Urban Stormwater Management in the United States (2009) / Chapter Skim
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4 Monitoring and Modeling
4 Monitoring and Modeling
Pages 257-338
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From page 257... ...
Finally, it makes numerous technical suggestions for improving the monitoring of MS4s, building on the data already submitted and analyzed as part of the National Stormwater Quality Database. Policy recommendations about the monitoring of both industries and MS4s are found in Chapter 6.
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From page 258... ...
. The regulations require the MS4 program to identify five to ten stormwater discharge outfalls and to collect representative stormwater data for conventional and priority toxic pollutants from three representative storm events using both grab and composite sampling methods.
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From page 259... ...
Because there were multiple sources of information, quality assurance and quality control reviews were very important to verify the correctness of data added to the database, and to ensure that no duplicate entries were added. The NSQD includes sampling location information such as city, state, land use, drainage area, and EPA Rain Zone, as well as date, season, and rain depth.
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From page 260... ...
An EMC is intended to represent the average concentration for a single monitored event, usually based on flow-weighted composite sampling. It can also be calculated from discrete samples taken during an event if flow data are also available.
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From page 261... ...
Maestre and Pitt (2006) reviewed the NSQD data to better explain the variability according to different site and sampling conditions (land use, geographical location, season, rain depth, amount of impervious area, sampling methods, antecedent dry period, etc.)
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The use of average EMC values will represent the larger values better, although they will still not represent the variability likely to exist. If all of the variability cannot be further explained adequately (such as being affected by rain depth)
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From page 263... ...
As an example, the model WinSLAMM (Pitt, 1986; Pitt and Voorhees, 1995) uses a Monte Carlo component to describe the likely variability of stormwater source flow pollutant concentrations using either lognormal or normal probability distributions for each constituent.
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From page 264... ...
For example, Figure 4-4 (total phosphorus in residential areas) shows that most of the data fall along the straight line (indicating a lognormal fit)
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The use of the lognormal distribution also has an advantage over the other distribution types because it can be easily transformed to a normal distribution and the data can then be correctly examined using a wide variety of statistical tests.
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From page 266... ...
This is the only method, however, that will characterize the changes in pollutant concentrations during the event. Time-based composite sampling refers to samplers being programmed to take an aliquot after a set period of time (as short as every 3 minutes)
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Nutrients in EPA Rain Zone 2 collected in residential, commercial, and industrial areas showed no significant differences using either compositing method. The only exceptions were for ammonia in residential and commercial land-use areas and total phosphorus in residential areas where time-based composite samples had higher concentrations.
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From page 268... ...
is a reasonable objective for most stormwater projects. This is especially important when monitoring programs attempt to distinguish test and control conditions associated with SCMs.
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MONITORING AND MODELING 269 FIGURE 4-5 Number of samples to characterize median (power of 80% and confidence of 95%)
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270 URBAN STORMWATER MANAGEMENT IN THE UNITED STATES FIGURE 4-6 Number of paired samples needed to distinguish between two sets of observations (power 80% and confidence of 95%)
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From page 271... ...
Figure 4-7 is a plot of monitoring data from the Villanova first-flush study (Batroney, 2008) showing the flows, rainfall, TSS concentration, TDS concentration, and TDS and TSS event mean concentrations for the inflow to an infiltration trench.
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This reduced frequency of observed first flushes in areas most likely to have first flushes is probably associated with the varying rain conditions during the different events, including composite samples that did not represent the complete runoff duration.
