Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
METHOD FOR TREATING CONTAMINATED SOIL BY
BIOLOGICAL DEG~ADATION ON A SLOPED SURFACE
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Back~round of the Invention
1. Field of the Invention
The present invention relates to the elimination of
contaminants from soils and, in particular, to the
elimination of organic compounds by biological --
degradation.
2. Brief Description of the Prior Art
It is known in the art that soils contaminated with
various organic compounds can be treated with certain
"land farming" techniques to eliminate the contaminants
by biological degradation. That is, the soil to be
treated is layered over a base surface and the growth of
microorganisms capable of degrading the contaminants is
encouraged in the soil by providing favorable moisture,
nutritional and aeration conditions.
Precipitation may result in excess moisture which may
tend to form in ponds on the surface of the soil layer.
Excess moisture may also migrate vertically downwardly
toward the water table and thereby spread contaminants
with it. Depending on the amount of local precipitation
and the particular characteristics of the soil being
treated, it is, therefore, often necessary to provide a
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plastic liner beneath the soil being treated to prevent
such downward migration of moisture
Summary of the Invention
In the present invention contaminated soil to be treated
is arranged in a sloped surface. The shape of the
treatment cell and the steepness of the slope are
selected so that downward migration of water from the
contaminated soil is minimized. The slope, however, is
not so great that the treatment cell will be
significantly eroded. Soil additives may also be used to
help achieve a condition in which downward migration of
water is minimized.
Brief DescriPtion of the Drawings
The invention is further described with reference to the
accompanying drawings in which:
Figure 1 is a plan view of a soil treatment bed
in which a preferred embodiment of the method of the
present invention may be practiced;
Figure 2 is a cross sectional view of the soil
treatment bed shown in Figure 1 taken through line II-II
in Figure 1; and
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Figure 3 is a cross sectional view of the soil
treatment bed shown in Figure 1 taken through line III-
III in Figure 1.
Detailed Descri~tion
Referring to Figures 1 to 3, contaminated soil to be
treated is arranged in a sloped structure for treating
such soil which is referred to herein as a "treatment
bed". As will be discussed hereafter a length in
direction of flow, L, and a width, W, may be selected to
help optimize conditions for treating the soil. It will
be observed that length in direction of flow L, is the
distance from the highest point in the txeatment bed to a
side trench. It would also have a height, H, selected
for the same reason, and it would have a downward slope
of 1-3%. The upper limit of 3% is selected to minimize
erosion. The treatment bed has two sloped sides 10 and
12 and two sloped ends 14 and 16. Around the edge of the -
treatment bed there is a trench 18, and adjacent the
trench the treatment bed slopes more steeply, at about a
1 foot ~ertical drop per 3 lateral feet. The sloped
treatment bed may be entirely formed from the
contaminated soil to be treated, or the contaminated soil
may be positioned in a layer of generally uniform
thickness on a sloped base surface that is equally or
less water permeable than the contaminated soil. The
trench serves to intercept, collect and channel water to
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a central point. The trench will preferably have a
gradual slope of about 0.0025 ft./ft. so that runoff
water can be collected from the trench at the central
collection point. There is also an access ramp 20 across -
the trench. The water thus collected can be discharged
under permit or collected in a tank or lined pond for
recycling. This water diversion and collection system
should be designed to accommodate between a 10 to 25 year
frequency storm. Moisture and nutrients will be provided
to encourage growth in the treatment bed of
microorganisms capable of degrading the organic compound
contaminants. The treatment bed will also be tilled to
provide aeration to encourage growth of such
microorganisms. Mechanically changing the land surface
of the treatment bed to drain surface runoff may be
accomplished by smoothing or leveling. Land grading for
drainage consists of shaping the land surface by cutting,
filling and smoothing to planned continuous grades. The
purpose of establishing continuous grades is to ensure
that runoff does not pond and thus infiltrate into the
treatment bed. The U.S. Department of Agriculture
published SCS National Engineerina Handboo~ (1971) may be
used in conjunction with state standard specifications as
a guide in land grading.
In the method of the present invention the dimensions of
the above described treatment bed are selected and the
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soil type adjusted so that nearly all precipitation will
run off the treatment bed. Slope is kept in a low enough
range (1-3 percent) so that undue erosion of the
treatment bed does not occur. Because nearly all
precipitation runs off the treatment bed, little if any
water migrates downwardly toward the water table.
The amount of rainfall that runs off the treatment cell
is influenced by precipitation, antecedent moisture
condition, soil hydrologic condition and topography and
shape in the manner discussed below.
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Precipitation
Precipitation generally occurs as rain or snow, is
potentially penetrated to the subsurface to combine
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groundwater flow or it may run off the surface of the -~;
treatment bed. The soil type has a major effect on the
amount of precipitation that runs off. Mechanical
treatment on the treatment bed along with the topography
and the shape, affect the rate at which water runs off.
The highest rates of runoff from small treatment beds are
usually caused by intense rainfall. The intensity of
rainfall affects the rate of runoff. The melting of
accumulated snow in the mountains or northern plains may
result in greater volume of runoff, but usually at a
lesser rate caused by rainfall. Three typical 24-hour
storm distributions, Type IA, Type I, and Type II were
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developed from U.S. National Oceanic and Atmospheric
Administration data. Type IA and I storm distribution is
characteristic of the coastal side of the western United
States. The Type II storm distribution is typical of
more intense storms occurring over the remainder of the
U.S. continent, Puerto Rico and Virgin Islands. Data on
the depth, aerial distribution, and water content of the
snow on small watersheds are rarely available. Sometimes
transposition of data from another area is very likely to
lead to erroneous estimates, since local topography and
ground cover will greatly affect the drifting and
distribution of snow. Probability analysis and
prediction are often used when precipitation is in the
form of snow.
