METHODOLOGIES FOR QUANTIFYING
NON-POINT SOURCE
CONTAMINANT LOADING
TO PUGET SOUND
Submitted to:
U.S. Environmental Protection Agency
Region 10
Prepared by:
Cooper Consultants, Inc.
1750 112th Avenue NE. Suite C-225
Bellevue, Washington 98004
and
Envirosphere Company
10900 NE 8th. Fifth Floor
Bellevue, Washington 98004
OCTOBER 1985
0589a
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1.0 INTRODUCTION
Nonpoint sources of pollution may carry levels of contaminants similar
to point sources. For example, urban runoff has been compared to
discharge from secondary treatment systems in the concentrations of
some pollutants. If the quantity of runoff is large, this source can
constitute a major input to the environment, particularly over the
short term, such as during storm events. Nonpoint source pollution is
much more difficult to monitor than pollution from point sources due to
its diffuse nature, therefore indirect methods of measurement and
various modelling approaches have been devised to obtain a quantitative
understanding of this contaminant source.
The objective of this report is to evaluate available methods for
estimating loadings to Puget Sound from nonpoint sources and to assess
whether simple and straightforward methods can provide accurate loading
estimates.
Three methods which can be used to estimate nonpoint source (NPS)
loading will be discussed in order of increasing complexity. These
methods include:
o Estimation of nonpoint source loadings from historical
measurements of flows, and concentrations
o Estimation of nonpoint source loadings from source inventories
and release rates (water quality assessment method)
o Estimation of nonpoint source loadings from computer
modelling of sources and drainage hydrology
For the first two methods, quantitative estimates of nonpoint source
loadings for two prototype embayments have been developed for a few
conventional and metal contaminants. For the more complex modelling
methodologies, available models, their input data needs and
implementation requirements are presented.
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1.1 PROTOTYPE DEMONSTRATION SITES
Nonpoint sources are conventionally classified according to locale as
urban or agricultural. The main differences between these two types
are the sources of pollution and their transport mechanisms. Urban
hydrology is characterized by rapid runoff from mainly impermeable
surfaces (e.g., streets) through systems designed to discharge their
flows as quickly as possible. Rural hydrology generally involves sheet
flow across soil surfaces into natural channels and is therefore a
slower process.
Since urban and rural sources are considered differently in modelling
procedures, two prototype embayments were chosen for initial
application of the methods presented in this report, one urban in the
character of its contributory drainage basin and the other rural.
These embayments serve as test cases for the simpler computations to be
discussed and as focal points for the discussion of more complicated
modelling methodologies. It is our recommendation that, if work is
undertaken to implement sophisticated computer modelling procedures,
these embayments should be considered as the test cases for that effort
as well.
The two embayments selected were:
o Elliott Bay, receiving runoff from the highly urban Seattle
and Duwamish Waterway/Kent Valley areas;
o Skagit Bay, receiving runoff from the predominantly rural
Skagit River Valley.
These embayments were chosen because they are representative of each
source type and because water quality data are available to apply and
confirm the methods. These embayments are described in more detail
below.
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1.1.1 Urban Embayment: Elliott Bay
Elliott Bay is a bight indentation in the eastern side of central Puget
Sound. The shores of the Bay are entirely within the city of Seattle.
The only substantial natural watercourse which flows into the Bay is
the Duwamish River, called the Green River upstream of a short distance
inland. The Duwamish drains an area of approximately 487 sq. mi.
(Seaber et al. 1984) and median annual flows are 1,400 cfs (Williams
1981). The Duwamish or Green is bordered by the cities of Seattle
(population 489,700, according to WSOFM 1984), Tukwilla (3600), Renton
(32,700), Kent (25,500), and Auburn (29,000). Land use along the Bay
is commercial (Seattle Central Business District), medium density
residential, and light industrial, with shipping and shipbuilding
immediately adjacent to the Bay. The land use in the Duwamish/Green
River valley is predominantly light industrial (office park or
warehouse) with some medium residential densities in the uplands above
the valley. Upstream of about Auburn the population density is much
less, with land use becoming agricultural. Howard A. Hanson Reservoir
was constructed somewhat further upstream in 1961 to allow flood
control.
Elliott Bay is probably the second most studied portion of Puget Sound
(Commencement Bay appears to be the only site likely to have more water
quality data). The availability of these studies simplified some of
the difficulties of data collection. The significance of Elliott Bay
for this evaluation is that it borders on the largest population in the
state and is an area with some of the best available information. Its
drainage area is small relative to other urbanized river basins and
there are no extensive lakes which would capture sediment and
complicate analysis procedures. In addition, Elliott Bay is an
embayment already showing signs of impact from Man's presence in terms
of elevated concentrations of some contaminants in the open water and
has loadings of pollutants from both point and nonpoint sources.
