SECTION 5: Determining Pollutant Loads

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SECTION 5: DETERMINING POLLUTANT LOADS

Section 5 Contents

Example Models for Marina Flushing Assessment........................................................5-2

Selection Criteria ..........................................................................................................5-2

Models Selected ...........................................................................................................5-3

Simple Model......................................................................................................................5-3

Mid-Range Models ............................................................................................................5-5

Tidal Prism Model .........................................................................................................5-5

NCDEM DO Model ........................................................................................................5-6

Complex Models ................................................................................................................5-6

WASP4 .............................................................................................................................5-6

EFDC Hydrodynamic Model .........................................................................................5-7

Water Quality Monitoring in Marinas (for modeling applications)................................5-8

Sampling Guidelines for Existing Marinas .................................................................5-8

Spatial Coverage...........................................................................................................5-8

Constituents Sampled ..................................................................................................5-9

Sampling Locations.....................................................................................................5-10

Sampling Time and Frequency ..................................................................................5-10

This section is included for those interested in the

technical information used to determine the

dynamics of water flow and water quality variations. Although numerical models provide an

effective approach to evaluate design parameters,

marina developers may use their own discretion in

employing modeling techniques.

The use of an area for a marina might infringe on

or preclude other uses of the resources, and it is

this potential conflict that can be evaluated by

using of water quality modeling. Marina basins

can contain pollutants ranging from sanitary

wastes to toxic metals leached from hulls and

petroleum products discharged in engine exhaust.

These wastes pose a variety of potential problems

for water quality, including microbiological contamination of adjacent shellfish and swimming

areas, depletion of dissolved oxygen in the water

column or sediments, and toxic effects on estuarine biological resources. Water quality monitoring

can be used before marina construction or

expansion to determine the design (including basin

shape and entrance locations and runoff controls)

that will be the least disturbing to the surrounding

aquatic environment. It can be used after marina

construction to determine compliance with water

quality criteria and what, if any, changes in design

are necessary to meet any water quality criteria

that have been violated.

Water quality criteria are based on pollutant

concentrations. Concentrations of water quality

constituents (such as dissolved oxygen [DO] or

petroleum hydrocarbons) can be used to assess

instantaneous conditions (water quality when the

sample is taken) and conditions over time

(samples taken daily for a week or a month).

Concentrations of pollutants in water can be

measured in storm water runoff before the runoff

reaches a waterbody or in the waterbody of

interest. If concentrations are measured in runoff,

the timing is important. Pollutant concentrations

usually vary widely during a rainstorm, typically

being higher during the first wave or “first flush”

of storm water, when pollutants accumulated

since the previous storm are washed away, and

lower later in the storm.
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National Management Measures Guidance

Concentrations also vary from storm to storm.

Longer periods between storms allow more

pollutants to accumulate on surfaces, whereas a

storm that occurs shortly after a previous storm

might carry very few pollutants in its runoff.

Time of year is also important. A storm that

occurs during a week of peak boat maintenance

activity is likely to carry more pollutants than a

storm that occurs in the spring before the boating

season begins. If nothing else, the pollutants

carried by the storm runoff will be different. A

storm in spring might carry more sediment and

salt from winter road treatments, whereas one in

summer might have more oil and debris from hull

maintenance activities.

Pollutant loads in a marina basin can be measured

by collecting samples at various times, depths, and

places in the basin. For a simple assessment of

water quality, samples of dissolved oxygen, fecal

coliform bacteria, and perhaps water clarity (using

a Secchi disk) might be performed. If sampling

for assessment of meeting state water quality

standards, samples for the constituents required

by the state have to be taken and the samples

might have to be analyzed by a state-approved

laboratory.

Samples can be taken once for an indication of

instantaneous water quality or over a period of

time to assess average water quality conditions or

trends in water quality (for example, whether

water quality is worse over busy boating weekends or in particular seasons, or just after a storm

and for how long after a storm has occurred).

