Soil Kitchen Participants:
Thank you so much for participating in the soil sampling segment of the 2011 Philadelphia Soil Kitchen project. You kept the EPA analysts very busy! Just over 350 samples were analyzed for lead (Pb) by the X-Ray Fluorescence (XRF) instruments. Arsenic (As) and cadmium (Cd) also were recorded but, as discussed later, the XRF instrument is not as effective in detecting these metals at low concentrations. The tabulated results appear in both the Results Index and Soil Map. You should be able to identify your specific result by the sample identification number provided to you on the day you dropped off the soil. Results are expressed as parts per million (ppm) – particles of the metal being tested (lead, cadmium or arsenic) for each one million (1,000,000) soil particles. For soil sample results, sometimes you will see ppm expressed as milligrams of some substance per kilogram of soil (mg/kg).
The following narrative provides perspective for interpretation of the values recorded for your sample and cites reference materials discussing recommended measures. Unfortunately, qualifications must be stated given the limitations of the sampling event, the lack of universal consensus on interpretive standards for metals in soil, and ongoing research to assess how metals in soil are accumulated in plants. We hope this doesn’t discourage you. The information we provide should give you added insight into the potential implications of your results and we are providing links for further reading.
If, after reading the summary, you have concerns and want to ask questions, you may contact Bob Helverson of the Agency for Toxic Substances and Disease Registry Region III (ATSDR) at 215-814-3139 or firstname.lastname@example.org or Jack Kelly of EPA Region III at 215-514-6792 or email@example.com. We both spent considerable time at the Soil Kitchen event. If we cannot answer your questions, we’ll seek assistance from co-workers or contacts in other agencies. If you have very specific questions about the potential for plant uptake of metals or other contaminants, we may need to suggest that you contact organizations like Cornell University’s Waste Management Institute or Cooperative Extension Service, the Penn State Cooperative Extension Service, the University of Massachusetts (Amherst) Department of Plant and Soil Sciences or the Kansas State Agronomy Soil Testing Laboratory.
Limitations for Data Interpretation
1. Before addressing the data, it is important to note that the time and resource constraints of the project and EPA’s involvement could only allow for one sample from each participant. The results for one sample will likely not be representative of the soil for your entire property. For example, a recent research study in Chicago reported results collected from multiple soil samples in seventeen urban gardens. The maximum difference in lead results between the highest and lowest values within a single property was 3700 ppm! A study in Boston where multiple samples were collected from residential yards also showed a wide variation in lead results. Results generally were dependent on sampling distance from houses painted with exterior lead-based paint. The point is that the values recorded for your one soil sample might not tell a complete story.
2. Also note that EPA performed XRF analyses, not laboratory analyses. The attached link (see document #1 here) provides a brief description of the XRF. EPA routinely employs and accepts the accuracy of this instrument for assessing properties for heavy metals, particularly lead, but any samples you send off to a laboratory will likely be performed by a more sensitive methodology. The attached links (see documents #2A and B here) are purchase forms from the University of Massachusetts Soil and Plant Testing Laboratory should you wish to arrange for additional analyses of your soil. It is one Eastern U.S. lab we know of that provides soil metal analyses for the general public at a reasonable cost. The Kansas State Agronomy Soil Testing Laboratory also is now providing low cost soil analyses for metals.
EPA analyzed a total of 354 samples for lead, cadmium and arsenic by XRF. The analytes lead, cadmium and arsenic were chosen because they are often found in urban environments and may be at concentrations of human health concern. Lead, because of its use in a wide variety of industries, can also be a good indicator of localized industrial activity.
While XRF can accurately detect lead at rather low concentrations, it is more difficult and time-consuming to do so for cadmium and arsenic. You will see that the reporting limit (RL on the data table - for our purposes, the lowest level at which the XRF can reliably detect the element) for cadmium was 80 ppm. The RL fluctuates for arsenic depending on the concentration of lead because lead interferes with the XRF’s ability to detect arsenic. The higher the lead concentration, the higher the reporting limit for arsenic. This is a function of how the XRF operates and “sees” these two elements. Suffice it to say it was easiest for the XRF to detect lead. (Note the large number of “NDs” (non-detect) results for arsenic and cadmium.)