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1.4.3 Rural Embayment: Skagit Bay
Skagit Bay is a branch of the Whidbey Basin of Puget Sound, bounded on
the west by Whidbey Island, on the south by Camano Island, and on the
east by the mainland (Skagit County). The Bay is connected to the rest
of Puget Sound through two outlets, Deception Pass to the northwest and
Saratoga Passage to the southwest. The areas bordering the Bay are
predominantly agricultural. The greatest runoff into Skagit Bay is via
the Skagit River, and land use along the lower portions of the Skagit
is agricultural, mainly dairy, vegetables, fruits, and flowers. Cities
in this portion of the Skagit Valley include Mount Vernon (population
13,600 according to WSOFM 1984), Burlington (3,820), Sedro Woolley
(6,225), Lyman (240), Hamilton (220), and Concrete (570). La Conner
(645) is a slight distance north of the mouth of the Skagit River near
the Bay. The upper reaches of the Skagit basin are mountainous with
extensive forests (the Mt. Baker National Forest) and logging. There
are only a few small, unincorporated villages in this area.
The Skagit River has a drainage area is 3218 sq. mi. (Seaber et al.
1984) and median annual flow is 18,000 cfs (Williams 1981), and is the
largest river emptying into Puget Sound. Major tributaries include the
Sauk, Suiattle, and Baker Rivers. Three major dams produce
hydroelectric power from upper Skagit River flows. A portion of the
Sauk River drainage is in Snohomish County to the south, and the Baker
and upper Skagit flow down from portions of Whatcom County and British
Columbia to the north, but most of this area is upstream of the dams
and therefore not contributing sediment to Skagit Bay.
The Skagit River has a national stream-quality accounting network
station (gage number 12200500 near Mount Vernon) with varying levels of
water quality data available back to 1962 (best since 1974) and flow
data available from 1940; presently sampling is conducted on a two
month interval schedule. Skagit Bay has the advantage of having almost
all its drainage in a single county (Skagit) with only minor
contributions from Whatcom and Snohomish counties; this reduced the
number of agency contacts which had to be made in data collection and
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coordination. The significance of Skagit Bay to this evaluation is
that it is appropriate for application of NFS methodologies which were
developed for rural areas. Land use and soil type data are fairly
scarce, however, and may require more extensive collection schemes
(e.g., aerial or satellite photography) for future studies. The water
quality data available for the Skagit River is fairly comprehensive and
will allow verification of the WQA model results.
1.2 APPLICATION TO OTHER SITES
The development of methodologies appropriate to urban and rural sites
should allow the results to be combined for basins which are both urban
and rural as long as care is taken to avoid counting an area's
contribution under both methods. Embayments which are predominantly
rural may allow small population centers to be left out of the analysis
without significant error in the calculation. Where possible, results
should continue to be checked for their accuracy using downstream water
quality data, but if a methodology is proved to be correct for several
areas it should be acceptable to apply it to areas without
corroborating data. Some areas (e.g. Bremerton on Sinclair Inlet) do
not have substantial freshwater drainage channels which could be used
to confirm model predictions but these areas should not be any the less
applicable for the methods presented in this report.
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2.0 ESTIMATION OF NONPOINT SOURCE LOADINGS FROM HISTORICAL
MEASUREMENTS OF FLOWS AND CONCENTRATIONS
Nonpoint sources are diffuse and are located at widely dispersed sites
throughout the Puget Sound region. Because most nonpoint source
pollution is waterborne it can often be gaged in aggregate by observing
the quantities of contaminants which are carried by rivers which drain
an area of concern. This is only feasible, of course, when the
drainage system is concentrated in a single, or at most a few, rivers
where the data collection process can be conducted for the entire
area. There are Puget Sound embayments which receive their pollutant
loads mainly from direct runoff into the bay rather than via collecting
river systems; the estimation procedure discussed in this chapter
cannot be used in such circumstances. A large majority of the drainage
into Puget Sound, however, is river-borne and so this method has
considerable application for the initial screening of nonpoint loadings
and their comparison to other sources of pollution. It must be pointed
out, however, that the total riverborne contaminant load at the river's
mouth will be the result of both point and nonpoint contributions
occurring upstream. Estimation of loading from contaminant
concentrations at the river mouth does not distinguish between point
and nonpoint source inputs.
This estimation procedure is based on historical records of flowrates
and contaminant concentrations. When records of frequent, simultaneous
measurement of flowrate and water quality for catchments containing
major nonpoint sources are available, the accuracy of these estimates
can be quite high. However, a lack of historical data necessitates
approximation of data or selection of data not truly representative of
actual loading, resulting in less accurate estimates. For areas which
drain directly into an embayment, it is necessary to supplement any
riverine contribution'with the direct runoff of nonpoint pollution.
Direct NPS pollution may be estimated using runoff calculations applied
to the drainage area and some estimate of contaminant concentrations in
that runoff.
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River-borne mass loading may adequately approximate nonpoint loading
when the river is directly or indirectly the major nonpoint transport
pathway to the embayment. It is desirable to have water quality and
streamflow data at reasonably frequent intervals (several times a year
for several years) taken at a point close to the mouth of the river.
When simultaneous measurements of streamflow and contaminant
concentration are available, the product of the two variables gives an
instantaneous mass loading value applicable to the particular pollutant
at that place and time. Time-weighted or loading-weighted averages of
an annual mass flux may be calculated and multiplied by a year's time
to give annual loading.
