Heavy metal contamination occurs in many marine and freshwater environments as a result of industrial activities. Primary metals of concern in these environments include copper, zinc, cadmium, mercury, and lead followed by chromium. The mobile, soluble forms of these metals are generally considered toxic as they easily pass through cell walls, are bioavailable to benthic community members, and can bioaccumulate. Precipitation of these heavy metals can reduce the toxicity of contaminated samples by moving the metal from the aqueous phase to a solid, often a less bioavailable phase.
The toxicity of these metals is not well defined given that metal bioavailability and toxicity varies among macro and micro organisms and between plant and animal species. Determining toxicity must account for a variety of combinations of these metals and environmental factors such as pH, salinity, and general hardness of the water. In general, it is known that constant exposure to one or combinations of these bioavailable metals often leads to detrimental environmental effects.
The objective of this project was to evaluate toxicity and bioaccumulation in several benthic community surrogates exposed to sediments with and without amendments currently used for remediation of mixed heavy metal contamination at Department of Defense sites.
Based on a literature review, apatite, an inorganic calcium phosphate amendment, and additional organic components such as chitin and acetate were evaluated. In the first year, geotextile mats containing apatite and the organoclay, bentonite, were incorporated into the toxicity examinations in non-contaminated sediments. All amendments were examined singly and in combination for their potential ecological impacts and for their capacity to sequester and immobilize metals.
To assess impacts to benthic communities, a suite of laboratory marine toxicity and bioaccumulation tests were used. The benthic community members were the marine amphipods (Eohaustorius estuarius), marine polychaetes (Neanthes arenaceodentata), and purple sea urchins (Strongylocentrotus purpuratus), as these species are commonly employed as surrogates for general benthic community health. Larval sheepshead minnows (Cyprinodon variegatus) are a fish species known to be associated with surficial sediments and represent potential impacts to vertebrates.
A combination of biological (standard bioassay and bacterial data) and chemical (mineralogical and elemental composition) measurements were employed to detect how metals are partitioned to assess metal bioavailability, immobilization, and sequestration. The microbiological benthic community was examined to determine how the bacteria potentially affected the bioassay data as well as to examine bacterial species prevalent under varying sediment amendments.
Laboratory toxicity testing used in this project successfully demonstrated that apatite, organoclay, and geotextile mats containing those materials were non-toxic in sediments at expected field application concentrations. Exposures were conducted on four different species (encompassing six different endpoints) representing a range of organism classes, life histories, and feeding approaches. Increased lethality and even beneficial effects (e.g., enhanced growth) were observed with acetate and chitin. Further analysis demonstrated that the static nature of the bioassay exposures likely contributed towards some of the observed negative effects. For example, increased ammonia concentrations were associated with increased bacterial growth due to acetate and chitin. This observation of enhanced ammonia in standard toxicity tests also has been observed in the evaluation of other amendments such as coal fly ash used for the removal of polycyclic aromatic hydrocarbons. Nevertheless, questions still remain as to whether these kinds of effects are representative of what might occur in the field.
Subsequent testing with decreased concentrations of chitin demonstrated that chitin may stimulate a smaller increase in bacterial numbers that can potentially increase the effectiveness of the apatite amendment for apatite sequestration of the heavy metal zinc.
Based on this research, the amendments apatite, chitin, and geotextiles containing apatite and organoclay (in suggested concentrations) are considered non-toxic to marine invertebrates in marine sediments. The project was innovative in that it examined both the macro and micro biological benthic community after amendment additions, and it helped determine the amendment combinations with the least harmful effects on benthic communities.