Showing papers on "Turbidity published in 2003"

Effects of spatial and temporal variability of turbidity on phytoplankton blooms

Abstract: A central challenge of coastal ecology is sorting out the interacting spatial and temporal components of environmental variability that combine to drive changes in phytoplankton biomass For 2 decades, we have combined sustained observation and experimentation in South San Francisco Bay (SSFB) with numerical modeling analyses to search for general principles that define phyto- plankton population responses to physical dynamics characteristic of shallow, nutrient-rich coastal waters having complex bathymetry and influenced by tides, wind and river flow This study is the latest contribution where we investigate light-limited phytoplankton growth using a numerical model, by modeling turbidity as a function of suspended sediment concentrations (SSC) The goal was to explore the sensitivity of estuarine phytoplankton dynamics to spatial and temporal variations in turbidity, and to synthesize outcomes of simulation experiments into a new conceptual framework for defining the combinations of physical-biological forcings that promote or preclude development of phytoplankton blooms in coastal ecosystems The 3 main conclusions of this study are: (1) The timing of the wind with semidiurnal tides and the spring-neap cycle can significantly enhance spring- neap variability in turbidity and phytoplankton biomass; (2) Fetch is a significant factor potentially affecting phytoplankton dynamics by enhancing and/or creating spatial variability in turbidity; and (3) It is possible to parameterize the combined effect of the processes influencing turbidity —and thus affecting potential phytoplankton bloom development —with 2 indices for vertical and horizontal clearing of the water column Our conceptual framework is built around these 2 indices, providing a means to determine under what conditions a phytoplankton bloom can occur, and whether a poten- tial bloom is only locally supported or system-wide in scale This conceptual framework provides a tool for exploring the inherent light climate attributes of shallow estuarine ecosystems and helps determine susceptibility to the harmful effects of nutrient enrichment

Removal of natural populations of marine plankton by a large-scale ballast water treatment system

Abstract: Large-scale experiments using a hydrocyclone, a self-cleaning 50 µm screen, and a UV unit were undertaken to evaluate the treatment efficiency of these commercially available units for preventing the transfer of unwanted species via ships' ballast water. The water flow through the treatment system was approximately 5.7 m 3 min -1 . The effect of increased suspended solids on these processes was the focus of this research. During each experimental run, 760 l samples were obtained and passed through 35 µm plankton nets for zooplankton collection. Samples were also collected for phytoplankton, microbiological, ATP and protein analyses. After the initial samples were obtained, a second set of samples was held for 18 h to determine the effects of storage on the effectiveness of treatment processes. Screening the seawater at 50 µm removed most of the zooplankton and a small percentage of the microphytoplankton, but hydrocyclonic separation was not effective. Initially, UV treatment reduced the viable count of microorganisms to an undetectable level; however, bacterial regrowth was observed in the samples held for 18 h. Statistical evaluation showed that increased tur- bidity (5 to 90 nephelometer turbidity units; NTU) had no effect on the treatment regime, even on the UV unit. At the highest turbidity (90 NTU), the UV dose was lowered to approximately 35 mW s cm -2 ; however, this dose was still sufficient to inactivate microorganisms. Overall, it was observed that only the 50 µm screen was effective in the removal of organisms, especially potential invading organisms such as large zooplankton or invertebrate larvae.

An integrated limestone/lime process for partial sulphate removal

Abstract: This paper will focus on two topics, namely (1) an innovative process for the neutralization and partial sulphate removal of acid streams produced during coal mining and processing and (2) optimization of this integrated neutralization process by adding small amounts of synthetic organic polymers. The integrated lime and limestone process has been developed to neutralize acid mine water and to remove sulphate (to less than 1 200 mg/l), magnesium and metals. Limestone and lime treatment is the most cost-effective technology for neutralization and partial sulphate removal of acidic/sulphate-rich water to sulphate levels of less than 1 500 mg/l due to precipitation of magnesium and removal of the associated sulphate fraction (through gypsum crystallization). Neutralized mine water of this quality may be suitable for irrigation. The process consists of the following stages: 1. Limestone (CaCO3) neutralization to raise the pH to 7 and CO2-production. 2. Lime (Ca(OH)2) treatment to pH 12 for Mg(OH)2 precipitation and gypsum (CaSO4.2H2O) crystallization. 3. pH adjustment with CO2 recovered from stage 1 and CaCO3 precipitation. Retention times of 1 hour, 4 hours and 1/2 an hour were required by stages 1, 2, and 3 respectively. The sulphate level was reduced to 1 094 mg/l, which is less than the original aim of 1 200 mg/l. Chemical costs associated with neutralization, using limestone instead of lime, are reduced by 69% in the integrated limestone/lime process. The overflow water from the clarifier of each of the above stages generally contains a wide variety of colloidal impurities that may cause the water to appear turbid or may impart colour. This results in very slow sludge settling rates that cause overflow water with a high degree of turbidity and colour. There are a number of successive or simultaneous stages involved in the agglomeration of particles. To get optimum neutralisation of the acid water and partial sulphate removal to below the saturation level of gypsum (i.e. 1 500 mg/l), maximum sludge recovery is needed. The higher the concentration of seed crystals, the larger the surface area. Therefore, effective removal of suspended and colloidal matter from the overflow of each stage is required and can be achieved by coagulation and flocculation. The polymers PAC6 and 3095 were effectively used as coagulant and flocculant respectively. The addition of these polymers resulted in a clear overflow in each of the above stages with a very low turbidity.

