C. Raised Beds vs. In-Ground Gardens

Overall, raised bed gardens were found to have less metals than in-ground gardens (gardens where crops are planted directly into the existing ground).  Specifically, the average of all metals was lower in raised beds, with the median (middle value) being lower in raised beds for all of the metals but zinc.  Also, the gardens with the greatest concentration (or ppm, parts per million) of each metal were in-ground gardens.  Surprisingly, for the metals that have their lowest concentrations above "undetectable," in-ground gardens also had the lowest concentrations for 7 of these 8 metals.

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69 raised bed & 22 in-ground gardens were tested.  The remaining 9 gardens are not included in the table above because
the soil collected from these gardens came from both in-ground and raised bed areas. Beryllium, mercury, molybdenum, selenium, silver & thallium are not shown because 96% or more of the gardens had levels less than the amount that the laboratory could detect. For gardens with levels that were "undetectable," i.e., those with a concentration less than what the laboratory could detect, the concentration was considered to be 0ppm.  

Averages & "Safe" Levels by Garden Type

The document below includes the standard error, how averages were calculated, Human Health Screening Levels (HHSL), and other technical details.  Refer to this page to see other "Safe" levels as determined by the U.S. EPA and the State of California. 

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More Project Details:

A.  Project Summary

B.  Project Demographics

C.  Raised Beds vs. In-Ground Gardens

D.  Distribution of Metals by Zip Code

E.  Addressing Environmental Justice