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MONITORING AND MODELING FIGURE 4-8 Pollutant variations during rain period (0.4-ha drainage area, mostly paved parking with small 273 fringe turf area, Tuscaloosa, Alabama)
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274 URBAN STORMWATER MANAGEMENT IN THE UNITED STATES TABLE 4-2 Significant First Flush Ratios (First Flush to Composite Median Concentration) Commercial Industrial Institutional Parameter n sc R ratio n sc R ratio n sc R ratio Turbidity, NTU 11 11 = 1.32 X X COD, mg/L 91 91 2.29 84 84 1.43 18 18 2.73 TSS, mg/L 90 90 1.85 83 83 = 0.97 18 18 2.12 Fecal coliform, 12 12 = 0.87 X X col/100mL TKN, mg/L 93 86 1.71 77 76 1.35 X Phosphorus total, 89 77 1.44 84 71 = 1.42 17 17 = 1.24 mg/L Copper, total, µg/L 92 82 1.62 84 76 1.24 18 7 = 0.94 Lead, total, µg/L 89 83 1.65 84 71 1.41 18 13 2.28 Zinc, total, µg/L 90 90 1.93 83 83 1.54 18 18 2.48 Open Space Residential All Combined Parameter n sc R ratio n sc R ratio n sc R ratio Turbidity, NTU X 12 12 = 1.24 26 26 = 1.26 COD, mg/L 28 28 = 0.67 140 140 1.63 363 363 1.71 TSS, mg/L 32 32 = 0.95 144 144 1.84 372 372 1.60 Fecal coliform, X 10 9 = 0.98 22 21 = 1.21 col/100mL TKN, mg/L 32 14 = 1.28 131 123 1.65 335 301 1.60 Phosphorus, 32 20 = 1.05 140 128 1.46 363 313 1.45 total, mg/L Copper, total, 30 22 = 0.78 144 108 1.33 368 295 1.33 µg/L Lead, total, 31 16 = 0.90 140 93 1.48 364 278 1.50 µg/L Zinc, total, 21 21 = 1.25 136 136 1.58 350 350 1.59 µg/L Note: n, number of total possible events; sc, number of selected events with detected values; R, result; X, not enough data; =, not enough evidence to conclude that median values are different; , median values are different.
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From page 275... ...
Other seasonal maintenance operations benefiting stormwater quality should also be scheduled before these initial rains. Rain Depth Effects An issue related to first flushes pertains to the effects of rain depth on stormwater quality.
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From page 276... ...
SOURCE: NSQD. resulting outfall stormwater concentration patterns for a large area having various surfaces is therefore complex and rain depth is just one of the factors involved.
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Second, flow-monitoring instrumentation also needs to be used at all water quality monitoring stations. The lack of flow data greatly hinders the value of the chemical data.
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The detection limits for filtered heavy metals should also be substantially less than shown on this table. Seasonal Effects Another factor that some believe may affect stormwater quality is the season when the sample was obtained.
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. FIGURE 4-10 Fecal coliform concentrations in stormwater by season.
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From page 280... ...
In addition, flow monitoring and on-site rainfall monitoring need to be included as part of all stormwater characterization monitoring. The additional information associated with flow and rainfall data will greatly enhance the usefulness of the much more expensive water quality monitoring.
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From page 281... ...
An even smaller subset of MSGP industries that are subject to numerical effluent guidelines under 40 C.F.R. must, in addition, collect grab samples of their stormwater discharge after every discharge event and analyze it for specific pollutant parameters as specified in the effluent guidelines (see Table 2-6)
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From page 282... ...
The following discussion is organized around the reasons why monitoring at industrial sites may be conducted. Regional Monitoring of Many Facilities An important monitoring objective would be regional monitoring to calibrate and verify stormwater quality models, to randomly verify compliance at facilities not normally requiring monitoring, and to establish benchmarks for compliance.
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From page 283... ...
Whether these data were collected each year and for each industry was highly variable. The analysis of the data from Los Angeles and Ventura counties revealed that stormwater monitoring data are not similar to the types of data that the environmental engineering field is used to collecting, in particular wastewater data.
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From page 284... ...
For drinking water treatment, monitoring is done to ensure the quality of the product, while for wastewater, there is a permit that requires the plant to meet a specific quality of water. Unlike these other areas of water resources, there are few incentives that might compel an industry to increase its frequency of stormwater monitoring.
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From page 285... ...
