Comparison of the Monod and Droop Methods for Dynamic Water Quality Simulations

The Monod method is widely used to model nutrient limitation and primary productivity in water bodies. It offers a straightforward approach to simulate the main processes governing eutrophication and it allows the proper representation of many aquatic systems. The Monod method is not able to represent the nutrient luxury uptake by algae, which consists of the excess nutrient uptake during times of high nutrient availability in the water column. The Droop method, which is also used to model nutrient limitation and primary productivity, takes into account the luxury uptake of nutrients. Because of the relative complexity of the Droop method, it has not been systematically adopted for the simulation of large stream networks. The Water Quality Analysis Simulation Program (WASP) version 7.1 was updated to include nutrient luxury uptake for periphyton growth. The objective of this paper is to present the new nutrient limitation processes simulated by WASP 7.1 and to compare the performance of the Droop and the Monod methods for a complex stream network where periphyton is the main organism responsible for primary productivity. Two applications of WASP 7.1 with the Droop and Monod methods were developed for the Raritan River Basin in New Jersey. Water quality parameters affecting the transport and fate of nutrients were calibrated based on observed data collected for the Raritan River total maximum daily load. The dissolved oxygen and nutrients simulated with WASP 7.1, obtained with the Droop and Monod methods, were compared at selected monitoring stations under different flows and nutrient availability conditions. The comparison of the WASP 7.1 applications showed the importance of using the Droop method when periphyton was the main organism responsible for primary productivity. The data simulated with the Droop method resulted in good agreement with the observed data for dissolved oxygen, ammonia-nitrogen, nitrate-nitrogen, and dissolved orthophosphate at the selected stations. The Monod method was not able to capture the diel dissolved oxygen variation when nutrients were scarce, and it resulted in unrealistic diel variations of nutrients at times of strong primary productivity at some locations.     

Stream Classification System Based on Susceptibility to Algal Growth in Response to Nutrients

We developed a stream classification system that is based on stream’s susceptibility to algal growth using a two-step approach. The model portrays algal biomass as a result of stream’s response to nutrient concentrations and the response is governed by various stream factors. In the first step, a nutrient-chlorophyll a relationship was developed to characterize nutrient’s effects on algal biomass. Residuals of the relationship were attributed to stream’s susceptibility to algal growth in response to nutrients and referred to as “observed” susceptibility. In the second step, conditions of other contributing factors were used to explain the variation in the residuals and the developed relationship was used to generate “predicted” susceptibility. Existing data compiled from various monitoring projects of Illinois streams and rivers were used to illustrate the approach. Streams were classified into three (high, medium, and low) categories based on their observed and predicted susceptibility values, respectively. With the available data, the model showed a 40-50% success rate for classifying the streams based on three observed and predicted susceptibility categories. Model entropy also was calculated for selecting the best model. The results show the important role of both nutrients and other contributing factors in explaining the variation of algal biomass. The study also suggests ways to fine tune the model and improve its accuracy, which would make the presented model a more viable tool for stream classification for establishing nutrient criteria to prevent surface streams from eutrophication.     

Modeling the Effects of Macrophytes on Hydrodynamics

A computer model was created as a scientific and management tool for understanding the effects of macrophytes on hydrodynamics and water quality. A model was required that could simulate macrophytes in a complex water body and could be coupled to a multicompartment water quality model of phytoplankton, dissolved oxygen, nutrients, pH, and organic matter. This would permit the investigation of water resource issues where macrophyte growth, phytoplankton growth, nutrient loadings, and flood control were all contributing factors. The model was added as a compartment to the U.S. Army Corps of Engineers two-dimensional, laterally averaged, dynamic water quality model, CE-QUAL-W2 (Corps of Engineers, water quality, width averaged, two dimensional) and applied to the Columbia Slough, Ore. Features of the macrophyte model include the capability to simulate multiple submerged macrophyte species; transport of nutrient fluxes between plant biomass and the water column and/or sediments; growth limitation due to nutrient, light and temperature; simulation of the spatial distribution of macrophytes vertically and horizontally; the modeling of light attenuation in the water column caused by macrophyte concentration; and the modeling of open channel flow with channel friction due to macrophytes. The macrophyte model was tested through mass balances and sensitivity analyses. The modeling of channel friction was evaluated by comparing predicted water levels with data from tests conducted in a laboratory flume. Use of the model in the Columbia Slough showed reasonable predictive capability regarding estimated biomass and water level dynamics.     

