Improving Soil Testing Methods for Lead Contaminated Soils

<p>This project seeks to build upon the momentum of a current project called "Growing Health Soils for Heathy Communities (GHSHC)" being conducted in Milwaukee, WI.</p><p> The objectives of that research project are to</p><p>1) demonstrate the effectiveness of soil and landscape interventions in reducing total lead, and</p><p> 2) expand environmental health literacy education and access to soil testing.</p><p> However, the GHSHC project does not attempt to characterize the chemical forms of lead in the soil or attempt to evaluate estimated changes in bioavailability. The proposed research seeks to further characterize the contaminated soil samples collected from the Lindsey Heights and KK River neighborhoods during the GHSHC project. The residents are currently being recruited by project leaders and soil sampling will occur under the budget of the GHSHC project with 50 samples being collected during the 2015 growing season and another 50 collected during the 2016 growing season. A pilot study found the majority of soils in these neighborhoods to be slightly alkaline with pH values ranging from 7.6 - 8.2. Total lead concentrations ranged from 7 - 3200 mg/kg which will be sufficient for establishing correlations among the soil testing methods we plan to evaluate.Upon receipt of the soils, each sample will be split in half with one half receiving a phosphate treatment to induce formation of lead-phosphate minerals with low bioavailability; the other half of the sample will remain un-amended. The phosphate treated soils will receive phosphoric acid at a phosphorus: lead molar ratio of 5: 1 based on the total lead content of the soil as determined by EPA 3050. The phosphate amended soils will be kept at 80% of field moisture holding capacity at room temperature for 14 days. After 14 days, the pH will be adjusted using calcium oxide to return the soil to its original pH and the allowed to equilibrate for another 14 days before being allowed to air dry and then oven dried at 40°C. Previous research has shown this method of phosphate addition to be an effective way to rapidly shift the bioavailability of lead in soils (Scheckel and Ryan, 2004).Soil lead will be characterized using the standard EPA method for total lead (EPA 3050), conventional soil nutrient methods, modified soil nutrient methods, and in-vitro bioavailability methods. The EPA 3050 method is the current regulatory standard used for soil lead assessment in the US. It extracts all lead in a soil sample regardless of chemical form. The following methods all extract a portion of the soil and are hoped to better represent bioavailability.Bioavailability Methods:In-vivo testing using swine exposed to contaminated soil is the gold standard for estimating the bioavailability of lead to humans. However, these methods are expensive at approximately $30,000 per soil sample (Zia et al., 2011). Therefore, in-vitro chemical extraction methods have been developed which correlate with the in-vivo bioavailability. One of the first of such tests that found wide use is called the Physiologically-Based Extraction Test (PBET), which employs a simulated gastric solution at body temperature with mixing and other parameters set to mimic the human digestive tract (Ruby et al., 1999). The USEPA follows a modified PBET test which was found to be well correlated with soil lead bioavailability fed to swine (Drexler and Brattin, 2007).While in-vitro bioavailability tests can be performed for significantly lower cost than in-vivo testing, they remain essentially inaccessible to the general public. Therefore, there is a great need to develop an even simpler and less expensive test that is reproducible and can accurately correlate with bioavailability.The in-vitro extraction solution will consist of 0.4 M glycine adjusted to pH 2.5 with HCl. One gram of soil will be added to the HDPE bottles and 100 mL of extraction fluid added. The extractor is turned on and the solutions will be rotated end-over-end at 30 rotations per minute for 1 hour. Immediately following extraction, the pH of the extraction solution will be measured. If the pH differs by more than 0.5 pH units, the extraction will be re-run and the pH will be monitored at 5, 10 ,15, and 30 minutes into the extraction. If the pH differs by more than 0.5 pH units the solution will be adjusted drop wise with HCl. After a successful extraction (with appropriate pH), the solution will be filtered through a 0.45 µm cellulose acetate disk filter and stored at 4 °C for lead analysis via ICP-AES.Conventional and Modified Soil Nutrient Methods: Conventional soil nutrient methods are commonly performed by soil testing labs to provide information on soil nutrient status to farmers and gardeners for a cost of $10 - $20 per sample. They are designed to extract a small fraction of the total nutrients in the soil - called labile, plant available, or bioavailable nutrients. Most of these chemical tests were developed in the early to mid-1900s, and consist of dilute acid or acids (pH 1.2 - 2.5) and short extraction times (5 minutes). While the Drexler and Brattin method attempts to precisely quantify the total bioavailable lead, an effective test need only correlate well with the Drexler and Brattin. Because soil nutrient tests are already conducted widely used across the US, if one could be identified that correlates well with the Drexler and Brattin (2007) bioavailability method, the barriers to more accurate soil lead hazard would be decreased resulting in an increase in knowledge of soil lead hazard by farmers, farm workers, and the general public. In this research we propose to evaluate how the lead extracted by four very commonly used soil nutrient tests compares to the standard in-vitro bioavailability test of Drexler and Brattin (2007) as modified by Zia et al. (2011). We also propose to evaluate a small modification of the Mehlich-3 test based on previous findings in the literature which are described below.The most widely used soil nutrient test in Wisconsin is the Bray P-1 (Bray and Kurtz, 1945). It works well for extracting phosphorus on slightly acidic to slightly alkaline soils. The method involves shaking 2 g of soil in 20 mL of the extracting solution (pH 2.6, 0.025 M HCl in 0.03 M NH4F) for five minutes at room temperature at 200 cycles/minute. The Mehlich-1 test (Mehlich, 1953) is commonly used to assess the plant available nutrient status of soils in the Southeastern US which tend to be lower in organic matter with lower pH than Midwestern soils. However, the Mehlich-1 extraction solution contains 0.0125 M H2SO4 and 0.05 M HCl and its acidic nature (pH 1.2) may be useful for simulating gastric release of lead from soil, and is therefore deserving of evaluation. The Mehlich-3 was developed to effectively assess soil nutrient status across a broad spectrum of soils. Because of this, it is commonly used across the US and Canada. The Mehlich-3 is the most complex of the soil nutrient tests and contains acetic acid, ammonium nitrate, ammonium fluoride, nitric acid and EDTA. Finally, the DTPA extraction for micronutrients in soil will be evaluated (Lindsay and Norvell, 1978) against the Drexler and Brattin method for estimating lead bioavailability. This test was developed to extract Cu, Fe, Mn, and Zn from neutral and calcareous soils via chelation with 0.005 M DTPA adjusted to pH 7.3. This test has promise because it was developed to address the over-estimation of micronutrients by acidic extracts and is commonly employed at every major soil testing laboratory in the US.</p>