Indian ocean

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(Indian ocean): modeling of plumes, distribution of trace metals in sediments and effects of .... treatment plant with a new marine outfall in the vicinity of the Pointe ...

Impact of an industrial and urban sewage off a coral fringing reef at Mauritius (Indian ocean): modeling of plumes, distribution of trace metals in sediments and effects of the eutrophisation on coral reef communities. THOMASSIN B.A. (1), GOURBESVILLE Ph. (2), GOUT B. (1,3) and ARNOUX A. (4) (1) Centre d’Oéanologie de Marseille, CNRS-UMR n° 6540 “DIMAR”, Univ. de la Méditerranée, F-13007 Marseille (2) CNRS UPRESA n° 6046 “Analyse Spatiale”, Univ. Nice-Sophia Antipolis, F-06204 Nice Cedex 3 (3) SMSRB, DIRCEN, F-91310 Montlhéry (4) Lab. Hydrologie et Molysmologie aquatique, Fac. Pharmacie, Univ. de la Méditerranée, F-13005 Marseille ABSTRACT: Impact of industrial and urban sewage outflows off tropical high islands is an important environmental problem for many developing countries of the Indopacific and Caribbean regions. This problem is yet more accurate when coral reefs surround those and furthermore when the beach and marine tourism represents one of the main economic richness of these islands. Problem was studied at Mauritius I. analyzing the effects of the main sewer outflow of the island in front of a coral fringing reef. Pluridisciplinary technics were applied. Dispersion of the plume was modelized using a bidimentional code. Seawater physical and chemical classic parameters, plus fecal bacteriological ones, were also recorded and used for the modeling of oceanic waters and reefal ones. Sediment parameters of the inner reef flat were analyzed for grain-size indices and silt ratio, living and detrital organic matter contents, and then trace metals composition (Cu, Zn, Ni, Pb, Mn). ACP statistical analyzes performed allow to map the sewage outflow influences, showing a clear decreasing gradient from this point. 57° 45'E

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Qualitative analyzes of the distribution of macrobenthic species (algae and invertebrates) living of the reef, mainly those known in the SW Indian Ocean as biologic indicators of some edaphic conditions, were also done. Main edaphic facies induced by the sewage outflow plume was the growth of a high and dense carpet of the green algae, Ulva reticulata, creating anoxic conditions at the sediment-water interface. High densities of seaurchins, up to 100 ind./m2 seem more the consequences of an overfishing on the reef flat, destroying the carnivorous fishes (young seaurchin predators) to the benefit of the herbivorous ones, that a consequence of the proliferation of other algal assemblages. Nevertheless these seaurchins outbreaks are characteristic of degraded stages of the coral reef communities as it is well known in the SW Indian Ocean. Better than few seawater analyzes, comprehensive sediment analyzes (sediments being a better integrator of the environmental conditions in time) appear as the best technic for mapping quickly this kind of sewage influences, and for their monitoring.

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Fig.1. Maps of the study area in Mauritius. I. INTRODUCTION Mauritius (Mascarene archip.) has known during the last decade an important economical development. The fast increase of the urban population has introduced a new risk of pollution for the coastal environments. Port Louis, the major city of the island, concentrates more than 600,000 people and the majority of the industrial production units. Since the last decade, all the waste waters are collected and

0-7803-5045-6/98/$10.00 © 1998 IEEE

return to the ocean by two marine outfalls. The waste waters are rejected without treatment. One of these pipes is located since 1974 at the Pointe aux Sables site (Fig.1), on the NW leeward coast. The sewage network gathers Port Louis area, and Coromandel/Rose Hill industrial valley. A pipe of 1,000m length and 600mm diameter crosses the boat channel (“lagoon”) and whole reef flat before to open on the upper outer slope (2m deep). The effluent about 12,000m3.j-1 associated with several beach domestic seepages induces a

