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IL SUOLO
DEFINIZIONI
Un suolo eredita caratteristiche chimiche dalla roccia
madre dal quale si e’ evoluto.
Per esempio suoli nei quali i backgrounds di Ni, Co, V,
Cr, Fe, sono elevati riflettono rocce madri costituite
minerali femici, ad esempio peridotiti, pirosseniti,
gabbri, basalti.
Un suolo povero nei menzionati elementi, e
caratterizzato da elevati backgrounds di U, Th, Zr, Nb,
Ta, La, Ce si e’ invece plausibilmente formato da rocce
madri di tipo granitoide.
Analisi dei suoli, vengono quindi effettuati di prassi
durante prospezioni minerarie, spesso concentrando le
analisi nella frazione fine dove generalmente si
concentrano gli elementi metallici che vengono ripartiti
preferenzialmente nei minerali argillosi e in
ossidi/idrossidi di neoformazione.
Anche l’approccio geobotanico viene talvolta utilizzato
anche nelle prospezioni minerarie, in quanto le radici di
alcune piante bioaccumulano campionando aree e
volumi superiori a quelli osservabili in superfice e
dando cosi’ notevoli informazioni sulla natura geologica
del substrato.
Al fine agricolo il miglioramento delle caratteristiche dei
suoli e’ praticato con la somministrazione di nutrienti
addizionali (fertilizzanti).
Fra le pratiche in uso si annoverano anche
spargimento di letame, fanghi degli impianti di
depurazione, percolati da discarica, sedimenti dragati
da porti e canali.
Questi possono contenere un ampio spettro di elementi
tossici come As, Cd, Hg, Mo, Co i quali possono poi
essere mobilizzati ed entrare nell’ecosistema.
Per esempio i fanghi provenienti da discariche, o i
sedimenti dragati da canali inquinati si trovano spesso
in condizioni riducenti. La mobilita’ dei metalli in essi
contenuti diventa notevole quando sono pero’ esposti
in ambiente ossidante.
Ne emerge che il potenziale ossido riducente Eh e il pH
sono fattori chiave nel determinare il comportamento
chimico di queste sostanze.
Le piante possono mobilizzare e
trasportare metalli bioaccumulando,
trasferendo cosi’ le sostanze inquinanti
a livello superiore nella catena
alimentare di cui gli esseri umani sono
generalmente gli ultimi consumatori.
Negli ambienti naturali vengono definite
piante accumulatrici quelle che
concentrano metalli sino a 100 1000
ppm (sul peso secco) e come piante
iper cumulatrici quelle che contengono
oltre 1000 ppm di metalli.
Alcune piante possono quindi
rappresentare dei biomarkers
(biocontrollori) del contenuto di elementi
potenzialmente tossici nei suoli.
Per confrontare dati relativi ad aree
diverse, le analisi devono essere riferite
alla medesima specie, ed inoltre alla
stessa parte della pianta (es. foglie,
aghi, ramoscelli), in piante aventi la
medesima eta’.
Tali proprieta’ delle piante possono
tuttavia essere usate per risolvere
problemi ambientali su terreni
contaminati.
Si parla di fitodepurazione
(phytoremediation) quando si impiegano
specie vegetali in opere di bonifica
sfruttando cosi’ complesse relazioni che
avvengono fra apparato radicale delle
piante, microorganismi e suolo. La
biomassa raccolta, ricca di contaminante
puo’ essere poi trattata in sicurezza
mediante operazioni di essiccamento,
polverizzazione e stoccaggio. Il volume
dei rifiuti prodotti risulta di gran lunga
inferiore rispetto ad altre metodologie di
bonifica che sono anche piu’ costose.
L’efficienza dei processi di fitodepurazione e’
determinata da tre fattori:
● biomassa prodotta dalla pianta
● fattore di bioaccumulazione
Fa= (Me)pianta/(Me)suolo
● volume del suolo esplorato/campionato dagli
apparati radicali
Si devono pero’ considerare alcuni limiti specifici
di tale metodologia, che derivano dalla necessita’
di individuare condizioni ottimali per lo sviluppo e
crescita dei vegetali, talora difficilmente ottenibili
nelle condizioni chimico fisiche dei siti inquinati,
nonche’ dalla necessita’ di avere a disposizione
ampi margini di tempo per il completamento delle
operazioni di bonifica.
Una altra limitazione della fitodepurazione e’
inoltre inerente alla profondita’ massima alla
quale puo’ spingersi l’apparato radicale.
• Teresa Fan at UC Davis is studying how plants can be used to remove toxic wastes from soil.
