Sewage Sludge:
Factors Affecting the Uptake of Cadmium
by Food-Chain Crops Grown on Sludge-Amended Soils
This document (SW-882) was prepared by the
W-124 SEA-CR Technical Research Committee
and is reproduced as received.
U.S. ENVIRONMENTAL PROTECTION AGENCY
1980
-------
The following 1s in response to a request by Albert Peter, Jr.,
U.S. EPA, for the H-124 SEA-CR Technical Research Committee to respond
to the September 13, 1979, Interim Final Criteria as they pertain to the
items contained in his letter, dated December 7, 1979. This document
was drafted January 17, 1980, following the Annual VI-124 SEA-CR Technical
Committee meeting and:is the result of discussions which occurred during
the P.M. on the 16th and A.M. on the 17th. Therefore, the time constraint
limited the"details in the response.
I.. Background on the Use of Cation Exchange Capacity (CEC) in Conjunction
with Cumulative Cd Addition to Soils
The following is a summary of the development of metal limitations' •
by NC-118/W-124 for application of^sewage sludge on agricultural viand. .
The approach has always included Pb, Zn,- Cu, Ni, and Cd rather than just
Cd as adopted in the Criteria.
1. 1971. Chumbley (ADAS No. 10, 1971) in England suggested.use of
a In equivalent approach (Zn eq) to limit Zn, Cu, and Ni additions to
soils in sludge. The-.equation used was Zri eq = Zn + 2 Cu + 8 Ni, with
the coefficients reflecting the potential phytotoxity of Zn, Cu, and Ni.
Sludge Zn eq added was limited to 500 Ibs/acre and soil pH > 6.5. Personal
communications with research personnel from England involve? revealed
that"the Zn eq was based upon solution and pot cultures and limited
field data. It was designed for protection of sensitive crops grown on
acid soils. It was. intended for use as an internal document to provide
guidance for local extension personnel.
2. 1972. Leeper (Report to Army Corps of Engineers) proposed that
soil CEC could be combined with the Zn eq approach because numerous
field and greenhouse studies have suggested that phytotoxicity from Zn,
Cu, and Ni was related to soil CEC. Leeper suggested adding sludges at
a Zn eq rate equal to 5% of the CEC."
3. 1974. The U.S. EPA published a draft technical bulletin which
proposed a modified version of the Zn eq concept to limit sludge Zn, Cu,
and Ni additions to soils. Since total sludge applications would be
quite low, and research indicated that phytotoxicity problems were not
occurring, the Zn eq added to soils was increased from 5 to 10% of the
CEC. In addition, the sludge Cd/Zn, presumably to minimize Cd uptake by
crops, was suggested not to exceed 1/100. Also soil pH was specified as
pH * 6.5 and, based upon research data, the relative phytotoxicity
coeTficient for Ni was decreased from 8 to 4.
4. 1974. A NC-118 subcommittee met in Chicago (August 1974) to
respond to the draft U.S. EPA sludge application guidelines. The
consensus of the group based upon research data suggested:
a) The Zn eq approach implied more knowledge than existed
concerning the relative phytotoxicity of Zn, Cu, and Ni.
b) Pb and Cd additions to soil should al.so be limited due
to phytotoxicity and human health considerations respectively.
c) Soil pH should be 6.5 at the time of sludge application
and maintained at > 6.2 after application.
-------
d) The annual rate of sludge application should be
controlled by either (1) the nitrogen required by the plant
(minimize N03~ leaching, or (2) 2 Ibs Cd/acre/yr, whichever is
•lower.
e) The annual Cd application of 2 Ibs/acre was based,, in
part, on field data from.Wisconsin where corn was grown and no
increase in grain Cd occurred at this rate (Soil pH 6.0-6.2).
