HEAVY METAL DISCHARGE FROM OSELLINO CANAL TO THE VENICE LAGOON: PRELIMINARY RESULTS OF THE “DRAIN” PROJECT.

Roberto Zonta, Flaviano Collavini, and Luca Zaggia (National Research Council - I.S.D.G.M., S.Polo 1364, Venezia - 30125, Italy e-mail: zonta@flux.isdgm.ve.cnr.it); Cristina Marinelli and Omar E. Fagarazzi (SELC Inc., Via Castellana 40, Mestre - Venezia- 30174, Italy)

ABSTRACT

Data presented are the result of the investigation on total and dissolved heavy metals in the Osellino Canal (Venice Lagoon – northern Italy) for the period between June and October 1998, when extremely different flow regime, from base flow to peak flood, occurred. This provided an overview on processes governing the behaviour of total and dissolved metals under varying river discharges. An estimate of the corresponding load transferred from the studied basin to the lagoon during the monitoring period is also given.

INTRODUCTION

The drainage basin of the Venice Lagoon (northern Italy - Figure 1 a,b) is developed on a highly populated (about one million of inhabitants) and industrial floodplain. About 70% of its 1800 km² surface area is also occupied by cultivated lands. The estimate of annual freshwater and contaminants load from the basin is the main focus of the DRAIN project (“DeteRmination of pollutAnt INputs from the drainage basin”).

Figure 1. (a) Location of the Venice Lagoon (northern Italy). (b) Map of drainage basin of the lagoon; the Marzenego River basin is shadowed. (c) Map of the lower Marzenego basin; the location of the stations for discharge and physico-chemical measurement are indicated.

The research started in May 1998 on behalf of the Consorzio Venezia Nuova - Magistrato alle Acque di Venezia (Italian Ministry of Public Works) and will be concluded in August 2000. Fresh water discharge in the eleven main tributaries of the lagoon is continuously measured and heavy metal, nutrient and organic micro-pollutant concentrations are determined. Among these tributaries, the Marzenego River - Osellino Canal (Figure 1b) is of particular interest, because it receives the effluents from the large urban district of Mestre (200,000 inhabitants).

The river system, which originates on the spring belt of the Venetian floodplain, drains a crop area of 47 km² and has an overall length of 47 km. In the upper course, the stream is denominated Marzenego River and, after traversing the urban centre of Mestre, it is named Osellino Canal. It the follows a 3 km straight course (Figure 1c) finally discharging in the Venice Lagoon through Le Rotte Canal. The present contribution describes the main results of the study of circulation and heavy metal behaviour in the first 5 months of monitoring, corresponding to the Phase 1 of the DRAIN project (from 18^th of May to 17^th of October 1998).

METHODS

The location of the sections for the acquisition of discharge and physico-chemical variables are shown in Figure 1c. Section 1 is positioned immediately downstream of Mestre, to monitor the input of the urban wastewater not collected by the sewer system. Starting from the second half of July 1998, a self-recording current meter was employed in section 2; the recorded time series of current speed were calibrated by manual discharge evaluations with the normal velocity-area technique, obtaining a continuous time series of freshwater discharge. Two other sections (3 and 4) are respectively located in the Le Rotte Canal and in the eastern Osellino branch.

The sampling strategy was designed in order to observe hourly-to-monthly variations along the vertical profile, according to the tide excursion and different hydrologic regimen. Water samples, collected in sections 1 and 2 for the determination of total and dissolved concentrations (Fe, Mn, As, Cd, Cr, Cu, Hg, Ni, Pb and Zn) were treated according to the EPA method 3005A/92. The metal analyses were performed by ICP-MS, following the EPA method 6020/94 rev.0.

RESULTS AND DISCUSSION

The trend of the measured freshwater discharge is reported in Figure 2. Since the continuous record started on July 21^st, an estimate based on a large set of point measurements is given for the previous period. The average rainfall on the Osellino-Marzenego basin is also reported on top of the figure.

Until the first half of July 1998, the freshwater discharge was appreciable ("base flow", average discharge, Q_A = 1.6 m³s^-1), because of the frequent rainfall events over the catchment area. The low discharge of the following period ("drought", Q_A = 0.8 m³s^-1) was occasionally interrupted by a few relative increases of the flow, induced by regulation manoeuvres in small upstream tributaries. In September, a new period characterised by rainfall events restored the base flow (Q_A = 1.8 m³s^-1), until a flood event (October 5^th through 9^th) dramatically raised the average river discharge up to a value of 14 m³s^-1. Return periods as high as 40 years in the upper and middle Marzenego basin, and about 3 years in the district of Mestre were evaluated from rainfall data by the Gumbel extreme value method (Gumbel, 1958; Richards, 1982).

