Pollutant fate and spatio-temporal variability in the choptank river estuary: Factors influencing water quality

Restoration of the Chesapeake Bay, the largest estuary in the United States, is a national priority. Documentation of progress of this restoration effort is needed. A study was conducted to examine water quality in the Choptank River estuary, a tributary of the Chesapeake Bay that since 1998 has been classified as impaired waters under the Federal Clean Water Act. Multiple water quality parameters (salinity, temperature, dissolved oxygen, chlorophyll a) and analyte concentrations (nutrients, herbicide and herbicide degradation products, arsenic, and copper) were measured at seven sampling stations in the Choptank River estuary. Samples were collected under base flow conditions in the basin on thirteen dates between March 2005 and April 2008. As commonly observed, results indicate that agriculture is a primary source of nitrate in the estuary and that both agriculture and wastewater treatment plants are important sources of phosphorus. Concentrations of copper in the lower estuary consistently exceeded both chronic and acute water quality criteria, possibly due to use of copper in antifouling boat paint. Concentrations of copper in the upstream watersheds were low, indicating that agriculture is not a significant source of copper loading to the estuary. Concentrations of herbicides (atrazine, simazine, and metolachlor) peaked during early-summer, indicating a rapid surface-transport delivery pathway from agricultural areas, while their degradation products (CIAT, CEAT, MESA, and MOA) appeared to be delivered via groundwater transport. Some in-river processing of CEAT occurred, whereas MESA was conservative. Observed concentrations of herbicide residues did not approach established levels of concern for aquatic organisms. Results of this study highlight the importance of continued implementation of best management practices to improve water quality in the estuary. This work provides a baseline against which to compare future changes in water quality and may be used to design future monitoring programs needed to assess restoration strategy efficacy.     

Assessing the improvement of the Bilbao estuary water quality in response to pollution abatement measures

To evaluate the success of almost 20 years of pollution abatement in the Bilbao estuary watershed in northern Spain, we analyzed temporal trends in pollution discharges and water quality from 1993 to 2003. Over that period a great portion of the raw wastewater discharge was intercepted and treated, leading to a significant reduction in the pollution load to the estuary (51.8% in biochemical oxygen demand, 70.9% in ammonia nitrogen and 81.9% in faecal coliforms). Temporal trends of mean annual levels of water quality variables showed statistically significant increases in dissolved oxygen saturation (between 2.04 and 4.11%/year) and decreases in ammonia nitrogen (between -4.15 and -175.75 microM NH3/year) and faecal coliforms concentrations (from 2.55 x 10(5) to 2.13 x 10(4) CFU/100ml). The improvement of the Bilbao estuary water quality reported in this paper is primarily attributed to the pollution abatement measures accomplished by the local water authority. Finally, as a result of these pollution control efforts, European bathing water quality standards were met at local beaches.     

基于有限单元算法的三峡水库水质模拟

三峡水库建成蓄水后，随着库区经济的发展，污染物排放量有进一步增加的趋势。为全面了解水库建成后污染物排放对水质的影响及水质变化趋势，将二维水质模型和有限单元算法引入到水质模拟预测中，开展了二维水质模型模拟三峡库区水质的适用性研究。经验证，NH3-N模拟值与实测值的误差最大，为23．8％；BOD5模拟值与实测值的误差最小，为5．3％。说明所采用的模型具有较高的预测精度，能够用来预测三峡水库蓄水后的水质变化情况。预测结果表明，三峡水库建成后，重庆至三峡大坝的污染物浓度总体上呈现出逐渐减小的趋势，但是受城市工业废水、生活污水和农业非点源排放的影响，城市江段和支流河口区的岸边污染物浓度明显高于长江中心处的污染物浓度。     

Seasonal water quality variations in a river affected by acid mine drainage: the Odiel River (South West Spain)