This analysis was performed for the two prototype embayments. The
particulars of the methods used varied according to the availability of
data. The procedures and results are discussed in the following
sections.
2.1 SKAGIT BAY
Since 1974, water quality and streamflow data have been measured at a
USGS station located 15.7 miles upstream of Skagit Bay on the Skagit
River near Mount Vernon. The drainage area for the gage is estimated
by the USGS to be 3093 sq. mi., which includes all of the river's
drainage basin except about 125 sq.mi. (4 %). Metal concentrations
were measured 3 to 4 times a year and conventional pollutant parameters
were typically measured 9 to 12 times annually, both at reasonably
regularly spaced intervals. Using these data, instantaneous mass
loadings were calculated, arithmetically averaged over the (water)
year, and multiplied by the time factor to obtain annual loadings for
selected constituents. Annual loading values were then averaged to get
an overall mean and standard deviation of the annual values. The
results are-presented in Table 2.1.
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TABLE 2.1
RIVER-BORNE LOADING OF SELECTED CONTAMINANTS
IN THE SKAGIT RIVER i/
Mass Loading Coefficient Years of
Parameter (mt/yr) —' of Variation of Data
Measured flow (109 ft3) 526 21
a
Ammonia (as N) 1,100 64% 8
N03 + N02 (as N) 2,000 17% 9
Total Kjeldahl Nitrogen (as N) 2,300 22% 3
Total Nitrogen 5,300 43% 4
Organic Nitrogen 4,100 51% 4
Total Phosphorous 520 29% 9
Total Organic Carbon 33,000 30% 4
Dissolved Solids 510,000 18% 9
Suspended Solids 1,800,000 89% 9
Arsenic 24 67% 9
Cadmium, total 51 69% 9
Copper, total 150 61% 9
Lead, total 370 73% 9
Mercury, total 1.6 75% 9
Zinc, total 400 80% 9
a/ Average annual values for water years 1974-1982 USGS gage 122005500,
Skagit River near Mt. Vernon, WA.
y Values include "not detected" entries at one-half the detection limit.
Source: USGS (1974-1982).
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2.2 ELLIOTT BAY
For Elliott Bay a majority of the inflow and pollution also enters via
a single river, in this case the Duwamish River. There is no USGS
water quality gage on the Duwamish (or its upper reaches, called the
Green River there) but the Washington Dept of Ecology (WDOE) and the
Municipality of Metropolitan Seattle (Metro) have done extensive water
quality sampling at many locations in the river. Harper-Owes, Inc.
(1982), carried out an extensive synthesis of this sampling data, and
other values contained in background literature, and their results have
been used for this analysis.
The methods employed in the Harper-Owes Study to obtain and combine
flows and concentrations varied according to the availability of data.
The study methods are summarized briefly below. Sources were divided
into the Black River, the Upper Green River, and stormwater runoff.
Pollution loads from point sources, such as the Metro Renton Treatment
Plant, the Combined Sewer Overflows, and industrial discharges, and
other flux components such as atmospheric deposition, advective
transport to and from Elliott Bay, and deposition in the Estuary, were
also estimated in the Harper-Owes study in order to estimate the total
loading rate to the Duwamish Estuary. The study developed a complete
mass balance for the different contaminants considered, and the
residuals left between the inputs and outputs were also estimated.
Since data on metal concentrations were scarce for the upper Green
River, the mass fluxes were estimated based upon the sediment flux and
background levels of metals in Puget Sound sediment. Flow in the Black
River was estimated based on a drainage-area ratio of Green River
flows. Mass fluxes from stormwater runoff were calculated using runoff
coefficients to calculate volume and multiplying by representative
chemistry values. The contaminant loadings calculated in the
Harper-Owes study are shown in Table 2.2.
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TABLE 2.2
RIVER-BORNE LOADING OF SELECTED CONTAMINANTS
IN THE DUWAMISH RIVER
(Metric tons per year)
Parameter
Measured flow (TO9 ft3)
Nonpoint
Input Loads
56
Total
Input Loads
64
Residual Load
Unaccounted
0%
Ammonia (as N)
Total Organic Carbon
Oil and Grease
Suspended Solids (sediment)
Arsenic
Cadmium
Copper
Lead
Mercury
Zinc
Total PCBs
Total Pesticides
173
220+
18+
243,000
0+
1.
5.
.02
,7
6.9
0.75+
42
0.0007+
0.0008+
770+
1200+
370+
244,000
1,200+
1
5
3
7
9.8
0.76+
48
0.241+
0.0035+
ca
81%
58%
1%
19%
82%
75%
91%
24%
45$
98%
73%
Oa
*
Source: Harper-Owes 1982
Notes: Nonpoint input loads include Upstream Green River, Black River,
Regional Stormwater
Residual load unaccounted
(Estimated outputs - estimated inputs)
/(Estimated outputs)
indicates that some components of total could not be estimated
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Another estimation method was also applied for the Elliott Bay site.
