Technique Used to Distinguish Natural Background from Human-Caused Enrichment of Trace Elements in Soils

Science Center Objects

Human activities can enrich toxic trace elements like uranium and arsenic in the environment, but these elements also are from natural sources and occur at background levels. Scientists utilized a technique that identifies the background and the elemental fingerprint of human-caused enrichment and tested the new technique on data collected near uranium mines in Arizona.

 

Mineral extraction provides important raw materials for a variety of products in modern society and is often perceived as a source of contaminants to ecosystems. However, it can be challenging to distinguish what portion of the trace elements in the environment are from natural sources or from human activities. The enrichment factor is a measure used for determining how much the presence of an element has increased relative to average natural abundance because of human activity.

U.S. Geological Survey employees sampling soil

Soil sampling near the Kanab North uranium mine site in Arizona.

(Credit: Katie Walton-Day, U.S. Geological Survey. Public domain.)

 

Calculation of an enrichment factor requires the selection of a natural background composition and a reference element, but these choices can strongly affect the result of the calculation. Previous efforts to understand sources of trace element enrichment were challenged by the lack of representative natural background composition to compare to samples that may be altered by human activities.

 

U.S. Geological Survey employees sampling the soil

Soil sampling near the Kanab North uranium mine site in Arizona.

(Credit: Katie Walton-Day, U.S. Geological Survey. Public domain.)

To address this challenge, U.S. Geological Survey (USGS) scientists used compositional data analysis as a tool to determine the selection of natural background compositions and reference trace elements in soils. The methodology involves examining patterns using multiple subsets of data to understand enrichment caused by human activities and to identify one or more elements that most efficiently trace the cause of the enrichment.

 

The methodology was used on data from samples collected near uranium mines in Arizona to demonstrate the approach. The data came from three sources: existing breccia pipe uranium ore deposit data from seven sites, composite regional geochemical background soil data from the literature, and data from surface soil samples collected for this study. Through a series of analyses of 39 elements, uranium was identified as the best individual tracer for these samples to help trace the enrichment back to its source. Sulfur, arsenic, molybdenum, and copper were also enriched but to lesser degrees.

 

This method provides an approach that integrates information from multiple trace elements and demonstrates practical benefits including the ability to identify one or more elements best suited to distinguish enrichment caused by human activities from background composition. This methodology can be applied as a tool to inform mitigation efforts to reduce human and wildlife exposure to contaminants. This study was funded by the USGS The Environmental Health Program (Contaminants Biology and Toxic Substances Hydrology) of the U.S. Geological Survey Ecosystems Mission Area.