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Chemosphere 52 (2003) 1781–1795
www.elsevier.com/locate/chemosphere
Self-purification ability of a resurgence stream
Roberta Vagnetti a, Paola Miana b, Mario Fabris b, Bruno Pavoni a,*
a Dipartimento di Scienze Ambientali, Universit�aa Ca’Foscari di Venezia, Calle Larga S. Marta, 2137-30123 Venice, Italyb VESTA S.p.A., Venezia Servizi Territoriali Ambientali, Palazzo Bonfadini, Cannaregio 462-30121 Venezia, Italy
Received 5 July 2002; received in revised form 10 April 2003; accepted 16 April 2003
Abstract
The self-purification ability of a resurgence stream has been investigated by taking samples along the course of a
channeled tract made up of a first part in beaten soil (3.3 km) and a second in concrete (7.2 km). The study has been
conducted by statistically processing pre-existent data, acquired monthly by analyzing waters at the beginning and at
the end of the whole canal for 6 years, from 1995 to 2000 (historic data), and by performing specific experiments (recent
data) to evaluate differently the self-purification capacity of the beaten soil section and that in concrete. A significant
abatement of concentrations has been observed from historic data for ammonium, phosphates, turbidity, heavy metals
and bacteria. From the recent data, all these parameters seem to decrease in the beaten soil tract. Whereas significant
further decreases in the concrete tract were observed only for ammonium, phosphates and bacteria. For other pa-
rameters, e.g. pH, dissolved oxygen, chlorides, fluorides, sodium, and sulfates, a significant increase was observed from
the historic data.
� 2003 Elsevier Ltd. All rights reserved.
Keywords: Self-purification; Resurgence stream; Nutrients; Metals; Turbidity; Bacteria
1. Introduction
The water environment reacts to the input of pol-
luting substances by means of a number of mechanisms
aiming to restore its original conditions. This process,
referred to as self-purification, actually consists of a re-
cycling of materials (Vismara, 1998). More precise defi-
nition for self-purification could be: ‘‘self-purification
means the partial or complete restoration, by natural
processes, of a stream pristine condition following the
introduction (usually through the agency of man) of
foreign matter sufficient in quality and quantity to cause
a measurable change in physical, chemical and/or bio-
logical characteristics of the stream’’ (Benoit, 1971). This
transformation produces compounds having a less neg-
*Corresponding author. Tel.: +39-41-234-8522; fax: +39-41-
2348582.
E-mail address: brown@unive.it (B. Pavoni).
0045-6535/03/$ - see front matter � 2003 Elsevier Ltd. All rights res
doi:10.1016/S0045-6535(03)00445-4
ative impact than the starting ones. The natural self-
purification process is therefore consisting of various
complex phenomena involving numerous physical, chem-
ical and biological factors acting and interacting more or
less effectively.
1.1. Physical processes
Dilution is an important component of self-purifica-
tion, for it allows the achievement of suitable concen-
trations for biological assimilation (Vismara, 1998).
Adsorption is the binding of molecules and ions which
are present in solution to solid particles. During the
adsorption process other ions are displaced from the
solid matrix into the solution (Benoit, 1971). Clays and
other colloidal particles (e.g. oxides–hydroxides of Fe
and Mn) can adsorb several organic and/or inorganic
solutes. Furthermore the solid phase can be a proper
support for bacterial degradation (Vismara, 1998). In
erved.
1782 R. Vagnetti et al. / Chemosphere 52 (2003) 1781–1795
particular, heavy metals are frequently adsorbed onto
the surface of suspended particulate matter instead of
being dissolved in the liquid phase as free or complexed
ions. This process plays a very important role in the
cycles of natural materials in the environment. Many
species are accumulated in the suspended solids or in
sediments, and partitioning between the solid and liquid
phase influences the transport and fate of contaminants
in the water bodies (Suzuky, 1997).
Sedimentation is one of the most important self-
purification mechanisms, especially in lakes, lagoons
and lentic waters. By this means suspended particles are
removed along with their adsorbed ions and molecules,
thus removing soluble materials from the water column.
Pollutants generally remain on the bottom, but can be
remobilized during period of increased water flow or
turbulence, with the concurrent activities of desorption
and ionic exchange, which are mainly favored by an-
aerobic conditions (Vismara, 1998). Sediment and sus-
pended particles are depositories of large amounts of
heavy metals, which can be present as distinct com-
pounds or associated with clays, insoluble humic sub-
stances or iron and manganese oxides. Accumulation of
heavy metals in bottom sediments is one of the most
effective factors in self-purification. However, since this
is a reversible process, metal accumulation can be con-
sidered a constant potential for secondary pollution,
caused by resuspension and release phenomena (Linnik
and Zubenko, 2000).
Volatilization (transfer of pollutants from water to
the gas phase) causes a permanent removal of com-
pounds from the liquid phase. From the vapor pressure
of the compounds, its tendency to volatilize can be in-
ferred, however its solubility and other features can also
be important (Brusseau and Bohn, 1996).
1.2. Chemical processes
Acid–base reactions maintain the natural water pH
by neutralizing acid and base pollutants. Water buffer
capacity is strongly related to alkalinity which is the
water�s content of carbonate and hydroxide species
(Vismara, 1998).
Within the very high number of redox reactions, very
important are the oxidation of organic matter which
consumes oxygen and the oxidation of ammonia to ni-
trate, which is important for nitrogen to be assimilated
by plants and therefore removed. It must be considered
that many of the most important redox reactions are
catalyzed by microorganisms or governed by other bio-
logical processes (Manahan, 1994).