As expected, lead was found consistently given its long history of industrial use, high concentrations in lead-based paint, presence in vehicle exhaust emissions until the late 1980s, and the XRF instrument’s ability to accurately detect lead even at low concentrations. Values ranged from non-detect (lowest RL was 40 ppm) to 3600 ppm.
Here is a brief breakdown of the lead results by concentration range:
|ND (below 40 ppm)||40 - 499 ppm||500 - 999 ppm||1000 - 1999 ppm||2000 - 2999 ppm||3000 - 3500 ppm|
Total = 354
Although there was a wide range in the concentrations, numerous studies of lead in urban soils over the years indicate this is not unusual. See the attached links (see documents #3 and #4 here) citing soil lead sampling efforts in several U.S. cities. For some of the studies, lead concentrations ranged from tens (10s) to tens of thousands (10,000s) of ppm. There are many factors for why this occurs, including sample proximity to homes, sheds, fencing or steel infrastructure (e.g. bridges) where lead paint was removed by weathering or hand or mechanical methods, proximity to past vehicle exhaust emissions from leaded gasoline, proximity to lead-emitting industrial facilities and demolished buildings; depth of soil sampled; whether or not soil was amended or mixed with “clean” soil, and age of buildings and building construction characteristics within the city. The historic legacy of industrial and other lead-emitting or lead-containing sources within our cities, especially older cities, has left us with lead concentrations in soils well above values that occur naturally (in the range of 20 ppm as an average background value).
Next Steps for the Urban Gardener/Homeowner with your XRF Soil Information
So what can you do? If you have elevated concentrations of lead, and especially if you have young children who routinely spend time playing in the soil, you may want to consider further soil testing and/or to take steps to reduce potential soil exposure by covering the soil or adding clean soil or organic amendments (e.g. compost). Good hygiene practices also are important. There are many fact sheets available from EPA, USDA, universities, State agencies, soil testing laboratories and gardening organizations offering recommendations and guidance regarding lead concentrations (and other contaminants) in garden soils. Most of the guidance is in accord, especially on protective measures to implement, but there is some divergence over what measures to consider at specific concentration ranges. Ideally, each property could be evaluated on a case-by-case basis regarding soil type, soil condition and cover, crops grown, presence and age of children, property use, etc. Of course, this is not possible given the wide audience for the fact sheets so they arguably tend to be a bit cautious and generic in content. We have identified five fact sheets (which can be found here) based on discussion with colleagues routinely involved in urban gardening issues (as you might expect, we are including EPA’s two page fact sheet prepared for the Soil Kitchen project and the eleven page fact sheet recently published by EPA Headquarters). Regarding those prepared outside of EPA, this is not to imply they are “the best” but we were partial to the common sense approach presented.
For lead in residential soils, it is generally believed that the greatest risk is posed to children, particularly those under the age of three who exhibit hand-to-mouth activity or thumb-sucking, by ingestion of the soil or dust while in direct contact with the soils through playing or other activities. The threat depends on the soil levels encountered and the duration and frequency of the exposure. In unusual circumstances (e.g. high lead concentrations, bare soils, frequent contact and disturbance of soils, significant hand to mouth activity, poor hygienic habits), acute risks may be present for children, but more commonly it is a chronic risk from ingesting small amounts regularly over a long time, not through occasional activity. Although some uptake into plants can occur, and research in this area is ongoing, the added risk posed is usually considerably less than that of the direct ingestion pathway. In addition, we simply cannot predict the quantity of lead transferred to the plant due to a host of variables including the species of plant, soil type, soil porosity and density, pH of the soil, season of harvest, soil organic content, the soil’s ability to provide required nutrients, and edible portion of the plant. Trying to predict how great a dose someone receives from eating urban garden plants is even more complicated because the frequency and duration of garden crop consumption by that person would be needed.