This method used the average values of contaminant concentrations in
urban runoff as determined in the National Urban Runoff Program (NURP,
USEPA 1983). The results (Table 2.3) are not entirely comparable to
those presented in Table 2.2 since some of the input loadings in the
latter table come from rural areas along the upper Green, rather than
being strictly urban sources. The runoff was estimated using an area
of 136 sq.mi. which includes the central business district of Seattle,
the lower Duwamish drainage, the Kent Valley, and urban upland areas
which drain into those areas, subtracting out the areas served by
combined sewers. The annual precipitation at Sea-Tac Airport (which is
the same as that at the Urban Climatology Station at the University of
Washington) of 38 inches, combined with a 75% runoff coefficient and
the area as derived above, gives a runoff quantity of nine million
cubic feet a year. This quantity was multiplied by the concentrations
reported in the NURP report (and shown in Table 2.3) to give the total
mass flux expected, assuming that the Elliott Bay area is similar to
other urban areas around the country. An additional average
concentration, for cadmium, was obtained from Tetra Tech (1985).
The contaminant concentrations used in the prediction of likely
contaminant loading levels were the median concentration measured in
the NURP study (i.e., the level which is higher than half of the sites
sampled, and lower than the other half). This is a reasonable
indicator of the contamination to expect at a site when there is no
other indication of runoff chemistry. Also included in Table 2.3 are
the concentrations and computed loadings that were found in the 90th
percentile of the samples taken in the course of the NURP study. This
gives an indication of the highest contaminant loading which would be
expected. The NURP study also analyzed for the organic priority
pollutants, but only the frequency of detection and range of detected
concentrations were reported. The available data were not sufficient
to develop representative concentrations of the priority pollutants in
urban runoff.
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TABLE 2.3
ELLIOTT BAY LOADING BASED ON PREDICTED RUNOFF
AND POLLUTANT CONCENTRATIONS FROM
NATIONAL URBAN RUNOFF PROGRAM (NURP) DATA
a,b/
Median Site
Parameter
Concentration Loading
90th Percent!le Site
Concentration Loading
BODS
COD
Phosphorus, Total
Phosphorus, Soluble
Total Kjeldahl Nitrogen
N03 + N02 (as N)
Total Suspended Solids
Cadmi urn
Copper
Lead
Zinc
lmg/1 )
9
65
0.33
0.12
1.50
0.68
100
0.002£/
0.034
0.144
0.160
(Mt/yr)
2,300
16,600
84
31
382
173
25,500
0.5
8.7
36.7
40.8
lmg/1 )
15
140
0.70
0.21
3.30
1.75
300
0.094
0.350
0.500
(Mt/yr)
3,800
35,700
180
54
841
446
76,500
24
89
127
Source: a/ USEPA 1983.
b/ Assumptions regarding surface runoff: Drainage area served by
separated sewers: 136 mi*; Estimated overall runoff
coefficient: 0.75; Precipitation:.38 in/yr; Resulting Annual
Runoff Volume = 9.0 x 10$ ft3
C/ Concentration for Cadmium from Tetra Tech, 1985.
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2.3 ACCURACY OF RIVER-BORNE AND RUNOFF NPS LOADING ESTIMATES
2.3.1 River-borne Loading
The methods of estimating river-borne nonpoint source loading, using
either instantaneous, average annual, or mean values from national
studies require relatively little effort once the data are obtained.
The accuracy of each method depends primarily on the quality of the
data. The more accurate and frequent the measurements on which the
estimate is based, the more reliable the estimate of river-borne
loading. However, for several reasons, river-borne loading may not
accurately approximate total nonpoint loading to the embayment. First,
measurements of river water quality include point as well as nonpoint
contributions. Second, nonpoint discharges directly into the embayment
are excluded. Third, nonpoint contributions may be made downstream of
the monitoring point if the measurements are not taken at the river
mouth; if the measurements are made too near the mouth, concentration
and flows will be inaccurate due to influences of tides and estuarine
conditions. Though river-borne loading may provide an easily-obtained
estimate of actual embayment loading, the values obtained must be
recognized as approximate and the contributions of point and nonpoint
sources will not be distinct.
2.3.2 Runoff Loading
If estimates are based on average values of concentration or runoff
obtained from samplings over a larger area than that being studied
(such as the NURP data), then there will be inaccuracies due to the
variability of the data base and the differences between the actual and
the representative sites. In addition, runoff volumes may be in error
if they are not based on actual measurements. Furthermore, no
provision is made for contaminant removal mechanisms prior to discharge
into the embayment. It is obvious that the more specific the runoff
flow and concentration data are to the study site, the more accurate
this kind of methodology will be for estimating nonpoint source
pollution loading.