Assessing the Impacts of Nutrient Load Uncertainties on Predicted Truckee River Water Quality

This study examined the effects of uncertain model boundary conditions on dissolved oxygen (DO) predictions for the lower Truckee River, Nevada using an augmented version of the EPA’s Water Quality Analysis Simulation Program Version 5 (WASP5) that included periphyton, or attached algae, in eutrophication kinetics. Uncertainty analyses were performed on selected organic nitrogen (ON) and carbonaceous biochemical oxygen demand boundary conditions using Monte Carlo techniques. The stochastic model was run using boundary concentrations assigned from observed probability distributions. Ranges of simulated values were used to construct confidence intervals, the magnitudes of which indicated the uncertainty associated with model predictions. Uncertainty in agricultural ditch return concentrations had minimal effects on in-stream model predictions, as predicted values of daily minimum and maximum DOs, daily average ON, and periphyton biomass all failed to show significant variability as a result of ditch concentration uncertainty. This result indicates that while ditch return nutrient loads are not trivial, their exact concentrations are not needed to make relatively accurate predictions of in-stream DO. However, uncertainty in the upstream ON boundary did result in significant uncertainty during summer months with regard to in-stream model predictions of ON, periphyton biomass, and DO. The model is clearly more sensitive to changes in this boundary than to changes in agricultural ditch concentrations.     

Long-Term Dynamic Modeling Approach to Quantifying Attached Algal Growth and Associated Impacts on Dissolved Oxygen in the Lower Truckee River, Nevada

Nutrient loads enter the lower Truckee River of western Nevada, affecting the growth of attached algae (periphyton) which causes depressed nighttime dissolved oxygen (DO) levels. The lower Truckee River is home to the endangered cui-ui and threatened Lahontan cut-throat trout, with DO standards being established to in part protect these species. Hydrodynamics, nutrient concentrations, periphyton biomass, and DO data spanning August 2000–December 2001 were used to calibrate and verify a modified version of the Water Quality Analysis Simulation Program Version 5 (WASP5). Under typical loading conditions the periphyton community is nitrogen limited, however nitrogen loading from an upstream wastewater treatment facility increased greatly during the analysis period due to approved site construction activities (discharge permit excursion) causing the periphyton community to temporarily become phosphorus limited. The developed modeling approach, with limited calibration, was able to accurately track dynamic system responses. Removing the impact of the noted discharge permit excursion resulted in a minimum computed DO value of 4.13?mg/L, occurring at the downstream end of the modeling domain on August 8, 2001. Additionally removing the impact of all nutrient loads from area agriculture resulted in a predicted minimum DO value of 4.54?mg/L, while also shifting its location significantly upstream and its timing to April 26, 2001. Meeting all prescribed DO standards required establishing a minimum in-stream flow value of 1.81?m3/s (64.0?ft3/s) downstream of Derby Dam.     

Integrated Hydrodynamic and Water Quality Modeling System to Support Nutrient Total Maximum Daily Load Development for Wissahickon Creek, Pennsylvania

This paper presents a hydrodynamic and water quality modeling system for Wissahickon Creek, Pa. Past data show that high nutrient levels in Wissahickon Creek were linked to large diurnal fluctuations in oxygen concentration, which combining with the deoxygenation effect of carbonaceous biological oxygen demand (CBOD) causes violations of dissolved oxygen (DO) standards. To obtain quantitative knowledge about the cause of the DO impairment, an integrated modeling system was developed based on a linked environmental fluid dynamics code (EFDC) and water quality simulation program for eutrophication (WASP/EUTRO5) modeling framework. The EFDC was used to simulate hydrodynamic and temperature in the stream, and the resulting flow information were incorporated into the WASP/EUTRO5 to simulate the fate and transport of nutrients, CBOD, algae, and DO. The standard WASP/EUTRO5 model was enhanced to include a periphyton dynamics module and a diurnal DO simulation module to better represent the prototype. The integrated modeling framework was applied to simulate the creek for a low flow period when monitoring data are available, and the results indicate that the model is a reasonable numerical representation of the prototype.     

