Modelling the impacts of Combined Sewer Overflows on the river Seine water quality

To achieve the objectives of the European Water Framework Directive (EWFD), the Seine basin Water Authority has constructed a number of prospective scenarios forecasting the impact of planned investments in water quality. Paris and its suburbs were given special attention because of their impact on the river Seine. Paris sewer system and overflow control is of major concern in future management plans. The composition and fate of the urban effluents have been characterized through numerous in situ samplings, laboratory experiments and modelling studies. The PROSE model was especially designed to simulate the impact on the river of both permanent dry-weather effluents and of highly transient Combined Sewer Overflow (CSO). It was also used to represent the impact of Paris at large spatial and temporal scales. In addition to immediate effects on oxygen levels, heavy particulate organic matter loads that settle downstream of the outlets contribute to permanent oxygen consumption. Until the late 90s, the 50 km long reach of the Seine inside Paris was permanently affected by high oxygen consumption accounting for 112% of the flux upstream of the city. 20% of this demand resulted from CSO. However, the oxygenation of the system is strong due to high phytoplankton activity. As expected, the model results predict a reduction of both permanent dry-weather effluents and CSOs in the future that will greatly improve the oxygen levels (concentrations higher than 7.3 mgO(2) L(-1), 90% of the time instead of 4.0 mgO(2) L(-1) in the late 90s). The main conclusion is that, given the spatial and temporal extent of the impact of many CSOs, water quality models should take into account the CSOs in order to be reliable.     

Impact of an intense combined sewer overflow event on the microbiological water quality of the Seine River

For a better understanding of the short and mid-term impacts of a combined sewer overflow (CSO) on the microbiological quality of the receiving river, we studied the composition of a CSO discharge and monitored during several hours the changes in the concentration of fecal indicator bacteria (FIB) in the impacted river water mass. The CSO occurred at the Clichy outfall (Paris agglomeration, France) in summer 2008 as a result of the most intense rainfall of the year. In 6h, 578, 705 m³ of sewage and 124 t of suspended matter (SM) were discharged into the Seine River. The CSO contained 1.5 × 10⁶E. coli and 4.0 × 10⁵ intestinal enterococci per 100 mL on average, and 77% of the E. coli were attached to SM. It was estimated that 89% of the CSO discharge was contributed by surface water runoff, and that resuspension of sewer sediment contributed to ∼75% of the SM, 10-70% of the E. coli and 40-80% of the intestinal enterococci. Directly downstream from the CSO outfall, FIB concentrations in the impacted water mass of the Seine River (2.9 × 10⁵E. coli and 7.6 × 10⁴ intestinal enterococci per 100 mL) exceeded by two orders of magnitude the usual dry weather concentrations. After 13-14 h of transit, these concentrations had decreased by 66% for E. coli and 79% for intestinal enterococci. This decline was well accounted for by our estimations of dilution, decay resulting from mortality or loss of culturability and sedimentation of the attached fraction of FIB.     

Production vs. respiration in river systems: an indicator of an "ecological status"

The Riverstrahler model of ecological functioning of large drainage networks validated on the Seine river system has been used for calculating the seasonal variations of Production and Respiration at various spatial scales (e.g. according to river orders). Based on the measurements of biological processes, the P/R ratio has led to an evaluation of the "ecological functioning", beyond the notion of "good ecological status". Furthermore, the effects, on the P/R ratio, of the geomorphological and climate factors characterizing the Hydro-Eco-Regions (HER) of the Seine watershed have been quantitatively explored with the model. Whereas one finds a typical upstream-downstream pattern of the P/R ratio variations under the traditional rural conditions that prevailed in the Seine basin until the end of the 18th century, this pattern is strongly affected by the changes in urban populations and the implementation of wastewater collection and treatment, more than by the specificity of the physical factors characterizing the different HER. We have also found that autotrophy (a P/R ratio >1) might leads to eutrophication symptoms when P exceeds 1-2 mg C m(-2) d(-1) and that heterotrophy of the system (P/R ratio< 1) would reveal organic pollution when R exceeds 1-2 mg C m(-2) d(-1), stocks and fluxes of organic matter being expressed in carbon unit. Consequently, the P/R ratio appears as a good indicator of the perturbations caused by human activities in the watershed. The Riverstrahler model is able to quantify this effect.     