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3.0 ESTIMATION OF NONPOINT SOURCE LOADINGS
FROM SOURCE INVENTORIES AND RELEASE RATES
(WATER QUALITY ASSESSMENT METHOD)
A somewhat more sophisticated procedure than simply monitoring
contaminant concentrations as flows leave the basin is to catalog the
land uses of the basin in terms of their potential as nonpoint sources
and the capacity of the hydrologic processes to deliver the
contaminants to the receiving water body. Results of previous studies
in which correlations have been found between contaminant loadings and
various measurements of land use, precipitation, or other factors are
used to develop equations and tabulated values with which to estimate
nonpoint source loading. The EPA had a standardized procedure of this
sort developed which has been published as the Water Quality Assessment
(WQA) procedure (Mills et al., 1982).
This procedure differs from those of the previous chapter in that
measurements of flow and concentration are not required. The method is
therefore suitable for embayments where no such measurements have been
made or, for lack of a main river system, can be made. In this sense,
the WQA procedure is similar to the calculation in the previous chapter
which used average concentrations from the NURP study. Although the
WQA method is simple to apply, the authors emphasize that whenever
possible local data should be used, if available, in lieu of the
'typical' loadings provided in the WQA manual.
The WQA method specifies separate analyses for rural and urban NPS
loads and allows for the computation of both annual and single event
load releases. The various contaminants which can be modelled using
the method include:
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o Rural watersheds:
Sediment
Nitrogen
Phosphorous
Organic Matter
Salinity (for irrigation runoff)
Agricultural chemicals
o Urban watersheds:
Suspended solids (sediment)
Biological Oxygen Demand (BOD)
Nitrogen
Phosphates
Volatile Solids
Other contaminants, available for single event calculation
The method is designed for hand calculation; however, it was
implemented for this study on a conventional microcomputer
spreadsheet. The WQA analyses for rural and urban sources are
discussed below in relation to the two prototype embayments.
3.1 RURAL WATERSHEDS: SKAGIT BAY
For non-urban watersheds the WQA method is based on the Universal Soil
Loss Equation (USLE), which is designed to predict sheet and rill
erosion from croplands. The equation can be expressed as the summation:
n
Y(S)E = / , Ai (R- K- LS- C -P 'Sd).
where:
is the sediment yield for a drainage basin
A. is the area in subarea "i"
R is a rainfall factor
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K is the soil-credibility factor
L is the slope-length factor
S is the slope-steepness factor (often combined with
"L")
C is the cover factor
P is the erosion control practice factor
Srf is the sediment delivery ratio
(all the factors are estimated for the subarea "i")
In its original form the USLE was based on data from a wide selection
of studies, all in areas east of the Rocky Mountains. More recent
studies have developed climatic and soil information for Western
sites. As the form of the equation implies, data which is required in
the WQA method for non-urban areas includes:
Rainfall characteristics
Land use
Cropland, woodland, pasture, etc
Soil characteristics
Canopy or ground cover
Area
Conservation practices
Characteristic slope (gradient and length)
Delivery ratios (based on distance to receiving water body)
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The parameters are developed from tables and maps which are given in
the WQA manual. It was necessary to confer with individuals who were
more familiar with the area and its land use and soils, specifically
from:
o Soil Conservation Service (Spokane, Portland, and Mt. Vernon)
o WSU Cooperative Extension Service (Anacortes)
o Skagit County Planning Department (Mt. Vernon)
The USLE simply calculates the amount of sediment which is eroded from
an area with the characteristics described by the parameters itemized
above. The WQA procedure for non-urban areas also predicts the amount
of nitrogen, phosphorus, and organic matter, based on two additional
factors: the contaminant concentration in the soil and an enrichment
ratio for the contaminant and the site. It would also be possible to
use the method to estimate salinity and toxic agricultural chemicals
carried in the runoff from an area but salinity is not a significant
contaminant under the climatic conditions of the Puget Sound region,
and there was not enough information to gage the amount of agricultural
chemicals used on the farms of the areas being studied.
The NPS loadings calculated for Skagit Bay using the WQA procedure are
presented in Table 3.1.
3.2 URBAN WATERSHEDS: ELLIOTT BAY
The Water Quality Assessment procedure for annual urban nonpoint
loadings is based on the Storm Water Management Model (SWMM) Level I
preliminary screening process, developed by the EPA in 1976. The basic
formula for each constituent is of the form:
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TABLE 3.1
NONPOINT SOURCE LOADINGS TO SKAGIT BAY
CALCULATED WITH THE EPA WATER QUALITY ASSESSMENT METHOD
Parameter Land use
Sediment:
Forest, 10-30% slopes
30-60* "
602
Field Crops
Row Crops
Pasture
City
Total
Area
(acres)
326,636
470,498
153,632
17,300
53,387
39,834
24,461
1,085,748
Rate
(Ib/ac)
12.6
19.2
66
206
260
5.4
79
Loading
(mt/yr)
1,867
4,098
4,600
1,610
6,277
119
882
19,430
Nitrogen (0.18}-/ 89
Phosphorus (0.26)-/ 127
Organic matter (2.82)-/ 1,387
a/ Rates back-calculated from total acreage, loading for nitrogen,
phosphorus, and organic matter is based on sediment.
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where: M. = loading of pollutant "j"
J
a = a units conversion factor
P = the annual precipitation
€.. - the area in land use "i"
°^ij ~ a P°llutant loading factor for land use "i" and
pollutant "j"
fp. = the population density function
PDrf = the population density
y « the street sweeping frequency factor.
