Applying the First-Order Error Analysis in Determining the Margin of Safety for Total Maximum Daily Load Computations

There are significant uncertainties associated with certain aspects of the total maximum daily load (TMDL) estimation. Selection of the “margin of safety (MOS)” term is typically made by subjectively assigning to it a small percentage (5–10%) of the TMDL load. To introduce some objectivity into the MOS estimation, the first-order error analysis (FOEA) was utilized to quantify the MOS term in the TMDL formulation. A case study, which was based on a previous study entitled “Nitrate TMDL Development for Muddy Creek/Dry River, Virginia,” is presented in this paper. Besides computational efficiency, one of the major advantages of FOEA is its capability of determining the relative importance of the various parameters that contribute to the overall variance of the model output. Precipitation was found by far to be the most dominant source of uncertainty. Furthermore, a relationship was established to link the pollutant loads with the FOEA output concentrations. The results from testing different TMDL allocation scenarios demonstrate that with the increase of relative percentage of nonpoint sources load reduction in the total load reduction, the portion needed to be reserved for MOS increases as well. The MOS term can be related to the variability in rainfall and therefore would be different for different locations in the country. In summary, as a practical, less subjective and reliable approach to TMDL uncertainty analysis, the use of the FOEA is considered as a viable alternative to the current simple explicit and implicit methods in estimating the MOS term for TMDL calculations.     

Decision Support System for Total Maximum Daily Load

A decision support system (DSS) was developed to calculate total maximum daily loads (TMDLs) of various pollutants for water quality limited sections (WQLS) within a river basin. The DSS includes a watershed simulation model, a database, a consensus building module, and a TMDL module that provides a worksheet for the calculations. The system can generate multiple combinations of waste load allocation and non-point-load allocation to meet the water quality criteria for the intended uses of the WQLS. Considering various possible solutions, the regulatory agency and local stakeholders can negotiate an option most agreeable to all parties. The methodology is demonstrated with an example application in the Catawba River Basin, which extends from North Carolina to South Carolina.     

Effect of non-point source pollution on water quality of the Weihe River

Non-point source pollution is an important factor that affects the water quality of the Weihe River. To study the effect of the pollution on the water quality of the Weihe River, five flood events and three normal discharge events during non-flood period were monitored from July to December in 2006, at the Lin-tong section. In order to identify how sediment influenced the water quality of the river, raw and supernatant samples taken from the monitored events were analyzed. Supernatant samples were siphoned from 5 cm below the water surface of the raw samples at the lab afterwards the raw samples were shaken up and laid for 30 minutes in the beakers. The results indicated that: 1) The concentrations of SS, COD, TP and TN in the raw samples from flood events were higher than those from normal discharge events. The higher values of the COD, TP and TN in the raw samples resulted from natural humic matters in surface soil. 2) The load transport rate of analyzed water quality indexes increased gradually to its maximum and then drop down, matching that in the discharge hydrograph. The concentrations of SS, NH3-N, NO2-N, NO3-N, COD, and TP in the raw samples increased initially and then decreased, while the concentration of dissolved orthophosphate and TN in the raw samples decreased gradually and then increased. The peak time of concentration and load transport rate of SS as well as COD, TP and TN in the raw samples were close to or lagged behind the time of peak flow.Generally, the peak time of concentration and load transport rate of dissolved orthophosphate,dissolved total-phosphate, NH3-N, NO2-N, and NO3-N occurred prior to the time of peak flow. 3) The mean concentration method was used to calculate the NPS pollution load at Lin-tong section, and the results are credible. In 2006, the proportions of the NPS pollution load to the total load for COD, TP,TN and inorganic nitrogen were more than 30%.     

Viewing Total Maximum Daily Loads as a Process, Not a Singular Value: Adaptive Watershed Management

This paper describes adaptive watershed management, which combines concepts for adaptive management and watershed management to address the various uncertain elements in a total maximum daily load (TMDL). The paper discusses how adaptive watershed management allows initial progress to be made while additional information is collected and incorporated in the TMDL. Adaptive watershed management differs from the conventional TMDL approach as a result of feedback loops, which allow managers to proceed with implementation of controls in a progressive manner, avoiding unproductive and irresolvable debate over uncertainty in the numeric value of the TMDL or the efficacy of the controls. Over time, improvements in monitoring, modeling, TMDL analysis, water quality targets, and control actions contribute to the improved effectiveness of the TMDL. The adaptive watershed management approach can be applied in situations dominated by nonpoint sources or having significant uncertainty in any number of issues. The paper includes examples of previous uses of adaptive approaches, a discussion of additional elements that need to be considered, and identification of regulatory and other obstacles.     

