WATERSHED-GENERATED POLLUTANT LOADS

9.3 WATERSHED-GENERATED POLLUTANT LOADS

Quantification of the relationship between the contaminant loads discharged into a water body and the quality of the receiving water is essential in determining the maximum con-

taminant loading that is consistent with meeting water-quality standards in the receiving water body. Such relationships between contaminant loading and water quality in the receiving body have been described in previous chapters for rivers, lakes, wetlands, estu- aries, oceans, and ground water. Contaminants of most concern in such analyses are typi- cally sediments, nutrients, metals, and pathogens (Caruso, 2005).

Contaminant loads on water bodies come from three main terrestrial sources: (1) point sources, (2) nonpoint sources, and (3) background (natural) sources. Point sources are

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FIGURE 9.4 Outfall pipe. (From University of Arizona, 2005 Photo by Barbara Tellman.)

primarily localized sources of contamination that enter water bodies via outfall pipes, and such an outfall pipe is shown in Figure 9.4. Nonpoint sources typically consist of areas over which contaminants are distributed and are subsequently transported to receiving water bodies. Urban roadways and parking lots are common examples of nonpoint sources, where various automobile-related contaminants are deposited and the drainage system collects the contaminant-laden runoff and transports it to a receiving water. A typ- ical example of a nonpoint source is shown in Figure 9.5. Contaminant loads resulting from nonpoint sources is collectively referred to as diffuse pollution.

The loading capacity of a water body is defined as the maximum contaminant load, including the background load (BL), that a water body can accept and still meet its

FIGURE 9.5 Typical nonpoint source of water pollution. (From University of Arizona, 2005.)

WATERSHED-GENERATED POLLUTANT LOADS

water-quality standards. The loading capacity typically includes a margin of safety. The total maximum daily load (TMDL) is the component of the loading capacity allocated to anthropogenic effects and loads and is given by

TMDL ⫽ LC ⫺ BL ⫺ MOS

where LC is the loading capacity, BL is the background load, and MOS is the margin of safety. The margin of safety takes into account uncertainties in the relationship between effluent quality and receiving-water quality, including inaccuracies in water-quality mod- els. The waste load allocation (WLA) is the portion of the surface-water loading capacity allocated to existing and future point-source discharges. The WLA serves as a basis for determining the water-quality-based effluent limitations on point-source discharges required by National Pollutant Discharge Elimination System (NPDES) permits and monitoring requirements. The load allocation (LA) is the portion of the surface-water loading capac- ity allocated to existing and future nonpoint sources, and this allocation is the basis for development of nonpoint-source restoration plans. Since the loading capacity (LC) is equal to the sum of the waste load allocation (WLA) and the load allocation (LA), the TMDL given by Equation 9.1 can be expressed in the form

TMDL ⫽ (WLA ⫹ LA) ⫺ BL ⫺ MOS

In cases where the receiving water is impaired, the contaminant load entering the water body exceeds the TMDL and contaminant-load reductions are required. These load reduc- tions must come from the point sources and/or the nonpoint sources. Reduction of con- taminant loading from point sources is fairly straightforward and can be made by adjusting (downward) the allowable loads in the (NPDES) discharge permit. On the other hand, required reductions in contaminant loading from nonpoint sources must be made by changing land-use practices in the contributing watershed. Understanding the relationship between the contaminant loading on a water body and land uses in the contributing water- shed is the basis for environmentally sound land management. Typically, best management practices are instituted for the land uses, and fate and transport models are used to relate the land-use practices to the nonpoint contaminant loads. These relationships are a primary focus of this chapter. Water-body restoration is not possible if the watershed sources of pollution are not controlled.

Models used to simulate pollution generation at the source and its movement from the source area to the receiving water body are called loading models, and such models are an integral part of the TMDL process. In most cases, the loading models provide flow (hydro- graph) and concentration (pollutograph) distributions at various points in the watershed and entry points into the receiving water body. Models used to simulate the fate and trans- port of contaminants within water bodies are called receiving-water models. Clearly, both loading models and receiving-water models must be used in determining effective strate- gies to control the quality of water in natural systems.

For simulating overland pollutant transport, hydrology must be calibrated and deter- mined first, followed by sediment, and finally, pollutant transport (Novotny, 2003). Any errors that appear in the hydrology or erosion component will be transferred and magnified in all subsequent components. Calibration of a model involves varying the parameters of

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the model within acceptable ranges until a satisfactory agreement between the measured and computed output values is achieved. Model calibration is a subjective process requir- ing experience, and an evaluation of what constitutes a calibrated model is purely judg- mental (USEPA, 1992b). Validation means demonstrating that the model output is realistic and in agreement with observations; however, data used in calibration must not be used in validation. Calibration and validation of models are essential components in TMDL cal- culations.