Using Plants for pollution cleanup
• Scientists are studying how plants can be used to bind up soil pollution found at national nuclear laboratories and nuclear power plants, where radioactive and other toxic wastes may reach groundwater.
• Some plants extract a variety of different chemicals into the soil, some of which act as signals to soil organisms.
• The challenge is to find out how plants release these chemicals and how these chemicals interact with microbes and soil.
• Source: UC Davis Magazine Spring 2002
• Plants, soil, and microbes in the soil work together to determine which metals and nutrients plants take up from the soil.
Anche la fauna puo’ fornire dei
biomarkers. Ad esempio i gusci di
gasteropodi e bivalvi sono generalmente
considerati dei biocontrollori della
qualita’ dei sedimenti al contatto dei
quali vivono.
A riguardo, studi epidemiologici hanno
talvolta trovato evidenze di alti valori di
concentrazione di elementi metallici (es.
Pb, Cd) in sangue ed urine di persone
che vivono in aree minerarie.
Anomalie geochimiche possono essere
trasferite nella catena alimentare di
uccelli, mammiferi, e ovviamente anche
nell’uomo.
Abstract
In this study, metals (Be, Cr, Mn, Fe, Ni, Cu, Zn, Ag, Cd, Pb and Hg) in
the fine-grained fraction of sediments(<63 μm) from 12 sites at different
locations in northern San Francisco Bay over a year period from March
2000 to March 2001 were analyzed after acid extraction. The results
showed that metal concentrations in the sediments varied from site to
site, whereas some of them were found elevated with respect to the
sediment of Tomales Bay, CA, which has little contamination history,
indicating an enrichment of the metals in the sediment samples
analyzed. Sediment toxicity and bioaccumulation evaluation by a clam
species, Macoma nasuta, exposed to the sediment samples collected
from the six sampling sites was carried out. The results showed that the
sediment samples tested significantly reduced clam survival. Toxicity of
the sediments to the clam was, in part, related to elevated metal
concentrations in the sediments. In order to examine geochemistry of
the metals and to understand potential correlations between metal
concentrations and geochemical matrix elements of the sediments,
bioavailability and toxicity of the metals, detailed analysis of metal
concentrations associated with total organic carbon and the Fe-oxy-
hydroxides in the sediment samples was performed. The analysis
showed that sediment geochemistry appeared to influence metal
bioavailability and may have important impacts on the toxicity of these
metals to the clam.
Introduction
Contamination in estuarine sediments by heavy metals has been a
problem since these environments often receive heavy metal wastes
generated naturally through the weathering of rocks and through a variety
of human activities. In estuarine environments, heavy metals discharged
from sources such as industrial and sewage effluents may accumulate in
bottom sediment as the suspended particles on which they are adsorbed
settle out.
Consequently, heavy metal concentrations in sediments near large
population centers are often significantly higher than those in the
sediments that have little history of contamination. Understanding of
ecological and environmental consequences of heavy metals in estuarine
sediments is a complex problem, but one that must be thoroughly
investigated to make good decisions for protecting exposed aquatic
habitats.
Bivalves such as mussels and clams have been used for heavy
metal monitoring programs in marine-estuarine ecosystems,
because of their ability to accumulate and tolerate exposure to
heavy metals and other pollutants in water and sediment. The clam
Macoma nasuta, ingesting sediment and extracting organic matter, is a
sediment feeder resident in San Francisco Bay and one of the dominant
clam species in the region. Its ubiquitous occurrence and relatively large
size makes the clam well suited for bioaccumulation studies ( US EPA,
1989). However, uptake of metals from sediment is highly dependent on
biological and geochemical factors. Among geochemical factors, organic
carbon, temperature, pH, dissolved oxygen, sediment grain size and
hydrologic features of the system are important in influencing the uptake
of metals from sediment.
The objectives of this study were (1) to investigate the concentrations of
heavy metals in the surface sediments in the northern San Pablo Bay
area (2) evaluate toxicity of the sediments, (3) quantify metal
bioavailability and bioaccumulation, and (4) identify geochemical matrix
elements, that have the greatest influence on the bioavailability of heavy
metals. Overall, this work examined the sources, pathways and sinks of
metals in the natural environment in northern San Francisco Bay and
quantified the impact of metal contamination on the ecosystem in the
region.
Sample preparation Sediment samples were wet
fractionated using 63-μm sieve in order to estimate the
percentage of fine-grained fraction (<63 μm). For metal
analysis, oven dried (50 °C) sediment samples were
crushed gently and sieved to collect the less than 63-μm
grain size fraction since this fraction contains more sorbed
metal per gram of sediment due to its larger specific
surface area. Metals in the sediment samples were
extracted by 0.5 M HNO3 in 250-ml polyethylene bottles
vibrated at 100 rpm at 25 °C for 24 h. The metals
extracted with this concentration of the acid are weakly
bound to sediment and thus are considered to be readily
bioavailable.