Each of the factors is evaluated separately for each of the following
land use types (ie., each "i"):
Residential
Commercial
Industrial
Other developed uses (parks, cemeteries, schools, etc)
The pollutant loading factor also varies according to whether the
sewers are separate storm sewers or combined, and depend on the
pollutant type. Pollutant loading factors are given for:
BioJogical Oxygen Demand
Nitrogen
Phosphates
Suspended solids, and
Volatile solids.
Data for the parameters is presented in the manual in the form of
tables or algebraic expressions.
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The Duwamish basin was analyzed according to the various census tracts
in the basin, a map of which was obtained from the King County Division
of Planning along with data giving population, area, and density of
each tract. Each tract was categorized according to land use and sewer
type. The categories were based on land use (zoning) and sewer maps
from the cities in the drainage area. The populations and areas of
tracts in each land use/sewer category were summed to allow an overall
density (PDd) to be determined for that land use. The summed area
data (£j) were incorporated into a computer spreadsheet which was
used to solve the above equation and also to evaluate the population
density factor (fpj) using equations in the manual.
The information about street sweeping was obtained from the city and
county road maintenance divisions. The specified frequency is used
with equations given in the manual to calculate the factor £f (this
equation is also calculated in the spreadsheet program). The values
of 0(.. which are given in the manual were also incorporated into the
spreadsheet program, and the NFS loading results for Elliott Bay , as
reported in Table 3.2, were calculated.
3.3 ACCURACY OF WQA METHOD
In order to evaluate the accuracy of loadings calculated with the WQA
procedure, the estimates obtained with this method were compared to
those obtai.ned using actual flow and concentration data as described in
Section 2. A comparison of the results is given in Table 3.3. In
general it can be seen that the WQA procedure greatly underestimated
the quantity of contaminant flow into the two embayments which were
studied. The amount by which the flux is underestimated by the WQA
method is on the order of a half to almost two orders of magnitude (a
factor of 4 to 93).
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TABLE 3.2
NONPOINT SOURCE LOADINGS TO ELLIOTT BAY
CALCULATED WITH THE EPA WATER QUALITY ASSESSMENT METHOD
Parameter/Land use
Biological Oxygen Demand
Industrial (cs)l/
(ss)*/
Commercial (cs)
(ss)
Residential (cs)
(ss)
Other (ss)
Total
Nitrogen:
Industrial (cs)
(ss)
Commercial (cs)
(ss)
Residential (cs)
(ss)
Other (ss)
Total
Phosphates (as P):
Industrial (cs)
(ss)
Commercial (cs)
(ss)
Resident!' al(cs)
(ss)
Other (ss)
Total
Sediment (suspended solids
Industrial (cs)
(ss)
Commercial (cs)
(ss)
Resident!' al(cs)
(ss)
Other (ss)
Total
Area
(acres)
(BOD):
6,726
3,776
1,171
7,219
3,930
67,347
8,365
98,534
6,726
3,776
1,171
7,219
3,930
67,347
8,365
98,534
6,726
3,776
1,171
7,219
3,930
67,347
8,365
98,534
):
6,726
3,776
1,171
7,219
3,930
67,347
8,365
98,534
Rate
(Ib/ac)
190
46
502
122
58
14.2
0.6
43
11
46
11.2
9.6
2.3
0.3
3.6
0.87
3.9
0.94
0.81
0.20
0.016
4560
1105
3288
844
1194
290
15
Loading
(mt/yr)
581
79
266
398
104
434
2
1,860
132
18
25
37
17
71
1
301
11
1.49
2.06
3.08
1.43
5.95
0.066
25
13,930
1,900
1,850
2,760
2,130
8,850
55
31,400
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TABLE 3.2 (Continued)
NONPOINT SOURCE LOADINGS TO ELLIOTT BAY
CALCULATED WITH THE EPA WATER QUALITY ASSESSMENT METHOD
Parameter/Land Use Acreage Rate Loadin
acreage Kate Loading
(acres) Ob/ac) (mt/yr)
Volatile Solids:
Industrial (cs) 6,726 2250. 6,850
(ss) 3,776 543. 930
Commercial (cs) 1,171 2200. 1,170
(ss) 7,219 532. 1,740
Residential(cs) 3,930 691. 1,230
(ss) 67,347 168. 5,130
Other (ss) 8,365 14. 53
Total 98,534 17,100
a/ (cs) s combined sewers
(ss) = separate sewers
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TABLE 3.3
COMPARISON OF ESTIMATES OF NONPOINT SOURCE LOADINGS
USING HISTORIC FLOWS AND CONCENTRATIONS (QxC)
AND
THE EPA WATER QUALITY ASSESSMENT (WQA) METHOD
Contaminant Loading (Mt/yr)
Parameter QxC method WQA method Ratio
Skagit Bay (Skagit River)
Nitrogen 5,300 89 60
Phosphorus 520 127 4.1
Organic Carbon 33,000
Organic Matter — 1,387
Suspended Solids (Sediment) 1,800,000 19,430 93
Elliott Bay (Duwamish River)
BOD(5) 2,300 (NURP) 1,860 1.2
Suspended Solids (Sediment) 243,000 31,400 7.7
Source: Tables 2.1, 2.2, 2.3, 3.1, and 3.2.
