Test Organisms Selected for Organic Contaminants in Dredged Material

ERDC TN-DOER-R3

September 2004

Four exposure pathways for HOCs are distinguished in plants (Wild et al. 1992): (1) passive root

uptake (with the transpiration flux) from the soil solution, (2) leaf uptake of compounds evapo-

rated from the soil surface, (3) direct contact between roots and/or leaves with contaminated soil

particles, and (4) active uptake through oil channel systems in the roots. The first two routes are

common, the last two routes pertain to specific conditions and/or plant species (Ryan et al.

1988). Uptake route 1 is relevant for HOCs with a low volatility, uptake route 2 for HOCs with a

high volatility (Ryan et al. 1988). Passive root uptake from the soil solution is similar to the par-

titioning of the compound between water and solid phase. Root uptake is usually related to the

measure for lipophily, the octanol/water partitioning coefficient (log KOW). Root-to-shoot trans-

location is generally related to the log KOW as well as the evapotranspiration rate. Translocation

of HOCs with a low log KOW (<4) is likely. HOCs with a log KOW ≥ 4 are well-adsorbed to soil

particles, organic matter and plant roots, and are not very mobile in the soil solution. The total

root uptake depends on the average concentration of the HOC during the whole exposure period

of the plant up to harvesting. Based on log KOW characteristics of various HOCs, Ryan et al.

(1988) conclude that PAHs (e.g. benzoapyrene, log KOW of 6.4) and PCBs (e.g. Arochlor 1254,

log KOW of 6.0) will bind strongly to the root surfaces, but may not be transported from roots to

shoots. No evidence currently exists on the relative sensitivity of mono- versus dicotyledonous

plants for HOCs (Fletcher et al. 1987, Hund and Traunspurger 1994).

Earthworms are suitable bioaccumulation and response indicators for metals as well as for HOCs

(ASTM 1998, Kula and Larink 1998, Lokke and Van Gestel 1998). Toxicological effects in this

test organism originate largely from direct skin contact with the toxic compounds in the intersti-

tial water. Chronic sublethal tests are preferred above acute toxicity tests since the former gener-

ate information not only on toxicity effects on biomass but also on body burdens. These tests

currently exist (Doube and Schmidt 1997, Van Gestel 1997).

Frequently used measures to describe toxicity are: the lethal concentration (LC), i.e. the concen-

tration of a toxicant that kills a specified percentage of the organism; the effective concentration

(EC), i.e. the concentration of a toxicant that produces an observable negative effect in the

organism; and the phytotoxicity threshold (PT), i.e. the contaminant tissue concentration of a

plant that corresponds with a defined growth reduction (ASTM 1994; ASTM 1998). The impor-

tance of phytotoxicity data for estimates of the risk posed by the soil contamination is evident

(Benenati 1990), but plant data are currently not widely used in setting policy for soil or sedi-

ment cleanup limits. For the beneficial use of dredged material, phytotoxicity should be mini-

mized since it inhibits the biomass production of vegetation on the soil.

Test Organisms Selected for Organic Contaminants in Dredged Material: The

monocotyledonous L. perenne (perennial ryegrass) was selected for its worldwide use and gen-

eral acceptance in standard test procedures (USEPA 1996; USEPA 1999). It has a wide geo-

graphical distribution, rapid growth, and profuse generative reproduction. In addition, its seeds

germinate simultaneously within several days and the species can be cultivated in the testing

environment. L. perenne is relatively tolerant towards HOCs, and is used widely as a response

and bioaccumulating indicator for HOC contamination of soils (Gorsuch et al. 1990; Van de

Leemkuile et al. 1998; Malmberg et al. 1998). A related species, Lolium multiflorum (ryegrass) is

widely used in Germany as a bioaccumulating indicator for organic contaminants