal lagoon subaqueous landscape, typically 

between the deeper lagoon bottom and the shallower washover fan, water 

currents have more energy than those in the lagoon bottom where the finest 

particles and organic matter settle out. During storm events, there is even 

greater energy due to an increase in waves, fetch, and nearby overwash 

events and coarser (sand-sized) particles are deposited. Where benthic 

organisms are continuously mixing the upper part of the soil, textures are 
Soil Survey Manual 515

Figure 10-7

Examples of coastal lagoon soils mapped

in Rhode Island and Connecticut

Estuarine 

over relict 

subaerial 

Histosol

Estuarine 

over relict 

subaerial 

soil

Fluid 

soil, 

high 

sulfides

Thick 

sandy 

soil with 

buried 

surfaces

Fluid 

estuarine 

silt

Freshwater 

peat

Loess

Lodgement 

till

Soil-landscape relationships across a coastal lagoon in Rhode Island and Connecticut.

loamy and fluidity is low. In this part of the landscape, subaqueous soils 

develop A horizons and sulfidic materials are not typically found in the 

upper horizons because of the mixing and elevation (shallowest water 

depth).

Adjacent to the barrier island that separates the lagoon from the open 

ocean, washover fans are the dominant landscape unit. The frequency of 

overwash events that reach the washover fan increase with decreasing 

distance to the barrier island, and the soils typically do not have A 

horizons. 

Freshwater systems such as ponds, reservoirs, and lakes are typically 

low energy. These environments typically do not have the highly 

depositional and erosional landscape units (such as deltas, washover 

fans, or current-formed channels) found in estuarine settings. Thus, the 

landscape patterns in the freshwater systems are much simpler than those 

in the estuarine systems. For example, Payne (2007) identified a total 

of 21 different landscape units in her study of three shallow estuarine 

embayments in Rhode Island. She noted that differences in geology, 

geomorphology, geography, wind, and tidal patterns accounted for much 
516 Chapter 10

of the variation in landscape units. In contrast, Bakken (2012) studied six 

natural and impounded freshwater systems and found only four landscape 

units (cove, shoal, lakeshore, and lakebed). 

There have been two studies of freshwater subaqueous soils (Erich 

et al., 2010; Bakken, 2012), and both found that soil characteristics 

in freshwater systems were more influenced by the geologic and 

anthropogenic history than by the landscape. The subaqueous soils in 

reservoirs were very similar to those prior to impoundment and flooding. 

The natural lakes were primarily kettle holes that slowly filled with rising 

ground water. In most cases, organic soil materials accumulated in the 

kettle holes when they were wetlands (subaerial soils) with little mineral 

inputs after permanent inundation and ponding. Thus, these soils typically 

were Histosols. In areas where there was significant anthropogenic input 

of nutrients, invasive species such as milfoil (Myriophyllum sp.) and 

fanwort (Cabomba caroliniana) proliferated and thick deposits of plantderived organic materials accumulated, forming present-day subaqueous 

Histosols. 

Survey Methods and Procedures

Special methods and procedures are used to develop base maps for 

conducting subaqueous soil surveys. Additional remote sensing tools and 

techniques can also be useful for preparing subaqueous soil survey maps.

Bathymetry

Developing soil-landscape relationship models is critical to the 

mapping of subaqueous soils (Bradley and Stolt, 2003; Osher and 

Flanagan, 2007). The first step is obtaining bathymetric data and 

creating a bathymetric map of the subaqueous landscape (Bradley 

and Stolt, 2002). The bathymetric map serves the same purpose as the 

topographic map. Aerial photos, if the water clarity was good on the day 

the photos were taken, can be helpful in the identification on the map of 

exact boundaries of landscape units, such as washover fans, shoals, and 

submerged beaches. 

In some areas, accurate bathymetric maps are already available; in 

other areas, they may need to be created. Bathymetric maps are created 

by systematically collecting water depth data while recording x and y 

coordinates. Water depth is typically determined using a fathometer 

mounted to the hull of a boat. The boat is slowly driven across the water 

body for a series of transects. Transects are spaced 20 to 30 m apart, and 
Soil Survey Manual 517

data are collected every 5 to 10 seconds, depending upon the speed of 

the boat. Each time a water depth is recorded, the x and y coordinates 

are also recorded with a global positioning system (GPS). A fairly simple 

fishfinder with GPS capabilities can collect the x and y data and the z 

(water depth) data at the same time. Caution is needed in areas with dense 

submerged aquatic vegetation (SAV) since the fathometer may read the 

top of the SAV as the soil surface. In these cases, manual readings of the 

water depth should be occasionally collected and checked against the 

fathometer values. Several transects are also completed parallel to the 

shoreline to augment the other transect data. In freshwater systems, the 

water depths can be corrected to the mean water level of the water body. 