Comparison of samples of storm water runoff and

samples of marina basin water quality might be

used to determine whether degraded water quality

during and shortly after storms is due to runoff

from the marina property or from surrounding

properties.

General water quality monitoring is discussed

under the Water Quality Assessment management measure in Section 4. A discussion of

models and monitoring, which supports their use

for in-depth analyses of water quality and water

quality changes that might occur from changes in

marina configuration or marina construction,

follows. The discussion is somewhat technical

because it is anticipated that if these models are

applied, they will be applied by persons trained in

their use and familiar with their implementation.

Those without a background in modeling can still

benefit from reading the discussion to gain a

general understanding of what modeling involves

and to help decide whether modeling is appropriate for a particular marina and situation.

Example Models for Marina Flushing

Assessment

Selection Criteria

To understand what is needed to apply a model, it

is essential to focus on the physical, chemical, and

biological processes that move water into and out

of the marina area, control mixing with adjacent

waters, regulate chemical reactions in the water

and sediments, and facilitate biological growth and

decay (die-off). A variable combination of winds,

tides, currents, and density differences is responsible for the physical movement of water volumes

and pollutants. The geometry of a site can also

have a major effect on flushing and dispersion and

is an important issue in selecting the model,

collecting the data, and attaining the required

water quality standards.

Biodegradation of organic material, growth and

decay of bacteria and other organisms, nutrient

uptake, and chemical transformations of various

kinds are typical of the biochemical processes that

affect contaminants. Physical, chemical, and

biological processes should be combined to form a

conceptual model of the site and its consequent

contaminant assimilation potential. After the site

in question has been conceptualized, the next step

is to choose a model that incorporates the appropriate physical processes and biochemistry to

predict water quality. Depending on the level of

sophistication at which the assessment is taking

place, the model selected might be a simple

screening calculation (e.g., Tidal Prism Analysis)

or a multidimensional numerical model (e.g.,

WASP4, DEM, WQM2D, or EFDC Hydrodynamic Model).

The models discussed here have been selected

for the following reasons:

• They are in the public domain.
SECTION 5: Determining Pollutant Loads

5-3

• They are available at a minimal cost from

various public agencies.

• They are supported to a varying extent by

federal or state agencies. The form of support

is usually telephone contact with a staff of

engineers and programmers who have

experience with the model and can provide

guidance (usually free of charge).

• They have been used extensively for various

purposes and are generally accepted within

the modeling profession.

• Together they form a sequence of increasingly more technically complex models; that

is, each model takes additional phenomena

into account in a more detailed manner than

the preceding model.

Selection from among these models should be

made on the basis of the model capabilities

needed.

In addition to model capabilities, the two most

important factors in the selection of a model are

the adequacy of the documentation and the

adequacy of the support available. The documentation should state the theory and assumptions in

adequate detail, describe the program organization, and clearly present the input data requirements and format. A well-organized data scheme

is essential. The support provided should include

user access via telephone to programmers and

engineers familiar with the model. Special support

(including short courses or informational or

personnel exchanges) might be available under

existing intra-agency or interagency agreements

or can be made available to the potential user.

The support agency might also be able to provide

the potential user with a list of local users who

could be contacted for information regarding their

past or current experience with the computer

program. Table 5-1 presents documentation and

user’s support available for some of the models

discussed in this section.

In addition to having adequate documentation and

user’s support, the selected model should address

all marina water quality problems of concern.

The following section provides an overview of the

best-qualified marina water quality model in each

of the selected categories. These models are

listed in Table 5-1, which provides information

related to the operational features of the models.

This information is provided to help in evaluating

the estimated cost associated with and the ease of

acquiring the model, getting the model running on

the user’s system, calibrating the model, and

finally applying the model. Table 5-2 lists the level

of effort involved in applying the models.

Models Selected

The most rigorous tools that can be used for

assessing marina impacts on water quality are

numerical models. Models range in complexity

from simple desktop calculations to full threedimensional models that simulate physical and

chemical processes by solving equations of motion

and rate equations for chemical processes.