Analysis of photosynthetic activity in the most polluted stretch of river Ganga

As a result of the increasing anthropogenic activities in the gangetic plain, Ganga water quantity as well as quality has declined over the years. A major effort to clean Ganga, named Ganga Action Plan (GAP) was instituted by the Government of India in 1984. The emphasis in GAP was on the reduction of organic load on the river through interception, diversion and treatment of wastewater reaching the river, thus maintaining the biochemical oxygen demand (BOD) and dissolved oxygen (DO) levels of river within the acceptable limits. A major criticism of GAP is that the significance of river ecology has not been addressed adequately during its conception and implementation. One of the important aspects from this perspective is the photosynthetic activity in the river Ganga. It has been postulated that photosynthetic activity plays an important role in maintaining high levels of DO in Ganga, and as a result the river can assimilate high organic loads without appreciable depletion in dissolved oxygen levels. Objective of the present study was to assess the photosynthetic activity and oxygen production rates in the river and correlate these values with various water quality parameters. Most polluted stretch of Ganga, which is known as the Kannauj-Kanpur stretch was chosen for this study. Based on the results of the study, it was concluded that despite implementation of phase I of GAP, and consequent diversion and reduction of organic loading to the river, both BOD and DO levels in the river has increased in the entire Kannauj-Kanpur stretch, except at Jajmau, where anaerobically treated effluent is discharged to the river. The nitrogen levels have also increased in the entire Kannauj-Kanpur stretch. Dissolved oxygen (DO) and alkalinity in the river water vary diurnally at all sites. Chlorophyll-a levels and oxygen production rates due to photosynthesis appear to be positively influenced by phosphate levels in the river water. Chlorophyll-a levels appear to be negatively correlated to the Ammonical and total Total Kjeldahl Nitrogen (TKN) content in the river water, suggesting the possibility of release of nutrients due to algal death and decomposition under certain circumstances.     

Experimental simulation of biodegradation in rivers: Oxygen, organic matter and biomass concentration changes

Dissolved oxygen (DO) and biochemical oxygen demand (BOD) concentration changes after an organic matter discharge into a river have been studied in the absence of oxygen transfer. According to these laboratory experiments, biodegradation of various organic compounds (glucose, glutamic acid, starch, ovalbumin and ethanol) in Seine river samples incubated at 15 30°C follow a biphasic behaviour. During a lag-phase of 10–20 h, DO decreases linearly (0.12 ppm h⁻¹ at 20°C), whereas BOD is constant. During a subsequent aerobic exponential phase, DO and BOD uptake are proportional and increase exponentially with time (0.13 h⁻¹ at 20°C). Using cell ATP as biomass indicator, the latter phase was shown to correspond to a cell division step. A kinetic model was developed for stimulating DO and BOD concentration changes after a waste water discharge at temperatures ranging between 15 and 30°C.     

Quality of dredged material in the river Seine basin (France). II. Micropollutants

Dredging rivers is needed to ensure safe navigable waters, rivers and waterways. To anticipate the management of dredged materials in the case of the river Seine basin, the quality of the sediments in the river is checked every 3 years before dredging operations. The river Seine Basin is heavily submitted to pollution pressure from nearby industrial activities and urban expansion of Paris and its region. Here, the micropollutant content of the sediment sampled in 1996, 1999 and 2000 before dredging is discussed compared to regulatory standards. The results indicate that most of the sediment samples from the river Seine basin are lightly to moderately contaminated with organic and inorganic micropollutants (heavy metals, PAH, PCB), which makes the management after dredging easier. This pollution is strongly correlated with the organic matter content and to the fine fraction (<50 microm) of the sediment. These results can lead to other management options than the ones already used in the river Seine basin: (1) dumping of lightly to moderately polluted sediments in quarries; and (2) physical treatment (sieving, hydrocycloning) of contaminated sediments issued from 'hot spots'.     