MONITORING AND MODELING 285 ip mp ww FIGURE 4-12 Number of cases needed to detect a certain percentage difference in the means, using COD as an example. SOURCE: Reprinted, with permission, from Stenstrom (2007)
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(1) Compiling available regional stormwater quality data and comparing the available data to the needs (such as calibration of a regional model; verifying compliance of facilities not requiring monitoring; and establishing regional benchmarks)
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Regional monitoring is also necessary to more accurately establish benchmarks for numeric permits. Geographical location, along with land use, is normally an important factor affecting stormwater quality.
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Finally, research was also conducted to examine possible water treatment methods for water being pumped from telecommunication manholes. Summary of Sampling Effort and Strategy The objective of the monitoring program was to characterize telecommunication manhole water and sediment.
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Samples from newer areas were also more toxic than those from older areas. Further statistical tests of the data indicated that the high toxicity levels were likely associated with periodic high concentrations of salt (in areas using de-icing salt)
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Very few organic toxicants were found in the water samples, but sediment sample organic toxicant concentrations appeared to be well correlated to sediment texture and color. About 10 to 25 percent of the sediment samples had relatively large concentrations of organics.
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The experimental design of this monitoring program allowed an efficient examination of factors affecting stormwater discharges from these operations. This enabled the efficient implementation of effective control programs that targeted specific site and operational characteristics.
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Box 4-4 provides a detailed example of industrial stormwater monitoring at individual sites in Wisconsin. In general, monitoring of industries should be tailored to their stormwater pollution potential, considering receiving water uses and problems.
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For the time-based composite sheet flow samples, a sheet flow sampler is programmed to take an aliquot of sheet flow after a set period of time (usually about every 5 to 15 minutes)
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The time-based composite sheet flow sample could not document the number of samples it collected, but it was set to collect a sample every few minutes. To judge the accuracy of the sampling methods, one method had to be selected as the most representative of the concentration and load affecting the receiving water.
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The sheet flow sampler could be the most cost effective approach to sampling an industrial site. A determination must be made of how many runoff events should be sampled in order to accurately characterize a site's water quality.
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Recommendations for Industrial Stormwater Monitoring Suitable industrial monitoring programs can be implemented for different categories of industrial activities. The following is one such suggestion, based on the likely risks associated with stormwater discharges from each type of facility.
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For these facilities, simple visual inspections with no monitoring requirements may be appropriate to ensure compliance with the basic stormwater regulations. A regionally calibrated stormwater quality model can be used to evaluate these basic stormwater conditions and to calculate the expected benefits of control measures.
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From page 298... ...
MODELING TO LINKING SOURCES OF POLLUTION TO EFFECTS IN RECEIVING WATERS Stormwater permitting is designed to regulate dischargers, develop information, and reduce the level of stormwater pollutants and impact on receiving waterbodies. An important assumption is that the level of understanding of the stormwater system, through a combination of monitoring and modeling, is sufficient to associate stormwater discharges with receiving waterbody impacts.
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From page 299... ...
A protocol linking pollutants in stormwater discharges to ambient water quality criteria should be based on conservation of mass, in which the major inputs, outputs, transformations, and stores of the pollutant can be quantified. Indeed, these are the components of hydrologic and watershed models used to simulate the fate and transport of stormwater and its pollutants.
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From page 300... ...
300 URBAN STORMWATER MANAGEMENT IN THE UNITED STATES FIGURE 4-14 The drainage area to Jordan Lake, a major drinking water reservoir in the Triangle area of North Carolina, is under nutrient-sensitive rules, requiring reductions in total nitrogen and phosphorus. Drainage flowlines and catchment areas are from NHDplus, and are shaded according to their percentage of industrial and commercial land cover from the NLCD.
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. Figure 4-15 shows a small tributary within the Jordan Lake watershed in Chapel Hill (outlined in Figure 4-14)
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Fundamentals of Stormwater Models Stormwater models are designed to evaluate the impacts of a stormwater discharge on a receiving waterbody. In order to do this, the model must have the capability of describing the nature of the source term (volumes, constituents)
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From page 303... ...
Major divisions between approaches are found in the representation of the watershed "geography" in terms of patterns and heterogeneity, and in runoff production and routing. Some stormwater models do not consider the effects of routing from a runoff source to a local waterbody directly, but may attempt to reproduce net impacts at larger scales through the use of unit hydrograph theory to estimate peak flows, and delivery ratios or stormwater control efficiency factors to estimate export to a waterbody.