massive degradation of the coral reef and boat channel environments. The Sewerage Master Plan (1993) has proposed to suppress the actual outlet and to build a treatment plant with a new marine outfall in the vicinity of the Pointe aux Sables. The present study was carried out to investigate the plume impacts on water quality, sediment associate pollution and reef communities. These data were integrated in a modeling approach for understanding the present evolution and for testing several technical hypothesis for waste water treatment (new outfall or reuse of waters for agriculture). II. REEF SITE AND COASTAL HYDRODYNAMICS In the continuity of the NW wave-beaten rocky shore, the fringing reef of Pointe aux Sables (3,750m of reef front) overlays a basaltic substratum down to 25m deep. Reef flat and boat channel increases north-eastwards to open largely on the Grand River Northwest bay. Depth of the boat channel never reaches more than 1.5m, except along the sand beach where waves dig a gully. Along outer surfs the reef crest, as flatten “cor-algal” detrital spreads, is 10 to 20m wide. According to tide range (0.1m in neap to 0.9m in spring tides), water currents over the reef vary according to height of the breakers flowing over the reef front or penetrating by the SW blind passage. Seawards, the circum-insular stream, changing orientation according to seasons, baths the shallow island slopes and the outer plateau (50m deep) while ebb tide currents (up to 0.3m.s-1) and those induced by land winds carry out terrigeneous inputs. III.METHODS A. Modeling approach Data were integrated in a 2D hydrodynamics model (MIKE 21, developed by the Danish Hydraulic Institute) in the aim to describe the plumes and the associated impacts from the actual outfall and from a future once. MIKE 21 is a comprehensive modeling system for 2 dimensional freesurface flows (DHI, 1993; Gourbesville, 1996). The hydrodynamic model simulates unsteady twodimensional flows in one layer - vertically homogeneous fluids. 3 basic equations (conservation of mass and momentum integrated over the vertical) describe the flow and water level variations. The hydrodynamic module uses a so-called Alternating Direction Implicit (ADI) technique to integrate the equations in the space-time domain. The equation matrices resulting for each direction and each individual grid line are resolved by a Double Sweep (DS) algorithm. Water quality module of MIKE 11 describes the resulting concentrations of bacteria which treaten bathing water quality, oxygen depletion due to release of BOD, excess concentrations of nutrients and degradation of chemical substances. The model evaluates the spreading and fate of total coliforms and faecal coliforms. Enteric bacteria

die-off can be modeled by a first order reaction (decay). This decay rate is highly variable due to interaction by environmental factors. Main factors are presumably light, temperature and salinity (Mancini, 1978). Light conditions in the water column is the most important factor affecting the decay coefficient and is described using Beer’s law. The coliform mortality rate is expressed as a row of terms each describing the depence of the environmental factors. The formulation is:

Kd = Kd 0 ⋅ Θ s with

( sal )

⋅ Θ I I ⋅ ΘT

( T −20 )

(1)

K d : decay rate of faecal coliforms (1/day) K d 0 : decay rate at 20°C, a salinity of 0 (°/oo) and darkness(1/day) Θ s : salinity coefficient for decay rate sal : salinity (°/oo) ΘT : light coefficient for decay rate I : light intensity integrated over depth (kWm2) ΘT : temperature coefficient for decay rate T : water temperature (°C)

The model input parameters of the Coliform mortality model are the maximum insolation (kWm2) at noon and the light attenuation coefficient of the water column, alternatively the Secchi disc depth (m). The mean light intensity is found by integrating over the depth. The decay rate at 20°C, fresh water and darkness were estimated to 0.8 (1/day) and the temperature coefficient to 1.07. The T90 value - the time elapsed until 90% of the faecal coliforms are dead - is related to the first order decay rate by:

Ln(10 ) (2) T90 The calculated values of T90 by the model are between 2 Kd =

and 4.5 hours. These periods are very similar to the experimental results obtained in Mauritius for the Masterplan. The very good light conditions associated with the high temperature form the very efficient process for the pathogenic microorganisms destruction. Two grids were created on the study area (Fig.1b) from depth data: 100m and 20m spacings. The first model was used for calibration, validation and to produce coherent boundary conditions, the second for details modeling. B. Field studies Samples and observations were performed along transectlines from the upper outer slope, or the outer reef crest backwards to the coast line (beach). 1). Seawater parameters: nutrients and organic contents. Classic in situ hydrological seawater data were recorded at the beginning of the austral winter, dry and cooler season (June 1994): temperature, salinity, dissolved oxygen content (using multiprobe YSI data-recorder), nutrients (N-NO3, NNO2 and P-PO4, by colorimetry on deep-frozen samples), particular materials (total, organic and mineral seston, after filtration on Whatman GF/F 550° C roasted glass filters) and chlorophyll pigment contents (chlorophyll a, phaeopigments, after filtration on Whatman GF/C filters and measures out by the Lorenzen’s method).

2). Sediments. They were sampled by skin or SCUBA diving. They were analyzed for grain-size parameters (percent of fine fraction 1000 cells*0.1 l-1). B. Reef seawater parameters Mainly along the NE transects in the vicinity of the sewage outfall, an increase of the total particular material was observed from the outer reef flat (5-9.10 -3 g.l-1,values close to that of the open sea) towards the littoral (11-20. 10-3 g.l-1), but this material is mainly composed by mineral suspensions. Chlorophyll a content, as phyto-planktonic or resuspended phytobenthic biomass indicator, showed also the same gradient, but with a peak above the dense green-

2A

algae, Ulva reticulata, fields (up to 0.9-1 mg.m-3). Nitrite was high close to the sewage output (0.16 mg.l-1). But from the reef crest leeward, nitrite and nitrate contents increased along the same gradient (from