Determining the domestic specific loads of two wastewater plants of the Paris conurbation (France) with contrasted treatments: a step for exploring the effects of the application of the European Directive

The effluents of wastewater treatment plants (WTTP) discharged into the rivers considerably affect the biogeochemical functioning of the system. In this paper, we characterize both raw and treated domestic wastewater from two WTTPs of Parisian agglomeration using different process treatments (Achères WWTP with a secondary treatment and Colombes WWTP with a tertiary one). In addition to the classical variables, we analyse the input of bacteria, both the heterotrophs and the nitrifyers. Tertiary treatment leads to significantly decrease ammonium-specific load (< 2 g KjN inhab equ.(-1) instead of 9 g KjN inhab equ.(-1) for secondary treatment) and notably reduces the one of organic matter (approximately 2.5 g biological oxygen demand (BOD) inhab equ.(-1) instead of approximately 7.5 g BOD inhab equ.(-1) for secondary treatment); it is therefore promising to improve oxygen status of both the Seine river and its estuary. In terms of total bacterial biomass abatement (the heterotrophs mostly), bioreactors (at Colombes WWTP) eliminate 12% more bacterial biomass than the activated sludge treatment (at Achères WWTP). Regarding the nitrifying bacteria, a tertiary treatment in bioreactors eliminates reverse similar 90% of both nitrifying bacteria and nitrogen pollution of wastewater. Bacterial populations are characterized by large size bacteria (> 1 microm) with a higher growth rate, that represent in the treatment plant effluents 70% of the biomass. These large size bacteria have therefore a strong impact in the organic matter degradation and oxygen consumption. Relationships between classical physical-chemical variables routinely analysed in WWTPs laboratory and bacterial biomass (heterotrophic and nitrifying) are established, in order to quantify the ecological role of the allochthonous bacteria brought into the river system. In addition, domestic specific loads are calculated for both raw and treated effluents of the two types of WWTPs. As the application of the European Water Directive requires to upgrade the wastewater treatment at Achères WWTP as soon as 2007 for 90% nitrification and 30% denitrification and in 2015 for further denitrification (up to 70%), the results of this study can be taken as point-source constraints into the modelling approach already developed for the Seine basin, and chosen to test the implementation of the Water Frame Directive.     

The spatial and temporal trends of Cd, Cu, Hg, Pb and Zn in Seine River floodplain deposits (1994-2000)

Fresh floodplain deposits (FD), from 11 key stations, covering the Seine mainstem and its major tributaries (Yonne, Marne and Oise Rivers), were sampled from 1994 to 2000. Background levels for Cd, Cu, Hg, Pb, and Zn were established using prehistoric FD and actual bed sediments collected in small forested sub-basins in the most upstream part of the basin. Throughout the Seine River Basin, FD contain elevated concentrations of Cd, Cu, Hg, Pb and Zn compared to local background values (by factors>twofold). In the Seine River Basin, trace element concentrations display substantial downstream increases as a result of increasing population densities, particularly from Greater Paris (10 million inhabitants), and reach their maxima at the river mouth (Poses). These elevated levels make the Seine one of the most heavily impacted rivers in the world. On the other hand, floodplain-associated trace element levels have declined over the past 7 years. This mirrors results from contemporaneous suspended sediment surveys at the river mouth for the 1984-1999 period. Most of these temporal declines appear to reflect reductions in industrial and domestic solid wastes discharged from the main Parisian sewage plant (Seine Aval).     

Short-term variability of dissolved trace element concentrations in the Marne and Seine rivers near Paris

The concentrations of dissolved trace elements (Li, B, Mn, Cu, As, Rb, Sr, Mo, Cd, Ba, Pb) in the Marne and Seine rivers in the Paris urban area were monitored over a 2-year period. The resulting data indicated moderate contamination of waters by the most toxic elements (Cu, As, Cd and Pb). The River Marne upstream and the River Seine downstream of the city of Paris displayed similar concentrations. However higher fluxes of trace elements were observed in the Seine than in the Marne due to their different discharges. Li, B, Rb, Sr and Ba concentrations were correlated with river discharge and concentrations were higher during high river flow. This was interpreted as a dilution by discharge from a major natural or anthropogenic source. Mn, Cu, Mo, Cd and Pb concentrations were not correlated with discharge. Dissolved Mn, Cu and Cd increased rapidly in summer, whereas the concentration of Mo decreased. These variations were attributed to redox processes. During summer when the dissolved oxygen concentrations decrease, Mn, Cu, Cd and Pb are released into solution whereas Mo is immobilised. Like metals, variations in arsenic contents were not linked with discharge. Its similarity with phosphate distribution suggests similar controls involving phytoplankton uptake and release from sediments through organic matter mineralization.     