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Two entries in Table 3.3 require explanation. First, since Organic
Carbon is a component of Organic Matter (in soil a ratio of about 2
will give an approximate estimate of one from the other) the total
organic content is underestimated with the WQA method by about an order
of magnitude. Second, the biological oxygen demand estimated for the
Duwamish River using median USA data from the NURP study was in good
agreement with that calculated using the WQA method. However, the NURP
data are not site-specific values, therefore, the calculated value is
not a strict measurement of actual BOD.
The reason for the discrepancy in values for all these contaminants
lies in the parameters chosen for the WQA model. The estimates could
be brought into agreement by adjusting the parameter values used. For
the non-urban WQA method (modelling the Skagit basin) the discrepancy
would be reduced by a considerable factor if the sediment analysis
alone were adjusted. The other contaminants are assumed to be carried
by, and thus are proportional to, the sediment mass. Therefore,
increasing the sediment flux by a factor of about 25 would bring the
other estimates into much better agreement with the measured values.
No effort was made to adjust model parameters to obtain agreement with
the loadings calculated from flow and concentration. At present, this
adjustment (or calibration) would defeat the purpose of assessing
whether the WQA method would provide a quick, simple, and accurate
estimate of NPS loadings.
Although preliminary results suggest that the WQA method does not
provide a simple solution it is possible the method could be calibrated
to Puget Sound embayments. This would be accomplished by applying the
equations to a number of embayments for which river-borne contaminants
are the primary loading source (and monitoring data are available for
the rivers). Particular emphasis should be placed on the sediment
transport representation. If similar parameter value adjustments were
found to provide satisfactory results for a number of embayments,
confidence in WQA model applicability to additional embayments would be
enhanced.
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4.0 ESTIMATION FROM CONTINUOUS MODELLING OF SOURCES
AND DRAINAGE HYDROLOGY
4.1 AVAILABLE MODELS
To estimate the release of contaminants from nonpoint sources in
greater detail than on an annual basis and with greater specificity to
the study site than is possible with the WQA procedure, it is
necessary, to use a computer-based modelling procedure for basin
hydrology and runoff quality. There are provisions in the WQA for
estimating releases from individual storm events, but this is a minor
component. The number of repetitious calculations involved in
hydrologic analyses makes this kind of procedure ideal for
computerization. Accepted procedures are sufficiently standardized
that several programs are available to model any basin without
modification of the program itself. The programs are generally well
tested, debugged, and supported, and adequate documentation is
available. Site specific input data are required and it is necessary
to exercise caution in the selection of model parameters and to
calibrate and verify for the specific model application.
Some of the more widely used models include:
o Storm water management model (SWMM), available from the EPA,
o Storage, treatment, overflow runoff model (STORM), from the
U.S. Army Corps of Engineers,
o Areal nonpoint source watershed environment response
simulation (ANSWERS) model, from Purdue University,
o Wisconsin hydrologic transport model (WHTM), from Oak Ridge
National Laboratory,
o Hydrocomp simulation program (HSP), from Hydrocomp, Inc.,
o Nonpoint source (NPS) pollutant loading model, from the EPA,
o Agricultural Runoff Management (ARM) model, also from the EPA,
o Hydrological Simulation Program—FORTRAN (HSPF), also from the
EPA.
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These models vary in their complexity, size, and input data
requirements. One (HSPF) even incorporates modules which accomplish
the work of two other models (NPS and ARM). A few other models have
been found in references but are either too specialized or are not
widely used and will not be discussed herein. A brief description of
each of the more widely used models follows.
SWMM is an extensive model developed for the EPA in 1971 by Metealf and
Eddy, Inc., the University of Florida, and Water Resources Engineers,
Inc., to predict urban sewer runoff. It has been modified considerably
in the years since and versions are advertised that will run on
microcomputers. The program models the whole rainfall/runoff cycle,
concentrating on flows (for design of storm sewer facilities), but
including some aspects of water quality. It was originally a single
storm event model but has had limited continuous modelling capability
added.
STORM is a 1976 product of the U.S. Army Corps of Engineers' Hydrologic
Engineering Center (HEC). It was designed to model both urban and
rural runoff and erosion for single events. Various contaminants,
besides sediment, are included in the model.
ANSWERS was originally produced by the Agricultural Engineering
Department at Purdue University in 1980. It is a single event model
and, though the name implies nonpoint source pollution in general, it
mainly concerns itself with soil erosion and so the model is more
applicable to rural than urban areas. There may also be limitations in
the size of the basin being modelled.