The complexity of the model used and the quality

of the input data determine the degree of resolution in the results. For example, in an early part of

a study, the Tidal Prism Analysis strategy is used

to obtain a general understanding of potential

impacts caused by pollutant discharged from a

proposed marina. It is likely that the simplified

strategy will predict substantial impacts on the

environment. Therefore, an advanced model is

needed to conduct further detailed analyses. A

mid-range model is used in situations where

steady-state conditions may be assumed and tidal

flushing is the predominant mode of flushing. A

complex model is used in dynamic environments

subject to complex circulation patterns and full

biochemical kinetics, with sources and sinks for all

dissolved constituents and for proposed marinas.

Simple Model

The methods listed here include desktop screening

methodologies that calculate seasonal or annual

mean pollutant concentrations based on steadystate conditions and simplified flushing time

estimates. These models are designed to examine

and isolate trouble spots for more detailed analyses. They should be used to highlight major water

quality issues and important data gaps in the early

stage of a study.

Methods presented in this section, particularly

some of the mathematical descriptions, are
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National Management Measures Guidance

simplifications of more sophisticated techniques.

These techniques, as presented, can provide

reasonable approximations for screening potential

impact problems when site-specific data are not

available. The Tidal Prism Analysis was selected

as the method of choice in this category. This

method is capable of addressing all marina water

quality issues of concern (e.g., dissolved oxygen

and fecal coliform bacteria) and comes with

excellent documentation. The primary strengths

and advantages of the screening procedures are

as follows:

• Excellent user documentation and guidance.

Table 5-1. Ease of application: Sources, support, and documentation.
SECTION 5: Determining Pollutant Loads

5-5

• No computer is necessary because the

procedures can be performed on hand

calculators.

• Relatively simple procedures with minimal

data requirements that can be satisfied from

the user’s manual when site-specific data are

lacking.

The Tidal Prism Analysis procedures can be

easily implemented in a computer program. This

allows the user to test model sensitivity and

determine the range of potential water quality

impacts from a proposed marina quickly and

efficiently.

Mid-Range Models

The recommended marina mid-range models are

the Tidal Prism Model and the NCDEM DO

Model. Both models are in the public domain, are

easy to apply, and are supported with good

documentation.

Tidal Prism Model

The Tidal Prism Model is a steady-state model

capable of simulating up to 10 water quality

variables, including dissolved oxygen and fecal

coliform bacteria. The user’s manual is well

written and includes input/output examples, as

well as guidance on how to calibrate and apply

the model. Based on constituents modeled, the

Tidal Prism Model is recommended as the bestqualified marina mid-range model. The primary

strengths and advantages of the Tidal Prism

Model are as follows:

• Excellent user documentation and guidance.

• Minimal computer storage requirements.

• Relatively simple procedures with data

requirements that can be satisfied from

existing data when site-specific time series

data are lacking.

The Tidal Prism Model is applicable only to

marinas where tidal forces are predominant with

oscillating flow (e.g., an estuary or a tidal river).

Therefore, the Tidal Prism Model can’t be applied

to marinas located on a sound, an open sea, or a

lake or reservoir. Because the Tidal Prism Model

is not applicable to most marina situations, the

NCDEM DO model is recommended as an

alternative best-qualified model for mid-range

applications where the Tidal Prism Model isn’t

applicable.

Table 5-2. Level of effort for best models.
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National Management Measures Guidance

NCDEM DO Model

The NCDEM DO model is a steady-state

program that is capable of predicting only DO

concentrations. The NCDEM DO model is

applicable to one-, two-, and three-segment

marinas. Model theory, assumptions, and input

parameters are presented in adequate detail.

Model documentation includes input and output

examples of several applications as well as a

listing of the model code. The model code is

written in BASIC.

The NCDEM DO model incrementally mixes the

ambient and marina waters as a function of the

average lunar tides. The tidal variation is assumed

to follow a sinusoidal distribution. For simplicity, a

12-hour tidal cycle is used. If this time-variable

model is run through a sufficient number of tidal

cycles, the average marina basin DO value

approaches a steady-state value.

