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Chapter 14
Soils for Environmental
Quality and Waste Disposal
What is a Contaminant?
• Nearly all materials we consider contaminants are
naturally-occurring elements or compounds, but they
become a hazard when present in soils or waters at
elevated concentrations.
• Some compounds such as pesticides are synthesized
by humans for their useful toxicity to target
organisms, but become contaminants when they are
found in food or water supplies.
• Most contaminants are similarly useful materials
that just get in the wrong place at too high a
concentration, due to human carelessness or mishap.
Risk Assessment
• An attempt to quantify the risks posed by a
certain level of some pollutant (contaminant)
in the environment, particularly to human
health.
• There are a number of pathways by which
contaminants might directly affect humans
Pollutant movement and risk pathways for soil contaminants.
A: Direct Soil Ingestion - Some metals, such as lead and arsenic
B: Leaching to Ground Water - Many organic contaminants
C: Runoff to Surface Water - Many organic contaminants
D: Plant Uptake - Some metals, such as cadmium, and radionuclides
E: Animal Uptake - Some metals, such as lead/mercury, organics/PCBs, radionuclides
• Food chain
– When contaminants enter plants or animals which constitute
our food supply, thereby transferring contaminants to
humans.
• Water supplies
– Either surface or ground water, are often contaminated
when pollutants leach through soil or run off the surface, as
well as direct discharge into streams (often the case with
industrial discharges).
• Direct ingestion
– Refers to direct eating of contaminated soil or waste
materials— mostly referring to children who, by constantly
putting their hands in their mouths, may eat up to 10 g of
soil per day!; if this soil is contaminated (as many urban
soils are), the children are potentially exposed to significant
amounts of pollutants.
• For any given contaminant, risk tends to be greater for certain
pathways rather than others, due simply to the chemical
nature of the contaminant.
• Organic contaminants are seldom are taken up by plants, but
may readily leach to ground water because they are not
adsorbed by soils.
• Some heavy metals (such as Cu and Zn) may be taken up by
plants, but often kill the plants at high concentrations, and
thus do not often enter the food chain.
• Other metals, as well as some radionuclides, can accumulate
in plants to high enough levels to threaten human health;
radioactive cesium (Cs) and strontium (Sr) may contaminate
milk grown on lands contaminated with radioactive fallout
(for instance, around the Chernobyl nuclear plant in Russia),
while metals like cadmium (Cd) can reach dangerous levels
in plants grown on contaminated soils.
Waste Disposal
• Many cases of soil and water contamination result from improper waste
disposal
• Prior to the 1970s, there were few regulations concerning waste disposal,
and highly toxic wastes were simply dumped anywhere - into rivers and
lakes, or buried in shallow trenches.
• In the late 1970s a subdivision in New York at Love Canal was found to
have been built directly on an older toxic waste dump containing
hundreds of drums of industrial solvents.
• During the same era Lake Erie, a huge water body and major fishery, was
essentially dead due to eutrophication resulting from discharge of raw
sewage into it.
• Today the EPA enforces federal laws that regulate how wastes can be
discharged or disposed of in order to protect human health.
• These laws are constantly changing as scientists better understand the
risks of various contaminants in the environment.
Ground-Water Travel Times
Landfills
• Most solid wastes are disposed of in landfills.
• Prior to 1980, most wastes were buried in unlined trenches.
• Nearly all such landfills have plumes of contaminated
groundwater below them containing both solvents and
metals.
• Current regulations (described in the Resource
Conservation and Recovery Act, RCRA of 1980) specify
that non-hazardous wastes like municipal garbage and some
industrial wastes must be buried in landfills that have
plastic and clay liners on the base to prevent leaching of
any contaminants to ground water, and must also have a cap
to reduce rainwater infiltration into the landfill.
• Because even household garbage may contain small
amounts of both heavy metals and organic contaminants,
these Class D landfills must prevent leaching of any
contaminants to aquifers below.
• Hazardous wastes, which contain high levels of
contaminants, must be buried in Class C landfills, which
must have even greater protection against possible
leaching.
• While Subtitle C landfills reduce leaching potential, they
are costly to build and have caused dramatic increases in
waste disposal costs for both cities and many industries.
• All landfills must obviously be built on deep, well-drained
soils, preferably with a clayey Bt that can be excavated and
later compacted into the bottom liner and cap.
• The liner and cap must have a low hydraulic conductivity
(K < 10-6 cm/sec), and are usually made of highly
compacted clay.
