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