This paper intends to analyse seasonal variations of the quality of the water of the Odiel River. This river, together with the Tinto River, drains the Iberian Pyrite Belt (IPB), a region containing an abundance of massive sulphide deposits. Because of mining activity dating back to prehistoric times, these two rivers are heavily contaminated. The Odiel and Tinto Rivers drain into a shared estuary known as the Ría of Huelva. This work studies dissolved contaminant data in water of the Odiel River collected by various organisations, between October 1980 and October 2002, close to the rivers entry into the estuary. Flow data for this location were also obtained. The most abundant metals in the water, in order of abundance, are zinc (Zn), iron (Fe), manganese (Mn) and copper (Cu). Arsenic (As), cadmium (Cd) and lead (Pb) are also present but in much lower quantities. The quality of the river water is linked to precipitation; the maximum sulphate, Fe, Zn, Mn, Cd and Pb concentrations occur during the autumn rains, which dissolve the Fe hydroxysulphates that were precipitated during the summer months. In winter, the intense rains cause an increase in the river flow, producing a dilution of the contaminants and a slight increase in the pH. During spring and summer, the sulphate and metal concentration (except Fe) recover and once again increase. The Fe concentration pattern displays a low value during summer due to increased precipitation of ferric oxyhydroxides. The arsenic concentration displays a different evolution, with maximum values in winter, and minimum in spring and summer as they are strongly adsorbed and/or coprecipitated by the ferric oxyhydroxides. Mn and sulphates are the most conservative species in the water. Relative to sulphate, Mn, Zn and Cd, copper displays greater values in winter and lower ones in summer, probably due to its coprecipitation with hydroxysulphates during the spring and summer months. Cd and Zn also appear to be affected by the same process, although to a lower degree than Cu, experiencing a slight reduction in summer with respect to Mn and sulphates.     

Influence of river discharge patterns on the hydrodynamics and potential contaminant dispersion in the Douro estuary (Portugal)

Freshwater input to estuaries is a fundamental feature of these ecosystems, which may be profoundly altered by river damming as human needs for water consumption, irrigation or energy production increase. The Douro estuary is limited upstream by a dam since 1985, which reduced its length by ca. 60%. Freshwater inputs to the estuary are now irregular and greatly dependent on hydroelectric power demand; values ranging from zero to over 1000 m³ s⁻¹, in a matter of hours, especially in summer are common. In the present study, a three-dimensional hydrodynamic model was applied to the Douro estuary. The model was calibrated and validated against water elevation, current velocity, salinity and temperature data. Thereafter, it was used to analyse the effects of different flow regimes and magnitudes on estuarine hydrodynamics and contaminant dispersion. Results obtained suggest that the highly variable flow regimes, currently observed in the Douro, tend to reduce water column stratification and to enhance seawater intrusion, when compared with flow discharges of similar average magnitude, but lower variability. Stable flows seem to be the most effective in dispersing contaminants eventually introduced into the estuary through its small river tributaries. Overall results suggest that flow management may have important effects on estuarine hydrodynamics through non-linear interactions between flow magnitude and variability.     

Geochemical and microfaunal proxies to assess environmental quality conditions during the recovery process of a heavily polluted estuary: the Bilbao estuary case (N. Spain)

This study explores the eventual environmental improvement of the Bilbao estuary (northern Spain), from 1997 to 2006, in order to assess current estuarine restoration being undertaken as part of a Revitalization Strategic Plan. The monitoring programme is based on spatial and temporal variation in the distributions of both benthic foraminiferal assemblages and heavy metals contained in surficial sediments from the polluted intertidal flats. The overall pattern shows a decreasing metal concentration; however, reversals to this trend are noticed in the middle estuary. From 2000 to 2003, a significant decrease in heavy metal concentration was observed which is most likely related to the implementation in 2001-2002 of the biological treatment at a central wastewater treatment plant. Although the metal concentration decreased by 85% for some elements, these values still remain considerably high. No significant change occurred between 2003 and 2006 in metal concentrations. During the monitoring period the estuary channel presented environmental conditions close to azoic in 3/4 of its watercourse. Only in the year 2006, the two lowermost estuarine samples exhibited an increase in microfaunal densities. These samples were dominated by pollution-resistant estuarine species. The fact that estuarine sediments quality is recovering very slowly seems to be caused mainly by the resuspension of accumulated contaminants in the sediments due to dredging and working activities in the estuary. Thus, local authorities should consider also the clean up of the pollutants stored in the sediments, not seeking only the achievement of the water quality standards, as these polluted sediments can act as source of contamination.     