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From page 304... ...
Distributed models include some scheme to represent spatial heterogeneity of the watershed environment pertinent to stormwater generation, including land cover, soils, topography, meteorological inputs, and stream reach properties distributed through a set of linked control volumes. Control volumes representing land elements, including vertically linked surface and subsurface stores, are connected by a representation of water and pollutant lateral routing through a network of flowpaths that may be predefined or set by the dynamics of surface, soil, and saturated zone water storage.
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From page 305... ...
Pollutant concentrations or loads are often estimated on the basis of look-up tables using land use or land cover. Land use- or land cover-specific EMC or unit area loading for pollutants can be developed directly from monitoring data or from local, regional, or national databases.
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From page 306... ...
Two statistically based relations were developed to drive the ratio, the first of which is based on a "curve number" which depicts the soil type, land cover, and initial moisture content. The second or initial abstraction is defined as the volume of losses that occur prior to the initiation of runoff, and is also related to the curve number.
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From page 307... ...
The NRCS curve number and dimensionless unit hydrograph were first incorporated in the Soil Conservation Service (SCS) TR-20 hydrologic computer model developed in the 1960s.
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From page 308... ...
In landscapes dominated by variable source area runoff dynamics in which runoff is generated from areas that actively expand and contract on the basis of soil moisture conditions, a fuller accounting of the soil moisture budget is required. Furthermore, event-based modeling is inappropriate for water quality purposes because it will not reproduce the full distribution of receiving water problems.
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From page 309... ...
The most common hydrology "engines" in simple stormwater models are the NRCS curve number method or a simple volumetric runoff coefficient -- Rv, the ratio of runoff to rainfall -- for either single rainfall events or the total annual rainfall depth. Runoff quality in the simple models is usually calculated based on published EMCs for similar land uses in the same geographical area.
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From page 310... ...
In general, stormwater models were designed to track and predict discharges from sources by surface water flowpaths into receiving waterbodies, such that infiltration was considered to be a loss (or retention) of water and its constituents.
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From page 311... ...
This is not synonymous with saying that the model can reliably represent the real system under any set of conditions. In general, the water quantity aspects of stormwater modeling are easier to calibrate and verify than the water quality aspects, in part because there are more water quantity data available and because chemical transformations are more complex to simulate.
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From page 312... ...
Stormwater pollutant concentrations are usually estimated from local or regional data, or from national data sources. The Simple Method estimates pollutant loads for chemical constituents as a product of annual runoff volume and pollutant concentration, as L = 0.226 R x C x A, where L = annual load (lbs)
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From page 313... ...
Officials are now in the process of implementing watershed goals to reduce nitrogen and phosphorus, with the reduction goals differentiated by geographic location within the basin. In support of the development of these rules as part of a TMDL effort, the North Carolina Division of Water Quality commissioned a water quality modeling study (Tetra Tech, 2003)
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From page 314... ...
. The model concentrates on the quality and quantity of urban stormwater, including detailed accounting of multiple SCMs acting within a treatment train and life-cycle costing.
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From page 315... ...
BOX 4-7 Using SWAT and WinSLAMM to Predict Phosphorus Loads in the Rock River Basin, Wisconsin Wisconsin Administrative Code NR 217 states that wastewater treatment facilities in Wisconsin must achieve an effluent concentration of 1 mg/L for phosphorus. Alternative limits are allowed if it can be demonstrated that achieving the 1 mg/L limit will not "result in an environmentally significant improvement in water quality" (NR 217.04(2)
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From page 316... ...
A threshold of 100 cfs mean annual flow is used to represent streams and rivers, and the one-to-one mapping of land segment to river reach produces large, heterogeneous land segments as the basic runoff-producing zones. SCMs are implemented either at the field or runoff production unit as distinct land segment types in terms of management or land cover, or as "edge-of-field" reductions of runoff or pollutant loads.
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From page 317... ...
