Methods for Deriving Pesticide Aquatic Life Criteria - Reviews of Environmental Contamination and Toxicology - page 69

(2.2)

MPC = MPC (1 + K × 0.001 × 0.03)

water _ total water _ dissolved ppm

where
0.001 = conversion constant (g/kg)
0.03 = content of suspended matter (g/L)
K ppm = partition coefficient for suspended matter/water
and

(2.3)

K = K × f

ppm oc oc

where
K ppm = partition coefficient for standard suspended matter (L/kg)
K oc = organic carbon-normalized partition coefficient (L/kg)
f oc = fraction organic carbon (standardized at 11.72%)
and,

MPC

water _ total

(2.4)

NC =

water _ total

100

where 100 is a safety factor to account for mixture effects.
Both the German and Dutch methods depend on an assumption of a standard
concentration and composition of solids for determining solid-water partitioning.
Unfortunately, partition coefficients are highly dependent on the composition of the
solids and on the nature of the contaminant (Schwarzenbach 1993). Solid-water
partition coefficients can be underestimated if colloids are not removed from the
solution phase (Wu and Laird 2004). If partition coefficients are not specific to
sediments in a given sample, calculations of dissolved versus bound pesticides may
produce erroneous results. For example, Wu and Laird (2004) determined that
partition coefficients for chlorpyrifos [ O,O -diethyl O -(3,5,6-trichloro-2-pyridinyl)
phosphorothioate] in aqueous mixtures of six different smectites ranged from 45
L/kg to 6,846 L/kg. Burgess et al. (2005) found partition coefficients for nonylphe-
nol that ranged from 21.3 for cellulose to 9,770 for humic acid, indicating that even
if values are normalized to organic carbon content they may not produce applicable
partition coefficients. Selecting a single value from such a wide range to represent
the partitioning behavior for solids of all compositions makes little sense.
The Dutch methodology recommends normalizing ERLs to a specific pH, or to
base the ERLs on the relevant chemical species, for chemicals whose speciation,
and thus bioavailability and/or toxicity, depend on pH (RIVM 2001). This adjust-
ment would apply to weak organic acids, such as phenols. Degradation of com-
pounds and metabolite formation are also considered in this methodology.
Compounds with half-lives less than 4 hr must have the MPC derived from stable
degradates or metabolites.
The USEPA (1985) provides detailed instructions for determining acute and
chronic criteria where toxicity to two or more species is related to a water quality
characteristic (hardness, pH, temperature, etc.). This method is not only regularly
applied to metals criteria but also applies to pesticides that have pH- or temperature-
dependent toxicity. The key is that a demonstrable quantitative relationship must