Methods for Deriving Pesticide Aquatic Life Criteria - Reviews of Environmental Contamination and Toxicology - page 70

exist between toxicity and the water quality parameter. In such cases, criteria are
expressed as mathematical formulae that describe that relationship. The USEPA
“Guidelines for Deriving Numerical Aquatic Site-Specific Water Quality Criteria
by Modifying National Criteria” describes the water-effect ratio (WER) technique,
designed to account for differences in bioavailability, that is dependent on chemical-
physical characteristics of site water (USEPA 1984a), as follows:

Site _ Water _ LC

Water _ Effect _ Ratio =

50

(2.5)

Laboratory _ Water _ LC

50

The site-specific maximum concentration is then equal to national maximum
concentration multiplied by the WER.
The site water to be used in WER determination is to be collected under typical
conditions (i.e., not during floods or storms). However, because pesticide loadings to
surface waters typically result from storms or agricultural runoff, and because sus-
pended solid content is higher than normal during runoff events, it is best to express
criteria in terms that reflect the covariance of pesticides and suspended solids at the
time a sample is taken. The simplest method would be to derive criteria based on
dissolved concentrations (as is typically done), and then to use solids data, together
with measurements of total concentrations and partition coefficients, to determine
compliance. This could be achieved using the following equation, which is given in
RIVM (2001), for converting total concentrations to dissolved concentrations:

C

C =

total

(2.6)

dissolved

1 + ( KS )

where
C dissolved = concentration of chemical in dissolved phase
C total = total concentration of chemical in water
K = solid-water partition coefficient (L/kg); may be expressed as K oc /f oc
S = concentration of sediment in water (kg/L)
The resulting dissolved concentration is then compared to the water quality
criterion, to determine compliance.
In the Central Valley of California, levels of suspended solids vary greatly. The
US Geological Survey reports levels ranging from 1 mg/L to 330 mg/L, in sam-
ples from various streams in the Sacramento River Basin, and from 1 mg/L to
5280 mg/L, in the San Joaquin River Basin (USGS 2005a, b). For pesticides with
high sediment-water partition coefficients, bioavailability may vary considerably
with solids levels and, ideally, this factor should be considered in deriving water
quality criteria.
In addition to bioavailability, effects of other water quality factors should be
considered in deriving criteria. For organic chemicals, these factors include pH and
temperature. As described in USEPA (1985), if data are available to establish quan-
titative relationships between water quality characteristics and toxicity, then criteria
should be expressed as equations reflecting that relationship.