Precipitation reactions, depending on the solubility
product of several compounds, are very important for
removing ions from the liquid phase. Many precipitation
reactions, such as the formation of salts of phosphates
or carbonates, involve the removal of cations from so-
lution.
Processes leading to the formation of aggregates
from colloidal suspensions are also very important and
result in sediment formation and water clarification
(Manahan, 1994). These processes are coagulation and
flocculation, which are complex phenomena of a physical
and chemical nature, and are not yet well understood.
1.3. Biological processes
Bacterial degradation is the most important removal
process for organic substances and some inorganic sub-
stances, especially nitrogen and phosphorus compounds.
Bacteria obtain the necessary energy for their survival
by redox reactions, which transform organic matter and
nutrients.
Plants can protect an ecosystem by assimilation and
removal of a portion of the macronutrients present in
the water body (Chambers and Prepas, 1994; Volterra
and Mancini, 1994; Cunningham and Davi, 1996; Borin
and Marchetti, 1997). Mostly nitrogen and phosphorus
compounds are involved, but many plants can accu-
mulate heavy metals and toxic substances. It is necessary
to distinguish between the phytoremediation of inor-
ganic elements and toxic organic compounds (Meagher,
2001): elemental pollutants are essentially immutable
by any biological and physical process, whereas organic
substances are mineralized into relatively non-toxic
constituents. Assimilation is higher in summer, when
nutrients are stored in expectation of the winter season
(Volterra and Mancini, 1994). Finally it must be con-
sidered that the rhizosphere hosts a very abundant
population of microorganisms and that vegetation itself
can act as a filter for the suspended particulate matter.
From a preliminary examination of historic analyti-
cal results, a considerable decrease of some substances
was observed in water samples taken at the beginning
and at the end of a canal. The aim of the present in-
vestigation was to establish, by means of a statistical
treatment of pre-existent data (historic data) and some
ad hoc confirmation experiments (recent data), for which
parameters the abatement is significant and to formulate
possible interpretations.
2. Materials and methods
2.1. Study site
The stream investigated in this study is an artificial
canal (averagely 2 m deep, 5 m large with a flow rate of
about 2.2 m3/s) feeding a municipal purification plant
for household (285 000 people served) and industrial use.
Main treatments carried out are coagulation, activated
carbon adsorption, clarification, filtration and disinfec-
R. Vagnetti et al. / Chemosphere 52 (2003) 1781–1795 1783
tion. The canal conveys water from the resurgence river
Sile, in Quarto d�Altino to Favaro Veneto (Veneto Re-
gion, North–East of Venice, Italy). Surrounding area is
only used for agriculture and along the course of the
canal (10.5 km) some water can be derived for irrigation.
The first section of the canal (3.3 km) is in beaten soil
and the second section (7.2 km) is in concrete. Various
plants species are present, mainly in the beaten soil tract:
Apium nodiflorum (abundance 0–1), Callitriche stagnalis
(ab. 2–3), Ceratophillum demersus (ab. 1–2), Elodea
canadensis (ab. 3–4), Potamogeton natans (ab. 3–4),
Potamogeton pectinatus (ab. 3–4), Ranunculus fluitans
(ab. 3–4), Vallisneria spiralis (ab. 1–2) (Marconato,
2003). The extended biotic index value for the canal
(beginning of the canal) is 8, i.e. the water is slightly
polluted (Marconato, 2003). Fish species present are
Anguilla anguilla, Cobitis taenia, Esox lucius, Padogobius
martensii, Rutilus erythrophthalmus, Scardinius erythro-
phthalmus (Marconato et al., 2000).
2.2. Statistical analyses
The historic data set consisted of pre-existent ana-
lytical results of water samples collected at stations lo-
cated at the beginning and at the end of the canal during
6 years (1995–2000) with a monthly frequency. These
data were processed with the Statistical t Test using
paired data (Piccolo, 2000) to discriminate significant
differences between the pairs of values obtained for any
single parameter at the beginning and at the end of the
canal in the same sampling session. A 5% significance
level was used. A parametric method was chosen after
checking data normality.
2.3. Sampling design
To validate the results obtained by means of the
statistical data processing, some additional experiments
were carried out. Sampling sessions took place on 27th
March, 27th June, and 12th September 2001 (recent
data).
The sampling stations were the following:
1. Quarto d�Altino (Head of the canal).
2. End of canal tract # 1 (Beaten soil).
3. Siphon.
4. Favaro Veneto (End of the canal).
A map of the canal is shown in Fig. 1.
Sampling times were established according to the
water velocity. Sampling station 2 was selected in order
to possibly distinguish the self-purification ability of
the canal tract bordered by beaten soil from that framed
by concrete. Sampling station 3 was chosen to detect
the influence of a siphon along the course of the canal
enabling the crossing of the river Zero. During the
samplings it was noticed that the siphon was partly
plugged with sediment and that an intense resuspension
was evident in the tract downstream. In order to es-
tablish the sampling times necessary to collect the same
water parcel at the beginning and at the end of its
travel along the canal, the water velocity was prelimi-
narily estimated. During each sampling session the flow
rate was measured by means of a float moving at a
water depth of about 20 cm below the surface to avoid
wind influence. Water samples (one sample for micro-
biological analyses, one for chemical analyses) were
collected by dipping the containers about 1 m below
the surface. Containers for microbiological analyses
were previously sterilized, whereas those for chemical
analyses were previously cleaned in the laboratory and
then simply rinsed with the water to be sampled.
Samples were then transferred to the laboratory by
means of a portable refrigerated container and kept at
4 �C before analyses, which were performed the same
or the following day.
