Table of Contents
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 and what we put on it determine, 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.
When a soil is well managed, it can be 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 (
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 aquatic plant
life 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
Sediment often carries organic matter, animal or industrial wastes, nutrients, and chemicals. The most troublesome nutrient element is phosphorus. In freshwater ecosystems developed under very low phosphorus conditions, large additions of phosphorus can stimulate the production of algae blooms. As the algae die, organisms in the aquatic system decompose the algae to use as a food source. In the process, they also use significant amounts of oxygen. If the oxygen level is initially low, the decomposition process can further reduce it to a point that "fish kills" can occur. Phosphorus may come from such sources as fertilizers, organic matter, and animal manure. Phosphorus is very immobile in most soils and concentrates in the top few inches of soil. It is very susceptible to erosion and likely to be present in sediment.
Sediment also may carry pesticidessuch as herbicides and insecticidesthat may be toxic to aquatic plants and animals. The varying chemical properties of pesticidesfor example, their solubility, toxicity, and chemical breakdown ratedetermine the potential damage to water quality.
Runoff water also can transport potentially
harmful dissolved chemicals from fields to bodies of water (Figure
1). Nitrogen, in the form of nitrate,
Certain dissolved nutrients and pesticides
can reach the groundwater by moving down through the soil (leaching).
Nitrogen in the nitrate form can move in this way. Results from a North
Carolina Cooperative Extension Service and EPA well testing program indicate
that the levels of nitrate nitrogen in groundwater are generally well
below the critical level of
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. More than one million housing
units in the state use on-site systems to dispose of their wastewater.
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, drainage, 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.
Alternatives to conventional septic systems need to be considered in these
Municipalities and industries are increasingly
interested in applying sludges from wastewater treatment plants to agricultural
land. (See Extension Service
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, one of the heavy metals such as cadmium, lead, or other
constituents such as sodium or calcium carbonate. 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
The soil's capacity to use, retain, or reduce the undesirable effects of waste varies significantly according to the physical, chemical, and biological properties of the soil and the characteristics of the wastes. Thus, 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 may disqualify a site: steep slopes; very clayey or sandy soils; nearby streams, wells, and property lines; a likelihood of flooding; and shallow depth to bedrock or the water table. Because 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.
An understanding of soil properties and how these interact with common management practices is essential for reducing the movement of pollutants from the land to our waters. These factors determine the types and amounts of water pollution risks in each situation.
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 Natural Resources Conservation Service
has developed a variety of practices that improve surface water quality.
Crops themselves as well as 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 (see
The benefits to water quality of several
widely used conservation practices are shown in
Soil properties and crop needs 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 absorbed materials
as can more clayey soils. Thus, the amount, frequency, and timing of nutrient
applications need to be adjusted for each situation. Furthermore, inputs
should be based on realistic crop yield expectations that vary with soil
properties. Proper management helps avoid excessive levels of elements
such as nitrogen and phosphorus that then
Contamination from home waste disposal systems can be prevented. This requires careful selection of soils for home sites and the installation of septic systems appropriate to the soil's characteristics.
The land application of wastes is now an
essential part of soil management programs. The composition and properties
of the waste need to be known before it is applied. When applying municipal
sludge, compost, or any other form of organic wastes to the land, match
the loading rate to the soil's capacity to safely accept the material
and the crop's ability to use the nutrients. 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
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 may result in surface water
and groundwater pollution. For more information on livestock and poultry
manure, see Extension Service publications
Many agricultural and industrial practices can threaten our water quality if soil properties and capabilities are poorly understood or are ignored. These threats are serious, but they also are manageable. Water quality can be improved while protecting the productivity and value of the soil for agricultural, industrial, and recreational uses. We can have both healthy 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