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Improving Soil Testing Methods, Interpretations and Education for Lead and Arsenic


Although most soil testing laboratories routinely measure total soil Pb and As, these tests are not consistent among states, and they are not specifically designed to estimate bioavailable fractions of these metals. In fact, tests for bioavailability are presently not offered in most cases. Even for the total metals test results, an interpretation is often not given, and those interpretations that are given by different testing labs are contradictory. For the different metal extraction procedures being used by different labs, there is usually no clear connection to plant uptake, bioavailability or potential toxicity. As a result, the effectiveness of soil amendments or other management practices in reducing Pb or As toxicity in the soil cannot easily be assessed. It is also not clear the extent to which soil properties affect the potential for toxicity and crop uptake. This situation has created considerable disagreement and uncertainty about the behavior, toxicity and best remediation practices for these metals in contaminated soils. <P>To help remedy this problem we will: a. Evaluate possible soil extractants for As and Pb, where the aim will be to develop an As and Pb soil test that is able to predict the potential for crop uptake or crop contamination. b. Assess the relative accuracies of the presently used "screening tests" for soil Pb presently in use among Northeastern states for estimating total Pb in the soil, and recommend the best method for estimating total soil Pb. c. Develop web-based educational materials and conduct outreach to ag. advisors, testing labs, farms and gardening communities, elucidating the behavior of toxic metals in soil, their potential for crop uptake and the pros and cons of different methods of soil testing for Pb and As in particular. We will provide interpretation of the potential health impacts of these tests and describe inherent limitations or uncertainties in the tests. Soil remediation approaches with proven effectiveness will be described and explained.