Determination of suspended particulate matter concentration from turbidity measurements: particle size effects and calibration procedures

Abstract: Measurements of suspended particulate matter concentration and turbidity point towards the possibility of a site-specific algorithm (SPM), relating SPM to nephelometric turbidity units (NTU). In this paper regression models are presented that account for changes in the relationship of SPM and NTU as a result of changes in particle properties. The models have been developed by the use of daily measurements of SPM concentration and a continuous record of turbidity for the period from June 1996 to February 2001 in the River Elbe, a major river in the eastern part of Germany. The effect of changes in the particle properties with increasing water discharge was taken into account by varying the slope of a linear regression equation according to a logistic function. Water discharge, Q, was defined to be the only variable of this function—as an adequate substitute of the parameter bottom shear stress, which cannot be measured directly. Measurements of flow velocity in the River Elbe show that bottom shear stress is related almost linearly to water discharge up to bankful discharges. Regression models with slopes varying continuously with hydraulic parameters may account for the effects resulting from changes of particle characteristics and thus may have some advantages compared with models with a constant slope or models calibrated for different seasons. Copyright © 2003 John Wiley & Sons, Ltd.

A comparison of water quality indices for coastal water.

Abstract: The present article discusses and compares five different water quality indices, viz arithmetic water quality index, multiplicative water quality index, unweighted arithmetic water quality index, unweighted multiplicative water quality index, and Harkin's water quality index, which were considered for characterizing the coastal water quality at the Jawaharlal Nehru Port Trust, Bombay, India. Dissolved oxygen, pH, biochemical oxygen demand (BOD), temperature, suspended solid, and turbidity were used as the parameters of water quality indices. The value function graphs used for above-mentioned variables were developed using harbour water quality standards and aquatic life. The product moment correlation coefficients for various water quality indices were determined using the SPSS software package to evaluate correlation among various indices. It was found that the unweighted arithmetic water quality index was higher than weighted arithmetic water quality index while the multiplicative water quality index was lower than unweighted multiplicative water quality index. All the indices were well correlated with each other except Harkin's water quality index. The Harkin's water quality index was different from other water quality indices. The comparison of different form of indices showed that the multiplicative water quality index was the most suitable water quality index for coastal waters.

How particles affect UV light in the UV Disinfection of Unfiltered Drinking Water

Abstract: This study assessed the impact of suspended particles on both ultraviolet (UV) dose transmission and spectrophotometric absorbance measurement for unfiltered raw water with turbidity up to 10 ntu. Results from bench-scale collimated beam experiments indicated that a low percentage of total incident UV light was attenuated by particles at 5 ntu turbidity. However, at a set applied UV dose, when particles were added to achieve turbidity from 1 to 10 ntu, the average dose in the collimated beam reactor decreased from 5 to 33%, respectively. This suggests that spikes in particles entering a UV reactor, if not adjusted and accounted for, may compromise lethal delivery of UV energy. UV absorbance coefficients, measured using direct and integrating sphere spectroscopy (which accounts for scattering effects by particles) were significantly different, suggesting that suspended particles in unfiltered water both absorb and scatter UV light. When absorbance, measured using the direct method, was used to model the average irradiance in a UV reactor, the average UV irradiance was underestimated by3to 20% at turbidity of 3 to 10 ntu, respectively, compared with irradiance calculations made using integrating sphere absorbance measurements. This analysis suggests that UV systems designed based on direct absorbance measurements of unfiltered waters with turbidity above 3 ntu may be overly conservative.