Construction of landfill cap.
Wastewater
• From our sinks, toilets, bathtubs, as well as
industry - is typically treated one of two
ways:
– in larger towns and cities, a central waterwater
treatment facility receives this water through a
sewer system, which is then treated and
discharged,
– in the country wastewater drains into a series of
perforated pipes buried in the yard of each
house — referred to as a septic disposal system.
Central Waterwater Treatment Facilities
• Various microbial digestion processes are used to remove
organic matter from the water, as well as nutrients such as N
(mostly through denitrification).
• Pathogenic organisms are also killed during this process.
However, some nutrients and dissolved organic materials
remain, which can lead to eutrophication of rivers receiving this
treated wastewater.
• Rather than using expensive methods to reduce these pollutants,
some cities spray this wastewater on land, allowing it to
percolate through soils where microbial action will degrade the
organics, and plants will utilize the nutrients.
• Clayton County (Atlanta) has such a treatment system
where up to 50" of wastewater is applied to several
hundred acres of pines and pasture. This system works
well in the summer, but in winter when plant water and
nutrient use is low, the soils stay too wet and there may be
leaching of nitrates through the soil profile.
• In central water treatment plants, the solids remaining after
digestion are collected and de-watered, and then must be
dealt with. This sewage sludge is organic- and nutrientrich, and can be used as a fertilizer. With septic systems the
solids must occasionally be pumped out of a holding tank
underground.
Septic Disposal Systems
• The soil is used as a bio-filter system to purify the wastewater
as it percolates through the solum.
• Water flow through the soil must be maintained at a sufficient,
but not too high rate all through the year.
• Soils with a seasonal water table (MWD or SPD soils) will
have a saturated zone within 3-4' of the surface part of the
year, which is the zone at which the drain pipes are buried.
• Septic systems on these soils may not drain during the winter
wet season, and sewage will back up into your house or seep
out in your yard!
• On the other hand, if the soil is very deep sand with a high
hydraulic conductivity, water may percolate too quickly with
little chance for organic degradation or nutrient removal.
Ground water contamination may result in this case.
On-site (septic) home water
treatment system
Recycling Solid Wastes
• Many millions of tons of solids wastes are currently buried
in landfills every year in this country, and most of those
landfills will eventually leak more-or-less toxic materials
into the underlying ground water.
• The reason is that clay liners and plastic sheeting can only
last so long before they crack and degrade.
• EPA realizes this, and is encouraging cities and industries to
begin looking for alternative waste disposal methods. One of
those is to recycle wastes containing organic materials and
nutrients back to the soil.
• The major types of wastes are from municipal sources and
from certain industries.
• Biosolids
– include the sewage solids from wastewater treatment
plants (a partially decomposed organic sludge with 2060% water remaining in it).
• Municipal Gargage
– Some cities also make a solid waste compost from the
organic components of municipal garbage (the stuff you
leave out at the curb every week).
– This material is currently being made at plants in Marietta
and Crisp County from organics (food scraps, paper, etc.)
remaining after plastic, glass, metal, etc. have been
removed from the garbage.
– Both contain a lot of organic carbon and some
macronutrients (1-3% each of N, P, and K, similar to
animal manures), and have been used on cultivated and
forest soils to add humus and nutrients.
Industrial Wastes
• Many industrial wastes are not suitable for land application
because they contain toxic materials, and under RCRA
they must be disposed of in approved landfills, or recycled
or incinerated.
• Some industrial wastes can be land-applied with a benefit
for agriculture. Pulp and paper mills, very common in
Georgia, produce waste pulp (essentially partially digested
wood) as well as wood ash and waste alkali (mixed
Ca(OH)2/CaCO 3). These materials are low in heavy
metals, although some have organic contaminants including dioxins - that must be measured to be a very low
levels before land application can be done.
• The ash and alkali wastes can be used on farm fields as
lime substitutes, and the waste pulp sludge is a good soil C
source (although it is pretty low in nutrients, which would
have to be added as fertilizer). Fly ash and gypsum are
mostly made by the electric power industry, and are low in
C and nutrients, but contain a lot of Ca and other nutrients.
• Gypsum is very high in Ca, and useful for adding soluble
Ca to subsoils, and fly ash is probably less useful; in
addition, most coal ash contains appreciable contaminants
such as As, and is still under study as a soil amendment.