Complex Models

Complex models consist of two components—

hydrodynamics and water quality. In this model

category, hydrodynamics may be represented by

numerical solution of the one-dimensional or the

full two-dimensional equations of motion and

continuity. Water quality conservation-of-mass

equations are executed using the hydrodynamic

output of water volumes and flows. The water

quality component of the models calculates

pollutant dispersion and transformation or decay,

giving resultant concentrations over time. These

models are very complex and require an extensive

effort for specific applications.

Water Quality Analysis Simulation Program

(WASP4)

The Water Quality Analysis Simulation Program,

WASP4, is a dynamic compartment modeling

system that can be used to analyze a variety of

water quality problems in one, two, or three

dimensions. WASP4 simulates the transport and

transformation of conventional and toxic pollutants

in the water column and benthos of ponds,

streams, lakes, reservoirs, rivers, estuaries, and

coastal waters. The WASP4 modeling system

covers four major subjects—hydrodynamics,

conservative mass transport, eutrophicationdissolved oxygen kinetics, and toxic chemicalsediment dynamics. The modeling system also

includes a stand-alone hydrodynamic program

called DYNHYD4, which simulates the movement of water. DYNHYD4 is a link-node model

that can be driven by either constantly repetitive

or variable tides. Unsteady inflows can be

specified, as well as wind that varies in speed and

direction. DYNHYD4 produces an output file of

flows and volumes that can be read by WASP4

during the water quality simulation. WASP4

contains two separate kinetic submodels,

EUTRO4 and TOXI4. EUTRO4 is a simplified

version of the Potomac Eutrophication Model

(PEM) and is designed to simulate most conventional pollutant problems. EUTRO4 can simulate

up to eight state variables, including dissolved

oxygen and fecal coliform. TOXI4 simulates

organic chemicals, metals, and sediment in the

water column and underlying bed.

The WASP4 model system is supported by the

EPA’s Center for Exposure Assessment Modeling

(CEAM) in Athens, Georgia, and has been

applied to many aquatic environments. The

WASP4 model can be obtained from the CEAM

web page (www.epa.gov/ceampubl/

softwdos.htm). The water quality component is

set up for a wide range of pollutants, and the

model is the most versatile and most widely

applicable of all models considered here. For

these reasons WASP4 is the model of choice in

this category. The primary strengths and advantages of the WASP4 model are as follows:

• Documentation: WASP4 has excellent user

documentation and guidance. Theory and

assumptions are presented in adequate detail;

program organization and input data requirements and format are clearly presented.

• Support: User access is available by telephone to programmers and engineers familiar

with the model. Occasional workshops,

sponsored by CEAM, are available. The

support agency (CEAM) can provide the

potential user with a list of local users who

could be contacted for information regarding

their past or current experience with the

computer program.
SECTION 5: Determining Pollutant Loads

5-7

• Flexibility: Model users can add their own

subroutines to model other constituents that

might be more important to the specific

application with minimal or virtually no

programming effort required. The user can

operate WASP4 at various levels of complexity to simulate some or all of these variables

and interactions.

CEAM maintains and updates software for

WASP4 and the associated programs. Continuing

model development and testing within the CEAM

community will likely lead to further enhancements and developments of the WASP4 modeling

system. In fact, CEAM is currently supporting the

development of a 3-dimensional (3-D) hydrodynamic model that will be linked to the WASP4

model.