The parameters selected for the analyses were those
showing a significant abatement. In addition, nitrates
and nitrites were also analyzed to estimate the total
inorganic nitrogen budget. In this study the organo-
chlorine compounds, such as 1,1,1-trichloroethane and
tetrachloroethylene were not included because they
were considered to be clearly lost to the atmosphere
by evaporation.
All microbiological and chemical analyses (historic
and recent data) were performed according to the pro-
cedures reported in the manuals ‘‘Standard Methods for
the Examination of Water and Wastewater’’ (AWWA,
1998) and ‘‘Analytical Methods for Waters’’ (CNR,
1994).
2.4. Microbiological analyses
Water samples were filtered through a membrane
with a pore size of about 0.45 lm, which traps most of
the bacteria on its surface. The membrane was then
placed in a pad saturated with a medium selected to
favor growth and differentiation of organisms. Media
used were M-endo agar for total coliforms, M-FC agar
for fecal coliforms, KF-streptococcus agar for fecal
streptococci and plate count agar for heterotrophic count
at 22 and 36 �C. The bacterial number was then reported
as colony forming units (CFU) per unit of water volume.
Precisions for microbiological analyses, calculated as
coefficient of variations from six replicates, were: 8% for
heterotrophic count at 22 �C, 13% for heterotrophic
count at 36 �C, 21% for total coliforms, 3% for fecal
coliforms, 2% for fecal streptococci and 15% for clo-
stridia spores. Increases and decreases (percent), for
each parameter, were considered significant when higher
than coefficients of variations.
Fig. 1. Map of the canal (North–East of Venice). Station 1 (head of the canal), station 2 (end of beaten soil tract), station 3 (siphon)
and station 4 (end of the canal, purification plant).
1784 R. Vagnetti et al. / Chemosphere 52 (2003) 1781–1795
2.5. Physical–chemical analyses
Turbidity was measured by the Nephelometric
Method using a HACH 2100 AN instrument, provided
with a tungsten source and a photoelectric detector.
Ammonium concentration was measured by the phe-
nol-hypochlorite method, using a spectrophotometer
UV–VIS Lambda 2, Perkin Elmer, operating at 660 nm.
Nitrates concentration was determined spectropho-
tometrically at 220 nm, after sample acidification to
eliminate carbonates.
Nitrites concentration was determined through for-
mation of a reddish purple azo dye (measured spectro-
photometrically at 540 nm) produced at a pH 2.0–2.5 by
coupling diazotized sulfanilamide with N-(1-naphtyl)-
ethylenediamine dihydrochloride.
R. Vagnetti et al. / Chemosphere 52 (2003) 1781–1795 1785
Phosphates concentration was measured after trans-
formation of polyphosphates into orthophosphates
with sulfuric acid and potassium persulphate. Ortho-
phosphates concentration was measured spectrophoto-
metrically at 650 nm, after reaction with ammonium
molybdate and potassium antimonyl tartrate in acid
medium to form phosphomolybdic acid, and further
reduction to the intensely colored molybdenum blue by
ascorbic acid.
Dissolved organic carbon (DOC) concentration was
determined potentiometrically as CO2 using a selective
Table 1
Average and percent abatement of the parameters analyzed at the be
data)
Average difference
pH 0.12
Conductivity )2 lS/cmTurbidity )3.3 NTU
Alkalinity 1 mg/dm3 CaCO3
Ammonium )0.16 mg/dm3
Sodium 0.70 mg/dm3
Potassium 0.04 mg/dm3
Calcium 0.49 mg/dm3
Magnesium )0.19 mg/dm3
Total hardness )0.5 mg/dm3 CaCO3
Dissolved oxygen 0.3 mg/dm3
Fluorides 0.01 mg/dm3
Chlorides 0.55 mg/dm3
Nitrites 0.05 mg/dm3
Nitrates 0.01 mg/dm3
Total phosphates )0.04 mg/dm3
Sulfates 0.57 mg/dm3
D.O.C. 0.1 mg/dm3
Al )65.9 lg/dm3
Cr )0.4 lg/dm3
Fe )38.2 lg/dm3
Pb )0.2 lg/dm3
Mn )3.9 lg/dm3
Ba )3.0 lg/dm3
B 0.8 lg/dm3
Cu )1.1 lg/dm3
Zn )3.8 lg/dm3
Chloroform )0.28 lg/dm3
1,1,1-Trichloroethane )0.07 lg/dm3
Trichloroethylene )0.01 lg/dm3
Dichlorobromomethane )0.11 lg/dm3
Tetrachloroethylene )0.18 lg/dm3
Dibromochloromethane )0.04 lg/dm3
Bromoform )0.01 lg/dm3
Heterotrophic count at 22 �C )8001 UFC/cm3
Heterotrophic count at 36 �C )6580 UFC/cm3
Total coliforms )19 661 UFC/100 cm3
Fecal coliforms )4382 UFC/100 cm3
Fecal streptococci )371 UFC/100 cm3
Sulfite-reducing clostridia spores )3 UFC/100 cm3
Negative difference¼ abatement; positive¼ increase. Results of t-test (5value.
electrode. Samples were previously filtered, acidified to
remove carbonates/bicarbonates and finally oxidized
with persulphate and UV irradiation.