Antecedent Moisture Condition
The amount of precipitation occurring in the five days
preceding a storm of interest is an indication of the
antecedent moisture condition of the soil. The average
is considered as between 1.4 and 2.1 inches during the
treatment period.
HYdrologic Soil Groups
In the U.S. Department of Agriculture, Soil Conservation
Service Technical Release 521 entitled "Procedure for
Computing Sheet and Drill Erosion Project Areas" (1977),
over 8,000 soils are classified into four hydrologic soil
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groups. These hydrologic groups, according to their
infiltration and transmission rate, are: -
A. Soil having high infiltration rates even when
it is thoroughly wetted. These soils have a
high rate of transmission in that water
readily passes through them.
B. Soil having moderate infiltration rates when
thoroughly wetted. These soils have a
moderate rate of water transmission.
C. Soil having slow infiltration rates when it
is thoroughly wetted. These soils have a
slow rate of water transmission.
D. Soil having very slow infiltration rates when
it is thoroughly wetted. These soils have a
very slow rate of water transmission.
Hydroloaic Conditions
The soil and its hydrologic condition affect the volume
of infiltration and runoff in the treatment cell. The
hydrologic condition of the soil is determined by its
moisture content at the time of the storm, its humus and
organic content and its te~perature, and whether or not
it is frozen.
Topography and Shape
The topography and the shape of the treatment cell have a
major effect on infiltration and runoff. The shape of
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the treatment cell is defined herein in terms of a shape
index, Sw, in which:
Sw = L/W = L /A
where L is the leng~h of the treatment cell from the
highest point to the trench, W is the average width of
the treatment cell, and A is the area of the treatment
cell. An increase in the shape index causes a reduction
of the peak discharge rate due to the longer time-of-
concentration, and vice-versa. Therefore in a particular
area which is exceptionally long and narrow, greater
infiltration would be expected. Conversely, a treatment
bed which has a minimum length would reduce the
infiltration. A treatment bed with a lesser length would
have a lesser time of concentration and thus less time
for infiltration to occur. The average slope can be
defined as the ratio of the difference in elevation
between the treatment cell discharge point and the
highest point to the approximate average length of the
treatment cell.
Time of Concentration
Time of concentration is the time it takes for water to
travel from the most distant point of a watershed to the
discharge point of the watershed. The time of
concentration can be calculated assuming an average
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Manning's n (roughness coefficient) and hydraulic radius.
The formula for determining time of concentration is:
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Tc = L1-15/7700 H0-38
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where Tc is the estimated time of concentration, L is the ~ :
length of the treatment bed in the direction of flow from
the highest point to the discharge point, and H is the
elevation difference between these two points. The
combined effect of soil, hydrologic condition,
precipitation, topography and shape, and antecedent
moisture condition on the amount of rainfall that runs
off the treatment cell can be represented by the runoff
curve numbers (CN). The CN can be expressed as:
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CN = 1000/ (S + 10)
where S is the potential infiltration, and runoff (Q) can
be calculated as follows:
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Q = (P - o.2s)2 / (p ~ 9.8S)
A detailed description on estimation of the peak flow
rate is presented in the U.S. Department of Agriculture,
Soil Conservation Service, Technical Release 55 of June,
1986 entitled "Urban Hydrology for Small Watersheds", the
contents of which are incorporated herein by reference.
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Those skilled in the art will, in view of the above,
appreciate that it will be possible to select the shape
of the treatment cell and/or adjust soil composition so
that runoff is approximately equal to precipitation.
Under such conditions there will be little, if any,
downward migration of moisture. Such selection of shape
and adjustment of soil type is illustrated in the
following example.
ExamPle
A treatment cell similar to the one shown in Figures 1-3
is constructed. Length in direction of flow ~L) is 100
ft., width (W) is 350 ft., and Height (H) is 2 feet.
Time of concentration (Tc) is thus calculated as follows:
Tc = L /7700 H
Tc = 199.53/10,020
Tc = 0.0199 hrs. - 2.19 min.
The soil in the treatment cell is soil Type D. From
pages 2-6 of the above mentioned Technical Release 55 the
curve number (CN) for a newly graded area with no
vegetation is 94. The potential maximum retention after
run off (S) is calculated as follows:
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S = 1000 _ 10 = 1000 _ 10 = 0.638
CN 94
For a design storm of 10 inches (25 year, 24 hour storm),
precipitation (P) is 10 inches and runoff (Q) is
calculated as follows:
Q = (P-0.2S) = [10 - 0.2 (0.638)] = 9 3
(P + 0.8S) = [10 + (0.8) (0.638)]
Thus out of 10 inches of precipitation 9.3 inches
runs off and only 0.7 inches infiltrates so that
approximately 93 percent runs off. By controlling the
type of soil additive to give a curve number (CN) of 94,
infiltration is thus limited.
It will be appreciated that a method for treating
contaminated soil is described in which downward migration
of moisture can be avoided without use of a plastic liner.
Although the invention has been described with a certain
degree of particularity, it is to be understood that the
present disclosure has been made only as an example and
that the scope of the invention is defined by what is
hereafter claimed.
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