Hydrologic and Water Quality Integration Tool: HydroWAMIT

A spatially distributed and continuous hydrologic model focusing on total maximum daily load (TMDL) projects was developed. Hydrologic models frequently used for TMDLs such as the hydrologic simulation program—FORTRAN (HSPF), soil and water assessment tool (SWAT), and generalized watershed loading function (GWLF) differ considerably in terms of spatial resolution, simulated processes, and linkage flexibility to external water quality models. The requirement of using an external water quality model for simulating specific processes is not uncommon. In addition, the scale of the watershed and water quality modeling, and the need for a robust and cost-effective modeling framework justify the development of alternative watershed modeling tools for TMDLs. The hydrologic and water quality integration tool (HydroWAMIT) is a spatially distributed and continuous time model that incorporates some of the features of GWLF and HSPF to provide a robust modeling structure for TMDL projects. HydroWAMIT operates within the WAMIT structure, developed by Omni Environmental LLC for the Passaic River TMDL in N. J. HydroWAMIT is divided into some basic components: the hydrologic component, responsible for the simulation of surface flow and baseflow from subwatersheds; the nonpoint-source (NPS) component, responsible for the calculation of the subwatershed NPS loads; and the linkage component, responsible for linking the flows and loads from HydroWAMIT to the water quality analysis simulation program (WASP). HydroWAMIT operates with the diffusion analogy flow model for flow routing. HydroWAMIT provides surface runoff, baseflow and associated loads as outputs for a daily timestep, and is relatively easy to calibrate compared to hydrologic models like HSPF. HydroWAMIT assumes that the soil profile is divided into saturated and unsaturated layers. The water available in the unsaturated layer directly affects the surface runoff from pervious areas. Surface runoff from impervious areas is calculated separately according to precipitation and the impervious fractions of the watershed. Baseflow is given by a linear function of the available water in the saturated zone. The utility of HydroWAMIT is illustrated for the North Branch and South Branch Raritan River Watershed (NSBRW) in New Jersey. The model was calibrated, validated, and linked to the WASP. The NPS component was tested for total dissolved solids. Available weather data and point-source discharges were used to prepare the meteorological and flow inputs for the model. Digital land use, soil type datasets, and digital elevation models were used for determining input data parameters and model segmentation. HydroWAMIT was successfully calibrated and validated for monthly and daily flows for the NSBRW outlet. The model statistics obtained using HydroWAMIT are comparable with statistics of HSPF and SWAT applications for medium and large drainage areas. The results show that HydroWAMIT is a feasible alternative to HSPF and SWAT, especially for large-scale TMDLs that require particular processes for water quality simulation and minor hydrologic model calibration effort.     

Improving Total Maximum Daily Loads with Lessons Learned from Long-Term Detailed Monitoring

Many thousands of impaired water segments in the United States will be the subject of total maximum daily load (TMDL) determinations in the next decade. Many of these load allocations will be established with access to only minimal local data. Long-term and detailed datasets from other locations can facilitate this process by offering general insights into the processes that interact to produce the chemistry observed in a particular waterbody over time. These insights can lead to more enlightened interpretation of sparse but locally relevant water quality data. They can also inform the design of implementation monitoring to evaluate success of TMDLs. Finally, study of such datasets reveals biases that may result from inappropriate sampling design or data interpretation algorithms, and may lead to erroneous conclusions about the success or failure of a TMDL program in a specific watershed.     

Development of a Simple Phosphorus Model for a Large Urban Watershed: A Case Study

Often, the initiation of a total maximum daily load (TMDL) program is delayed until intensive monitoring data can be collected—even in watersheds where large historical data sets exist. This paper provides a case study of a modeling effort that utilizes available historical data to fulfill an intermediate goal of a TMDL program for the Passaic River Basin. The subject model is developed to simulate total phosphorus concentrations (and loads) within the basin’s effluent-dominated streams. The model is based on the assumption that the primary process controlling in-stream total phosphorus concentrations is the dilution of the cumulative upstream effluent load—which was computed on a continuous (daily) basis. Model comparisons indicate a generally good fit to long-term river-monitoring data at several key sites. Model results, and data analyses, suggest that secondary processes have a relatively minor impact on total phosphorus (TP) concentrations in this relatively large, urbanized system. This finding is consistent with a previous QUAL2E model study of the system, and consistent with the relatively conservative behavior of TP reported in many medium-to-large river systems throughout the United States. Model results are used to facilitate TMDL planning efforts for a major water supply reservoir in the basin.     