Eutrophication Model for the Patuxent Estuary: Advances in Predictive Capabilities

A robust eutrophication and sediment diagenesis model has been developed for the Patuxent Estuary to study the impact of different nutrient loadings on phytoplankton biomass and dissolved oxygen (DO) levels. The modeling approach was to begin with an existing water quality model (CE-QUAL-W2) for the Patuxent Estuary (hereafter referred to as the Estuary). First, formulations for the water column kinetics were completely replaced with routines based on the WASP/EUTRO5 water quality model. Then, a sediment diagenesis component was added to simulate the accumulation and mineralization of organic matter in the sediment, the generation of sediment oxygen demand, and the flux of phosphate and ammonia from the sediment. Loadings from the tributaries for nutrients and flow were based on a combination of watershed modeling and sampling by scientists at the Smithsonian Environmental Research Center. The new model was able to reproduce the ambient water quality data from 1997 to 1999 by adequately simulating the high concentrations of phytoplankton and low DO levels in the Estuary. The model was then used to evaluate the response to various hypothetical nutrient loading scenarios. Model results show that phytoplankton growth in the upper Estuary is much more sensitive to nutrient loading from tributaries than in the lower estuary. Further, model results indicate that DO concentrations in the lower Estuary are largely influenced by levels of nutrients and organic carbon at the mouth of the Estuary.     

Polyurethane Foam Medium for Biofiltration. II: Operation and Performance

A polyurethane foam medium with characteristics described in Part I of this paper was tested in a toluene degrading biofilter to demonstrate its ability to support an active biofilm and to study feasibility of a novel nutrient addition and biomass wasting strategy. A laboratory-scale biofilter was fed a model waste stream containing toluene for more than 300 days using empty bed residence times ranging from 1 to 4 min and toluene concentrations ranging from 50 to 200 parts per million by volume. Results reported herein demonstrate that a polyurethane foam medium with high porosity, suitable pore size, low density, and an ability to sorb water was able to remove over 99% of the influent toluene after implementation of a nutrient addition and biomass removal strategy. The strategy, made possible by use of the foam medium, overcame problems such as clogging, high head loss, moisture content control, and nutrient limitation that are often associated with conventional biofilter operation.     

Diffusive Behavior of Bedform-Induced Hyporheic Exchange in Rivers

Solute transport in natural streams is a complex phenomenon that involves both in-stream dispersion and mass exchange with the porous zones surrounding the water body. Due to the complex nature of the riverine systems several models may be used to simulate and analyze the transport of solutes with different degrees of complexity. The bedform-induced hyporheic transport is a stream-subsurface exchange mechanism that can be reproduced in controlled systems, such as laboratory flumes. Application of a simple Fickian diffusion model to laboratory data obtained with passive solutes and stationary bedforms proves successful within a range of durations of the contamination process. A dimensionless form of the diffusion coefficient, scaled with dynamic, physical, and geometric properties of the system is derived by comparison with another physically based model. A prediction of the dimensionless diffusion coefficient is obtained as a function of the timescale of the exchange process and is validated with a few sets of results from laboratory tests.     