Heavy metals concentration was determined by an
Atomic Absorption Spectrophotometer (3030, Perkin
Elmer) after acidification of the non-filtered sample with
nitric acid. The following wavelengths were used: Al,
309.3 nm; Fe, 248.3 nm; Mn, 279.5 nm; Pb, 324.7 nm;
Zn, 213.19 nm. Precisions for chemical analyses, calcu-
lated as coefficient of variations from six replicates,
were: 2% for turbidity, 2% for ammonium, 3% for
ginning and at the end of the canal from 1995 to 2000 (historic
% abatement n t Significance
0.15 81 6.57 Significant
)0.4 81 )1.41 Non-sign.
)61 80 )7.26 Significant
0.5 79 0.26 Non-sign.
)69 81 )13.25 Significant
10 65 13.40 Significant
3 65 1.28 Non-sign.
0.7 81 0.52 Non-sign.
)0.8 81 )1.62 Non-sign.
)0.2 80 )0.64 Non-sign.
3 73 1.67 Non-sign.
10 79 2.16 Significant
6.6 80 9.20 Significant
31 80 6.21 Significant
0.07 80 0.40 Non-sign.
)25 77 )6.51 Significant
1.3 80 3.72 Significant
8 73 0.36 Non-sign.
)58.3 75 )4.41 Significant
)27 74 )1.79 Non-sign.
)50.2 74 )5.73 Significant
)25 74 )2.33 Significant
)43 70 )4.49 Significant
)6 22 )1.25 Non-sign.
2 19 0.42 Non-sign.
)46 31 )2.49 Significant
)35 28 )2.35 Significant
)82 72 )1.20 Non-sign.
)35 71 )6.40 Significant
)11 72 )1.78 Non-sign.
)92 72 )1.40 Non-sign.
)30 72 )10.64 Significant
)80 72 )1.26 Non-sign.
)4 72 )0.28 Non-sign.
)71.1 75 )5.10 Significant
)77.2 76 )3.32 Significant
)86.8 76 )10.20 Significant
)90.8 76 )10.02 Significant
)90.5 75 )11.11 Significant
)60 69 )3.84 Significant
% significance level): n ¼ number of observations, t ¼ statistic t
1786 R. Vagnetti et al. / Chemosphere 52 (2003) 1781–1795
nitrites, 1% for nitrates, 18% for phosphates, 2% for
DOC and 2% for heavy metals. Increases and decreases
(percent), for each parameter, were considered signifi-
cant when higher than coefficients of variations.
3. Results
Average abatement and the percent abatement for
historic data are shown in Table 1. For a clear identifi-
cation of the most important parameters involved in the
self-purification of the canal, these data were represented
in a Box Whisker Plot (Fig. 2). By means of the t Test
using paired data, the following parameters were found
as having a significant decrease between the beginning
and the end the canal (5% significance level; t values are
reported in Table 1): turbidity, ammonia, total phos-
phates, Al, Fe, Pb, Mn, Cu, Zn, 1,1,1-trichloroethane,
tetrachloroethylene, heterotrophic count at 22 and 36
�C, total and fecal coliforms, fecal streptococci, sulfite-
reducing clostridia spores. For some others (pH,
chlorides, fluorides, nitrites, sodium, and sulfates) a
significant increase was observed. Also dissolved oxygen
showed a significant increase with a 10% significance
level.
Statistical results are evident also in Fig. 3, which
shows the trends of pH (Fig. 3(a)), oxygen (Fig. 3(b)),
nitrates (Fig. 3(c)), nitrites (Fig. 3(d)) and phosphates
(Fig. 3(e)), from historic data (1995–2000). Dissolved
oxygen, nitrites concentration and pH were generally
higher at station 4 than at station 1 (particularly in
Box Whisker Plot o
-30
-20
-10
0
10
20
pH*1
0
Tur
bidi
ty
Am
mon
ium
Sod
ium
*10
Pot
assi
um*1
0
Oxy
gen
Flu
orid
e*10
0
Chl
orid
e*10
Nitr
ites*
100
Pho
spha
tes*
100
Sul
fate
s
Alu
min
um/1
0
Means+SDMeans-SD
Means+SEMeans-SE
Means
Fig. 2. Box Whisker Plot of differences for routinely acquired histori
Table 1 for units.
summer for nitrites). Phosphates were lower at station 4
than at station 1 and nitrates were also lower, but only
in summer. However for nitrates and nitrites a seasonal
trend was more evident.
Analytical results (recent data) are reported in Tables
2–4, and plotted in Figs. 4–6.
For ammonium, phosphates, turbidity, heavy metals
and bacteria a net abatement was observed from station
1 to station 2. Only ammonium, phosphates and bac-
teria decreased from station 2 to station 4. Whereas for
turbidity and heavy metals an increase was observed
from station 2 to station 4 and also from station 2 to
station 3 on 12th September. Concentration of nitrites
grew significantly from station 1 to station 4. Nitrates
and dissolved organic carbon did not show a particular
trend.
An estimation of the budget for all species of inor-
ganic nitrogen was attempted, in order to establish if
these species were simply transforming from one species
into another or if a total inorganic nitrogen decrease
also occurred. Concentrations of inorganic nitrogen for
historic data is reported in Table 5 and for recent data in
Table 6.
4. Discussion
4.1. Nitrogen
From historic data, a systematic decrease of total
inorganic nitrogen during the summer period (especially
f differences
Iron
/10
Man
gane
se
Cop
per
Zin
c
1,1,
1Tric
hlor
oet*
100
Tet
rach
loro
eth*
10
Het
erC
ount
22˚C
/100
0
Het
erC
ount
36˚C
/100
0
Tot
alC
olifo
rm/1
0000
Fec
alC
lifor
m/1
000
Fec
alS
trep
t./10
0
Clo
strid
iaS
pore
s
c data: negative difference¼ abatement; positive¼ increase. See
Fig. 3. Trends of pH (a), oxygen (b), nitrates (c), nitrites (d) and phosphates (e) from 1995 to 2000 (historic data obtained from
monthly analyses) at stations 1 and 4 (see Fig. 1, map of the canal).
