Soils and Water Quality
Prepared by:
Maurice G. Cook
Extension Soil Science Specialist
Published by: North Carolina Cooperative Extension
Service
Publication Number: AG 439-1
Last Electronic Revision: March 1996 (JWM)
How Soils Influence Water Quality
North Carolina has an abundant supply of clean water, a resource vital to
our high quality of life. Rivers, lakes, groundwater aquifers, and coastal
estuaries are crucial to public health, economic development, and recreational
opportunities. However, our water sources are constantly threatened with
degradation by such activities as imprudent development, improperly managed
agricultural and industrial activities, and unsound waste disposal practices.
The soil exerts an important influence on water quality. How we manage the
soil determines, in part, the level of treatment required to make our water
supplies safe and enjoyable. This fact sheet explains how soils influence
water quality and why efficient soil management helps protect water quality.
Erosion and Sedimentation
When a soil is well managed, it is an efficient receiver of rainwater. If the
soil is improperly managed, however, the water may run off the surface,
carrying soil particles with it. This process, called soil erosion, has been
a major cause of soil degradation in North Carolina for many years. Damage
to water quality occurs when the eroded soil settles out in a body of water,
a process called sedimentation. Sedimentation occurs when water carrying
eroded soil particles slows long enough to allow soil particles to settle out. The smaller the particle, the longer it stays in suspension. Larger, heavier particles such as gravel and sand settle out sooner than smaller, lighter particles such as clay. Clay may stay in suspension for very long periods, contributing significantly to water turbidity.
Sediment comes from many sources: agricultural fields, woodlands, highway
road banks, construction sites, and mining operations. By volume, sediment is
the largest water pollutant in North Carolina. It affects water quality
physically, chemically, and biologically. Damage from sediment is expensive,
both economically and environmentally. Sedimentation destroys fish spawning
beds, reduces useful storage volume in reservoirs, clogs streams, and makes
costly filtration necessary for municipal water supplies. Suspended sediment
can reduce photosynthesis and alter a stream's ecology. Because the
environmental damage from sediment is often additive, the ultimate effects and
costs may not be evident for years. The consequences of off-site sedimentation
can be severe, both for those immediately affected and for those who must cope
with subsequent problems. Sediment often carries organic matter, animal or
industrial wastes, adsorbed nutrients, and toxic chemicals. The most
troublesome nutrient element is phosphorus: it stimulates the production of
algae blooms that can choke out beneficial plants and smother aquatic animals.
Excessive phosphorus may come from such sources as fertilizers, organic
matter, and animal manure. Because phosphorus is concentrated in the top few
inches of soil, it is very susceptible to erosion and likely to be present in
sediment. Sediment may also carry pesticides—such as herbicides and
insecticides —that may be toxic to aquatic plants and animals. The varying
chemical properties of pesticides—for example, their solubility and chemical
breakdown rate—help determine the damage they inflict on wate quality.
Runoff and Leaching
Runoff water can also transport potentially harmful dissolved chemicals from
fields to bodies of water (Figures 1 and 2). Nitrogen can be particularly
damaging, especially in the form of nitrate, NO3. Concentrations exceeding
l0 milligrams (mg) of nitrogen per liter (44 mg of NO3) may induce animal and
human health problems. Nitrogen also stimulates algae growth in the manner
described earlier for phosphorus. Certain dissolved nutrients and pesticides
can reach the ground-water by moving down through the soil. Nitrogen in
nitrate form can move in this way. Preliminary results from an Agricultural
Extension Service well-testing program indicate that the levels of nitrate
nitrogen in groundwater are generally well below the critical level of 10 mg
of nitrogen per liter, or 10 parts per million (ppm). Fewer than 3 percent of
the wells sampled have nitrogen levels exceeding 10 ppm. Certain hazardous
pesticide chemicals are highly mobile and have been detected in the
groundwater of other states. Examples include aldicarb (Temik), alachlor
(Lasso), and the triazines (Atrazine).
Household Waste Disposal
About half of the citizens of North Carolina depend on septic tanks (and
hence on soil absorption) for the treatment and disposal of their household
wastewaters. Over one million housing units in the state use on-site systems
to dispose of their wastewater. At least 30,000 additional septic tank
systems are installed each year. Each day, septic tanks discharge more than
100 million gallons of sewage into the soils of North Carolina. Septic tanks
should be used only in soils that can filter, absorb, and treat waste
constituents. Key soil properties to consider include depth, texture,
structure, consistency, color, and the presence of restrictive layers. These
properties should be evaluated to a depth of at least 6 feet to reveal any
limitations. Each region of North Carolina poses potential problems for septic
tank installation. In the piedmont, problems occur with thin, shallow soils
over bedrock and with clayey soils whose mineral content causes them to swell
extensively when wet. In the coastal plain, problems result from a seasonally
high water table close to the soil surface. In the mountains, major soil
problems occur on steep slopes, in shallow soils, and at the base of long
slopes, where subsurface water can accumulate.
Land Application of Waste Materials
Municipalities and industries are increasingly interested in applying sludges
from wastewater treatment plants to agricultural land. (See Extension Service
publication AG-439-3, Health and Environmental Concerns for the Application of
Municipal Sludge to Agricultural Lands.) Applications of livestock and poultry
manure have also shown renewed popularity, both as a means of disposal and as
a source of nutrients. Land application is an appropriate technique, but
loading rates should be calculated carefully to avoid potential hazards.