Effects of Biomass Growth on Gas Pressure Drop in Biofilters

The effects of biomass accumulation and distribution on air pressure losses in biofilters were experimentally studied. Two bench-scale biofilters, one packed with inert porous pellets (Nova Inert) and the other with wood chips, were operated under similar conditions with excess nutrients to treat an airstream containing methanol, at loading rates of 100–150 g methanol∕m3 bed∕h. Localized biomass accumulation in the biofilter beds was the key factor increasing the pressure drop, which was caused by local bed clogging due to biomass growth. The highest pressure drops in the beds (wood chips: 2,600 Pa∕m; Nova Inert: 550 Pa∕m) occurred in sections where there were high biomass levels with high water content. The pressure drop varied nonlinearly with the amount of accumulated biomass and the amount of methanol consumed. Sixfold higher pressure drops were measured in the wood chip biofilter than in the Nova Inert biofilter because of more biomass growth and bed compaction. A model, based on the Ergun equation, was developed to predict biomass-affected porosity and pressure drop as a function of the biomass concentration in a bed packed with spherical pellets. A comparison of the experimental and the predicted pressure drops showed that the model provided good estimates of biomass-affected porosity and pressure drop in the biofilter packed with spherical porous pellets with even biomass distribution.     

Jackson River Modeling: 50‐Year Perspective

The development of water quality models, and also the nature of water quality impairment, is uniquely presented in the point source dissolved oxygen (DO) modeling completed in the Jackson River (Virginia) over the past 50?years. Various water quality modeling studies have been completed in the Jackson River over the years starting with the earliest of modeling frameworks, the Streeter–Phelps equation (1950s and 1960s); progressing to a biochemical oxygen demand–DO model (1970s and 1990s) including diurnal photosynthetic effects (DIURNAL); a Monte Carlo DO analysis using the DIURNAL model (1990s); to the most recent modeling that is currently developing a periphyton model to assess the impact of nutrient loadings on the periphyton community and ultimately DO levels (2000). These early modeling studies were completed by such modeling forefathers as Clarence J. Velz and Donald J. O'Connor, both completing their work at academic institutions (Manhattan College and the University of Michigan) and private consulting firms (Hydroscience and HydroQual, Inc.). Interesting to note is that Earle B. Phelps taught Clarence J. Velz, Donald J. O’Connor’s eventual professor at Manhattan College. Other work completed on the river by early environmental engineers included reaeration studies by Ernest C. Tsivoglou (1966) and the first activated sludge wastewater treatment design for a pulp and paper mill by Wesley Eckenfelder (1950s). The studies investigated: how to improve existing DO conditions in the river; the effects of color reductions on diurnal DO swings; proposed upstream flow regulation effects on water quality and river temperature; and the impact of instream oxygen addition.     

Parameter Estimation of the Transient Storage Model for Stream–Subsurface Exchange

The transient storage model (TSM) is the most commonly used model for stream–subsurface exchange of solutes. The TSM provides a convenient, simplified representation of hyporheic exchange, but its lack of a true physical basis causes its parameters to be difficult to predict. However, the simple formulation makes the model a useful practical tool for many applications. This work compares the TSM with a physically based pumping model. This comparison is advantageous for two reasons: Advective pumping is known to be an important hyporheic exchange process in many streams, and the pumping model can be used to derive dimensionless transient storage parameters that are properly scaled with important physical stream parameters. Transient storage model parameters are shown to be dependent on both the timescale of observation and the shape of the breakthrough curve, i.e., on the temporal evolution of the solute concentration in the surface water. This indicates that the transient storage model can, in practice, lead to incorrect predictions when model parameters are obtained without consideration of the stream flow dynamics, the properties of the stream bed, or the process timescale. This work emphasizes the limitations of simplified models for hyporheic transport, and indicates that such models need to be carefully applied.     

Metal Stimulation and Municipal Digester Thermophilic/Mesophilic Activity

Biomass from thermophilic and mesophilic digesters (four temperature-phased anaerobic digesters and one phased thermophilic digester) was assayed for potential methane production rate increases resulting from nutrient (Ni, Co, and Fe) addition. Furthermore, digester operations and biomass activities were compared. The majority (77%) of biomass samples benefited from nutrients, with propionate and acetate utilization rates increasing as much as 50 and 35%, respectively, after nutrient addition. Propionate utilization rates were more frequently stimulated by nutrient addition, demonstrating increased methane production rates of from 14 to 50% upon nutrient amendment. Others have observed difficulty achieving low propionate concentrations in municipal thermophilic digesters, especially at lower retention times. Trace nutrient supplementation is one method to increase propionate and acetate utilization in some municipal thermophilic as well as mesophilic digesters.     