Estimating Nonpoint Source Pollution in the Upper Yangtze River Using the Export Coefficient Model, Remote Sensing, and Geographical Information System

Recently, increasing nutrient (i.e., nitrogen and phosphorus) concentrations have been observed in the surface water of many countries and this nonpoint source (NPS) pollution has become an important factor in the deterioration of water quality in the upper reach of the Yangtze River Basin. In this paper, the NPS pollution loads in the upper reach of Yangtze River Basin in the year 2000 were estimated using export coefficient model and remote sensing techniques. The spatial distributions of the NPS loads within the watershed were then displayed using geographical information system. Results indicated that the total nitrogen load was 1.947×106?t and the total phosphorus load was 8.364×104?t. Important source areas for the nutrients were croplands in the Jinsha R. and Jialing R. watershed, as well as the Chongqing municipality.     

Dynamic Wave Study of Flow in Tidal Channel System of San Juan River

In this work the complete equations of one-dimensional unsteady flow in open channels in integral form, and compatibility equations at the junctions of a channel network, are solved numerically. Analytical integration in space is used between each pair of consecutive irregular sections of a channel, and the nonprismatic term is expressed in terms of uncoupled functions of the geometry at the sections. The linearized system of equations for each time interval is solved by an elimination method based on a double-sweep algorithm. The model is applied to the estuary of the San Juan River in Venezuela, where oscillating currents by effect of semidiurnal tides take place and the amplitude of the wave at the mouth is amplified toward the inland direction. Alternating drying and filling is simulated by means of slight modifications in the bed geometry of upper river sections. Measured water elevation and flow rates available at two stations are used to calibrate the model, and a very accurate adjustment of the tidal levels observed in the river is obtained.     

Considering Bacteria-Sediment Associations in Microbial Fate and Transport Modeling

The development of a total maximum daily load (TMDL) for water bodies impaired by elevated microbial levels (the second leading cause of impairment nationally) requires an understanding of microbial transport processes at the watershed scale. Continuous monitoring of impaired water bodies can be expensive, and models are typically employed, but most current models represent bacteria as single discrete (“free” phase) organisms with near-neutral buoyancy, subject to first-order decay resulting primarily from predation or die-off. Studies indicate, however, that a significant fraction of microbes are associated with sediment particles, both in the water column and bed-sediments, associations that can impact microbial transport behavior and survival rates. This work incorporates considerations of microbial partitioning and its impact on survival into microbial fate and transport modeling using a well-characterized watershed. Agreement between observed and modeled instream microbial concentrations is comparable to, or better than, that seen in similar studies. Nonetheless, differences in instream concentration between model runs that consider microbe-sediment association (with attendant survival differences) and those that assume all microbes exist in the free phase are relatively small. A sensitivity analysis of relevant model inputs further indicates the minor effects of incorporating these considerations. The low settling velocities of small particles with which microbes typically associate and the dominance of other inputs related to wet weather microbial loadings, when compared with resuspension, result in the reduced significance of microbial partitioning as a factor in water quality modeling.     

金华江小流域氨氮污染状况分析及控制对策探讨

通过对金华江小流域水质状况、污染成因、上游小支流氨氮污染沿程分布以及不同支流汇入交接断面污染现状分析和跟踪分析调查,以及对金华江近十年上下游氨氮污染变化趋势进行研究,据此提出具体氨氮污染控制对策以及可实施的流域管理技术与方法.     

Evaluating Four Storm-Water Performance Metrics with a North Carolina Coastal Plain Storm-Water Wetland

Storm-water best management practices (BMPs) are typically assessed using the performance metric of pollutant concentration removal efficiencies. However, debate exists whether this is the most appropriate metric to use. In this study, a storm-water wetland constructed and monitored in the coastal plain of North Carolina is evaluated for water quality and hydrologic performance using four different metrics: concentration reduction, load reduction, comparison to nearby ambient water quality monitoring stations, and comparison to other wetlands studied in North Carolina. The River Bend storm-water wetland was constructed in spring 2007 and was monitored from June 2007 through May 2008. Twenty-four hydrologic and 11 water quality events were captured and evaluated. The wetland reduced peak flows and runoff volumes by 80 and 54%, respectively. Reductions were significant. Concentrations for the following pollutants increased: total kjeldahl nitrogen (TKN), NH4–N, total nitrogen (TN), and total suspended solids (TSS); inflow and outflow concentrations did not change for total phosphorus (TP), while only NO2–3–N and orthophosphorus (OP) concentrations were lower at the outlet. Using a load reduction metric, results were strikingly different, showing positive load reductions of 35, 41, 42, 36, 47, 61, and 49% for these respective pollutants: TKN, NO2–3–N, NH4–N, TN, TP, OP, and TSS. When comparing the effluent concentrations from the wetland to ambient water quality in the Trent River, all effluent nitrogen species concentration were either similar or lower. TP and TSS concentrations leaving the wetland were higher than ambient water quality data. Finally, by comparing pollutant concentrations among different North Carolina wetlands, it is apparent the River Bend wetland received relatively “clean” water and released water with pollutant concentrations comparable to all other studies examined. Major conclusions from this study include: (1) storm-water wetlands sited in sandier soils (such as those of the North Carolina coastal plain) should be considered a low impact development tool and (2) the selection of performance metric has a pronounced bearing on how a BMP’s performance is perceived. Sole reliance on a concentration reduction metric is discouraged.     