For land application, the characteristics of the wastes determine the amounts
to use. Each waste will contain one constituent that limits the amount that
can be safely applied to land. This limiting constituent may be one of the
plant nutrients such as nitrogen or phosphorus or one of the heavy metals such
as cadmium or lead. Waste regulations administered by the state and
recommendations developed by North Carolina State University provide
information on correct loading rates. With most wastes generated in North
Carolina and applied to supply the nitrogen and phosphorus needs of a crop,
health hazards and crop toxicities will not be a problem if recommendations
are followed. The soil's capacity to dispose of, use, or treat waste varies
significantly according to the physical, chemical, and biological properties
of the soil and the characteristics of the wastes. Although general
information on waste reactions in soil is available and although the
environmental fate of many chemicals has been studied, the development of a
land treatment system must be tailored to the characteristics of the specific
site and the specific waste. The following are among the characteristics that
would disqualify a site: steep slopes; very clayey or sandy soils; proximity
to streams, wells, and property lines; a likelihood of flooding; and shallow
depth to bedrock or the water table. Since each site has a finite capacity to
accept certain waste constituents (for example, heavy metals such as zinc,
copper, and cadmium), a threshold may be reached beyond which land application
of wastes is no longer acceptable.
Importance of Good Soil Management
An understanding of soil properties and their management is essential for
reducing the input of water pollutants from the soil. Reducing soil erosion
is the key to reducing the damaging effects of sedimentation. Fortunately,
with current technology, erosion can be reduced to acceptable levels.
The challenge is to match the appropriate technology to each situation.
The Soil Conservation Service has developed a variety of practices that
improve surface water quality. Crops, crop residues, and structures are used
(alone or in combination) to hold the soil in place and allow water to move
into it rather than to run off the surface. Agricultural practices such as
strip-cropping, contour cultivation, and filter strips are both beneficial and
economical. Sometimes, more costly structures such as grassed waterways and
terraces are required to provide the necessary control. Conservation tillage,
which reduces soil disturbance and promotes residue cover on the land, is
another effective way to reduce erosion. The benefits to water quality of
several widely used conservation practices are shown in Table l. Certain
practices that enhance surface water quality do little to improve the
groundwater. In some cases, such practices may even prove harmful by
increasing the amount of water that moves through the soil. It is essential,
therefore, to determine whether the problem involves surface water or
groundwater. Practices that minimize both forms of contamination must be
developed. Soil properties determine the proper amounts of fertilizers and
pesticides to apply and the timing and method of their application. For
example, sandy soils cannot hold as large a quantity of nutrient elements and
other adsorbed materials as can clay soils. Thus, the amount, frequency, and
timing of chemical applications need to be adjusted for each situation.
Furthermore, fertilizer levels should be based on realistic crop yield
expectations, which vary with soil properties. Such monitoring helps avoid
harmful levels of critical elements such as nitrogen and phosphorus. When
applying organic wastes to the land, match the loading rate to the soil's
absorbent capacity and the crop's ability to use the nutrients. Contamination
from on-site waste disposal systems can be prevented by carefully selecting a
soil site and installing the system appropriate to the soil's characteristics.
Land application of wastes must play an integral part in the total soil
management program. The composition and properties of the waste need to be
known before it is applied. For example, manure must be thoroughly mixed into
the soil to maximize the effectiveness of the nitrogen: manure exposed on the
field's surface loses up to 25 percent of its ammonia nitrogen within two
days; 60 percent or more can be lost within one month. Manure application
rates should be based on the available portion of the nutrients and should not
exceed the nutrient requirements of the crop. Excessive loading squanders
valuable nutrients and may result in surface water and groundwater pollution.
For characteristics of livestock and poultry manure, see Extension Service
publications AG-439-4, Swine Manure as a Fertilizer Source, and AG-439-5,
Poultry Manure as a Fertilizer Source, and also the North Carolina
Agricultural Chemicals Manual. Soil testing should also be conducted to
determine proper loading rates. An example of how to compute application
rates is outlined in the worksheet on this page.
Table 1. Effects of Selected Conservation Practices on Water Quality
Practice Surface Water Groundwater
Chiseling and subsoil + N
Conservation tillage + N
Contour farming + N
Cover crop + +
Crop residue use + N
Field border + N
Filter strip + N
Grassed waterway +* N
Strip-cropping (contour) + N
Subsurface drainage N** +
Terrace + N
Note: The plus sign (+) denotes a positive effect; the N denotes a neutral or
possibly unfavorable effect.
*Chemical maintenance of vegetation may lower the quality of runoff water.
**The negative effect occurs where water table control is not applied.
Worksheet
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Crop: corn
Expected yield: 100 bushels per acre
Organic waste: broiler litter (soil incorporated)
Nutrient recommendations (based on soil test):
N 120 to 160 pounds per acre
P2O5 30 pounds per acre
K2O 30 pounds per acre
Nutrients per ton of waste (total per ton x availability coefficient):
N 75 x 0.6 = 45 pounds
P2O5 82 x 0.8 = 66 pounds
K2O 45 x 0.8 = 36 pounds
Waste required: 3 tons:*
N 45 x 3 = 135 pounds per acre
P2O5 66 x 3 = 198 pounds per acre
K2O 36 x 3 = 108 pounds per acre
*Note that 3 tons are needed so that the nitrogen level reaches
the amount called for in the soil test recommendations. One ton
would have been sufficient for the other nutrients.
Conclusion
Various agricultural and industrial practices threaten our water with contaminants from the soil.
These threats are serious, but they are also manageable. Water quality can be improved without
sacrificing the quality of the soil for agricultural, industrial, and recreational uses.
We can have both productive soil and clean water by applying good soil management
practices.
The use of trade names in this publication does not imply endorsement
or criticism of the products named or discrimination against similar
ones not mentioned.
Distributed in furtherance of the Acts of Congress of May 8 and June 30,
1914. Employment and program opportunities are offered to all people
regardless of race, color, national origin, sex, age, or disability. North
Carolina State University, North Carolina A&T State University, U.S.
Department of Agriculture, and local governments cooperating.
AG 439-1