Turbidity dynamics in karstic systems. Example of Ribaa and Bittit springs in the Middle Atlas (Morocco)

Abstract: The identification of the turbidity mechanisms in two karstic springs (Ribaa and Bittit) located in the Middle Atlas Plateau in Morocco was performed by means of correlation and spectral analyses applied to time series of rainfall, flow rates, and turbid- ity. Time series analyses of rainfall and discharges revealed high inertia and storage capacities of the karstic systems. However, the occurrence of turbidity in the springs proved independent of discharges. A causal relationship between rainfall and the occur- rence of turbidity in the form of waves was established. Accordingly, turbidity was assumed to be related to the hydrodynamic conditions prevailing in the karst. Turbulent quickflows in the karst transmissive conduits, following heavy rainfall, are thought to provoke the resuspension of solid particles deposited in the conduits, as well as their transport towards outlets. An external origin has also been contemplated, concerning infiltration waters may be loaded with suspended matters washed from the watershed.

A model study of turbidity maxima in the York River estuary, Virginia

Abstract: A three-dimensional numerical model is used to investigate the mechanisms that contribute to the formation of the turbidity maxima in the York River, Virginia (U.S.). The model reproduces the basic features in both salinity and total suspended sediments (TSS) fields for three different patterns. Both the prominent estuary turbidity maximum (ETM) and the newly discovered secondary turbidity maximum (STM) are simulated when river discharge is relatively low. At higher river inflow, the two turbidity maxima move closer to each other. During very high river discharge event, only the prominent turbidity maximum is simulated. Diagnostic model studies also suggest that bottom resuspension is an important source of TSS in both the ETM and the STM, and confirm the observed association between the turbidity maxima and the stratification patterns in the York River estuary. The ETM is usually located near the head of salt intrusion and the STM is often associated with a transition zone between upriver well mixed and downriver more stratified water columns. Analysis of the model results from the diagnostic studies indicates that the location of the ETM is well associated with the null point of bottom residual flow. Convergent bottom residual flow, as well as tidal asymmetry, is the most important mechanisms that contribute to the formation of the STM. the STM often exists in a region with landward decrease of bottom residual flow and net landward sediment flux due to tidal asymmetry. The channel depth of this region usually decreases sharply upriver. As channel depth decreases, vertical mixing increases and hence the water column is better mixed landward of the STM.

Use of continuous turbidity sensor in the prediction of fine sediment transport in the turbidity maximum of the Trent Estuary, UK

Abstract: Results from continuous monitoring of turbidity and water level at Burringham, on the tidal River Trent, UK, are presented for the period 18 May 1997 to 9 February 1998. These measurements, together with detailed readings of velocity and suspended sediment concentration over an individual tidal cycle near the opposite bank at Derrythorpe, help to describe the mechanisms and behaviour of the turbidity maximum (TM). It is demonstrated that there is a distinct pattern of fine sediment movement that reflects a predictable system response to changing hydraulic features. It is shown that the TM in this system is highly mobile, and its location depends on antecedent fresh water flow, and tidal range. Approximate representative flood and ebb tide suspended sediment concentrations of up to 13 g/l over this nine-month period have been derived from the data and plotted against fresh water flow and tidal range, in order to show the relationship between these parameters. Three semi-empirical polynomial regression models have been tested for goodness of fit against available data. It was found that a partitioning approach, whereby data are grouped into different categories depending on antecedent fresh water flow, yielded the lowest standard error for the period analysed. Analysis of detailed observations of suspended sediment concentration and velocity measured over an individual tidal cycle also help to elucidate the mechanism of tidal pumping within this system. These results also help to give an estimate of the relative magnitude of suspended sediment fluxes during typical low fresh water flow conditions. It is estimated that for low fresh water flow conditions, a typical spring tide can mobilise at least an order of magnitude more sediment than a neap tide.

Lake age and water level affect the turbidity of floodplain lakes along the lower Rhine

Abstract: 1. We sampled a set of 93 lakes situated in the floodplains of the lower River Rhine in search for morphometric and other factors that explain their variation in clarity. 2. Lakes with a drop in summer water level were less turbid at the time of sampling, mainly because of a lower concentration of inorganic suspended solids (ISS). 3. We also found that older lakes were more turbid than younger lakes and that this was largely because of an increase in phytoplankton. 4. Water clarity was positively related to lake depth and the presence of vegetation. 5. Model calculations indicated that the underwater light climate was strongly affected by chlorophyll and ISS, the latter being the dominant factor affecting Secchi depth. Dissolved organic carbon (DOC) was less important. 6. The high concentration of ISS suggests that intensive resuspension occurs in most of the lakes. Using a simple wave model, and assuming that vegetation protects sediments against resuspension, we could eliminate wind resuspension as an important process in 90% of the lakes, leaving resuspension by benthivorous fish as probably the most important factor determining transparency. 7. Chlorophyll a concentration showed a strong positive correlation to ISS concentration, suggesting that resuspension may also have a positive effect on phytoplankton biomass in these lakes. 8. In conclusion, in-lake processes, rather than river dynamics, seem to be driving the turbidity of floodplain lakes along the lower River Rhine.

