Use of toxicity assays for evaluating the effectiveness of groundwater remediation with Fenton’s reagent

A chemical dump site adjacent to the Danish North Sea holds a variety of constituents from pharmaceutical production including sulfonamides, barbiturates, aniline, pyridine, phenols, benzene, toluene, chlorinated solvents, lithium, copper, lead, mercury, etc. An on-going pilot scale project evaluates in situ chemical oxidation (ISCO) using modified Fenton’s reagent (H2O2 + chelated Fe2+) as a groundwater remedy.

Three injections were performed over a period to test treatment efficacy. Performance monitoring samples were collected from two depths both prior to and during treatment, and analyzed for toxicity and contaminant chemistry. Organisms exposed to a mixture will react to all contaminants present and, consequently, the toxic effect will represent a sum effect.

In contrast, chemical analyses yield information on individual or possibly groups of contaminants but not necessarily all the contaminants. Thus, using a combination of chemical analyses and toxicity assays yields a more robust understanding of the contaminated site and the risk it poses to the environment.

Ground water samples were tested via toxicity assay using algae, crustaceans, luminescent bacteria, nitrifying bacteria and yeast (estrogen screening test). Results from the baseline study showed that the two most sensitive species were the marine diatom Skeletonema costatum and the marine copepod Acartia tonsa.

It was found necessary to dilute untreated groundwater samples up to 3400 times to reduce the short-term toxicity to the LC10 level. Samples from the upper layer were 2-4 times more toxic than samples from the lower layer. Applying a safety factor of 10 on these results and combined with a dilution model for the recipient indicated that the ecosystem in an area of the North Sea of approximately 1x7 km is affected by groundwater flow from the contaminated site.

Chemical analyses showed that PCE and toluene concentrations up to 137 and 60 mg/L, respectively, in the upper layer Total hydrocarbons were up to 94 mg/L. Sulfonamides and barbiturates were found at 600 μg/L and 200-400 μg/L, respectively. After the second treatment with Fenton’s reagent the toxicity had increased and now needed 7100 times dilution to reduce toxicity to the LC10 probably due to mobilization of metals.

It is concluded that toxicity assay is a useful tool for evaluating samples from contaminated sites and that toxicity assays and chemical analyses supplement and support each other.