Slight but not statistically significant grain Cd increases were
observed at 4 Ibs Cd/acre.
f) The cumulative amount"of-Cd should be limited to 10 Ibs/
acre based on:
1) 5 years of sludge application at 2 Ibs/acre/yr would
result in an average soil concentration of 10 Ibs Cd/
acre in the plow la-yer (typically 0-6 inches) a level
within the range encountered for natural mineral soils
. (based upon.information published by Alloway, 1968), and
2) additional data would be available within this 5 year
period.
g) The amounts of Pb, Ni, Zn, and Cu added to soils should
be limited because of evidence for relative phytotoxicity. Data
were used from greenhouse and limited field studies.
h) The limits were chosen for medium textured soils in
the North Central states region. The NC regional committee proposed
the following limits for metal additions to soils in the form of
sludge. . •
Pb 1000 Ibs/acre
Zn . 500 Ibs/acre
Cu 250 Ibs/acre
Ni .100 Ibs/acre
Cd 10 Ibs/acre
Sludge applications should cease when any one of the above exceeded the
limit specified above.
5. ' 1976. The NC-118 Committee at the request of USDA (Cincinnati,
Jan. 1976) adopted a combination of the single metal limits and the Zn
eq at 10% of the CEC. Sludge applications should be lower for coarse
textured soils because of the potential for a lower pH buffering capacity,
lower metal adsorption capacity, and to minimize possible metal leaching.
Coarse, medium, and fine textured soils were selected based upon CEC.
The metal limits suggested as guidelines were
CEC (meg/100 g)
<5 5-15 >15
Pb 500 1000 2000
Zn 250 500 1000
Cu 125 250 500
Ni 50 100 200
Cd 5 10 20
-------
The 15 meq/100 g category was selected because of known trace
element deficiencies that can occur in high organic matter/high CEC.
soils. Also, metal adsorption was correlated with CEC. These metal
addition limits also assumed soil pH control at pH ^ 6.5 during and
after sludge application. The committee did not have research data to
show a relationship between Cd uptake by plants and CEC. Rather
CEC was chosen as an easily measured soil property that is 'directly
related to the ability of.a soil .to minimize Cd solubility and thus
reduce plant uptake of Cd. That .is, as CEC increases, the concentration
of soil-components responsible.for metal retention and metal buffering
capacity also increase, i.e., organic carbon, Fe and Al oxides and clay
content. Numerous exceptions exist to. this general statement..^ It
should be emphasized that the above metal additions approach was
1. Conservative in nature for Pb, Zn, Cu, and Ni because
of insufficient data at high application rates.
2.. Capable of providing adequate protection for the
continued productivity of agricultural soils.
3. Based on CEC because soil properties, in addition to
pH, were believed to be important in controlling uptake, including
Pb,.Zn, Cu, Ni, and Cd by plants and because CEC was a routinely
determined soil parameter in soil testing labs.
4. Designed for use as a guideline for state and'local
extension personnel in the NC and NE regions where soil organic
, carbon makes an important contribution to CEC. The limits were not
intended to be used as a regulatory tool.
5. By following these guidelines farmers utilizing sewage
sludge would only have to utilize their current soil testing programs
which include soil pH and lime requirements. No soil or ground-
water monitoring for N and metals was thought to be needed.
6. The consensus of the committee is that CEC serves as a
useful guideline to limit applications of Pb, Zn, Cu, and Ni. Its
application for limiting Cd application to soils cannot be supported
on a broad scale. .
II. Is CEC a viable soil factor controlling the uptake of Cd by crops
from soils amended with sewage sludge?
A consensus of the committee regarding this item follows.
1. The NC-118, W-124, USDA Committee did not propose that Increasing
the CEC would decrease Cd .uptake by crops in all cases.
2. Most studies involving soils with varying CEC have also had
varying pH and other soil properties which confound the interpretation
of the data and result in the inability to relate'Cd uptake by plants to
any single soil factor.
-------
3. Studies relating CEC to Cd uptake show conflicting results.
4. Not all soils should receive the same cumulative Cd loading
rates. CEC does not adequately reflect all soil factors which influence
the uptake of Cd by plants. Therefore it is not applicable as a
regulatory tool over a broad range of soils from the various regions
throughout the USA. Some examples where CEC is not applicable include:
a) Calcareous soils;which are generally insensitive to CEC
effects on Cd uptake by plants..(see glossary of Soil Science terms
(SSAJ).for definition of calcareous soils).
.b) Some soils in the Southeastern USA have low CEC but high
Cd sorption capacities due to Fe and Al oxides. Data available
suggests that when these soils are at pH _^ 6.5, Cd uptake by plants
is less than one would predict based upon soil CEC.