R. Vagnetti et al. / Chemosphere 52 (2003) 1781–1795 1787
Table 3
Concentrations of selected parameters obtained by analyzing water samples during sampling session II (27th June), at stations 1 (Quarto d�Altino), 2 (End soil tract) and 4 (Favaro
Veneto) (recent data)
St. Turbidity (NTU) Ammonium (mg/dm3) Nitrites (mg/dm3) Nitrates (mg/dm3) Total phosphates (mg/dm3) DOC (mg/dm3)
1 9.4 0.19 0.15 18.99 0.13 1.9
2 7.4 0.08 0.19 16.92 0.11 1.8
4 14.4 0.08 0.18 17.05 0.09 1.7
Aluminum (lg/dm3) Iron (lg/dm3) Lead (lg/dm3) Manganese (lg/dm3) Copper (lg/dm3) Zinc (lg/dm3)
1 42.5 87.6 <0.1 9.9 2.8 6.6
2 48.8 98.1 <0.1 11.0 2.5 4.7
4 266.0 207.0 <0.1 11.8 3.1 7.6
Heterotrophic count
at 22 �C (CFU/cm3)
Heterotrophic count
at 36 �C (CFU/cm3)
Total coliforms
(CFU/100 cm3)
Fecal coliforms
(CFU/100 cm3)
Fecal streptococci
(CFU/100 cm3)
Clostridia spores
(CFU/100 cm3)
1 80 000 20 000 97 400 7560 400 30
2 40 000 4000 39 000 3960 44 22
4 20 000 3000 24 800 780 19 20
Table 2
Concentrations of selected parameters obtained by analyzing water samples during sampling session I (27th March), at stations 1 (Quarto d�Altino), 2 (End soil tract) and 4 (Favaro
Veneto) (recent data)
St. Turbidity (NTU) Ammonium (mg/dm3) Nitrites (mg/dm3) Nitrates (mg/dm3) Total phosphates (mg/dm3) DOC (mg/dm3)
1 7.4 0.26 0.10 15.6 0.27 2.20
2 2.8 0.12 0.20 15.5 0.30 2.32
4 8.0 0.07 0.19 15.5 0.24 2.16
Aluminum (lg/dm3) Iron (lg/dm3) Lead (lg/dm3) Manganese (lg/dm3) Copper (lg/dm3) Zinc (lg/dm3)
1 162 145.0 <0.1 10.1 2.5 14.8
2 111 53.0 <0.1 6.0 1.8 10.2
4 352 147.5 <0.1 9.4 7.4 11.4
Heterotrophic count
at 22 �C (CFU/cm3)
Heterotrophic count
at 36 �C (CFU/cm3)
Total coliforms
(CFU/100 cm3)
Fecal coliforms
(CFU/100 cm3)
Fecal streptococci
(CFU/100 cm3)
Clostridia spores
(CFU/100 cm3)
1 150 000 100 000 33 400 6000 246 42
2 17 300 11 900 4750 700 71 15
4 50 400 50 000 10 000 360 3 30
1788
R.Vagnetti
etal./Chem
osphere
52(2003)1781–1795
Table
4
Concentrationsofselected
para
metersobtained
by
analyzing
watersa
mplesduring
sampling
session
III(12th
Sep
tember),
atstations1
(Quarto
d�A
ltino),
2(E
nd
soil
tract),
3(S
iphon)and
4(F
avaro
Ven
eto)(recen
tdata)
St.
Turb
idity(N
TU)
Ammonium
(mg/dm
3)
Nitrites(m
g/dm
3)
Nitra
tes(m
g/dm
3)
Totalphosp
hates(m
g/dm
3)
DOC
(mg/dm
3)
16.0
0.16
0.13
17.6
0.14
1.9
23.3
0.07
0.17
17.5
0.13
1.8
36.2
0.07
0.18
17.6
0.13
1.8
49.7
0.06
0.17
17.5
0.12
1.8
Aluminum
(lg/dm
3)
Iron
(lg/dm
3)
Lea
d(l
g/dm
3)
Manganese(l
g/dm
3)
Copper
(lg/dm
3)
Zinc(l
g/dm
3)
1103
93
<0.1
7.7
1.0
6.7
2100
76
<0.1
5.5
0.6
3.1
3332
155
<0.1
7.7
0.7
4.9
4487
238
<0.1
10.5
2.9
7.9
Heterotrophic
countat
22�C
(CFU/cm
3)
Heterotrophic
countat
36�C
(CFU/cm
3)
Totalco
lifo
rms
(CFU/100cm
3)
Fecalco
lifo
rms
(CFU/100cm
3)
Fecalstrepto
cocci
(CFU/100cm
3)
Clostridia
spores
(CFU/100cm
3)
13760
13200
82600
6000
750
83
22000
2440
12000
2440
100
56
32320
4680
5900
780
300
26
41680
2100
5100
550
70
32
R. Vagnetti et al. / Chemosphere 52 (2003) 1781–1795 1789
in June) was observed. This phenomenon can last until
the early autumn depending on the particular year. This
is evident from the data reported in Table 5. During the
other periods of the year, the inorganic nitrogen budget
can be considered closed: in other words the concen-
tration of total inorganic nitrogen remains constant.