In tidal systems, tidal fluctuations need to be recorded at the same time 

as the x, y, and z data to correct the water depth data. Any water-level 

recording device that has time-stamped data can serve this purpose. In 

areas where tidal fluctuations are complicated, as many as three waterlevel recording devices may need to be used at one time. In most cases, 

however, a single-water level measurement device can be used and 

moved to new locations as the bathymetric data are collected. Because of 

recent advances in Light Detection and Ranging (LiDAR) for underwater 

applications, accurate bathymetric maps may eventually be available for 

most coastal environments.

The bathymetric data are imported into a GIS program to create the 

contour maps. They are typically kriged, and a stop line is set at the 

tidal datum chosen as the elevation where the water meets the land. All 

bathymetric data collected in tidal systems are normalized to NAVD-88 

or new vertical datum if available. McVey et al. (2012) discussed the 

use of various datums for a bathymetric contour map. To normalize the 

data, the bottom depth (elevation) of each tide gauge must be determined 

using traditional land survey techniques from an order 1 benchmark. 

Changes in the shallow subaqueous landscapes are rarely abrupt; thus, 

contour intervals of the bathymetric maps are commonly on the order of 

20 to 30 cm. Using the contour maps and aerial photographs, the various 

landscape units within the mapping area are delineated. Landscape units 

in coastal systems include lagoon bottom, bay bottom, flood tidal delta, 

washover fan, inlet, and shoal (Schoeneberger and Wysocki, 2012; 

USDA-NRCS, 2016). Landscape units in freshwater lakes and ponds 

include submerged shoreline, lakebed, cove, and shoal (Bakken, 2012).

The bathymetric contour maps serve as the base maps for the 

subaqueous soil survey. The landscape unit is the primal factor in the 

delineation of soil distribution on the landscape. Each landscape unit 

should be visited in the field and a preliminary assessment done with a 

bucket auger and peat sampler. To develop initial metrics of soil variability 
518 Chapter 10

within landscape units, larger units may need to be investigated at three 

or four random locations. Which units will need additional investigations 

will depend on the scale of mapping (order 1 or 2). Preliminary assessments 

can be used to establish initial soil-landscape relationships. Delineations 

that do not follow these relationships should be reassessed in the field, 

and additional delineations (soil polygons) added where appropriate. 

Representative soil types should be identified from the preliminary 

studies, and their locations established for vibracoring. Transects can be 

run across the largest units to assess variability within delineations. For 

stony or bouldery mapping units, a push probe can be used to quickly 

estimate boulders and stones and better identify boundaries between 

different phases.

Remote Sensing

Although not necessary, several remote sensing tools can help improve 

mapping accuracy. In estuarine systems, side scan sonar is effective 

in identifying surface textures and features, such as stones, boulders, 

macroalgal cover, SAV, bottom type, and anthropogenic features (Oakley 

et al., 2012). The side scan is towed behind a boat sending signals in 25- 

to 50-m swaths. Transects are set apart by the size of the swath to ensure 

complete coverage. Soundings are recorded and translated in the lab 

with the appropriate software into a map showing the mosaic of swaths. 

Surface samples from preliminary soil survey efforts are used as ground 

truth for the signals that are mapped. Easily identifiable features from the 

side scan data include extent of stones or boulders and surface textures. 

A simple tool that works well in areas with excellent water clarity 

(low turbidity) is the underwater video camera. The video camera can 

be towed slowly by the boat and tracked with a GPS. The images are 

projected onto a laptop. Points of interest (e.g., changes in surface texture 

or presence, absence, or abundance of coarse fragments or stones and 

boulders) are noted and the GPS coordinates recorded. Limitations for 

the video camera are the field-of-view and the possibility of fouling by 

algae or seagrasses. The field-of-view is often limited to a width of 25 

cm because the camera needs to be close to the subaqueous soil surface 

to clearly observe it. Thus, many transects are necessary to delineate 

between two mapping units (e.g., soils with a high content of rock 

fragments and soils with a low content of rock fragments). The closer 

the camera is to the soil surface the more likely the lens will be fouled by 

detritus, algae, or seagrasses. 

Ground-penetrating radar (GPR) can be invaluable in freshwater 

systems (it cannot be used in saltwater because the salts attenuate the 
Soil Survey Manual 519

signal). The GPR antenna can be towed slowly behind a boat in a rubber 

or plastic raft or across ice. GPR is effective at identifying surface and 

subsurface stones and boulders, shallow depths to lithic materials, and 

the depth and distribution of organic soil materials (fig. 10-8). Chapter 6 

has additional information about the use of GPR in soil survey.

Figure 10-8

GPR output for a freshwater lake with thick organic materials. Water, mineral soil 

materials, and organic soil materials are easy to identify from the output.

Significance of Subaqueous Soil Information

Subaqueous soil survey interpretations can have a wide variety of 

uses. This section provides a few examples.

Water depth characteristics.—For use of a shallow water 

body, one of the most important spatial data layers is the bathymetry. 