EFDC Hydrodynamic Model

The environmental fluid dynamics code (EFDC)

model was originally developed at the Virginia

Institute of Marine Science (VIMS) for estuarine

and coastal applications and is considered public

domain software. It is a general-purpose modeling

package for simulating three-dimensional flow,

transport, and biogeochemical processes in

surface water systems, including rivers, lakes,

estuaries, reservoirs, wetlands, and coastal

regions. In addition to hydrodynamic and salinity

and temperature transport simulation capabilities,

EFDC can simulate cohesive and noncohesive

sediment transport, near-field and far-field

discharge dilution from multiple sources, eutrophication processes, the transport and fate of toxic

contaminants in the water and sediment phases,

and the transport and fate of various life stages of

finfish and shellfish. Special enhancements to the

hydrodynamic portion of the code, including

vegetation resistance, drying and wetting, hydraulic structure representation, wave-current boundary layer interaction, and wave-induced currents,

allow refined modeling of wetland marsh systems,

controlled flow systems, and nearshore waveinduced currents and sediment transport. The

EFDC model has been extensively tested and

documented for more than 20 modeling studies.

The model is currently being used by a number of

organizations, including universities, governmental

organizations, and environmental consulting firms.

The structure of the EFDC model includes four

major modules: (1) a hydrodynamics model, (2) a

water quality model, (3) a sediment transport

model, and (4) a toxics model (see Figure 5-1).

The EFDC hydrodynamic model itself is composed of six transport modules—dynamics, dye,

temperature, salinity, near-field plume, and drifter.

Various products of the dynamics module (water

depth, velocity, and mixing) are directly coupled to

the water quality, sediment transport, and toxic

models.

• Documentation: Extensive documentation of

the EFDC model is available. Theoretical and

computational aspects of the model are

described by Hamrick (1992a). An excellent

user’s manual (Hamrick, 1996) is available

and includes input file templates. A number of

papers describe model applications and

capabilities (Hamrick, 1992b, 1994; Hamrick

and Wu, 1996; Moustafa and Hamrick, 1994;

and Wu et al., 1996).

EFDC Model

Hydrodynamics Sediment

Transport

Water

Quality Toxics

Figure 5-1. Structure of and modules associated with the EFDC model.
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National Management Measures Guidance

• Support: User access is available by telephone to programmers and engineers familiar

with the model. VIMS can provide the

potential user with a list of local users who

could be contacted for model information.

• Flexibility: The EFDC model can be configured to execute all or a portion of a model

application in reduced spatial dimension mode,

including two-dimension depth or width

averaged and one-dimension cross section

averaged. The number of layers used in the

three-dimension mode or two-dimension width

averaged mode is readily changed by one line

of model input. Model grid sections specified

as two-dimension width-averaged are allowed

to have depth-varying widths to provide

representations equivalent to those of twodimension width-averaged estuarine and

reservoir models, such a CE-QUAL-W2.

Water Quality Monitoring in Marinas (for

modeling applications)

Sampling Guidelines for Existing Marinas

General guidance is presented to develop the

framework for a site-specific water quality

sampling program suitable for an existing marina.

A monitoring study at an existing marina may be

requested by regulatory agencies if it is suspected

that the marina is causing degradation of water

quality standards. An overall monitoring program

can consist of three phases or levels. In Level 1,

preliminary screening is conducted to gather

baseline information on the marina. If historical

data on the marina are available, this level might

not be needed or the quantity of data needed

might be reduced. Based on the historical or

Level 1 data, if it is established that the marina

may be causing impacts on water quality, Level 2

sampling, which incorporates additional sampling

of the receiving waters, would commence. If

evaluation of Level 2 data also indicates that the

marina is affecting water quality, marina design

changes may be recommended and eventually

implemented. Level 3 sampling would be initiated

to evaluate the performance of any implemented

marina design changes. Examples of potential

marina design changes include removal of sills,

which tend to trap water in the lower depths of a

marina, and improvement of flushing by altering

sharp corners within the marina or by enlarging

the marina entrance.

Spatial Coverage

An intensive spatial coverage of the marina and

the adjacent waterbody for some indicator or

surrogate water quality parameter, such as salinity

or turbidity, is generally needed to estimate spatial

variability and to determine the model type and

the segmentation required.

Generally, the spatial coverage of the modeled

marina should extend away from the marina site

to the extent that normal background levels for

DO are encountered. At this location, model

boundary conditions (i.e., surface elevations or

current velocities) can be established. In this

manner the total effect of the marina can be

measured.