Biogeochemical value of managed realignment, Humber estuary, UK

We outline a plausible, albeit extreme, managed realignment scenario ('Extended Deep Green' scenario) for a large UK estuary to demonstrate the maximum possible biogeochemical effects and economic outcomes of estuarine management decisions. Our interdisciplinary approach aims to better inform the policy process, by combining biogeochemical and socioeconomic components of managed realignment schemes. Adding 7494 ha of new intertidal area to the UK Humber estuary through managed realignment leads to the annual accumulation of a 1.2 x 10(5) t of 'new' sediment and increases the current annual sink of organic C and N, and particle reactive P in the estuary by 150%, 83% and 50%, respectively. The increase in intertidal area should also increase denitrification. However, this positive outcome is offset by the negative effect of enhanced greenhouse gas emissions in new marshes in the low salinity region of the estuary. Short-term microbial reactions decrease the potential benefits of CO(2) sequestration through gross organic carbon burial by at least 50%. Net carbon storage is thus most effective where oxidation and denitrification reactions are reduced. In the Humber this translates to wet, saline marshes at the seaward end of estuaries. Cost-benefit analysis (CBA) was used to determine the economic efficiency of the Extended Deep Green managed realignment. When compared to a 'Hold-the-Line' future scenario, i.e. the present state/extent of sea defences in the estuary, the CBA shows that managed realignment is cost effective when viewed on >25 year timescales. This is because capital costs are incurred in the first years, whereas the benefits from habitat creation, carbon sequestration and reduced maintenance costs build up over time. Over 50- and 100-year timescales, the Extended Deep Green managed realignment scenario is superior in efficiency terms. The increased sediment accumulation is also likely to enhance storage of contaminant metals. In the case of Cu, a metal that currently causes significant water quality issues, Cu removal due to burial of suspended sediment in realigned areas translates to a value of approximately pounds sterling 1000 a(-1) (avoided clean up costs). Although this is not formally included in the CBA it illustrates another likely positive economic outcome of managed realignment. Although we focus on the Humber, the history of reclamation and its biogeochemistry is common to many estuaries in northern Europe.     

Self-cleaning in an estuarine area formerly affected by 226Ra anthropogenic enhancements: numerical simulations

A numerical model of the Odiel-Tinto estuary (Spain) has been developed to study the self-cleaning process that was evidenced from 226Ra measurements in water and sediments collected in the period 1999-2002, after direct releases from a fertilizer complex ceased (in 1998). The hydrodynamic model is first calibrated, and standard tidal analysis is carried out to calculate tidal constants required by the dispersion code to determine instantaneous water currents and elevations over the estuary. In this way, long-term simulations may be carried out. The dispersion code includes advective/diffusive transport of radionuclides plus exchanges with bottom sediments described through a kinetic approach. The dispersion model is first tested by comparing computed and measured 226Ra concentrations over the estuary resulting after releases in the Odiel and Tinto rivers. Next, it is applied to simulate the self-cleaning process of the estuary. The time evolution of radium concentrations in bed sediments is in generally good agreement with observations. The computed sediment halving time of the estuary is 510 days, which also is in good agreement with that estimated from measurements.     

The contribution of science to risk-based decision-making: lessons from the development of full-scale treatment measures for acidic mine waters at Wheal Jane, UK

The use of risk-based decision-making in environmental management is often assumed to rely primarily on the availability of robust scientific data and insights, while in practice socio-economic criteria are often of considerable importance. However, the relative contributions to decision-making made by scientific and socio-economic inputs are rarely assessed, and even less commonly reported. Such an assessment has been made for a major remediation project in southwest England, in which some 300 l/s of highly acidic, metalliferous mine waters are now being treated using oxidation and chemical neutralisation. In the process of reaching the decision to commission the treatment plant, a wide range of scientific studies were undertaken, including: biological impact assessments, hydrogeological investigations of the effect of pumping on the flooded mine system, and hydrological and geochemical characterisation, together with integrated catchment modelling, of pollutant sources and pathways. These investigations revealed that, despite the spectacular nature of the original mine water outburst in 1992, the ecology of the Fal estuary remains remarkably robust. No scientific evidence emerged of any grounds for concern over the estuarine ecology, even if mine water were left to flow untreated. However, a rare ecological resource known as "maerl" (a form of calcified seaweed) is harvested annually in the estuary, providing significant revenue to the local economy and underpinning the 'clean' image of local sea water. Social and environmental benefit surveys revealed strong public perceptions that any visible discoloration in the estuary must indicate a diminution in quality of the maerl, to the detriment of both the public image and economy of the area. This factor proved sufficient to justify the continued pump-and-treat operations at the mine site. Although the decisive factor in the end was socio-economic in nature, robust assessment of this factor could not have been made without robust scientific evidence. It is concluded that investment in investigating and contributing to the formation of public perceptions is just as important as investing in scientific investigations per se.     