Proceedings of the Federal Interagency Workshop on Turbidity and Other Sediment Surrogates, April 30-May 2, 2002, Reno, Nevada

Abstract: As part of the Subcommittee on Sedimentation’s “Turbidity and Other Sediment Surrogates Workshop,” April 30-May 2, 2002, a questionnaire on uses of turbidity was submitted to water-quality coordinators for all State and some Tribal agencies. The questionnaire was designed to address key issues related to turbidity, including water-quality standards, technology, ranges observed, water bodies, seasonal variability, calibration and sampling protocols, and use of other measures of fluvial and suspended sediment. All but 5 of the 40 agencies that responded indicated having established either narrative or numeric standards for turbidity under their jurisdictions. In addition to water-quality standards, several agencies are using either turbidity or TSS to identify sediment-impaired streams or stream reaches with application to developing sediment TMDLs. Water clarity was identified by the agencies as the parameter of primary interest when measuring turbidity. Several agencies have correlated either turbidity or TSS with habitat or aquatic life. Several agencies indicated having noticed a seasonal variability in turbidity that is possibly related to an increase in plankton in the water column and not runoff. Reported ranges in turbidity vary widely, ranging from below detection limits to over 10,000 NTU. The large majority of agencies use instruments operating on the bulk optical properties of watersediment mixtures, including turbidimeters, optical backscatter meters (OBS), and optical transmissometers to infer turbidity, and analyses of grab samples to provide the comparative SSC or TSS data. Some agencies are using flow-integrated sampling techniques depending upon the project objectives. The majority of the agencies that measure turbidity use formazin as a calibration standard and USEPA Method 180.1 for analysis (U.S. Environmental Protection Agency, 1999). The majority of the agencies used either Oracle, STORET, or a “local” database or spreadsheet for data storage and analysis. The agencies identified several technical needs related to turbidity, including improving the relation between turbidity, TSS, SSC, channel stability, and biological impairment; establishing reference fluvial sediment conditions and means of measuring significant departure from reference conditions; improving depthintegrated isokinetic samplers; and developing a consistent procedure and less expensive probes that can be rapidly deployed and are stable in the field. In addition, most agencies agreed that additional longterm, stream-discharge, suspended and bedload data are needed, and that the USEPA should revise Method 180.1 on turbidity (U.S. Environmental Protection Agency, 1999) to include state-of-the-art instrumentation capable of measuring higher concentrations of turbidity without making sample dilutions.

Relationship of pH and Floc Formation Kinetics to Granular Media Filtration Performance

Abstract: In drinking water treatment, destabilization of suspended particles through effective coagulation processes is critical for optimizing filtration performance This study examined the impact of coagulation pH (60−74), ζ-potential, and floc formation kinetics on particle removal during settling and filtration Increasing pH improved turbidity removal in the settling process and significantly increased the ζ-potential range in which filtration performance was optimized Although it was possible to optimize filtration (ie, attain total particles < 10/mL) at each of the three pH levels examined, the ζ-potential range in which this was possible was approximately +2 to +4 at pH 60 and −4 to +4 at pH 74 Overall, these results indicate that the use of a higher coagulation pHand thus higher alum dosesmay be particularly advantageous during periods of rapidly changing water quality conditions, such as high-NOM runoff events Results from PDA experiments indicated that the rate of floc formation (measured imme

Behaviour of Inorganic Coagulants in Secondary Effluents from a Conventional Wastewater Treatment Plant

Abstract: An experimental laboratory scale study was conducted to compare the effectiveness of three inorganic coagulants — aluminium sulphate, ferric chloride and polyaluminium chloride (PAC) — in reducing the turbidity of secondary effluents from a conventional wastewater treatment plant. For each inorganic coagulant, the dose and the pH of coagulation were optimized. The results enabled the selection of the most appropriate coagulant, its dose and the optimal operational conditions to obtain the required water quality (3–5 NTU) for re-use in agricultural drip irrigation systems. PAC was identified as the most suitable coagulant/flocculant for reducing the turbidity of this type of water. The optimal conditions corresponded to pH 6 and a dose of PAC of 20 mg/l, but the recommended conditions are 50 mg/l for pH 8 (the initial pH of the water) to obtain a turbidity reduction of 90%, which is independent of the initial turbidity of water (i.e. 5–32 NTU).