Dissolved and bioavailable contaminants in the Seine river basin

Diffusive Gradient in Thin Films (DGT) and Semi-Permeable Membrane Devices (SPMDs) were deployed in the Seine river basin in order to assess labile metals and truly dissolved Polycyclic Aromatic Hydrocarbons. We show that the tools are reliable in aquatic environments to assess the speciation of dissolved contaminants and hence provide a good insight into the potential bioavailability of contaminants. The deployment of the DGT and SPMDs in contrasting environments in the Seine river basin allowed distinction to be made of availability of contaminants between headwater streams and much more impacted river reaches and an assessment of bioavailability. At the stations under urban influence, the impact of dissolved organic matter on both copper and PAHs bioavailability is less pronounced than at upstream stations, where humic substances dominate.     

Relationship of the raw water quality to mutagens detectable by the Ames Salmonella/microsome assay in a drinking-water supply

Mutagenic activity of non-volatile compounds in dichloromethane extracts of a drinking-water supply derived by conventional treatment (flocculation, sedimentation, sand filtration and chlorination) of river water, was related to the concentration of volatile halogenated hydrocarbons. Limited relationship also existed between mutagenic activity and the total organic carbon content of both the raw and treated water. The dry mass of extracts from neither raw nor treated water correlated with these parameters. Mutagenic activity of the treated water increased during spring, possibly as a result of the first showers of the rainy season washing mutagen precursors into the river, rising temperature, and actively growing algae increasing the concentration of mutagen precursors in the river. After increased river flow, mutagenic activity of the treated water decreased and reached a stable summer level. Mutagenic activity was not detected in the raw water, which indicates that mutagens in partially purified wastewater effluents discharged into the river about 40 km upstream, were removed or inactivated in the river environment. Properties of the drinking-water extracts showed that the mutagens formed during treatment of the water supply consisted mainly of neutral organic compounds inducing frameshift mutations without liver activation. Although the specific activity of mutagens in the extracts does not indicate that the treated water constituted a health hazard, the results illustrate that regular surveillance of potential carcinogens in drinking-water derived by conventional treatment of water subject to pollution may be important.     

Critical budget of metal sources and pathways in the Seine River basin (1994-2003) for Cd, Cr, Cu, Hg, Ni, Pb and Zn

River basin metal pollution originates from heavy industries (plating, automobile) and from urban sources (Paris conurbation: 2740 km(2), 9.47 million inhabitants). The natural sources of metal have been found to be limited due to sedimentary nature of this catchment and to the very low river sediment transport (10 t km(-2) y(-1)). Several types of data have been collected to build the metal budget within the whole Seine River basin: field surveys, economical statistics and environmental models. Environmental contamination and related fluxes have been measured on atmospheric fallout, rural streams particles, and Seine River particles upstream and downstream of Paris and at river mouth. Metal pathways and budgets have been set up for (i) a typical cultivated area, (ii) a Paris combined sewer system, (iii) Paris conurbation and (iv) the whole catchment metal retention effect in floodplain and dredged material. Metal fluxes to the estuary have been decomposed into natural, urban domestic and other sources. The latter are within 1-2 orders of magnitude larger than waste water fluxes directly released into rivers according to an industrial census. These fluxes have been further compared to the annual use (1994-2003) of these metals. Metal excess fluxes exported by the river are now a marginal leak of metal inputs to the catchment (i.e. "raw" metals, metals in goods, atmospheric fallout), generally from 0.2 to 5 per thousand. However, due to the very limited dilution power in this basin, the contamination of particles is still relatively high. The Seine River basin is gradually storing metals, mostly in manufactured products used in construction, but also in various waste dumps, industrial soils, agricultural and flood plain soils.     

Carbon source utilization‐based metabolic activity of Salmonella Oranienburg and Salmonella Saintpaul in river water

The presence of introduced or non‐native bacteria in river water can create a selective pressure due to their ability to overcome fluctuations in physicochemical conditions and carbon source availability. The carbon source concentrations (monosaccharides and polysaccharides), physicochemical parameters (turbidity, total dissolved solids, biochemical oxygen demand, temperature and pH) and metabolic activity of Salmonella Oranienburg and Saintpaul were determined in water from the central Sinaloa state rivers. This study's results revealed that the carbohydrate availability and physicochemical conditions of river water make it a suitable niche for the establishment of Salmonella serotypes. Metabolic profiles showed that Salmonella Typhimurium could consume a wide variety of carbon sources but only at moderate levels; in contrast, Salmonella Oranienburg and Salmonella Saintpaul demonstrated intense carbon source utilization of a limited diversity of carbon sources. Carbohydrates, amino acids and carboxylic acids were the metabolites most utilized by the environmental Salmonella strains, demonstrating their superior ability to adapt to and survive in river water.     