Modification de la marée par le creusement du chenal d'un estuaire

The present study compares the tide measurements on the left and right banks of the mouth of the river Loire (France) which flows into the Atlantic Ocean and two others points on the French Brittany coast (Fig.?1, 2), for the last twentyfive years. In order to simplify the location of the sites, the points have been numbered from the north to the south as follows: ??Point 1?:?Le Conquet ??Point 2: Port Tudy, île de Groix ??Point 3: St.-Nazaire ??Point 4: Pointe de St.-Gildas The observed tide levels in the estuary of the river Loire show important changes since 1903 (Fig.?3). These modifications can be linked to the dredging works in the navigation channel between Paimboeuf and Nantes (55?km upstream) where the bottom level has been excavated from –2.50?m to –3.00?m in 1929 and to –5.50?m in 1969. Whereas the chart datum (hydrographic zero reading), which in France corresponds to the lowest possible seas for safe navigation, had been set at the end of the last century at 3.03?m under the terrestrial chart datum, on January 1st 1996, the SHOM, Naval Hydrographic and Oceanographic Service, indicated that on all the nautical documents it publishes this chart datum was now being set at 3.43?m under the terrestrial chart datum for the right bank of the mouth of the estuary (corresponding to the northern side of the estuary) while the one for the left bank, although only 15?km away, remained unchanged. The statistical study of surges for a period of twenty five years (1965–90) on both sides of the external estuary shows that the surges are significantly greater on the north-eastern side. It also highlights the incoherence of certain results. The study of level readings on 3 500 high and low tides effected by 4 tide gauges spread out along 300?km of coastline highlighted that a single badly zeroed tide gauge could lead to false results with regards to the average tide level at point?3. The work shows that this discrepancy is neither due to the blocking of fresh river water nor to a set of compartment faults but to the displacement and dredging of the channel along the right bank of the mouth of the estuary. The storm surges have also been studied between 1985 and 1990 using a dynamic model where surge levels depend on the duration of the meteorological disturbances. In this model the surge associated with a stationary meteorology S_∞ depends on the wind (velocity and direction) and the atmospheric pressure. The theoretical law for the surge due to atmospheric pressure is valid in the 4 points studied; for each hectoPascal of lowering of barometric pressure, the sea level increase is 1?cm. For a wind of constant velocity and direction, the model shows that the surges are greater at points 3 and 4 than at points 1 and 2. This fact can be explained by the local morphology of the external part of the estuary; it also gives an explanation for the coastal erosion of this area. This model confirms that the reduction of the extreme low water level observed at point 3 is a local disturbance. In fact, the increase of the mean sea level is approximately 0.10?m per century at points 1 and 2 as in Brest harbour where level measurements have been undertaken since 1808. In contrast, this level appears to be stable at the two points situated on the estuary while the amplitude of the tide increases. This phenomenon is better marked at point 3 in the north of the estuary. Finally, this study shows that the installation of a mobile dam in the Loire estuary (similar to the one existing in the Thames estuary) would permit regulation of the upper water levels above the dam although the amplitude of the tide could be enhanced downstream of the dam.     

Geomorphological impacts of Inanda Dam on the Mgeni estuary, north of Durban, South Africa)

 Completed in 1988 at a site some 32 km from the Mgeni river mouth, the Inanda Dam may have influenced bed load transport and deposition. The aims of this study were to ascertain whether the dam had affected fluvial sediment delivery to the lower channel, whether the sediment characteristics and bedform of the estuary had been modified and, within this context, to predict the likely effects of continued sand mining on the Mgeni estuary. Re-examination of estuarine cross sections originally surveyed prior to dam completion, comparison of present estuarine sediment characteristics with those of pre-dam sediments and a bottom survey of the Inanda Dam itself were key elements in the analysis. Mean annual discharge of the Mgeni river has reduced by 4% since dam construction and changes in discharge regime caused by the water release policy resulted in an increase in competent discharge events. Retention of sediment behind the dam wall had led to reduction in available downstream sediment. Sediment volume in the estuary had not changed significantly since 1989, although sediment calibre is now much finer. These changes will probably signal the end of sand mining in the lower river and will ultimately cause the estuary to deepen as residual pre-dam sediment is flushed through and not replaced by new material from upstream. Since Mgeni sediment is a source of material for natural beach nourishment, it is likely that beaches will become narrower. Received: 20 November 1999 · Accepted: 3 April 2000     