The document below, taken from the 1998-2000 Boston Lead-Safe Yard Project (identified as EMPACT Lead Safe Yard Project) provides recommended actions for ranges of soil lead levels found in yards in older residential neighborhoods but was designed for different purposes based on neighborhood wide sampling efforts. It is a guideline only to introduce different tiers of exposure and possible risk reduction actions! The EMPACT project was well-funded and allowed for the collection of more than thirty samples per property, thus permitting a detailed sampling assessment of each property. Recall that we could only arrange for the analyses of one sample (grab or composite) from each yard for Soil Kitchen.
If you access additional fact sheets on “safe Pb levels in yards” or similar phrasing, you will see that organizations provide somewhat different soil lead ranges and recommended actions based on those ranges; some more, and some less protective. For instance, raised beds using “clean” soil could be used in areas with soil lead values in the 2,000-5,000 ppm range provided the soil where you will be standing or kneeling is adequately covered with mulch, woodchips, gravel, pavers, stone or some other cover to prevent dust and tracking of soil. Common sense is important.
It is not surprising that the EMPACT recommendations, like many fact sheets, are generally considered cautious since, as stated, they are prepared for wide distribution, must be relatively short in content and attempt to address the most susceptible scenarios. In addition, there may have been local factors involved in the setting of the EMPACT recommendations (We fully acknowledge that there are likely some who do not view the EMPACT table as cautious). Because of XRF limitations regarding arsenic and cadmium, we will discuss cadmium and arsenic more qualitatively.
Cadmium was not detected above the RL (80 ppm) in any of the samples. Any detection would have been surprising since typical cadmium levels in U.S. urban areas, barring close proximity to cadmium-releasing industries or other sources, are in the 1 to 5 ppm range. Cadmium exposure is of human health concern, regulatory risk-based screening levels in soil for direct human ingestion are generally below 80 ppm, and it can accumulate in plants. Had detections been found we would have urged the individual to call EPA or ATSDR to discuss the need for preventative measures based on soil sample location, soil cover, crops grown, property use, household demographics, and the like.
Arsenic was detected in only four samples above the RL. Values ranged from 38 to 110 ppm. The source for these elevated concentrations is unclear since urban arsenic sources are not as easily identifiable. One potential source of arsenic in soil is preserved wood treated with “CCA”, or chromate copper arsenate, a wood preservative to prevent rotting when exposed to weathering outdoors. These treatment chemicals are found to migrate from the wood to the soil. Regarding uptake of arsenic into plants, the four results reported in this sampling event should not present a plant uptake problem since arsenic, especially after “aging” in soil, does not easily transfer from soil to crops. Unfortunately, there are no established soil arsenic threshold levels denoting what might be harmful to different types of plants. What uptake does occur would likely be very difficult to measure.
From a direct soil exposure standpoint, the four soil arsenic XRF results reported are elevated and we would recommend the prudent public health measure that children should not frequent these areas. Residents with these four elevated arsenic values in their soil sample may want to call ATSDR or EPA to discuss the readings and incorporate the measures to minimize exposure appearing on the accompanying fact sheets.
In conclusion, we recommend that you take a practical approach on what measures to take based on your results. By far, pre-school age children are the most vulnerable to lead exposure. This is due to greater exposure to soil via play activities, greater hand-to-mouth behavior, increased absorption of lead into the body compared to adults, and the greater potential for having nutrient deficiencies that might increase lead absorption. Children’s developing neurological and other organ systems are very sensitive to absorbed lead. If your soil lead levels were in the high range, and you have young children frequently playing in the yard, seriously consider implementing protective measures that will reduce your family’s and especially your child’s exposure to the soil.
Thank you for the opportunity to be involved in Soil Kitchen. We truly found it rewarding.
View Soil Kitchen Results in a larger map
|Sample ID||Arsenic (As)||Cadmium (Cd)||Lead (Pb)|