Characterization and treatment of waste discharged from high-density catfish cultures

Warm condenser cooling water that is discharged from a steam-electric power plant was used to achieve optimum temperature levels for high-density production of catfish. The heated water extended the catfish growing season, enhanced growth rates, and increased production poundage. Characteristics of the wastewater from the high-density warm water culture (90 to 160 g of fish per liter of water and average water temperature of 28.2°C) were evaluated including evaluation of settleable solids, suspended solids, biochemical oxygen demand, chemical oxygen demand, total and soluble phosphates, ammonia nitrogen, organic nitrogen, nitrate and nitrate nitrogen, and fecal coliform bacteria for hourly and daily variations and for the effects of feeding on waste production. Unit waste loads were computed and compared to limits promulgated by the U.S. Environmental Protection Agency. A pilot-scale sedimentation basin was evaluated for reducing settleable solids, suspended solids, chemical oxygen demand, and phosphates. Difficulties in reducing fecal coliforms are discussed.     

Impact of combined sewer overflow on urban river hydrodynamic modelling: a case study of the Chicago waterway

Combined sewer overflow (CSO) can be a critical inflow source for urban rivers during storm events. This paper presents a case study of the Chicago waterway. A three-dimensional (3D) river hydrodynamic model was developed and integrated with an urban rainfall-runoff model using the Open Modelling Interface (OpenMI). Both the effects of CSO discharge on river and river water levels on CSO outlets were considered by the integrated model. A historical storm, which was similar to a 100-year return period rain event, was simulated and compared with field measurements. This study highlights the necessity of quantifying CSO for hydraulic modelling of urban rivers under extreme storm event conditions, and shows that an integrated hydrologic and hydraulic approach can be used to address this challenge. The 3D river hydrodynamic model can deal with the complex hydrodynamics at river confluences and provide better hydrodynamic results for water quality modelling in the future.     

Fractionation of dissolved organic matter from surface waters using macroporous resins

To obtain fractions enriched with biodegradable dissolved organic carbon (BDOC) or with organic compounds responsible for the chlorine demand (CID) and for trihalomethane formation potential (THMFP), Seine river water samples were percolated on various macroporous resins (anionic, cationic and non-ionic) and compared with granulated activated carbon (GAC). In addition, measurement of UV absorbance at 254 nm and the fluorescence index (λ_excitation 320 nm) had allowed to follow up the retention of dissolved organic matter by the different adsorbants. In contrast to cationic and non-ionic resins, anionic resins confirm their excellent retention capacity of organic compounds responsible for UV 254 absorbance and fluorescence index. The relative values of BDOC/DOC ratio (mg-C/mg-C) are slightly increased in the effluents of anionic resins, indicating that they retain a little preferentially the refractory fraction instead of the biodegradable fraction. There is no significant difference between the ratio of CID/DOC (mg-Cl₂/mg-C) in influent and effluent of anionic resins. Cationic resin has a low capacity for retention of DOC, but they seem to retain significantly the organic compounds responsible for CID. The capability of anionic resins to retain THMFP is similar to that of GAC.     

PCB pollution behaviour in the River Seine

The contamination of the River Seine (France) and its main tributaries (Yonne, Marne, Oise) have been studied, under different hydrological conditions, at the end of spring before the low watermark and at the beginning of autumn after the first swelling of flow.The sector under study consists of the Seine basin, centered on the Paris area; the variability of PCB concentration in water, apart from flow dependence, shows that important lateral discharges are involved. The pollution level is particularly noticeable at Paris and downstream from Paris and the median concentration in sediments reaches 3800 ng l⁻¹ (ppb). Data on particulate organic carbon and various grain size fractions show the PCB to be preferentially adsorbed on organic-rich sedimentary particles.Partition coefficients between particulate matter and liquid phase have been computed and show that the liquid phase has the main role in micropollutant transport.     

