Environmental Engineering Reference
In-Depth Information
detailed ecological risk assessment is not warranted. This overlooks the pos-
sibility, and in extreme cases, the likelihood that there are no resources present
because site contamination prevents their presence and survival. Comparison
of the site to a reference area with similar physical attributes, except for con-
tamination can correct this fallacy, and in some cases if the sites are structurally
similar but sensitive resources are plentiful at the reference site and absent at
the other site, it can reveal elevated risk.
The second type of screening addresses the level of contamination and
is more quantitative. The first step in the screening-level exposure estimate
(Figure 7.1) is to assemble the analytical data for each media (e.g., water, soil,
sediment). There should be contaminant concentration data to represent the
exposure pathways for each ecological measurement endpoint, and if there
is none, additional data collection is necessary to accomplish the screening.
The next step is to identify a generic and conservative (i.e., tendancy to over-
estimate rather than underestimate the risk) ecological benchmark for each
environmental media that represents the maximum concentrations protec-
tive of the measurement endpoint. For example, if the measurement endpoint
is sustainable brook trout population, the benchmark for exposure to sur-
face water would be the ambient water-quality criteria for chronic exposure
(U.S. EPA 2013). The benchmarks for each endpoint and each media are then
compared with the measured site concentrations to determine which, if any,
contaminants pose no risk; pose a severe risk; or warrant more investigation.
These comparisons can be made and presented as calculations of bench-
mark or hazard quotients. The quotients represent the measured site con-
centration divided by the benchmark; thus if the measured concentration is
equal to the benchmark, the quotient will be 1.0. Values substantially higher
than 1.0 indicate significantly elevated risk, and values well below this mark
indicate that the particular media, contaminant, and pathway do not pose a
risk and can be screened out from further consideration in the detailed eco-
logical risk assessment. Benchmark quotients between these extremes typi-
cally warrant additional investigation (see Section 7.2.6). The cutoff quotient
value between no elevated risks, need for investigation, and substantial risks
are site, receptor, contaminant, and quality of data specific and dependent. If
there is a large data set covering the entire site at a high density, a compari-
son of the benchmark with the mean site concentration revealing a quotient
below 1.0 frequently indicates the contaminant can be screened out. In contrast,
if the data set is sparse and there is uncertainty in the appropriateness of the
benchmark to the specific site, a quotient comparing the benchmark with the
maximum site concentration just slightly above 1.0 could indicate the need for
additional investigation.
Table 7.2 presents a hypothetical screening of contaminants for a specific
media (water) and endpoint (survival, growth, and reproduction of a fresh-
water fish community). The appropriate benchmark is the chronic water-
quality criteria because the endpoint addresses all life history aspects. If
survival was the only endpoint, the acute water-quality criteria might be
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