Toxicity tests were initiated within 10 days of sediment collection. M.
nasuta clams were obtained from John Brezina and Associates, Dillon
Beach, CA, USA. Clams were maintained in the laboratory for 48 h
before test initiation. Seawater from Bodega Bay Marine Laboratory was
filtered through 1-μm filters and mixed with distilled water to obtain the
required salinity. Tests followed the procedure described in US EPA
Draft Protocol no. 600/x-89/30, ERLN-N111 (1989). Twenty clams
were exposed to the sediments in each of three replicate 20-l
aquaria for 28 days at a temperature of 14–15 °C. Sediment depth
was >4 cm. Overlying water was aerated and 80% was changed twice
a week. Salinity, pH and dissolved oxygen of the overlying water were
monitored. Salinity ranged from 25 to 26 ppt, pH was from 7.5 to 8.1
and dissolved oxygen ranged from 95.1% to 98.4% of saturation. After
28 days, surviving clams were counted and samples were
preserved for metal and biomarker analysis. Freeze-dried clam
tissues were digested using concentrated HNO3 in a Teflon digestion
vessel at 120 °C for more than 6 h until completely digested ( Nasci et
al., 1999).
Sediments collected from the sampling sites exhibited
some toxicity since clam survival is below 100 percent.
In particular, the toxic effect of the sediment collected
from M14 is significant since more than half of the
clams exposed to the sediment died during the
experimental period.
In order to estimate the degree of metal pollution in the
sediments, an enrichment factor was calculated using
the following equation ( Covelli and Fontolan, 1997):
EF= (M/Al)sample/(M/Al)reference
where EF is the enrichment factor, (M/Al)sample is the
ratio of metal and Al concentrations of the sample, and
(M/Al)reference is the ratio of metal and Al concentrations
of a reference. Reference concentrations were
provided by Hornberger et al. (1999) based on a study
in Tomales Bay, CA, where sediment has little history
of contamination.
EF values around 1.0 indicate that the element is
primarily from lithogenous sources, whereas EF values
greater than 1.0 indicate that the sources are more
likely to be anthropogenic.
Conclusions
Metal concentrations in the sediments were
significantly elevated at all sites investigated
in the northern San Pablo Bay area. Positive
correlations were obtained between
concentrations of Be and Mn in clam tissues
and sediments. This suggests that these
metals are not toxic and readily bioavailable
at the measured concentrations.
Negative correlations were observed between
enrichment factors for Cr, Cu, Zn and Ni and
clam survival, suggesting that these metals
are at least partially responsible for the
observed toxic effect. Some metals exhibited
low bioavailability to clams, possibly because
of their association with Fe-oxy-hydroxides or
sediment organic matter, or due to metabolic
regulation/excretion.
Ill. EPA employees wearing level "C" protective gear take soil sample in south Chicago's "cluster sites" area. Source: Ill.
EPA.
La contaminazione di un suolo, e’ l’introduzione
di sostanze, organismi biologici, o energia che
inducono deterioramento nella qualita’ del
suolo precludendone un suo utilizzo, spesso
creando inoltre rischi per la salute umana.
wearing level “B" protective gear
wearing level “A" protective gear
Drilling to determine pollution extent
wearing level “D" protective gear
Prima di intraprendere ogni attivita’
di bonifica e’ necessario conoscere:
La natura fisica del contaminante
La natura chimica del contaminante
La quantita’ del contaminante
Fra i possibili approcci per bonificare un suolo
inquinato esiste la bioremediation
Trattamenti che usano microorganismi (funghi
batteri) per abbattere la sostanza inquinante...
Per favorirne la crescita, si puo’ fornire
ossigeno, aumentare la temperatura, o cercare
di creare il giusto rapporto fra le sostanze
nutrienti (N, P, K ecc ecc).
In-situ-Bioremediation
Biostimulation (stimulates biological activity)
Bioventing (Inject air/nutrients into unsaturated zone – good for midweight petroleum, jet fuel)
Biosparging (Inject air/nutrients into unsaturated and saturated zones)
Bioaugmentation (inoculates soil with microbes)
Biosparging
Biostimulation
Ex-situ -Bioremediation
– Soil combined with water/additives in tank, microorganisms, nutrients, oxygen added
– Land-farming: soil put on pad, leachate collected
– Soil biopiles: soil heaped, air added
•Easier to control •Used to treat wider range of contaminants and soil types •Costly •Faster