This trend has been confirmed by the results of the ex-
periments (Table 6). The only total inorganic nitrogen
decrease was monitored in the sampling session of 27th
June. In the other sampling sessions the concentration at
station 1 is not different from the concentration at sta-
tion 4. The summer abatement of nitrogen species was
observed by several investigators (Elosegui et al., 1995;
Bratli et al., 1999; Jing et al., 2001) and it can be rea-
sonably ascribed to a greater assimilation by living or-
ganisms, particularly plants. In fact, in this season the
most efficient nutrient and pollutant uptake occurs, be-
cause higher air and water temperatures and a more
intense light irradiation favor higher plant productivity
(Volterra and Mancini, 1994). The vegetale species pre-
sent in the canal include also some which are used for
phytoremediation: e.g. Elodea canadensis, Potemogeton
natans, Ceratophyllum demersus (Volterra and Mancini,
1994). In addition, nitrogen can be definitively removed
by denitrification, a biological process which is depen-
dent on temperature and favored in the summer (Bratli
et al., 1999; De Crespin De Billy et al., 2000; Haag and
Kaupenjohann, 2001). It is evident from the recent data
(Table 6) that this reduction of nitrogen in summer is
mostly due to a reduction of nitrate concentrations. In
fact, nitrates accounted for more than 98% of the total
nitrogen species.
Considering the nitrogen species separately, a sig-
nificant ammonium abatement was observed both in the
historic data and in the experimental data. Considering
these latter, it can also be observed that this decrease is
more evident at the end of the beaten soil tract. This
phenomenon is considered normal in natural oxygen-
ated waters, and it was observed in all considered self-
purification studies (Elosegui et al., 1995; Lam-Leung
et al., 1996; Bratli et al., 1999; Jing et al., 2001). It
is caused by several mechanisms, including oxidation
(nitrification) and biological assimilation.
In parallel to an ammonium drop, a significant
increase of nitrites was observed both in the historic
data and in the experiments. In this case, the more
important increase of nitrites was monitored between
the beginning of the canal and the end of beaten soil
tract. It is known that the overall process of nitrogen
oxidation from ammonia to nitrate can be considered
a two step process, i.e. the oxidation from ammonia
to nitrite and from nitrite to nitrate. The concentra-
tion of the intermediate species nitrite depends on the
relative rates of the two steps. When the step from
nitrite to nitrate is somehow slowed down, the nitrite
concentration increases. According to Von der Wiesche
Sampling session I - Stations 1,2,4 - Nutrients
0.04
0.08
0.12
0.16
0.20
0.24
0.28
0.32
Station 1 Station 2 Station 4
NH4 (mg/dm3)NO2 (mg/dm3)NO3 (mg/dm3)*10-2
PO4 (mg/dm3)DOC (mg/dm3)*10-1
(a)
Sampling session I - Stations 1,2,4 - Turbidity and metals
-5
0
5
10
15
20
25
30
35
40
Station 1 Station 2 Station 4
Turbidity (NTU)Al (mg/dm3)*10-1
Fe (mg/dm3)*10-1
Mn (mg/dm3)Cu (mg/dm3)Zn (mg/dm3)
(b)
Sampling session I - Stations 1,2,4 - Bacteria
0
50
100
150
200
250
300
350
Station 1 Station 2 Station 4
Heter. count 22°C (CFU/cm3)*10-3
Heter. count 36°C (CFU/cm3)*10-3
Total coliforms (CFU/100cm3 )*10-2
Fecal coliforms (CFU/100cm3 )*10-1
Fecal streptococci (CFU/100cm3)Clostridia spores (CFU/100cm3)
(c)
Fig. 4. Recent data obtained by analyzing water samples during sampling session I (27th March) at stations 1 (Quarto d�Altino), 2
(End soil) and 4 (Favaro Veneto): (a) concentrations of ammonium, dissolved organic carbon, nitrates, nitrites and phosphates, (b)
turbidity, aluminum, copper, iron, manganese, and zinc, (c) heterotrophic count at 22 and 36 �C, total coliforms, fecal coliforms, fecal
streptococci, clostridia spores.
1790 R. Vagnetti et al. / Chemosphere 52 (2003) 1781–1795
Sampling session II - Station 1,2,4 - Nutrients
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
Station 1 Station 2 Station 4
NH4(mg/dm3)
NO2 (mg/dm3)
NO3 (mg/dm3)*10-2
PO4 (mg/dm3)
DOC (mg/dm3)*10-1
Sampling session II-Stations 1-2-4- Turbidity and metals
0
6
12
18
24
30
Station 1 Station 2 Station 4
Turbidity (NTU)Al (µg/dm3)*10-1
Fe (µg/dm3)*10-1
Mn (µg/dm3)Cu ( µg/dm3)Zn (µg/dm3)
Sampling sessionII-Stations1-2-4-Bacteria
-10
10
30
50
70
90
110
Station1 Station2 Station4
Heter. count 22°C (CFU/cm3)*10-3
Heter. count 36°C (CFU/cm3)*10-3
Total coliforms (CFU/100cm3)*10-3
Fecal coliforms (CFU/100cm 3)*10-2
Fecal streptococci (CFU/100cm3)*10-1
Clostridia spores (CFU/100cm3)
(a)
(b)
(c)
Fig. 5. Recent data obtained by analyzing water samples during sampling session II (27th June) at stations 1 (Quarto d�Altino), 2 (End
soil) and 4 (Favaro Veneto). Same parameters as Fig. 4.