Wash Load and Bed-Material Load Transport in the Yellow River

It has been the conventional assumption that wash load is supply limited and is only indirectly related to the hydraulics of a river. Hydraulic engineers also assumed that bed-material load concentration is independent of wash load concentration. This paper provides a detailed analysis of the Yellow River sediment transport data to determine whether the above assumptions are true and whether wash load concentration can be computed from the original unit stream power formula and the modified unit stream power formula for sediment-laden flows. A systematic and thorough analysis of 1,160 sets of data collected from 9 gauging stations along the Middle and Lower Yellow River confirmed that the method suggested by the conjunctive use of the two formulas can be used to compute wash load, bed-material load, and total load in the Yellow River with accuracy.     

赣江上游河道氮污染分析评价

为研究赣江上游-桃江流域水质污染分布状况,基于平水期在桃江流域采集的167个水样,通过现场和室内分析测定,并运用内梅罗综合污染指数和单因子污染指数评价法对流域的硝氮和氨氮污染状况进行评价。结果表明:①桃江干流和大部分支流pH值都在地表水环境质量标准限值范围内,但濂江17个采样点中,有4个采样点受小型电站和酿酒厂的影响,pH值低于地表水标准限值; ②亚硝氮极易被氧化,是流域总体亚硝氮含量较低的原因,干流上游部分地区亚硝氮浓度稍高,渥江和濂江水质差,河水处于缺氧或厌氧状态,浓度较高; ③流域总体硝氮超标率为31.7%,龙迳河和濂江受矿区及畜禽场的影响,硝氮变异系数大且超标严重,超标率分别为73.3%,70.6%;④流域内稀土矿区分布广、数量大,采矿、选矿过程中大量使用的硫酸铵、碳酸氢铵等化学药剂的使用,使氨氮污染最严重,且均超出地表Ⅲ类水氨氮限值,其中介于Ⅲ~Ⅳ类水占51.5%,Ⅳ~Ⅴ类水占8.4%,劣Ⅴ类水占40.1%。      

Influence of Physical Forcing on Bottom-Water Dissolved Oxygen within Caloosahatchee River Estuary, Florida

Environmental Fluid Dynamics Code, a numerical estuarine and coastal ocean circulation hydrodynamic and eutrophication model, was used to simulate the distributions of dissolved oxygen (DO), salinity, water temperature, and nutrients in the Caloosahatchee River Estuary. Modeled DO, salinity, and water temperature were in good agreement with field observational data from the Florida Department of Environmental Protection and South Florida Water Management District. Sensitivity analyses identified the effects of river discharge, atmospheric winds, and tidal forcing on the spatial and temporal distributions of DO. Simulation results indicated that vertical mixing due to wind forcing increased the bottom DO concentration. River discharge enhanced stratification in deep locations but propagated vertical mixing in the shallow upper estuary. Finally, tidal forcing heavily influenced bottom layer DO concentrations throughout the whole river estuary.     

Field Experiment Study of Transient Stratified Flow in an Estuary

An experimental study of stratified flow was conducted in this study. An electromagnetic measurement instrument, the S4 current meter, was used in field data collections of salinity and currents in the lower Apalachicola River estuary, Florida. The S4 current meter has an advantage in field deployments for long periods of time due to its preprogrammable capability for automatic data sampling and recording of fluid flow. Time series of surface and bottom salinity and currents obtained from field experiments were used to characterize the stratified flow at the measurement location in the Apalachicola River. Analysis of field data indicated that the river was strongly stratified. The stratification was affected by the upstream river flow and the downstream tidal variations. Stratification was stronger at high tide than at low tide. By removing the tidal signal using low-passing filtering, subtidal salinity, and currents were obtained to investigate the salinity stratification and currents responses to the changes of fresh water input. Subtidal vertical salinity and velocity profiles were presented at different flow conditions. At high flow conditions, both surface and bottom subtidal currents were in the seaward direction. At low flow conditions, the bottom subtidal currents were in the upstream direction due to the strong effects of density gradients. Empirical regression equations were obtained to quantify the effects of river flow on the subtidal salinity and the bottom currents. Regression analysis indicated good linear relationship between subtidal salinity stratification and the bottom currents.     

