Bioavailability Studies to Improve Risk Assessment at SRS

What is Bioavailability?

Bioavailability: Is the degree of ability to be absorbed and ready to interact in organism metabolism. (U.S.EPA)

Previous studies have confirmed that bioavailability is a major factor in toxicity of environmental samples (Umbreit et al., 1988). The soil matrix is an important factor in determining a contaminant’s bioavailability. CRESP is developing methods to use bioavailability information to improve risk assessment at DOE sites.

Bioavailability Definition

Why Study Bioavailability

Bioavailability data can:

• provide realistic information on potential health effects of contamination
• modify defaults using site specific data
• change proposed clean-up levels saving time and money to reach acceptable levels
• help prioritize sites for subsequent evaluation

Methods to Estimate Bioavailability

• Site specific analysis can provide important information that can impact a risk assessment
• In soil, a contaminant’s bioavailability can be estimated in several ways:
  • Extraction using acids and bases
  • Extraction using biological fluids
  • In vitro assays using bacterial systems
  • In vivo assays using aquatic organisms or mammals
  • Statistical estimation methods

Savannah River Site

• L-Area Rubble Pile (LRP) was a surface depository of general trash. The unit consists of multiple piles of rubble. Visible contents includes scrap metal, construction debris, railroad ties, electrical wiring, and unlabeled cans and bottles.
• LRP was sampled in a preliminary site assessment and found to contain metals, PAHs and PCBs.
• CRESP took parallel samples of the seven definitive locations to examine bioavailability and bioaccessibility of metals located at LBRP.

CRESP STUDIES

• Hypothesis: bioavailability of soil contaminants will impact risk assessment?
• Site specific analysis can provide important information that can impact decisions in a risk assessment such as:
  • Selection of chemicals of concern
  • Recommended level of risk
  • Level of risk
  • Remedial solution
  • Regulatory decisions
• Can have broader implications in identifying most efficient and cost effective method for bioavailability assessment
• Can provide information to identify and prioritize sites most impacted by bioavailability data

Study Design: Four assays performed on site samples

In Vitro Procedures

Gastric -vs Intestinal Solubility

In Vitro Bioaccessibility

• EPA standard methods use high concentrations of harsh acids and bases to extract metals from soil (Fed Reg, 1995)
• This approach generally overestimates bioavailability
• CRESP and EOHSI have developed a two step procedure to extract metals using gastric fluids and saliva

In Vitro Bioaccessibility Data Points

• Solubility of metals within gastric fluid
• Solubility of metals in intestinal fluid, where absorption is likely to occur
• Total mass balance for recovery verification
• Availability of metals from contaminated soil using biological fluids

Soil Analysis Data Points

• Total amount of metals in soil
• Maximum availability of metals
• Baseline for determining percent bioavailable using gastric extraction
• Provides basis for comparison

Importance of Bioaccessibilty Data

• More realistic estimate of bioavailability than chemical extraction
• Less laborious and less expensive than in vivo animal bioavailability studies
• Previous studies have demonstrated that this is a conservative estimate that shows consistency with known bioavailability of metals in soils.

In vitro Bioavailability Procedures

• Soils are diluted
• Flourescence of organisms is measured
• Light output is related to toxicity and bioavailability

Bioavailability Data

• The Microtox® Solid-Phase Test evaluates bioavailability by relating the concentration that comes off a soil particle in aqueous solution to toxicity
• This can be used to screen sites to determine which risk assessments may be impacted by more thorough bioavailability evaluation

In-vivo Bioavailability Data Points

• The Japanese medaka embryo larval assay (ELA) is designed to examine the toxicity of soil.
• Toxic endpoints include lesion occurrence, embryonic development and survival.
• Soils can be tested directly, or eluted at various pH’s or using common solvents

Chemical Analysis of Soil

• Samples were filtered and acidified in nitric acid.
• Samples were analyzed for heavy metals using a Graphite Furnace Atomic Absorption Spectrometer

Importance of Bioavailability Data

• This test can provide information on the impact of pH and solvent extraction on toxicity
• This provides information on the concentration of contaminants likely to be bioavailable.
• This can be used to identify sites that have both toxic concentrations of contaminants which are modified by bioavailability data.

Risk Assessment Using Bioavailability Data

• Information: Comparison of these four methods will provide a range of information regarding bioavailability of metals at LBRP
• Method Evaluation: Will enable determination of efficient and cost effective screen for site bioavailability
• Decision Points: Integration of bioavailability will allow assessment decisions to be compared with and without this information

Metal Availability