The preceding approach is appropriate when

using complex models. Sampling stations for

complex models should be spaced throughout the

model grid system, with the spatial coverage being

governed by the gradients in velocities and water

quality constituents. For existing marinas, adjacent

waterbodies are divided into a series of reaches

for complex model application, with each reach

described by a specific set of channel geometry

dimensions (cross-sectional dimensions) and flow

characteristics (flow rates, tidal range, velocities,

and biochemical processes). The models assume

that these conditions are uniform within each

reach. Each reach is in turn divided into a series

of model segments or computational elements to

provide spatial variation for the water quality

analysis. Each segment is represented by a grid

point in the model where all water quality variables are computed. For the WASP4 model, the

segment length is dependent on the degree of

resolution desired and the natural variability in the

system. Enough detail should be provided to

characterize anticipated spatial variation in water

quality.

The hydrodynamics of the Tidal Prism Model are

based on the tidal prism volume at each segment.

Therefore, the spatial coverage of a marina, using

the Tidal Prism Model, includes the entire estuary/

river where the marina is located. The length of
SECTION 5: Determining Pollutant Loads

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each segment is defined by the tidal excursion, the

average distance traveled by a water particle on

the flood tide, because this is the maximum length

over which complete mixing can be assumed.

A sampling station for each model segment is the

minimum requirement to calibrate the returning

ratios of the Tidal Prism Model. Sampling stations

should generally be located along the length of the

estuary and in the main channel. The returning

ratio is defined as the percentage of tidal prism

that was previously flushed from the marina on

the outgoing tide.

Constituents Sampled

The specific constituents that must be sampled, as

well as the sampling frequency, depend to some

extent on the particular modeling framework to be

used in the analysis. The selected model should

include all of the processes that are significant in

the area under investigation without the unnecessary complexity of processes that are insignificant. A few preliminary measurements might be

useful to define which processes are important.

The minimum sampling requirements for all

dissolved oxygen studies should include dissolved

oxygen, temperature, carbonaceous biochemical

oxygen demand (CBOD), and total Kjeldahl

nitrogen (TKN), because these parameters are

fundamental to any dissolved oxygen analysis.

Biochemical oxygen demand (BOD) is typically

measured as 5-day BOD, but a few measurements of long-term BOD are also necessary. The

Tidal Prism Model considers only the CBOD

component, and therefore the model should be

used only in situations where the nitrogenous

components are known to be unimportant.

In addition to TKN, ammonia (NH3

) and nitrate

(NO3G) (or nitrite [NO2G] plus nitrate) should be

measured for dissolved oxygen investigations for

both the Tidal Prism and WASP4 models. Even if

ammonia, nitrate, and nitrite are not modeled, the

data are useful for estimating the nitrogenous

BOD decay rate or ammonia oxidation rate.

Concentrations of algal dry weight biomass or

chlorophyll a should be measured because both

the complex models and the Tidal Prism Model

simulate algae growth for dissolved oxygen

analysis. Light extinction coefficients (or Secchi

depths) are also needed for the algal growth

computations in dissolved oxygen analysis if the

complex models are used.

In situ sediment oxygen demand (SOD) should be

measured in situations where it is expected to be

a significant component of the oxygen budget.

This is most likely to occur in shallow areas

where the organic content of the sediments is high

or in deep marina basins where flushing is minimal. In developing a strategy for SOD measurement, it is logical to assume that those factors

important in establishing model reaches or segments are also relevant to selecting SOD measurement sites. The more important of these

factors are

• Geometry: depth and width.

• Hydraulics: velocity, slope, flow, and bottom

roughness.

• Water quality: location of point sources,

nonpoint source runoff, and abrupt changes in

DO/SOD concentrations.