. cj Organic matter, Fe and Al oxides, texture, and pH
buffering capacity were identified as factors which probably
regulate Cd uptake by plants, but these are interrelated and our
knowledge is not presently adequate to quantify their independent
effects.
III. Priorities for alternative soil factors to use as a regulatory
tool to limit Cd uptake by plants.
1. From the standpoint of Cd uptake by plants, soil pH is the soil
factor which has the greatest and most consistent effect on Cd uptake in
various studies, and is most readily supported by available research
data.
2. Other soil factors identified were: organic matter content,
content of Fe and Al oxides, texture, and buffering capacity. Because of
the interrelationships of these factors, lack of research data to describe
their effects on Cd uptake, and the lack of routine procedures for their
measurement (except texture), the committee did not feel that any single
factor could be used, at this time, to set Cd loading limits for all
soils in all regions of the USA.
3. Research indicates that calcareous soils greatly limit Cd
uptake over a range of soil CEC. Field research data show that a
cumulative Cd application of 20 kg/ha results in a Cd concentration in
crops which falls in the range for crops grown on natural (not amended
with Cd) non-calcareous soils.
4. Cd loading rates should not be the same on all non-calcarous
soils. Low organic matter sandy soils should have lower cumulative Cd
loading limits than high organic matter medium textured soils. There is
insufficient information at this time to differentiate between these two
broad groups .of soils for regulatory purposes.
-------
IV. Based on research data of W-124 committee representatives and the
published data of other researchers, the W-124 Technical Research
Committee would summarize the current knowledge of annual and
cumulative Cd additions to soils, and the resulting Cd uptake by
plants, as follows:
At any given level of Cd in soil, whether that soil Cd is naturally-
occurring or has been applied by annual or cumulative additions, Cd
uptake by plants will be dependent on several soil, plant; and environ-
mental factors. Because several factors can affect the amount of Cd
taken up by plants, it is difficult to make generalizations about Cd
uptake that will hold for all soil-plant systems. However* with a given
soil and plant variety, some generalizations can be made as to how
annual and cumulative additions of Cd will affect Cd uptake. (One must
recognize that with similar Cd additions the actual amount of Cd taken
up will be different with a different soil type, a different plant
species, or a different climate.)
1) The pH of a soil will influence the Cd uptake by a given
plant variety from annual or cumulative Cd additions,
A) Cd uptake is more pronounced at acid pH's than at
slightly-acid, neutral or alkaline soil pH's.
B) Cd uptake into vegetative tissues is greater than
in the fruit, grain, or tubers of plants.
2) Annual Cd loadings are important because Cd uptake by
plants is related to the annual application rate.
3) Total Cd (or cumulative Cd) level in soils is important
because Cd uptake by plants is dependent on the soil Cd level, but
the availability of Cd to plants may be different than with annual
Cd loadings.
4) For an operating site, plant uptake of Cd will depend
on annual and cumulative Cd additions to soils, as summarized above.
5) For a closed site, plant uptake of Cd will be dependent
on the cumulative Cd present in the soil rather than on the rate of
annual Cd additions made to reach that soil Cd level. However, the
rate of annual Cd loadings will have an effect on how fast the Cd
uptake by plants will decline to a level that is dependent on the
total Cd level in soil, following site closure. The plant uptake
level of Cd eventually reached may or may not return to the normal
background level in plant tissue, depending on the soil, 'plant, and
climatic factors indicated above.
6) The background document accompanying the criteria is not
considered adequate for completely addressing annual vs. cumulative
Cd'additions to soils because: (1) limited field data were used
without"adequate discussion of their limitation; (2) differences in
uptake rate as a function of annual or cumulative rate of application
was not sufficiently documented; and (3) there was not adequate
support for a phase reduction of the annual loading rate. However,
-------
we support the use of field data for establishing these guidelines.
New research data and forthcoming data from continuing research
projects will provide further insights on annual vs. cumulative Cd
additions to soils. The forthcoming CAST report on this topic
should contain the current state-of-the-art on this subject.
ya 1981
SW-882
------- |