Mixed Contamination of Polyaromatic Hydrocarbons and Metals at Manufactured Gas Plant Sites: Toxicity and Implications to Bioremediation - Environmental Contamination: Health Risks and Ecological Restoration - page 362

PAHs.and.metals.on.soil.urease.activity.depended.largely.on.the.incubation.
time..However,.concentrations.of.0.1.mg/kg.phenanthrene.and.10.mg/kg.Cd.
applied.to.soil.is.insuficient.to.arrive.at.a.conclusion.on.the.toxicity.toward.
urease.activity.
Strongly.complexed.metals.are.less.toxic.than.weakly.complexed.forms,.
which.in.turn.are.less.toxic.to.biota.than.the.free.ions.(Adriano.2001)..The.
co-occurrence. of. Cd. with. Zn. or. Pb. (Cd-Zn. or. Cd-Pb). is. common. in. cer-
tain.contaminated.environments.due.to.their.origin.from.similar.sources..
However,.the.interaction.of.these.metal.combinations.on.PAH.remediation.
is. not. yet. known.. Also,. the. inluence. of. heavy. metals. on. biodegradation.
of. PAHs. depends. upon. the. fraction. of. bioavailable. metal. concentration.
rather. than. the. total. concentration.. Determining. the. metal. speciation. is.
therefore.important.to.understand.the.real.toxicity.of.metals.to.biota.and.
thereby. bioremediation. in. PAH-metal. cocontaminated. soils.. Thavamani.
et.al..(2011).used.visual.MINTEQ.(Ver.2.52).computer.software.(Gustafsson.
2009).and.determined.for.the.irst.time.the.metal.speciation.of.soil.solutions.
(water-extractable).from.a.former.MGP.site..The.water-extractable.Zn.was.
the.highest.followed.by.Pb.and.Cd..More.than.80%.of.total.Pb.complexed.
with.DOC.and.the.inorganic.ligand.sulfate,.and.the.calculated.speciation.
showed.the.lowest.free-ion.activity.for.Pb 2+ ..On.the.other.hand,.Zn.and.Cd.
that.are.characteristic.in.having.high.mobility.showed.the.same.pattern.of.
speciation.irrespective.of.soil.pH..These.results.warrant.the.consideration.
of.free-ion.activity.of.metals.while.assessing.risk.of.the.MGP.sites.
NecessityforRemediationofMGPSites
The. MGP. sites,. which. were. once. welcome. additions. to. the. cities. and.
towns,. are. now. sources. of. complex. environmental. contamination. prob-
lems.and.are.slowly.being.cleaned.up..Industrialization.led.to.urbanization,.
and. city. growth. eventually. placed. heavy. demand. for. residential,. recre-
ational,. and. commercial. areas. to. be. developed. in. the. erstwhile. industrial.
zones.. The. estimated. land. use. patterns. around. MGPs. are. the. following:.
industrial/commercial. (50%),. residential. (30%),. and. recreational. (20%). (US.
EPA. 2004).. The. abandoned. MGPs. were. therefore. revisited,. and. the. waste.
material.contamination.in.those.sites.was.well.recognized..The.presence.of.
mixed.organic.(e.g.,.PAHs).and.inorganic.(e.g.,.metals).contaminants.in.the.
sites. limits. the. application. of. various. remediation. technologies. to. cleanup.
because.of.the.complex.nature.of.the.contaminants..Excavation.and.disposal,.
containment.and. in situ .technologies,.and.soil.washing.are.the.three.broad.
categories.of.remediation.techniques.used.often.in.MGP.sites..Remediation.
of. MGPs. had. commenced. in. the. past. 30. years,. and. much. focus. has. been.
on.single.contaminants.such.as.PAHs,.BTEX,.and.chlorinated.hydrocarbons..