Total mercury in Pema perna mussels from Guanabara Bay--10 years later

Guanabara Bay (GB) is an important estuary in regard to the productivity of the southeastern Brazilian coast. It is heavily polluted by oil, land runoff, and sewage, had large areas reclaimed, and its basin has been seriously deforested. The objective of the present work is to compare the total mercury levels in mussels from the bay, measured at the beginning and at the end of a 10-year interval (1988 and 1998). The commonly occurring species of bivalve in the bay is the mussel Perna perna. The total mercury content in mussels from GB has probably remained constant for the last 10 years (17.3-74.1 microg Hg kg(-1)), with possible isolated fluctuations attributed to occasional changes in water quality. Mussels have a good potential as biomonitors of water quality in GB. The need for the establishment of long-term biomonitoring programs is also commented.     

The Mersey Estuary – Back from the Dead? Solving a 150‐Year Old Problem

The Mersey Estuary has suffered a legacy of abuse and neglect since the beginning of the Industrial Revolution. The discharge of effluents from manufacturing processes, together with wastewater from the burgeoning centres of population, resulted in the estuary gaining the unenviable reputation of being one of the most polluted rivers in Europe.
As a result of the long-awaited remedial action which has been implemented over the last fifteen years, there is now unequivocal evidence that the water quality of the river and the biology of the system have improved significantly and will continue to do so as further planned alleviation schemes are completed.
This paper reviews the achievements which have been made at the half-way stage in the 25-year multi-billion pound 'clean-up'campaign.     

Strategies to Combine the Functions of Ecology and Navigation in the Scheldt Estuary

During recent years, water management in The Netherlands has been broadening its scope by: (a) including aspects of economics, sociology and physical planning, and (b) introducing interactive policy making; this paper illustrates these developments using the Westerschelde estuary as an example. The Westerschelde strategy aims at optimizing the multi-national use of this estuary. The main functions are navigation and ecology, and the paper shows how these functions can be combined. An interactive management model, the 'COSMO-Westerschelde'is described, which assists decision makers in comparing strategies for estuarine management under different scenarios.     

鄂尔多斯盆地安塞地区长6油层沉积相研究

文章选取长6油层组这一典型的曲流河三角洲作为例案,分析该地区发育的微相类型:水上分流河道、分流间沼泽、水下分流河道、河口坝、分流间湾等,其中以三角洲前缘水下分流河道为主。研究表明了相是沉积物形成条件的物质表现。(1)曲流河三角洲平原弯度大的分流河道与平原相近,分流河道许多向外延展;(2)曲流河三角洲前缘水下分流河道河道短,由于水体浅所以导致了大量的堆积物沉积,河口处冲击力变大,水携带者冲积物缓慢运移,多在水下分流河道的两侧堆积成指状;(3)受曲流河三角洲平原的影响,在其末端垂向沉积上,上粗下细在沉积粘土沉积之上,形成河口坝等厚度的砂体富集体。在沉积相砂体展布中体现了砂体长61发育最强,对于长6油层的开发有指示意义。     

Restoration of the Golden Horn Estuary (Halic)

Restoration of the iconic Golden Horn Estuary in Istanbul, Turkey was a substantial political, logistical, ecological, and social challenge. Forty years of uncontrolled industrial and urban growth resulted in thick layers of anoxic sediment, toxic bacteria, strong hydrogen sulfide odor, and ecologically unlivable conditions. The major components of restoration, spanning two decades, have included (1) demolition and relocation of industries and homes along the shore, (2) creation of wastewater infrastructure, (3) removal of anoxic sludge from the estuary, (4) removal of a floating bridge that impeded circulation, and (5) creation of cultural and social facilities. Although Turkey is not known as an environmental leader in pollution control, the sum of these efforts was largely successful in revitalizing the area through dramatic water quality improvement. Consequently, the estuary is once again inhabitable for aquatic life as well as amenable to local resource users and foreign visitors, and Istanbul has regained a lost sense of cultural identity. This paper focuses on literature review and personal interviews to discuss the causes of degradation, solutions employed to rehabilitate the estuary, and subsequent physicochemical, ecological, and social changes.     