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Non-Technical Summary:<br/>
The increasing interest in 'greening' of inner-city urban areas and growing healthy foods locally has led to numerous new urban gardening initiatives in inner cities of New York state and elsewhere in the Northeast. Because city sites typically available for development as urban gardening centers are contaminated from historical uses such as heavy industry and traffic, numerous questions have arisen with respect to the potential for exposure of gardeners to toxic substances. This situation has pointed to the need for soil testing procedures and interpretations specifically designed for toxic metals such as lead, arsenic and cadmium in garden soils. Historically, soil testing offered by land grant universities has emphasized measurement of parameters of soil fertility rather than potential for human exposure to toxins. With increasing awareness of the neurotoxic effects of lead to young children even at low exposure levels, and evidence that ambient soil Pb levels affect children's blood Pb levels, limiting children's exposure to lead-contaminated soils has become more urgent. There is also emerging evidence that dietary arsenic is more toxic than had been believed in the past, and stricter limits for As levels in food crops may be imposed. Soil tests need to be designed and calibrated to estimate risk of human exposure to toxic metals, particularly lead, arsenic and cadmium, both through the consumption of food crops and by direct exposure to soil particles. A key research and extension priority is to make agriculture and food systems responsive to human health needs. With the present lack of consistency and transparency in soil testing among the Northeastern states for toxic metals, there is a great deal of confusion and concern among gardeners and homeowners. Soil testing laboratories may provide very little, or inconsistent, information on how to interpret test results for toxic metals. This project will develop interpretative materials for gardeners and growers that will be used in support of soil testing for toxic metals.
The first two objectives will be addressed in part by comparative laboratory testing of soils in collaboration with directors of other soil testing labs in the Northeastern states who have an interest in improving testing methods for toxic metals. We now have a large collection of archived urban garden soils variably contaminated with Pb, As and Cd (>500 samples) that will be made available for interlaboratory comparison of different screening tests presently in use in the Northeast. The objective of a simple and inexpensive screening test for soil Pb has practical importance for many homeowners and gardening groups with limited resources for soil testing. A screening test can be cheaper and faster than standard total Pb measurement in soil by acid digestion. We will evaluate several simple extracting solutions in use by soil testing labs in the Northeast for their ability to estimate total Pb as determined by the EPA 3051-6010 method. Field-based research will be conducted in support of the first two objectives in order to provide verification for the soil tests being evaluated. We have already established small-scale field plots on the Dilmun Hill student farm, an area that was previously part of the Cornell apple orchard, and is variably contaminated with Pb and As. These plots provide a "natural" experimental site for careful measurement of vegetable crop uptake of Pb and As, as well as measurement of soil properties that may affect the plant availability of the metals. Vegetable crop uptake studies will be conducted in 2011-12 for lettuce, carrots, beans and tomatoes on these plots, with the measurement of Pb and As concentrations in the edible portions of the crops by ICP-MS. Concomitant with the uptake measurements, soil properties in each of the plots will be measured, including pH, organic matter content, total and extractable Pb and As. An effort will be made to find soil factors that most strongly predict Pb and As uptake into crops. The last objective, developing web-based and other educational materials and outreach for soil testing, will be achieved by using existing knowledge about Pb, and As as well as new results from the research component of this project to categorize the level of concern to human health based on total and/or estimated bioavailable metal concentrations in the soil. This approach will lead to the establishment of a range of impact scale from essentially none, to low or moderate risk to high risk that is related to measured concentrations of toxic metals in the soils and to the particular activity or crop grown on the soil. Collaboration with other Northeastern state soil testing labs will ideally allow us to reach consensus on soil test interpretations, so that inconsistencies in interpretations and guidelines for gardeners are minimized. Dissemination of guidelines for each of the potentially toxic metals ( Pb, As, Cd) will be posted on the website of the Cornell Waste Management Institute.
2011/10 TO 2012/09<br/>
OUTPUTS: Research in this project continues to focus on the challenges presented by testing of soils for heavy metals in the urban environment, with most of the emphasis being on the potentially toxic metals lead (Pb), cadmium (Cd), zinc (Zn) and arsenic (As). We have evaluated simple and inexpensive screening tests for estimating total Pb, As, Cd and Zn concentrations in contaminated urban garden soils. We have found that 1M HNO3 extraction compares favorably with other commonly used "screening test" for Pb in urban soils, and total soil Cd and Zn can also be estimated reliably by the same test. In fact, we have demonstrated that our estimate of soil Cd by this screening method, using flame atomic absorption to measure Cd in the acid extracts, provides a more reliable measure of total soil Cd than the widely used microwave acid digestion of soil (EPA Method 3050/3051) with ICP-emission analysis of the digests. Recently, the EPA has recognized that "false positive" or "biased" results for analysis of heavy metals in soils by ICP-emission is a severe problem (particularly for arsenic and thallium), indicating that alternative testing methods may be needed for some toxic metals in soils. Our recent extensive surveys of soils in urban areas, particularly private yards and community gardens, show that Pb is the toxic metal most frequently at levels of concern for human exposure. Unfortunately, for many gardeners, the cost of having soil testing done for toxic metals by standard methods (e.g., 3050 EPA) is an impediment to the adequate assessment of soil contamination because Pb is typically very heterogeneously distributed in gardens and private yards, requiring intensive sampling and testing in order to establish its spatial distribution. Consequently, our simple and inexpensive screening test using 1 M nitric acid to estimate total metal concentrations, particularly Pb, in contaminated urban garden soils, will provide gardeners with a practical and affordable alternative to standard soil testing. The results for the Pb screening test have been summarized in two manuscripts, one already published and the other recently accepted for publication. A third manuscript evaluating the screening test for Cd and Zn is in preparation. We are presently working on evaluating the 1 M HNO3 screening test for soil arsenic (As), a metalloid of increasing concern for human health. Soil Pb presents a serious challenge for remediation because phytoremediation is not feasible ( there are no known strong Pb hyperaccumulator plants), and attempts to stabilize Pb in soils and thereby reduce its bioavailability have not been widely successful. However,The removal of Pb from soils using chelating agents to solubilize the metal has been demonstrated by other researchers , and we are beginning experiments to determine whether Pb in urban garden soils is readily susceptible to dissolution and removal. There are numerous obstacles and disadvantages to this chemical remediation approach, however, and we intend to evaluate these potential pitfalls. <br/>PARTICIPANTS: Dr. Metka Udovic spent 6 months in my laboratory in 2010-2011, conducting research on correlating soil test methods for Pb and As with bioavailability assays in soil invertebrates. In addition, two visiting scientists from the South China Botanical Garden have been involved in soil toxic metals research in my laboratory since early 2012. One of them (Yi Ping) is a Ph.D. candidate, and the research she has completed here will become part of her graduate thesis when she returns to China.
<br/>TARGET AUDIENCES: Our interactions with community stakeholders (e.g., through gardening events and discussion forums in NYC and Ithaca, urban farming workshops in Buffalo, responding to information requests by email and phone) have indicated a need for comprehensive educational programs addressing diverse topics, including: 1) Training on site assessment for contaminants and soil sampling and testing protocols; 2) Information about and access to reliable, affordable, certified soil testing labs; 3) Simple guidelines for interpretation of soil test results that allow for site-specific considerations; 4) Assessment of contaminants in municipal compost and available soil/fill, and access to these materials; and 5) Possible liability issues or closure or avoidance of gardens if soil tests reveal contamination. Additional fact sheets, workshops, etc. to address these topics continue to be developed to augment existing resources available at our CWMI website under the heading "Soil Quality".
<br/>PROJECT MODIFICATIONS: Not relevant to this project.
IMPACT: The results from the soil lead (Pb), cadmium (Cd) and zinc (Zn) test comparisons have produced a highly useful observation in practice - extraction of a wide range of soil types with 1 M nitric acid provides a very good approximation of total lead, cadmium and zinc in the soil without the added effort and expense of measuring total metals using an acid soil digestion process. Furthermore, the Pb, Cd and Zn in the acid extracts are easily measured by either standard flame atomic absorption or by ICP emission with equivalent results. Because many laboratories, particularly in underdeveloped countries, have greater access to atomic absorption equipment than to ICP because of lower cost, these simpler "screening" tests may be adopted by many laboratories for practical reasons. We have used the screening test for Pb to advantage in assisting urban gardeners to get rapid spatial mapping of soil Pb concentrations for their gardens, allowing them to best site locations for vegetable-growing areas. This screening test is also now offered as a less expensive soil test for lead by the Cornell Nutrient Analysis Laboratory. Our observation of extreme Pb spatial heterogeneity in urban soils has informed our recommendations on the minimum number of discrete soil samples needed to characterize gardens and yards, and the methods best used to combine and homogenize these samples prior to analysis. Our results indicate that it is inadvisable to base management of contaminated yards and gardens on the results from one or two composited soil tests. In addition, thorough grinding of urban soils has the effect of reducing Pb soil test variability. Our understanding of heterogeneity in Pb concentrations found at several spatial scales in urban soils is helping to guide future sampling designs in New York City community gardens, and is also influencing our recommendations to homeowners who wish to test their own yards for Pb and other toxins.

McBride, Murray; Vatamaniuk, Olena; Russell-Anelli, Jonathan
Cornell University
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