Hydraulic Study of the Onset of Hypoxia in the Tone River Estuary

Field measurements were conducted to investigate the onset and growth of hypoxia in the Tone River Estuary. Vertical profiles of dissolved oxygen (DO), salinity, and temperature were measured along the deepest line of the estuary. Rates of oxygen consumption by water and sediment in a salt wedge were obtained using laboratory tests. The measurements showed that hypoxia frequently occurred in the front part of the salt wedge and expanded its area toward the river mouth during the summer when the river flow rate was small. The data also suggested that the onset of hypoxia was delayed by the estuarine circulation which supplied oxygen-rich seawater to the salt wedge. To simulate this phenomenon, a two-dimensional flow model in the vertical-longitudinal plane was constructed by transversely integrating the k–ε model equations. The results of model simulation for three months in the summer of 1997 closely matched the field data. The model simulation proved that DO degradation is highly correlated with the residence time of salt water in the estuary.     

Bacteria Load Estimator Spreadsheet Tool for Modeling Spatial Escherichia coli Loads to an Urban Bayou

The model developed in this paper, the bacteria loading estimator spreadsheet tool (BLEST), was designed as an easy to use indicator bacteria model that can overcome the shortcomings of many of the simpler total maximum daily load (TMDL) modeling approaches by integrating spatial variation into load estimates. BLEST was applied to the Buffalo Bayou watershed in Houston, Texas and incorporated loading from point and nonpoint sources, such as wastewater treatment plants, sanitary sewer overflows, septic systems, storm sewer leaks, runoff, bed sediment resuspension, and direct deposition. The dry weather Escherichia coli load in Buffalo Bayou was estimated using BLEST to be 244 billion MPN/day and would require an overall 48% reduction to meet the contact recreation standard, while wet weather loads would need to be reduced by 99.7%. Dry weather loads were primarily caused by animal direct deposition, septic systems and discharges from storm sewers under dry weather conditions, while wet weather loads were mostly attributable to runoff and resuspension from sediment. Unlike most simple TMDL load allocation strategies, BLEST can be used to evaluate spatially variable load reduction strategies. For example, septic system load reductions implemented in less than 10% of the subwatersheds resulted in a decrease in bayou loading of more than 20%.     

3D Numerical Model for Pearl River Estuary

A 3D numerical model with an orthogonal curvilinear coordinate in the horizontal direction and sigma coordinate in the vertical direction has been developed. This model is based on POM (Princeton Ocean Model). In this model a second moment turbulence closure submodel is embedded and the stratification caused by salinity and temperature is considered. Furthermore, to adapt to estuary locations where the flow pattern is complex, the horizontal time differencing is implicit with the use of a time-splitting method instead of the explicit method in POM. This model is applied to the Pearl River estuary, which is the largest river system in South China, with Hong Kong at the eastern side of its entrance. The computation is verified and calibrated with field measurement data. The computed results mimic the field data well.     

Optimization of Watershed Control Strategies for Reservoir Eutrophication Management

A vital key to the development of a reservoir eutrophication management strategy is to link the watershed-nutrient model to the model of reservoir water quality. To develop a cost-effective optimization model, a coupled watershed-reservoir model with an optimization model has been developed to design control strategies in the watershed in a planning time horizon. This methodology can help reduce the phosphorus concentration of a reservoir to the standard level. In this study, the weather data for the next 10 years was generated using downscaled GCM data to simulate the watershed phosphorus load using the SWAT model. Then an optimal model for selection and placement of best management practices (BMP) at watershed scale is developed by linking the coupled watershed and reservoir models with a genetic algorithm. This model is able to identify the minimum present cost design (type and location) of BMP structural alternatives. The objective of water quality is obtained using a system dynamic model for reservoir phosphorus concentration to determine a permissible phosphorus load as the main agent of eutrophication in a reservoir. Structural BMPs in this study include, filter strips, parallel terraces, grade stabilization structures, and detention ponds. The optimum solution was obtained through a trade-off curve between cost and exceedance magnitude from the standard of reservoir phosphorus concentration. The case study is the Aharchai River Watershed upstream of the Satarkhan Reservoir in the northwestern part of Iran.