Although bathymetric maps created for soil survey are not meant for 

navigation purposes (nautical charts include locations of buoys and 

other boating information), these maps can be used to understand the 

effects of storms on coastal systems or identify rapid changes in water 

depth for recreational purposes (mostly fishing). In most soil surveys of 

subaqueous soils, water depth is provided as a phase of each soil mapped 

(similar to slope phases in subaerial soil surveys). Although not perfect 

for navigation purposes, these data can help in identifying shallow areas 

boats should avoid or in identifying shallow areas suitable for wading 

activities, such as clamming.

Mitigation of dredging effects.—Areas of subaqueous soils in 

estuarine or freshwater settings are dredged to allow boats and ships 

to move freely in and out of the system or for the development and 

construction of marinas. Issues related to dredging are primarily disruption 

of the benthic ecology, re-release of the contaminants and nutrients in 
520 Chapter 10

the soils, and disposal of the dredge materials. If the dredged materials 

are placed someplace in the water, another subaqueous area is disturbed. 

The dredging process alone may re-suspend nutrients or contaminants 

that are stored in the subaqueous soils. Pruett (2010) showed that soils 

with high fluidity (Hydrowassents) and soils with sulfidic materials 

(Sulfiwassents) contained significantly higher concentrations of heavy 

metals (Pb, Zn, As, Cu, and Cr) than simple soils (Haplowassents) 

and sandy soils (Psammowassents). Hydrowassents are typically finer 

textured and contain high levels of soil organic carbon. The soil organic 

carbon forms a complex with the metals in the water column, and the 

complex is later deposited on the soil surface. In Sulfiwassents, the 

metals may also complex with the sulfides. Pruett (2010) found in some 

cases that the metal concentrations in Sulfiwassents were high enough 

to negatively affect the benthic ecology. In freshwater systems, Bakken 

(2012) found that there were significant differences in P concentrations 

among soil types. Where extractable P concentrations were greater than 

200 μg g-1, there was a significantly greater chance of the presence of 

invasive plants such as milfoil and fanwort. 

Commonly, the dredge materials are placed on the land surface. 

Knowing whether or not those materials contain contaminants or sulfides 

is critical to managing the dredge materials. When exposed to the air, 

sulfides will oxidize, release sulfuric acid, and may create acid sulfate 

conditions or soils. These acid sulfate soils may release metals if reaction 

approaches extremely acid (pH of 4 or 3). They are also extremely 

difficult to vegetate. Salisbury (2010) showed that soils in low-energy 

environments, such as coves, bayfloors, and lagoon bottoms, that contain 

high concentrations of sulfide may drop in pH from 7.5 to 3.0 in as little as 

3 weeks following dredging and placement in an oxidized environment. 

Suitability for moorings, docks, and other structures.—Surabian 

(2007) showed how subaqueous soil properties, primarily fluidity and 

depth of fluid soil materials, affect the types of moorings that can be used 

to secure boats. If the soil materials are non-fluid, heavy moorings are 

required. If they are fluid, mushroom-type moorings are used. Similarly, 

areas for docks require soil materials that are non-fluid and can support 

pilings.

Aquaculture.—Salisbury (2010) showed that certain subaqueous 

soils are more productive for aquaculture than others (in this study, 

hard shell clams and oysters). For example, an average of 58 percent 

of the oysters grown on sandy non-fluid soils reached harvestable size 

in two growing seasons while less than 12 percent of the oysters grown 

on soils dominated by sulfidic materials reached that size in the same 
Soil Survey Manual 521

period. Aquaculturists are now using subaqueous soil maps to identify 

new places to lease for growing oysters in coastal lagoons.

Ecological assessment and restoration.—Seagrasses such as 

eelgrass (Zostera marina) are an important component of the estuarine 

ecosystem. They provide several ecosystem functions and services, 

including trapping sediment and pollutants in the water column and 

serving as habitat for a variety of shellfish, finfish, shrimp, and other 

benthic fauna. Because eelgrass habitat has been declining worldwide, 

there have been numerous studies focused on eelgrass distribution and 

restoration. Restoration of eelgrass has been difficult, with an average 

success rate of 30 percent (Fonseca et al., 1998). Because a major issue is 

site selection (Calumpong and Fonseca, 2001), subaqueous soil surveys 

can provide spatial soil data to use in site selection models for eelgrass 

restoration.

Carbon accounting.—Another important current issue in soil 

science is carbon accounting. Subaqueous soils are typically not sampled 

and analyzed when carbon stocks are assessed. However, studies have 

shown that, although soil organic carbon concentration in subaqueous 

soils varies widely, some subaqueous ecosystems may have as much 

carbon as wetlands and forested ecosystems (Jespersen and Osher, 2007; 

Payne, 2007; Balduff, 2007; Pruett, 2010; Bakken, 2012). These potential 

sinks for carbon should be inventoried, characterized, and considered in 

regional carbon budgets.

References

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Balduff, D.M. 2007. Pedogenesis, inventory and utilization of subaqueous 

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Bradley, M.P., and M.H. Stolt. 2002. Evaluating methods to create a base 

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[Accessed 8 August 2016]