Reviews of existing contaminant-transport models applicable to diffuse-pollution mod- eling of urban and agricultural watersheds can be found in Donigan et al. (1995) and

USEPA (1997e), and new models are constantly being developed (e.g., Donohue et al., 2005). There are two commonly used approaches to modeling diffuse pollution: lumped-

parameter modeling and distributed-parameter modeling. Lumped-parameter models treat the watershed, or a significant portion of it, as one unit, while distributed-parameter mod-

els divide the watershed into smaller homogeneous units with uniform characteristics. In distributed-parameter models, each subarea is described by a mass-balance equation, and mass-balance equations for the entire system are solved simultaneously at each time step. Both lumped- and distributed-parameter models can be used inside or attached to a geo- graphical information system (GIS) shell. Watershed models may range from modeling small uniform segments of less than 1 ha to entire watersheds with areas of several hun- dred square kilometers. Models can be designed to run on either an event or continuous basis. Single-event models simulate the response of a watershed to a single (rainfall) event, while continuous models simulate the response of the watershed to several consecutive events. These models typically use time steps ranging from a fraction of an hour to a day, with water and pollutant mass balanced at each time step.

The formulation of toxic standards in water bodies does not allow a simple application of steady-state models, since toxic criteria are related to the probability (frequency) of excursions of concentrations. For example, the 1-hour average concentration of a priority pollutant can exceed the maximum concentration criterion (CMC) no more than once in 3 years, and for chronic toxicity the 4-day average concentration may exceed the continuous concentration criterion (CCC) no more than once in 3 years. Consequently, modeling must include these statistical considerations.

Methods used to control contaminant loads on water bodies can be divided into: source- control, hydrologic modification of source areas, reduction of transport of pollutants from

the source area to the receiving water, and end-of-pipe removal of pollutants. These approaches are described briefly as follows:

1. Source-control reduces or eliminates sources of pollution and includes such meas- ures as restricting the use of polluting and harmful chemicals, reducing discharge rates (such as requiring wastewater reuse), litter-control programs in urban areas, and soil con- servation in agricultural areas.

2. Hydrologic modification of the source area is used primarily to control pollution that is carried by surface runoff. Reduction of imperviousness, detention and retention of sur-

face runoff in urban areas, and soil-conservation best management practices in agricultural areas are common examples of hydrologic modification.

3. Reduction of contaminant transport is determined primarily by the type of drainage system in the watershed. Typical urban sewerage and storm-sewer collection systems do not provide opportunities for attenuation of pollutants, while natural drainage systems and overland flow provide many opportunities and places for sedimentation and filtration of

WATERSHED-GENERATED POLLUTANT LOADS

pollutants in surface runoff. Pollutant attenuation can be enhanced by installation of buffer strips along drainageways and the use of catch basins before urban stormwater enters a

sewer system.

4. End-of-pipe removal of pollutants is typically accomplished by some form of treatment. Sedimentation ponds and wetlands at the drainage outlet point are common examples.

The approaches described above are applicable to diffuse pollutant loads from nonpoint sources. For point sources of pollution, abatement generally consists of end-of-pipe

control. Example 9.1 The analysis of a water-quality-impaired lake indicates that the maximum

phosphorus load consistent with the lake meeting the phosphorus standard is 800 kg/yr. Background atmospheric loading of phosphorus is estimated to be 50 kg/yr, and the cur- rent NPDES permits for discharges into the lake allow an annual loading of 700 kg/yr. (a) Determine the TMDL of the lake and the acceptable loading from nonpoint sources in the watershed. (b) If the current diffuse loading from nonpoint sources is estimated to be 300 kg/yr, recommend an equitable reduction in the load allocations. Assume a 10% fac- tor of safety in the estimated loading capacity of the lake.

SOLUTION (a) From the data given, LC ⫽ 800 kg/yr, BL ⫽ 50 kg/yr, and MOS ⫽

0.10 (800) ⫽ 80 kg/yr (corresponding to a 10% factor of safety). According to Equation 9.1, the total maximum daily load (TMDL) is given by

TMDL ⫽ LC ⫺ BL ⫺ MOS ⫽ 800 ⫺ 50 ⫺ 80 ⫽ 670 kg/yr

The loading capacity (LC) of the lake is the sum of the waste-load allocation (WLA) from point sources and the load allocation (LA) from nonpoint sources according to the relation

LC ⫽ WLA ⫹ LA

Taking LC ⫽ 800 kg/yr and WLA ⫽ 700 kg/yr yields

800 ⫽ 700 ⫹ LA

which yields LA ⫽ 100 kg/yr. Therefore, the acceptable phosphorus loading from nonpoint sources is 100 kg/yr.

(b) Since the current diffuse loading is 300 kg/yr, reduction in phosphorus loading is needed. If r is the percent reduction in the WLA and LA that will yield an acceptable total

phosphorus load of 800 kg/yr,

which yields r ⫽ 20%. Therefore, the NPDES permits should be revised downward to a waste-load allocation of (1 ⫺ 0.2)700 ⫽ 560 kg/yr, and the load allocation from nonpoint sources should be reduced to (1 ⫺ 0.2)300 ⫽ 240 kg/yr. The reduced loading from non- point sources could be accomplished by instituting best management practices.

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