The observed changes of the river regimen are accompanied by relevant variations in the concentration of metals. Figure 3 shows the trend of average total and dissolved Mn and Zn concentrations in freshwater samples, which are representative of the behaviour of the major part of the analysed elements. If compared to the base flow conditions, the dissolved concentrations in the drought period are higher for almost all metals. This increase, which is limited for Zn, As, Cd and Pb, is instead considerable for Mn, Cu and Ni, and is possibly related to the release of metals from the anoxic sediment of the channel bottom. This process is favoured by both the slack dynamics and the progressive decay of the water quality in summer months, which is reflected by decreasing trend of redox potential, dissolved oxygen and pH. Once in the water column, metals remain in solution as far as reducing conditions are maintained.

The flood event of October caused a 10-fold average increment of suspended particle matter, determining a sharp increase of metal concentrations. This peak of concentrations occurred not only for metals associated to the lattice of clay particle and amorphous hydrous-oxides (Fe, Mn, Cr and Ni), but also for anthropogenic elements such as Zn, Pb, Cu and, to a minor extent, As. Despite the dramatic changes in the river regime, dissolved concentrations did not show significant variations in flood conditions.

Table 1 shows the estimated average values of total and dissolved heavy metal loads. Data refer to the three different flow regimen (base flow, drought and flood) and are also presented as the total load for the entire monitoring period. Finally, the table reports the daily loads and the ratios between values of flood versus both the other two regimen.

Generally, it can be observed that, with the exception of As and Hg, the flood transports a percentage greater than 50% of the total metal load (from 52% to 78% for Fe). The high discharge, combined to increased concentration of the particulate metal fraction, yield by soil erosion and re-mobilisation of channel materials, strongly affected the rate of total metal transport.

If compared to daily values computed for the flood conditions, the daily load of total Fe and, to a minor extent, of Pb and Zn, for the drought period, is practically negligible. Although the dissolved concentrations do not differ during the flood, the considerable increase in flow magnitude determines a proportional increase of dissolved load. Anyway, with respect to the whole monitoring period, the base flow transfers the larger proportion of the dissolved load (from 50% for Ni to 65% for Cr).

Total		Fe	As	Cd	Cr	Mn		Hg		Ni		Pb		Cu		Zn
Base flow (91 days)		8322 (19)	100 (53)	1.1 (35)	18 (29)		677 (36)		1.0 (69)		44 (36)		38 (22)		71 (27)		327 (31)
Drought (41 days)		1240 (3)	29 (15)	0.2 (6)	5 (8)		224 (12)		0.2 (18)		11 (9)		9 (5)		17 (6)		53 (5)
Flood (5 days)		34248 (78)	59 (32)	1.9 (59)	40 (63)		975 (52)		0.2 (13)		66 (55)		125 (73)		173 (67)		680 (64)
Load (137 days)		43810	188	3.2	63		1876		1.4		121		172		261		1060
daily	Base flow	91	1.1	0.012	0.2		7		0.01		0.5		0.4		0.8		3.6
	Drought	30	0.7	0.005	0.1		5		0.01		0.3		0.2		0.4		1.3
	Flood	6850	11.8	0.375	8.0		195		0.04		13		25		35		136
	Flood/Base	75	11	31	39		26		3		28		60		45		38
	Flood/Drought	226	17	78	69		36		6		51		120		85		106
Dissolved		Fe	As	Cd	Cr		Mn		Hg		Ni		Pb		Cu		Zn
Base flow (91 days)		643 (57)	93 (63)	0.40 (60)	15 (65)		528 (63)		-		24 (50)		7 (56)		29 (51)		84 (62)
Drought (41 days)		155 (14)	26 (18)	0.05 (7)	3 (15)		176 (21)		-		7 (14)		3 (23)		8 (14)		25 (19)
Flood (5 days)		333 (29)	28 (19)	0.22 (33)	5 (20)		135 (16)		-		18 (36)		3 (21)		20 (35)		26 (19)
Load (137 days)		1131	147	0.67	23		839		-		49		13		57		135
daily	Base flow	7	1.0	0.004	0.2		6		-		0.3		0.1		0.3		0.9
	Drought	4	0.6	0.001	0.1		4		-		0.2		0.1		0.2		0.6
	Flood	67	5.6	0.044	0.9		27		-		3.5		0.5		3.9		5.2
	Flood/Base	9	5	10	6		5		-		13		7		12		6
	Flood/Drought	18	9	39	12		6		-		21		7		20		9

Table 1. Total and dissolved metal loads (kilograms) discharged in the Venice Lagoon from the Osellino-Marzenego tributary basin. The percentages of total load are reported in parentheses. For dissolved Hg, which was always below the analytical detection limit, the loads are not computed.

The monitoring activity of the research project is still in progress and the continuous input of data determines a progressive upgrade of the results and a refinement of load estimates. However, the information acquired during the Phase 1 can provide useful indications on the dynamics of pollutant transfer from the drainage basin. The occurrence of an exceptional flood event in a period mainly characterised by low-to-base flow conditions permitted, in fact, the comparison of freshwater discharge and pollutant load from the studied system to the lagoon in extremely different flow regime.

REFERENCES

Gumbel E J (1958), J. Inst. Water Eng. 12: 157-184.

Richards K (1982), Rivers, Form and Processes in Alluvial Channels. New York, Meuthen.