Establishing the Relationship between Turbidity and Total Suspended Sediment Concentration

Abstract: This laboratory work examines the relationship between turbidity (NTU) and total suspended sediment concentration (TSS, mg/L) for a Cecil Ap soil, a common Southeastern Piedmont soil type. We show a 1:1 correlation between NTU and TSS for the silt and clay fractions, but a smaller ratio for clay and bulk-soil samples. These results suggest that NTU measurements can be used to estimate sediment concentrations for fine soil fractions, but underestimates the total sediment concentration when sand-size fractions are present.

Impact of black bullhead ( Ameiurus melas ) on turbidity in a diked wetland

Abstract: Benthivorous fish in shallow, aquatic systems have been correlated with increased turbidity and declines in macrophyte production and wildlife use. Bullheads have been credited with increasing turbidity, but this has been seldom tested and has not been studied in a diked marsh. To assess the relationships of black bullhead (Ameiurus melas) and turbidity, we assembled mesocosmsin the Show Pool Management Unit of The Ottawa National Wildlife Refuge, OH, U.S.A. We stocked treatment enclosures with different biomasses of black bullhead at weekly intervals. Mean turbidity within treatment enclosures was significantly higher than within controls but remained lower than that of the open marsh. Both surface and bottom turbidity increased with adult and juvenile black bullhead biomass. Turbidity increased with fine sand concentration only in the presence of juvenile fish. Wind speed and direction were significant influences on the turbidity of the open marsh, but not within control enclosures. That treatment turbidity – even at extreme biomasses – remained significantly lower than the turbidity of the open marsh implicates fetch in having a greater influence on a marsh's turbidity than the presence of black bullhead. The greater impact of benthivorous fish on turbidity within shallow systems may be an indirect one through the destruction of macrophytes and subsequent destabilization of unconsolidated substrates.

Factors Controlling Extremely Productive Heterotrophic Bacterial Communities in Shallow Soda Pools

Abstract: Dilute soda lakes are among the world’s most productive environments and are usually dominated by dense blooms of cyanobacteria Up to now, there has been little information available on heterotrophic bacterial abundance, production, and their controlling factors in these ecosystems In the present study the main environmental factors responsible for the control of the heterotrophic bacterial community in five shallow soda pools in Eastern Austria were investigated during an annual cycle Extremely high cyanobacterial numbers and heterotrophic bacterial numbers up to 307 × 109 L−1 and 268 × 109 L−1 were found, respectively Bacterial secondary production rates up to 738 µg C L−1 h−1 and specific growth rates up to 165 h−1 were recorded in summer and represent the highest reported values for natural aquatic ecosystems The combination of dense phytoplankton blooms, high temperature, high turbidity, and nutrient concentration due to evaporation is supposed to enable the development of such extremely productive microbial populations By principal component analysis containing the data set of all five investigated pools, two factors were extracted which explained 625% of the total variation of the systems The first factor could be interpreted as a turbidity factor; the second was assigned to as concentration factor From this it was deduced that bacterial and cyanobacterial abundance were mainly controlled by wind-induced sediment resuspension and turbidity stabilized by the high pH and salinity and less by evaporative concentration of salinity and dissolved organic carbon Bacterial production was clustered with temperature in factor 3, showing that bacterial growth was mainly controlled by temperature The concept of describing the turbid water columns of the shallow soda pools as “fluid sediment” is discussed

Determination of the residence time of suspended particles in the turbidity maximum of the Loire estuary by 7Be analysis

Abstract: The aim of the present work was to evaluate the half life of suspended particles in the Loire estuarine turbidity maximum by analysis of 7 Be budgets. The methodology was based on in situ sampling and further measurements aiming at quantifying 7 Be sources (atmospheric deposition and river inputs) and 7 Be stock in the water column of the turbidity maximum. 7 Be river inputs were determined by monthly 7 Be measurements performed upstream of the estuary. 7 Be atmospheric deposition was estimated by using an empirical relation between 7 Be deposition and rainfall. 7 Be in particles of the estuarine turbidity maximum was measured at eight different dates corresponding to different tidal and hydrological conditions. 7 Be sources and stocks thus determined have been compared to a mathematical model. Results allow to quantify the standard half life of suspended particles in the Loire estuarine turbidity maximum and show that it depends on the season (6–10 months in summer and about 0.7 month during flood periods). Furthermore, a rather good linear correlation was observed between the standard half life of particles and the sum of flow rates in the Loire river during 60 days before each sampling date. The kinetic evolution of the mass of particles within the turbidity maximum could be estimated by this method and appeared to be consistent with previous studies. Moreover, the method proposed in this study could presumably be used for estimating 60 Co concentrations in the estuarine turbidity maximum. 2003 Elsevier Science B.V. All rights reserved.