Spatial and temporal variability of priority volatile organic compounds in the Scheldt estuary

The occurrence and spatial-temporal variability of 25 volatile organic compounds (VOCs) was studied from May 1998 to November 2000 in the Scheldt estuary. Target VOCs were selected from lists of priority pollutants to the North Sea, and included chlorinated short-chain hydrocarbons (CHCs), monocyclic aromatic hydrocarbons (MAHs) and chlorinated monocyclic aromatic hydrocarbons (CMAHs). Samples were taken between Vlissingen and Temse over a 95 km trajectory, and analysed by purge-and-trap and high resolution gas chromatography-mass spectrometry. Data were attended by analyses deemed 'in control' by a rigorous quality assurance/quality control program, as proposed by Quality Assurance of Information for Marine Environmental Monitoring in Europe (QUASIMEME). CHCs were among the most commonly found VOCs in the estuary, ranging from a few ng l(-1) to several microg l(-1). Most analytes were traceable to a single input source in the upper part of the estuary, which is densely populated and highly industrialized. By contrast, the occurrence of MAHs resulted from contributions of a wide spectrum of sources. The CMAHs were less abundant in the water column, and were mainly detected as chlorobenzene and 1,4-dichlorobenzene. Concentrations of several priority VOCs have markedly decreased since 1995 owing to emission reduction efforts within international frameworks for the protection of the North Sea.     

Hydro-environmental modelling for bathing water compliance of an estuarine basin

In recent years, considerable investment has been committed to sewerage infrastructure and new sewage treatment plants in the catchment surrounding an estuarine basin along the north-west coast of England. Although this capital investment has resulted in a marked reduction in the input of bacterial loads, relatively high counts of faecal indicator organisms are still being encountered in the coastal receiving waters, and the local bathing waters continue to fail on occasions to comply with the European Community (EC) Bathing Water Directive (1976) mandatory standards. Details are given herein of a comprehensive modelling study aimed at quantifying the impact of various bacterial inputs into the estuary and surrounding coastal waters on the bathing water quality. The model domain includes the coastal area and the entire estuary (namely the Ribble) up to the tidal limits of its tributaries. Faecal coliforms have been used as the main water quality indicator organisms. The numerical model developed for this study combines a depth integrated two-dimensional coastal model and a cross-sectionally integrated one-dimensional river model, and is capable of predicting water surface elevations, velocity fields and faecal coliform concentration distributions across the entire model domain. The hydrodynamic model was calibrated using water level and velocity measurements from three surveys and then validated against measured data from three other surveys. In order to predict the faecal coliform concentration distributions, variable faecal coliform decay rates were used, i.e. different values of decay rates were applied to the coastal and riverine waters, for day- and nighttime, and for wet and dry weather conditions. The maximum and minimum decay rates used were 2.32/day and 0.71/day for the dry and wet weather surveys, respectively. The model was then applied to (i) assess the impact of previous discharge strategies and investigate the effectiveness of future capital investment works and (ii) predict the impact of a range of strategic options, including: the effects of adding UV treatment, constructing storm water storage tanks and incorporating various combined sewer overflow (CSO) discharge scenarios for different weather conditions.     

Transport and sources of metal contaminants over the course of tidal cycle in the turbidity maximum zone of the Hudson River estuary

Transport and source of metal contaminants (Ag, Cd, Cu, Pb and Zn) in the turbidity maximum zone of the Hudson River estuary were studied over the course of a tidal cycle in November 1994 and August 1995. This study showed that the metal/Fe ratios in suspended particles varied more widely than those in the local sediments, implying that sources of metal contaminants to the water column are not only from local sediment resuspension but also from lateral advection. Although the metal/Fe ratios oscillate with time at the anchor stations, a general increasing trend with salinity was found over the course of a tidal cycle, suggesting that the lower estuary could be a source of metal contaminants to the upper estuary regions. These results support that sediment resuspension and lateral advection are important factors responsible for transporting the lower estuary contaminants up river and redistributing metal contaminants in the estuary.