R. Vagnetti et al. / Chemosphere 52 (2003) 1781–1795 1791
Sampling session III - Stations 1,2,3,4 - Nutrients
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
Station 1 Station 2 Station 3 Station 4
NH 4 (mg/dm3)
NO2 (mg/dm3)
NO3 (mg/dm3)*10-2
PO4 (mg/dm3)
DOC (mg/dm3)*10-1
Sampling session III - Stations 1,2,3,4 - Turbidity and metals
0
10
20
30
40
50
Station 1 Station 2 Station 3 Station 4
Turbidity (NTU)Al (µg/dm3 )*10-1
Fe (µg/dm3)*10-1
Mn (µg/dm3)Cu ( µg/dm3)Zn (µg/dm3)
Sampling session III-Stations 1,2,3,4 - Bacteria
0
20
40
60
80
100
120
140
Station 1 Station 2 Station 3 Station 4
Heter. count 22°C (CFU/cm3)*10-2
Heter. count 36°C (CFU/cm3)*10-2
Total coliforms (CFU/100cm3)*10-3
Fecal coliforms (CFU/100cm3)*10-2
Fecal streptococci (CFU/100cm3)*10-1
Clostridia spores (CFU/100cm3)
(a)
(b)
(c)
Fig. 6. Recent data obtained by analyzing water samples during sampling session III (12th September) at stations 1 (Quarto d�Altino),
2 (End soil), 3 (Additional point) and 4 (Favaro Veneto). Same parameters as Fig. 4.
1792 R. Vagnetti et al. / Chemosphere 52 (2003) 1781–1795
Table 5
Total inorganic nitrogen (N–NH4 + N–NO2 + N–NO3) at the beginning and at the end of the canal (station 1 and 4) from 1995 to 2000
(historic data)
Date Tempera-
ture (�C)
Total nitrogen
(mg/dm3)
Difference
station 4� 1
(mg/dm3)
Date Tempera-
ture (�C)
Total nitrogen
(mg/dm3)
Difference
station 4� 1
(mg/dm3)1 4 1 4
16/01/95 8.1 4.17 4.15 )0.02 15/12/97 9.3 3.57 3.41 )0.1613/02/95 10.6 3.93 3.78 )0.15 12/01/98 10.1 3.64 3.46 )0.1827/02/95 10.3 4.09 3.92 )0.16 09/02/98 10.0 3.65 3.36 )0.2913/03/95 11.0 3.43 3.21 )0.23 09/03/98 13.2 3.75 3.50 )0.2510/04/95 14.0 4.12 3.93 )0.19 06/04/98 13.7 3.35 3.26 )0.0922/05/95 15.0 4.06 4.13 0.07 19/05/98 16.3 3.16 2.79 )0.3605/06/95 16.3 3.61 3.37 )0.24 01/06/98 17.4 3.24 2.95 )0.2903/07/95 18.3 4.09 3.46 )0.63 30/06/98 19.9 3.26 3.00 )0.2624/07/95 19.8 4.01 3.68 )0.33 27/07/98 19.6 3.33 3.31 )0.0328/08/95 18.0 4.25 3.56 )0.69 24/08/98 16.9 3.58 3.36 )0.2325/09/95 16.5 4.33 4.10 )0.23 21/09/98 15.7 3.53 3.26 )0.2730/10/95 13.6 4.50 4.46 )0.04 19/10/98 14.1 3.86 3.65 )0.2120/11/95 10.5 3.65 3.70 0.05 14/12/98 8.7 3.62 3.39 )0.2318/12/95 10.4 3.42 3.58 0.16 08/02/99 8.5 3.47 3.29 )0.1815/01/96 11.2 3.73 3.83 0.10 08/03/99 10.9 3.97 3.77 )0.1912/02/96 8.0 4.08 3.87 )0.21 12/04/99 13.9 3.50 3.37 )0.1311/03/96 10.1 3.96 3.88 )0.08 03/05/99 16.0 3.29 2.96 )0.3309/04/96 11.9 3.50 3.01 )0.48 31/05/99 19.2 3.04 2.61 )0.4306/05/96 14.0 3.33 3.02 )0.32 28/06/99 18.0 3.25 3.00 )0.2503/06/96 17.4 3.32 2.92 )0.40 26/07/99 18.5 3.42 3.00 )0.4101/07/96 17.4 3.30 2.98 )0.32 23/08/99 19.1 3.39 3.17 )0.2229/07/96 18.2 3.77 3.46 )0.31 18/10/99 13.0 3.73 3.13 )0.6026/08/96 18.0 3.66 3.27 )0.39 15/11/99 11.2 3.72 3.55 )0.1723/09/96 14.5 3.68 3.41 )0.27 13/12/99 9.6 3.71 3.50 )0.2121/10/96 13.7 3.68 2.81 )0.88 10/01/00 8.7 3.58 3.21 )0.3818/11/96 13.2 3.42 3.07 )0.36 07/02/00 11.4 3.56 3.51 )0.0516/12/96 9.9 3.51 3.32 )0.19 06/03/00 14.4 3.96 3.72 )0.2414/01/97 9.5 3.80 3.59 )0.21 17/04/00 16.5 3.26 3.14 )0.1217/02/97 9.6 3.65 3.35 )0.31 15/05/00 17.2 3.31 3.19 )0.1210/03/97 13.7 3.59 3.38 )0.21 05/06/00 18.6 3.50 3.32 )0.1905/05/97 16.3 3.32 2.89 )0.44 10/07/00 16.6 3.71 3.40 )0.3102/06/97 14.3 3.27 2.79 )0.48 07/08/00 15.9 3.46 3.44 )0.0201/07/97 18.5 3.11 2.92 )0.19 02/10/00 13.7 3.52 3.25 )0.2728/07/97 19.2 3.41 3.18 )0.22 30/10/00 11.9 4.20 4.11 )0.1025/08/97 18.2 3.55 3.35 )0.20 07/11/00 12.9 3.62 3.92 0.30
22/09/97 21.3 3.48 3.09 )0.40 08/11/00 12.3 4.10 3.67 )0.4317/11/97 11.7 3.58 3.34 )0.24 20/11/00 11.9 4.25 4.23 )0.02
Table 6
Nitrogen concentration in ammonium, nitrites, nitrates species and total inorganic nitrogen concentration (mg/dm3) from recent data
St. 27th March 27th June 12th September
N–NH4 N–NO2 N–NO3 Tot N–NH4 N–NO2 N–NO3 Tot N–NH4 N–NO2 N–NO3 Tot
1 0.20 0.03 3.52 3.76 0.15 0.05 4.29 4.48 0.13 0.04 3.98 4.14
2 0.09 0.06 3.50 3.66 0.06 0.06 3.82 3.94 0.05 0.05 3.95 4.06
3 0.05 0.05 3.98 4.08
4 0.05 0.06 3.50 3.61 0.06 0.05 3.85 3.97 0.05 0.05 3.95 4.05
R. Vagnetti et al. / Chemosphere 52 (2003) 1781–1795 1793
and Wetzel (1998), who observed a similar trend, this
phenomenon can occur when the temperature exceeds
the range 10–17 �C, in which the balance between the
two forms is maintained. At higher temperatures,
inhibition of nitrate formation is greater than the in-