Food processing generates a range of liquid and solid
wastes, most of which are high in C and nutrients, and can
be readily land-applied.
• EPA has a set of rules that must be followed in order to
land-apply these kinds of wastes. The major potential
hazards are:
– pathogens in the waste (especially sewage sludge);
– runoff or leaching of contaminants or nutrients;
– excess loading of the soil with heavy metals.
• Wastes generally need to be incorporated into the soil
directly after application to prevent disease organisms from
spreading, and grazing animals have to be kept off treated
pastures for certain time periods.
• The key factor for nutrients and metals is to calculate the correct
application rate so as to not overload the soil's capacity to handle
either nutrients or metals.
• For N and P, which can cause environmental problems, you
should treat them like any other fertilizer – add what the crop
needs, and no more. N is usually tied up in organic forms in
organic- based fertilizers, and extra needs to be added to account
for mineralization.
• Usually only about 20-30\% of the N in these wastes will
mineralize the first year. Thus, continued applications will build
up nutrients (and humus) in the soil, which is good– but too
much long-term application could build up soil levels too much,
resulting in water contamination.
• Probably no more than 200-300 kg/ha of N should be applied in
any one year; for P, if soil tests show P levels in the very high
range, applications should be stopped.
• A real controversy surrounds heavy metals in wastes right now.
• EPA has established total loadings (in kg/ha) of different metals
that can be added to soils in wastes – lifetime loadings that are
cumulative over time.
• But many people (scientists and otherwise) think these levels are
too high, and eventually public health will be endangered by
food chain or water contamination.
• Sewage sludges - also called biosolids - from industrial cities
(Atlanta, Birmingham, Augusta) can have significant heavy
metal levels, and long-term application can build up metals in
soils – and they can't be removed once in there . . .
• Maintaining pH levels in the 6.0-6.5 range helps reduce metal
solubility, but is not a guarantee metals will not move in all soils
(remember that anionic contaminants such as As are more
soluble at higher pH).
• Land managers need to keep track of how much waste and metals
have been applied to their lads, and stay well below EPA limits to
safeguard the productivity of their lands.
• For most people, the term water quality brings to mind chemical
concentrations and healthy bounds for those concentrations.
• For example, they might know that the U.S. Environmental
Protection Agency (EPA) recommends that drinking water contain
no more than 10.0 mg/L of nitrate (as N). Or they may know that
most fish begin to suffocate when dissolved oxygen levels drop
below 4 mg/L.
• Agencies like the EPA and the World Health Organization have
developed guidelines and regulations regarding safe
concentrations of around 100 inorganic and organic chemicals
within drinking water.
• These guidelines have been developed through laboratory toxicity
testing, epidemiological studies, and even taste and odor tests. In
fact, regulations of certain chemicals are not targeted at human
health but rather at preventing pipe corrosion.
A Few Examples
• A wastewater contains 200 ppm plant-available N.
• Calculate the inches of water that can be spray-applied in order to
supply 300 lbs-N/acre to a growing crop over a one-year period.
• Try this again with water containing 50 ppm available-N, and
supply 500 lbs N/ac/yr.
• A sewage sludge contains 25% biosolids (that is, 25 lbs solid
material per 100 lbs wet sludge).
• The total N content of the solids is 4%; about 25\% of this N will
mineralize in the first year after application.
• The Cd content is 22 ppm on a dry weight (solids) basis.
• Calculate the tons of wet sludge that should be applied per acre in
order to supply 250 lbs of available N to a crop during the first year
after application.
• Calculate the Cd (in lbs/acre) that would be applied.
• Indicate whether this is an acceptable load, given that the
maximum EPA load rate for Cd is 2 lbs/ac/yr.
• Calculate how many year this sludge could be applied if the EPA
maximum lifetime loading for Cd is 40 lbs/acre.
• Try this problem again with different sludges (10% solids, 3%
N, Cd = 12 ppm), using the same N mineralization rate.
• It can be easy, and fun. Just keep track of your units, and identify
what units you want to achieve in the end.
Chapter 14 Quiz
1. Match:
_____ Class C landfill
_____ Class D landfill
a. Municipal Waste
b. Hazardous Waste
2. What two pathways pose the greatest risks for organic
contaminants?
3. What two pathways pose the greatest risks for metals?
4. Besides metals and organics, what other class of
contaminants should we be worried about?
5. What are the two primary risk pathways for this class?
6. [True / False] DNAPLs are found above LNAPLs.