Continuous measurement of suspended-sediment discharge in rivers by use of optical backscatterance sensors.

Abstract: Optical sensors have been used to measure turbidity and suspendedsediment concentration by many marine and estuarine studies, and optical sensors can provide automated, continuous time series of suspended-sediment concentration and discharge in rivers. Three potential problems with using optical sensors are biological fouling, particle-size variability, and particlereflectivity variability. Despite varying particle size, output from an optical backscatterance sensor in the Sacramento River at Freeport, California, was calibrated successfully to discharge-weighted, cross-sectionally averaged suspended-sediment concentration, which was measured with the equal discharge-, or width-increment, methods and an isokinetic sampler. A correction for sensor drift was applied to the 3-year time series. However, the calibration of an optical backscatterance sensor used in the Colorado River at Cisco, Utah, USA, was affected by particle-size variability. The adjusted time series at Freeport was used to calculate hourly suspended-sediment discharge that compared well with daily values from a sediment station at Freeport. The appropriateness of using optical sensors in rivers should be evaluated on a site-specific basis and measurement objectives, potential particle size effects, and potential fouling should be considered.

Suspended sediment transport in flash floods of the semiarid northern Negev, Israel

Abstract: Our aim is to provide insight into various suspended sediment transport phenomena of upland ephemeral streams in a semiarid environment. Information about suspended sediment concentration is derived with the help of a programmable pump sampler and the continuous record of a turbidity sensor. Suspended sediment concentrations are high; the mean during six years of measurements was 34 000 mg l -1 and regression of suspended sediment concentration on water discharge takes the form SSC = 10 4.41 Q 0.42 . During individual flash floods, the suspended sediment-water discharge relation may be hysteretic (clockwise or counterclockwise) and/or monotonic. In spite of complicated intra-event behaviour, there is a good, deterministic (R 2 > 0.9) relation between total suspended sediment yield and flood volume.

Effects of β‐Glucan, Shearing and Environmental Factors on the Turbidity of Wort and Beer

Abstract: The presence of added β-glucan in wort caused increased turbidity levels, which increased at higher molecular weights and concentrations of the polymer. Levels of pH, maltose and ethanol, and shear experienced in a brewery also influenced the turbidity of wort and beer. Haze levels of beer after 0.45 μm membrane filtration were found to decrease due to the removal of non-β-glucan particles. Cold storage at 4°C for two weeks was found not to lower the turbidity caused by high concentrations of high molecular weight β-glucan polymers.

A Model Study of Turbidity Maxima in the York River Estuary

Abstract: A three-dimensional numerical model is used to investigate the mechanisms that contribute to the for- mation of the turbidity maxima in the York River, Virginia (U.S.). The model reproduces the basic features in both salinity and total suspended sediments (TSS) fields for three different patterns. Both the prominent estuary turbidity maximum (ETM) and the newly discovered secondary turbidity maximum (STM) are simulated when river discharge is relatively low. At higher river inflow, the two turbidity maxima move closer to each other. During very high river discharge event, only the prominent turbidity maximum is simulated. Diagnostic model studies also suggest that bottom resuspen- sion is an important source of TSS in both the ETM and the STM, and confirm the observed association between the turbidity maxima and the stratification patterns in the York River estuary. The ETM is usually located near the head of salt intrusion and the STM is often associated with a transition zone between upriver well mixed and downriver more stratified water columns. Analysis of the model results from the diagnostic studies indicates that the location of the ETM is well associated with the null point of bottom residual flow. Convergent bottom residual flow, as well as tidal asymmetry, is the most important mechanisms that contribute to the formation of the STM. The STM often exists in a region with landward decrease of bottom residual flow and net landward sediment flux due to tidal asymmetry. The channel depth of this region usually decreases sharply upriver. As channel depth decreases, vertical mixing increases and hence the water column is better mixed landward of the STM.