hibition of nitrite formation. In the studied canal, tem-
peratures were often above that range in summer (Table
5). Observing the seasonal trend of nitrite concentra-
tion, we can notice winter minima and summer maxima
(Fig. 3(d)).
1794 R. Vagnetti et al. / Chemosphere 52 (2003) 1781–1795
Considering all of the data, no significant difference
of nitrate concentrations was observed between the be-
ginning and the end of the canal, apart from the summer
period when a significant decrease of concentrations was
detected (Fig. 3(c)).
4.2. Phosphates
From the statistical processing of historic data (Table
1), it is evident that total phosphate concentrations
significantly decrease along the course of the canal (Fig.
3(e)). This is a typical phenomenon frequently observed
in natural purification processes (Benka-Coker and
Ojior, 1995; Elosegui et al., 1995; Lam-Leung et al.,
1996; Bratli et al., 1999; De Crespin De Billy et al., 2000;
Jing et al., 2001). For phosphorus removal the physical–
chemical processes, e.g. precipitation with calcium, ad-
sorption and sedimentation, are dominant. Phosphates
can be eliminated from water also by uptake by primary
producers or microbial decomposers (Elosegui et al.,
1995; Bratli et al., 1999). The abatement, for recent data,
was significant (percent higher than coefficient of varia-
tions) only on 27th June: in fact in the summer period
biologic assimilation is greater.
4.3. Bacteria
The concentration of bacteria in the water sharply
dropped from the beginning to the end of the canal as
a consequence of sedimentation and natural decay, but
an important abatement factor is also the filtration by
aquatic plants (mainly by rhizosphere) (Benka-Coker
and Ojior, 1995; Elosegui et al., 1995; Borin and
Marchetti, 1997).
4.4. Turbidity and heavy metals
The observed behavior for turbidity and heavy metal
concentrations was difficult to explain. A net abatement
was observed from the data collected for six years
(1995–2000). This decrease was attributed to sedimen-
tation or to a combination of adsorption and sedimen-
tation (Linnik and Zubenko, 2000). However data
investigated from the second half of 2000 only, showed
remarkable increases of these parameters. This phe-
nomenon non-remarkable overall, being limited to the
last period, is confirmed by all recent data. From the
analytical results of the experiments, it can be seen that
a decrease occurs in the beaten soil tract, whereas in
the sample corresponding to the siphon located 1 km
downstream at the end of this tract (station 3), some
resuspensions occur that causes turbidity and heavy
metal concentrations to exceed the values at the begin-
ning of the canal. The above observation is further
emphasized by a remarkable drop in concentrations
observed at the end of the beaten soil tract for other
parameters more sensitive to the self-purification pro-
cess, namely ammonia, phosphates and all bacteria.
For other parameters e.g. pH, chlorides, fluorides,
sodium, sulfates and dissolved oxygen, a significant in-
crease from the beginning to the end of the canal was
observed from the historic data. Whereas it is easy to
explain the increase of pH and dissolved oxygen (Fig.
3(a) and (b)) due to more photosynthesis occurring in
the canal, the explanation for increases of chlorides,
fluorides, sodium and sulfates is unclear. It can be rea-
sonably hypothesized that these parameters, which are
typical of marine waters, would increase as a conse-
quence of deposition of marine spray (Wilson, 1975)
transported from the Venice Lagoon, which is less than
5 km from the canal.
5. Conclusions
Along the course of the canal, a self-purification
process was observed to occur which was significant for
ammonium, total phosphates and bacteria. The largest
extent of this process took place in the first tract of the
canal (beaten soil) in which a net abatement of turbidity
and metals occurred as well. Evidence of an increase of
turbidity and metals in the second tract and the re-
markable increase of some parameters such as chlorides,
fluorides, sodium, sulfates in the whole tract are under
further investigation. It can be therefore concluded that
a more natural condition, such as that in the beaten soil
tract, can significantly favor natural self-purification
processes leading to improved water quality.
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Recommended