Coastal Marine Environment Sustainability - Sustainable Practices in Geoenvironmental Engineering - page 287

as 2,3,7,8-TCDD or more simply as TCDD (2,3,7,8-tetrachlorodibenzo- p -dioxin) is consid-
ered to be one of the most toxic. The WHO (World Health Organization) has designated
this as a known human carcinogen. In general, CDDs ind their way into the land and
marine environment as waste discharges from some processes that employ chlorine, such
as paper mills that use chlorine bleaching processes (see Section 7.4 in Chapter 7), manu-
facturing of chlorinated organic intermediates, and water treatment plants. Generically,
CCDs are known as dioxins. They also occur naturally and are discharged when materi-
als containing such substances are degraded and combusted, as in forest ires. Dioxins
released as atmospheric emissions from forest ires, combustion of fossil fuels, and from
incinerators serving industrial plants and municipal and hazardous waste facilities will
subsequently be deposited onto land and water bodies at points distant from their sources.
In water bodies, they will be attached to the suspended and sedimenting particles and will
form part of the sediment. A limit of 3 × 10 −5 μg of 2,3,7,8-TCDD per liter of drinking water
(30 picograms per liter; i.e., 30 pg/L) has been set by the EPA.
Dioxins generally occur in the environment with chlorinated dibenzofurans (popu-
larly known as furans). They are ubiquitous, and have been found in all media: air,
water, soil, sediments, animals, and food (Johnson, 1992). They fall into the class of POPs
(persistent organic pollutants) and are bioaccumulative. Since they have a strong afinity
for soil and sediment particles, their presence in the media containing these POPs pose
signiicant health threats. To determine the risks resulting from exposure to these POPs,
many environmental regulatory bodies, such as the U.S. Environmental Protection
Agency (USEPA) have adopted an evaluation technique that uses t oxicity equivalency
factors (TEFs). This technique compares the toxicity of designated dioxins and furans
to that of the most toxic dioxin, i.e., the 2,3,7,8-TCDD. With this technique, the TCDD
establishes the height of the toxicity bar and is given a TEF value of 1, and all other
organic chemical compounds being scrutinized as assigned TEF values according how
toxic they are perceived to be. Determination of toxicity intake (or uptake) of a particular
organic chemical contaminant requires the use of toxicity equivalents (TEQs). A TEQ for
a particular organic chemical contaminant ingested is determined by the product of the
contaminant weight (in grams) and its assigned TEF, and the units are grams TEQ. As an
example, Makiya (1997) stated that 60% to 70% of daily intake of dioxins (approximately
3.5 TEQ pg/kg/day) is obtained from ish and shellish. The tolerable daily intake (TDI)
established in Japan is 4 TEQ pg/day per kg weight. The Japanese standard value for
soils contaminated with dioxins is 1000 pg-TEQ/g or less, whereas the corresponding
standard value for sediments is 150 pg-TEQ/g. This value drops to 1 pg-TEQ/L or less for
water—in accordance with the Ministry of Environment (Japan) standards. Sediments
containing dioxins that are more than the standard value are required to be treated. The
classical remediation techniques used are dredging and disposal on land—procedures
that are most often costly and sometimes prohibitive when dredged materials have to be
treated before land disposal.
8.5 Rehabilitation of Coastal Marine Environment
Several techniques have been tried and used in the rehabilitation of coastal marine envi-
ronments. Table 8.4 gives a short summary of some of the recent procedures used to
treat contaminated seawater and sediments and also measures taken to create coastal