The most important factor for SOD is likely to be

the location of abrupt changes in DO/BOD

concentrations, such as areas surrounding the

entrance channels of marinas and in the marina

basin proper. The final point to consider is that

SOD can vary with season. This observation is

particularly relevant to marinas and adjacent

areas dominated by algal activity and/or oxidation

of organic and inorganic nutrients by benthic

microorganisms, both of which can occur seasonally. The modeler should thus be aware of this

potential concern and structure the SOD measurement times accordingly.

In addition to sampling for the constituents to be

simulated, measurements are also necessary to

help quantify the various coefficients and parameters included in the model equations. Coefficient

values can be obtained in four ways: (1) direct

measurement, (2) estimation from field data,

(3) literature values, and (4) model calibration.

Model calibration is usually required regardless of

the selected approach. However, coefficients that

tend to be site-specific or that can take on a wide

range of values should be either measured directly
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National Management Measures Guidance

or estimated from field samples. These could

include the following parameters:

• CBOD decay rate

• CBOD settling rate

• NH3

 oxidation rate (nitrogenous BOD decay

rate)

• SOD

In addition to the preceding model parameters,

which are determined primarily from the results of

field sampling surveys, several other rate coefficients can be measured in the field. For example,

stream reaeration rates for the WASP4 model and

returning ratios for the Tidal Prism Model can be

measured using tracer techniques. WASP4

provides several options for the reaeration rate

equation because many of the equations are

applicable to only certain ranges of depth and

velocity.

Sampling Locations

Water quality data should be collected at the

downstream boundary of the study area for model

calibration. Adjacent waters both upstream and

downstream should also be sampled to determine

background concentrations of water quality

constituents. Although a single downstream

station is the minimum requirement for short

channel sections, additional sampling stations are

desirable to provide more spatial data for calibrating the model. Logical locations for additional

stations are sharp corners and dead end segments

in the marina basin proper. If the marina is

segmented for a complex model application, each

segment should be sampled. However, water

quality variations might be negligible at stations

located upstream and downstream immediately

outside marinas.

In the Tidal Prism Model, water quality is assumed to be well mixed and uniform over each

segment of the stream. Therefore, samples taken

immediately downstream of the marina would

probably not match conditions in the model unless

they were taken far enough downstream for

complete cross-sectional mixing to occur. In

general, increased sampling should be allocated to

those areas of the marina and the adjacent water

that have the most impact (along the shoreline). In

general, all of the major water quality parameters

of interest (DO, CBOD, TKN, NH3

, NO3, fecal

coliform bacteria, temperature, and so forth)

should be measured at each station in the sampling network.

Rate coefficients and model parameters can be

estimated from literature values before sitespecific measurements are available. For important parameters such as the BOD decay rate,

sensitivity analyses can be performed to evaluate

the effects of different coefficient values in

formulating DO concentrations. These analyses

should provide enough information so that sampling stations can be located in critical areas.

Sampling Time and Frequency

The duration and frequency of water quality

sampling depend to a large extent on whether the

Tidal Prism Model or a complex model will be

used. The Tidal Prism Model computes water

quality conditions only at slack before ebb; thus,

sampling at a higher rate is not necessary. The

complex models have a user-specified time step,

which means that sampling should be more

frequent for shorter time steps.

Because the Tidal Prism Model assumes that

conditions remain constant with time, it is important to conduct the sampling program during a

period when this assumption is valid. Synoptic

surveys (e.g., sampling all stations over 2 to 3

days) should be conducted to the extent possible

so that water quality conditions at different

locations are not affected significantly by changes

in the weather or variations in the marina discharge that are not accounted for in the model.

However, since temperature varies diurnally and

temperature influences the process rates of most

biological and chemical reactions, some variability

in the sampling results will be inevitable. It should

be noted that the Tidal Prism Model uses the first

day of field data as initial and boundary condition

input to the model. Field data from succeeding

cycles are then used to compare the output

simulations at the same cycle.

Complex models compute continuous changes

that occur over time because of variations in

stream flow, temperature, nonpoint and point
SECTION 5: Determining Pollutant Loads

5-11

source loadings, meteorology, and processes

occurring within a marina and its adjacent waters.