Exchange Processes between a River and Its Groyne Fields: Model Experiments

The exchange of dissolved matter between a groyne field and a main stream influences the transport and distribution of a pollutant cloud in a river. In forecasting models, groyne fields are represented as dead zones with effective properties like exchange coefficients and exchanging volume. Despite its relevance for such practical applications, little research has been done on the exchange process between a groyne field and the main stream itself. Therefore, this study is aimed at examining this exchange process and validating the dead-zone prediction model, which treats the exchange process as a first order system. A schematized physical model of a river with groynes was built in a laboratory flume. The exchange process was visualized quantitatively with dye in adjacent groyne fields. In order to couple the exchange process to the velocity field, particle tracking velocimetry measurements were performed. Two different types of exchange were observed. First, exchange takes place via the mixing layer that is formed at the river-groyne-field interface. The large eddies formed in the mixing layer are the major cause of this exchange. Second, under certain conditions, even larger eddies are shed from the upstream groyne tip. Distortions in the flow field caused by such intermittent structures cause a much larger exchange than that by the mixing layer alone. The occurrence of large shed eddies depends on the presence of a sufficiently large, stationary, secondary gyre located at the upstream corner of the groyne field. The overall exchange of matter could be characterized as a first-order process, in accordance with the dead-zone-theory. The corresponding exchange coefficients agreed reasonably well with the results of earlier experiments and the effective coefficients as found in experiments in real river flows.     

3D Numerical Modeling of MICROCYSTIS Distribution in a Water Reservoir

A 3D computational fluid dynamics program was used to calculate the wind-induced accumulation of phytoplankton in Eglwys Nynydd, a water supply reservoir in Wales. The computational fluid dynamics model solved the Navier-Stokes equations for the water velocities using the SIMPLE method to calculate the pressure. Two turbulence models were tested: a zero-equation model and the k-ε model. An unstructured nonorthogonal 3D grid with hexahedral cells was used. The distribution of the blue-green algae Microcystis was calculated by solving the transient convection-diffusion equation for phytoplankton concentration, based on the modeled flow field. The numerical model included algorithms for calculating the growth rate of phytoplankton and simulating the response of the algae to changes in underwater light intensity. The model was validated by comparing the horizontal distribution patterns produced by simulation with those recorded during a field survey of surface concentrations. The results demonstrated reasonable agreement, particularly when using the k-ε turbulence model. The main parameter affecting the results was the effective diameter of the Microcystis colonies.     

Mathematical Modeling as a Tool in Aquatic Ecosystem Management

The capacity of an existing model to simulate the growth (biomass) of a reed [Phragmites australis (Cav) Trin. ex Stuedel] in fresh water habitats using published field data and the incorporation of a submodel to estimate seasonal variation in reed mineral–nutrient content was investigated. This new feature also enabled one to estimate plant removal of mineral–nutrients from sediments. Model-predicted and observed shoot, rhizome, and root biomass showed concordance correlation coefficients of 0.97, 0.52, and 0.99, respectively. The nutrient analysis study showed that the annual uptakes of nitrogen and phosphorus from sediment by P. australis in the Denmark Vejlerne Nature Reserve were 143.9 and 16.1 kg?ha?1, respectively. The simulated results also showed that at the time of peak standing stock of minerals, shoots contained 40 and 22.5% of whole plant N and P, respectively. This suggested that the use of the common reed in wastewater treatment plants allows removal of nitrogen more easily than phosphorus, because a higher percentage of nitrogen is bound with the easily removable shoot parts.     

Simulation of Flow past an Open-Channel Floor Slot

In the past, solutions to the problem of flow past a floor slot in a rectangular open channel used to divert flow from one stream to another were obtained mainly on the basis of model tests or through the development of simplified theoretical expressions. In the present study, the free-surface turbulence model is applied to obtain the flow parameters such as pressure head distribution, velocity distribution, and water surface profile. The predictions of the proposed numerical model are validated using previous experimental data. In particular, the model predictions agree well with the test data related to flow parameters. The study indicates that the free-surface turbulence model developed is an efficient and useful tool for predicting characteristics of free surface flows such as flow past a floor slot. For flow past an open-channel floor slot, a model that is properly validated can be used to predict the flow characteristics under various flow configurations encountered in the field, without resorting to expensive experimental procedures.     