Intertidal trace metal concentrations in some sediments from the Humber estuary

The trace concentrations of a number of metals including chromium, zinc, lead, vanadium and copper have been determined in intertidal sediments from the south bank of the Humber estuary. The values have been associated with the particle size and organic carbon content of the sediments and the levels of metal concentration are compared generally with levels found elsewhere in estuarine and river sediments. The high values and trends in metal content of the sediments compared with “natural” levels may be ascribed to industrial and domestic effluents discharged into the estuary from a number of outfalls within the area. Methods of analysis to distinguish between natural and effluent metal contents of sediments are discussed.     

Nitrobacter and Nitrospira genera as representatives of nitrite-oxidizing bacteria: detection, quantification and growth along the lower Seine River (France)

Pollution from agriculture and urban effluents influences the ecology and biochemical functioning of the Seine River. Nitrification dominates nitrogen transformations downstream of the effluents of the Paris wastewater treatment plant (WWTP) at Achères, treating, by activated sludge the wastewater of 6.5 million inhabitant equivalents from Paris and its suburbs, without nitrification and denitrification treatment. It discharges effluents containing large amounts of nitrogen, ammonium mostly (30 mg L⁻¹ N-NH₄⁺ L⁻¹), on average 45 mg L⁻¹ of suspended particulate matter, high quantities of total organic carbon (30 mg C L⁻¹) largely biodegradable (40%), and high concentration in total phosphorus (3 mg Tot P L⁻¹), as well as microorganisms. Ammonium, brought into the river system, is slowly nitrified in the lower Seine River and especially in the freshwater estuary. The nitrifying activities can be observed by measuring inorganic nitrogen compound concentrations and potential activities. To understand the contributions of the WWTP effluents, the upstream agricultural runoff water and the Seine tributaries, it is useful to investigate the bacterial community. Whereas ammonia oxidation has been widely studied, the second step, i.e. nitrite oxidation, is less well understood. We have previously analysed the ammonium-oxidizing bacterial (AOB) community in the Seine (Cébron, A., Berthe, T., Garnier, J., 2003. Nitrification and nitrifying bacteria in the lower Seine River and estuary (France). Appl. Environ. Microbiol. 69, 7091–7100; Cébron, A., Coci, M., Garnier, J., Laanbroek, H.J., 2004. DGGE analysis of the ammonia oxidizing bacterial community structure in the lower Seine River: impact of the Paris wastewater effluents. Appl. Environ. Microbiol. 70, 6726–6737), and focus here on the composition of the nitrite-oxidizing bacterial (NOB) community. As no general molecular probe targeting all known NOBs is currently available, we chose to target and quantify (by competitive PCR) the two genera Nitrobacter and Nitrospira assumed to be the major players in nitrite oxidation in freshwater environments. Nitrobacter species were dominant in the upstream Seine River basin but Nitrospira was the dominant NOB downstream of the WWTP. These two genera were equally represented in WWTP effluents. In the Seine River estuary, especially in the salinity gradient, the Nitrobacter proportion increases and that of Nitrospira disappears, possibly due dilution by seawater.     

The impact of combined sewer overflows on the dissolved oxygen concentration of a river

As described in the preceding paper by Harremoës (Harremoës, Water Res.16, 1093–1098, 1982) it is important to distinguish between removal and degradation of organic matter for non-steady-state discharges to rivers. These effects were investigated to determine the impact of combined sewer overflows on the dissolved oxygen concentration of a small river. Two different effects on the DO-concentration in the receiving river were observed during and after the passage of the bulk of combined sewage discharged at an existing outlet:

1. 1. An immediate effect caused mainly by degradation of the soluble BOD-fraction in the water body and by direct absorption and degradation of organic matter at the bottom.

2. 2. A delayed effect caused by degradation of adsorbed soluble, colloidal and fine particulate organic matter. After the passage of the bulk discharge a delayed effect on the DO-concentration in the river would be observed. This delayed effect lasted 12–24 hours after the discharge event.

Only 4% of the discharged organic matter was degraded during passage of the investigated stretch of the river, approx. 4 km. On the other hand about 35% of the discharged organic matter was removed by transfer to the bottom sediments. The rest was carried past the stretch of river investigated. This results in a rate of adsorption from the water phase of k = 9 m day⁻¹. The deposited organic matter was degraded with a first order reaction rate of K₄ = 0.75 day⁻¹.