All of the factors that are assumed constant for a

Tidal Prism analysis are free to vary continuously

with time in a complex model. This feature allows

an analysis of diurnal variations in temperature

and water quality, as well as continuous prediction

of daily variations or even seasonal variations in

water quality.

Application of a complex model requires a much

more detailed sampling program than that required

by a mid-range model. Enough data should be

collected to define the temporal variations in

water quality throughout the simulation period at

the model boundary conditions. Therefore, more

frequent data collection should be conducted at

the model boundary condition. Complex models

investigate the temporal variations in dissolved

oxygen and fecal coliform bacteria much better

than mid-range models. To achieve this resolution,

intensive surveys should be mixed with long-term

trend monitoring. The significance of the temporal

variations depends on the context of the problem.

For example, if the daily average dissolved

oxygen concentration is around 5 mg/L or less, a

diurnal variation of less than 1 mg/L could be very

important with respect to meeting water quality

standards; if the average dissolved oxygen

concentration is around 10 mg/L, diurnal variations are important and the sampling program

should include 2 or 3 days of intensive sampling

for dissolved oxygen and temperature at all of the

key stations. As a minimum, these stations would

include the stations designated as the model

boundary, as well as the stations surrounding the

marina and adjacent waters and stations within

the marina. These locations satisfy the minimum

requirements of defining the boundary and loading

conditions, plus a few calibration stations in the

critical areas for DO, SOD, and fecal coliform

bacteria.

Long-term dynamic simulations of seasonal

variations in stream water quality might be

impractical. Where seasonal variation is of

interest, the typical practice is to run the Tidal

Prism Model or a complex model (with short-term

simulations) several times for different sets of

conditions that represent the full spectrum of

conditions expected over the period of interest.

Enough data should be collected to characterize

the seasonal variations and to provide adequate

data for calibrating and applying the model. If

possible, enough data should be collected to cover

the full range of conditions of the model analysis.

As a minimum, these should include conditions

during the critical season for the water quality

variable of interest. For DO, for example, the

critical season occurs during the hot summer

months (July through September).

Two general types of studies can be defined—

intensive surveys, which are those used to

identify short-term variations in water quality, and

trend monitoring, which is used to estimate

trends or mean values. Intensive surveys are

intended to identify intertidal variations or variations that occur because of a particular event in

order to make short-term forecasts. Intensive

surveys should encompass at least four full tidal

cycles. They should usually be conducted regardless of the type of modeling study being conducted. Boundary conditions should be measured

concurrently with the monitoring of the marina

basin and the adjacent water. A record of all point

source waste loads located near the marina site

during the week before the survey is recommended. Variables that should be sampled during

the intensive surveys include tide, current velocity,

salinity, DO, fecal coliform bacteria, nitrogen, and

phosphorus, measured hourly.

Trend monitoring is conducted to establish

seasonal and long-term trends in water quality.

Trend sampling may take place on a biweekly or

monthly basis for a year at a time. Stations should

be sampled at a consistent phase of the tide and

time of day to minimize tidal and diurnal influences on water quality variations. Some stations

may be selected for more detailed evaluation

during the intensive survey. Long-term trend

monitoring should also be considered as a way to

track changes in water quality between the

intensive surveys.

Most states have water quality standards for the

24-hour average concentration and the instantaneous minimum concentration of DO. Therefore,

it is important to collect DO data throughout a

complete cycle, that is, from the high value, which

normally occurs at mid-afternoon, to the low
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National Management Measures Guidance

value, which usually occurs at dawn. This

approach will allow the DO range in the model to

be calibrated to specific field conditions. If the

waterbody is stratified, samples should be collected at the surface, mid-depth (above and below

the thermocline and pycnocline, if possible), and

bottom. In general, it is necessary to collect

samples at a 2-hour frequency over a 24-hour

period to adequately define the daily average and

the minimum DO concentrations.