南京生态科技岛海绵城市河道系统植物技术设施对水体重金属及营养盐净化能力评价

为探究海绵城市植物技术设施对重金属及营养盐净化能力,以江心洲南京生态科技岛河道系统为研究对象,对其上下游水体中重金属及营养盐含量进行分析,通过综合污染指数法对上下游重金属风险进行评估,采用冗余分析及Spearman相关系数探究水体环境因素对重金属含量的影响,利用河道区域四种植物技术设施(河岸缓冲带、植被过滤带、生态浮岛、台地式石笼护岸)探究不同植物组合对水体中污染物的净化能力。结果显示,江心洲河道上游重金属的枯水期、丰水期及平水期WQI值分别为1.85、1.74及2.90,分别对应为重金属轻度污染、轻度污染及中度污染。而河道下游重金属的枯水期、丰水期及平水期的WQI值分别为0.18、0.30及0.52,均未存在污染现象。河道上游水体中pH是影响重金属含量的最重要因素,pH与溶解氧(DO)、总氮(TN)、五日化学需氧量(COD₅)及总磷(TP)呈正相关。河道下游水体的pH也是影响水中重金属含量最重要的环境因素,其与溶解氧(DO)呈显著正相关。水体中营养盐净化能力大小为河岸缓冲带>植被过滤带>生态浮岛>台地式石笼护岸。相比其它植物组合,乔灌木群落栽植对水体中营养盐净化最具有潜力。     

Prediction of Nutrient Concentrations in Urban Storm Water

Excessive quantities of nutrients in urban storm-water runoff can lead to problems such as eutrophication in receiving water bodies. Accurate process based models are difficult to construct due to the vast array of complex phenomena affecting nutrient concentrations. Furthermore, it is often impossible to successfully apply process based models to catchments with limited or no sampling. This has created the need for simple models capable of predicting nutrient concentrations at unmonitored catchments. In this study, simple statistical models were constructed to predict six different types of nutrients present in urban storm-water runoff: ammonia (NH3), nitrogen oxides (NOx), total Kjeldahl nitrogen, total nitrogen, dissolved phosphorus, and total phosphorus. Models were constructed using data from the United States, collected as a part of the Nationwide Urban Stormwater Program more than two decades ago. Comparison between the models revealed that regression models were generally more applicable than the simple estimates of mean concentration from homogeneous subsets, separated based upon land use or the metropolitan area. Regression models were generally more accurate and provided valuable insight into the most important processes influencing nutrient concentrations in urban storm-water runoff.     

Spatial-Dynamic Modeling of Algal Biomass in Lake Erie: Relative Impacts of Dreissenid Mussels and Nutrient Loads

Over the past several decades, reductions in phytoplankton stocks and increased water clarity in Lake Erie have resulted from phosphorus load abatement and the introduction of zebra (Dreissena polymorpha) and quagga mussels (D. bugensis). The relative impacts of these developments and their implications for lake management have remained difficult to delineate. To address this issue, we numerically model the complex biophysical interactions occurring in Lake Erie using a two-dimensional hydrodynamic and water quality model that is extended to include dreissenid mussel and zooplankton algorithms. The model reasonably simulates longitudinal trends in water quality as well as the dynamics of central basin hypoxia. Phosphorus is the limiting nutrient through the euphotic zone and its control decreases the algal growth rate and biomass ( ～ 55–60%). Filter feeding by dreissenid mussels also decreases algal biomass ( ～ 25–30%), simultaneously stimulating increased net algae growth through enhanced algal consumption and subsequent phosphorus recycling. Effective recycling implies that algae stocks are ultimately regulated by external phosphorus loads. Returning phosphorus loads to pre-abatement 1960s levels, in the presence of dreissenid mussels, results in a western basin algae concentration of ～ 0.7?mg?dry?weight?L?1 with a potential for nuisance algae growth.