The description of build-up and wash-off is a critical component in urban stormwater models applied to areas with substantial impervious surfaces and is a good example of the need to match detailed and rigorous field sampling in
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From page 318... ...
and laterally from a drainage divide to the stream, including stream-channel and riparian segments. A set of models has been developed and applied to stormwater generation using this paradigm that can be applied at the scale of residential neighborhoods, resolving land cover and topography at the parcel level.
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From page 319... ...
The San Jose study also investigated the role of different street textures, which resulted in very different street dirt loadings. Although the accumulation and deposition rates are quite similar, the initial loading values (the permanent storage values)
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. FIGURE 4-20 Street dirt accumulation and particle size changes on good asphalt streets in San Jose, California.
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From page 321... ...
may approximate the deposition rate, resulting in relatively constant street dirt loadings. Wash-off of Street Surface Pollutants Wash-off of particulates from impervious surfaces is dependent on the available supply of particulates on the surface that can be removed by rains, the rain energy available to loosen the material, and the capacity of the runoff to transport the loosened material.
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From page 322... ...
The experimental factors examined included rain intensity, street texture, and street dirt loading. The differences between available and total street dirt loads were also related to the experimental factors.
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If the calculated wash-off is greater than the carrying capacity (such as would occur for relatively heavy street dirt loads and low to moderate rain intensities) , then the carrying capacity is limiting.
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From page 324... ...
However, if the predicted wash-off is greater than the values shown in this figure, then the values in the figure should be used. Accumulation and Wash-off Summary This discussion summarized street particulate wash-off observations obtained during special wash-off tests, along with associated street dirt accumulation measurements.
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From page 325... ...
has recently been used to integrate rural and urban stormwater production, including dissolved phosphorus source and transport in New York State. Alternatives to mass budget-based models include fully statistical approaches such as simple regressions based on watershed land use and population (e.g., Boyer et al., 2002)
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TABLE 4-7 Example Mathematical Models That Have Been or Can Be Used in Stormwater Modeling 326 URBAN STORMWATER MANAGEMENT IN THE UNITED STATES
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MONITORING AND MODELING 327 Note: CN, curve number
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From page 328... ...
In order to develop a more consistent capability to support stormwater permitting needs, there should be increased investment in improving model paradigms, especially the practice and methods of model linkage as described above, and in stormwater monitoring. The latter may require investment in a new generation of sensors that can sample at temporal resolutions that can adjust to characterize low flow and the dynamics of storm flow, but are sufficiently
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From page 329... ...
EPA needs to ensure continuous support and development of their water quality models and spatial data infrastructure. Beyond this, a set of distributed watershed models has been developed that can resolve the location and position of parcels within hydrologic flow fields; these are being modified for use as urban stormwater models.
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From page 330... ...
Data obtained from too few grab samples are highly variable, particularly for industrial monitoring programs, and subject to greater uncertainly because of experimenter error and poor data-collection practices. In order to use stormwater data for decision making in a scientifically defensible fashion, grab sampling should be abandoned as a credible stormwater sampling approach for virtually all applications.
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From page 331... ...
Thus, it is not yet possible to create a protocol that mechanistically links stormwater dischargers to the quality of receiving waters. The utility of models with more modest goals, however, can still be high -- as long as the questions being addressed by the model are in fact relevant and important to the functioning of the watershed to which that model is being applied, and sufficient data are available to calibrate the model for the processes
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From page 332... ...
2000. Urban Stormwater Management Planning with Analytical Probabilistic Methods.
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From page 333... ...
1999. Digital Data Used to Relate Nutrient Inputs to Water Quality in the Chesapeake Bay Watershed, Version 1.0.
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2006. Identification of significant factors affecting stormwater quality using the National Stormwater Quality Database.
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2004. Nonparametric statistical tests comparing first flush with composite samples from the NPDES Phase 1 municipal stormwater monitoring data.
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2008. The updated National Stormwater Quality Database (NSQD)
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1997. Regional interpreta tion of water-quality monitoring data.
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2000. Improving Urban Stormwater Quality–From Theory to Implementation.
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