Geochemical Evaluation of Metals: How to Distinguish Contamination from Natural Background

Answering the “Why”

At APTIM, we have a saying: “Statistics can tell you if concentrations are high, but statistics can’t tell you why.” When it comes to metals data, we use geochemical evaluation to answer the “why.”

When only using the standard statistical approaches to identify metals contamination at a particular site, or when only comparing site concentrations against regulatory screening levels, it is difficult to distinguish actual metals contamination from what may be high background concentrations that naturally occur in the area. Why does this matter? Property owners may be required to perform more rigorous investigations or even remediation at sites where it appears there is significant metals contamination, while in reality the concentrations could simply be naturally elevated and therefore not require additional investigation or remediation.

The problem lies not in the data collected but in the subsequent data analysis—by only using statistical methods or comparing to regulatory screening levels, the well-known geochemical processes that control metals concentrations in soil, sediment, groundwater, and surface water are ignored. But metals occur naturally in these media, sometimes at levels that may cause concern. Geochemical evaluation can reveal why we have elevated metals concentrations in our samples, and it may demonstrate that those concentrations are naturally occurring.

Understanding Metals in Environmental Media

APTIM’s Geochemical Evaluation Approach

Since 1998, APTIM geochemists have understood that trace elements are naturally associated with a limited number of minerals in the soil and sediment matrix (or with specific suspended particulates in groundwater and surface water) under a given set of environmental conditions. In uncontaminated samples, these associations yield positive correlations between trace elements and the major elements present in the host minerals (e.g., arsenic versus iron). Potentially contaminated samples are identified by their anomalously high elemental ratios. For groundwater and surface water, other factors are considered during the evaluations, such as pH effects, reductive dissolution, redox gradients, and salinity gradients.

Proven Results Across Complex Sites

APTIM’s geochemical evaluation techniques allow us to distinguish between naturally elevated metals concentrations and potential contamination without performing geochemical modeling or acquiring specialized analytical data. Over the past 28 years, APTIM has successfully applied these techniques to bring clarity and cost efficiencies for clients at hundreds of investigation sites across the US, US territories, Mexico, Canada, and Australia. This has enabled more targeted site investigations (toward actual site-related contamination); reduced unnecessary remediation; and helped clients make informed, defensible decisions based on sound scientific analysis.

Industries & Site Types Served

A wide spectrum of clients has been served using APTIM’s geochemical evaluation methodologies.

• Oil and gas industries
• Chemical manufacturing facilities
• Transportation departments
• Landfills
• Power-generating facilities
• Military installations (active and former)
• Department of Energy facilities
• Water treatment and wastewater treatment facilities
• National Park sites (including waste accumulation areas)
• Other industrial, federal, and commercial properties

Addressing Complex Geology & Regulatory Challenges

Many sites we have evaluated are characterized by complicated geology, hydrogeology, and groundwater geochemistry. APTIM considers all available data to identify the geochemical mechanisms controlling metals concentrations in environmental media. We place the data in context to provide defensible conclusions for further investigation, remediation, or site closure.

Our services have also been sought during the development of guidance documents (e.g., Interstate Technology & Regulatory Council; corporate clients) and international standard guides (ASTM International). Those documents highlight the importance of incorporating geochemical evaluations during background studies, site-to-background comparisons, and other applications.

Common Metals & Background Evaluation Considerations

APTIM’s geochemical evaluations answer many common, practical questions—like the ones below—that often require detailed technical considerations.

• Regulatory screening criteria for some metals are low; in many places, they are lower than natural background concentrations. This begs the question: do we really have metals contamination at our site, or just naturally elevated metals?
• Statistical tests claim that we have site-related contamination. How can we be sure about that?
• We need a facility-wide background data set. Are the collected background samples truly representative of background conditions? Can historical samples collected near or within investigation site boundaries be included? How do we justify excluding any candidate background samples as being unrepresentative of background?
• Statistical outlier tests flagged some metals concentrations in our data set as outliers. Are these outliers actually just naturally elevated background concentrations?
• Can we combine apparently different data sets (e.g., samples from different depths, different areas, and/or different soil types) into one larger data set?
• Have we fully characterized the extent of contamination at our site – both laterally and vertically?
• Have we excavated all the contaminated soil, such that we are confident only unimpacted soil remains in place?
• How can we compare new groundwater sample data against baseline data, beyond using standard statistical methods, to show that we have cleaned up groundwater?
• Within our long-term monitoring program, can certain metals be removed from the analyte list? Can any monitoring wells be removed from the program, by demonstrating that the groundwater at those locations is uncontaminated?
• Why are certain metals elevated in groundwater when the only known contaminants were organic compounds? Are redox conditions affecting the metals concentrations in site groundwater?
• The total chromium concentrations in our groundwater samples were conservatively assumed to represent hexavalent chromium. Can we confirm or rule out that assumption using our existing data?
• What types of analytical data should we collect to answer these questions?

When metals concentrations raise questions about background conditions or site-related impacts, APTIM’s geochemical evaluation approach helps bring clarity and technical defensibility to the process. Learn more about how APTIM’s Lab R&D capabilities support geochemical evaluations and strengthen regulatory decision-making.

Learn More About Our Lab R&D Capabilities