WHTM was produced originally at the University of Wisconsin but was
rewritten, documented, and expanded at Oak Ridge National Laboratory in
1974, making it more easily available for other computer systems in the
process. It was designed to model the transport of toxic materials
through the hydrologic process and may even be coupled with an air
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quality model which calculates the deposition aspects. It is general
for both pervious (rural) and impervious (urban) runoff and even some
consideration of subsurface flows.
HSP is a very extensive, continuous simulation, basin model first
available in 1976 from Hydrocomp, Inc. and a direct descendent (as are
many of the other models) from the Stanford Watershed Model (SWM) of
the mid 1960's. A number of water quality parameters are available for
inclusion in the model, appropriate to any land use. It had the
problem of being written in the IBM computer language PL/I and so was
less easily installed in other computer systems.
NFS and ARM are small watershed models (no channel transport is
modelled) produced by Hydrocomp, Inc., for the EPA in 1976 and 1973
respectively, but with later enhancements. They were designed for
similar general runoff situations, although ARM concentrates on
agricultural aspects such as nutrients, sediment, and pesticides while
NPS models general nonpoint pollution for a variety of contaminants.
HSPF is a rewriting of the HSP model in a more transportable form,
accomplished by Hydrocomp, Inc., in 1980. It can be used to model both
impervious and pervious runoff (incorporating the methods of NPS and
ARM for these processes). Because of its generality it is appropriate
to model any of the basins in the Puget Sound region and its input
requirements and implementation procedures will be discussed in the
following sections.
4.2 MODELING METHODOLOGY
The discussion in this section is based primarily on the general
watershed model HSPF. This model is recommended for any further work
on the question of Puget Sound nonpoint source delineation and
quantification. An advantage of using HSPF instead of less complete
models is that if further in-depth study of some pollutant and its
transport or degradation is desired, the existing model can be enhanced
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rather than setting up a whole new model. HSPF is a comprehensive
model that should reward the extensive data collection effort with
considerable information about both acute and chronic contaminant
fluxes of any pollutant of interest from any study site basin.
It is likely that only segments of the HSPF model would be required for
simulation of most Puget Sound embayments. Model segments that could
initially be simplified include channel effects. To include these
would require definition of channel properties (length, cross-sectional
area, roughness, slope), and data for whatever physical, chemical, or
biological transformations are considered important, such as
reaeration, volatilization, oxidation, nitrification, and hydrolysis.
With consideration of channel effects, the hydraulics (flow in the
channels) must be calibrated, using flow and contaminant data for
Individual storms. Another simplification that could be made is
consideration of snow impacts. However, these considerations may be
necessary for basins with significant flows and contaminant loadings
coming from snow deposits at higher elevations and would require more
meteorological data.
The documentation concerning HSPF (Donigian et al., 1984) describes the
following steps involved in a complete model application:
1) Study definition,
2) Development of a modelling strategy,
3) Learning the operational aspects of HSPF use,
4) Input and management of time series data,
5) Parameter development,
6) Calibration and verification, and
7) Analysis of alternate scenarios.
In general, similar steps are required for application of any of the
models described above. The first step (study definition) requires
identification of the need for the study and the appropriate scale
(level of detail) on which it should be addressed, investigation of
available data, and evaluation of time and money resources for the
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study. The second step (modelling strategy) should consider the
characterization of the area in climate, soils, topography, land use,
pollutant sources, and historical data and should plan the
investigation and overall form of the model. The third step (learning
HSPF) requires time with the model for familiarization; this can be
done with various sample problems which are supplied with the model.
All these steps can be considered preparation for the work of modelling
a particular basin, and will probably become less significant for
later simulations if the effort is a continuing one.
The fourth (input time series) and fifth (parameter development) refer
to data collection; the former involves climatological data
(precipitation) the latter concerns soils, stream channels, topography,
and land use. These steps are likely to be the most time consuming.
The sixth step (calibration and verification) is very important. It
involves comparing model predictions against field observations and
modifying the parameters in order to bring the model into better
agreement with reality. Verification comes after calibration and is
the final testing. Verification involves use of a data set independent
of that used for model calibration to see whether the model will be
accurate for future simulations.
The final step (analysis) is the actual use of the model. This
requires both understanding of likely and useful alternatives
(including continuing with the basin unchanged-), applying the model to
predict the results of the alternatives, and interpreting the model
outputs to explain the benefits of the alternatives.
4.3 DATA REQUIREMENTS AND AVAILABILITY
The HSPF model requires a great deal of input data to specify the
properties of the drainage basin, its hydrology, soils, land use, and
channels, the contaminants of interest, their chemistry, and
distribution around the basin, and the precipitation which drives the
whole system. The HSPF Application Guide (Donigian et al., 1984)
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points out that there are over 1000 parameters in the entire HSPF
system. However, only a small fraction of these parameters are used
for most applications. The actual amount of data required depends on
which options are of importance to the purpose of the project. For
modelling nonpoint source pollutant loadings to an embayment of Puget
Sound, required parameters would minimally include:
o Precipitation,
o Potential Evapotranspiration,
o Land use,
o Soil properties,
o Contaminant loadings from land uses,
o Chemical properties of contaminants,
o Historical measurements of streamflows, chemical concentrations,
and sediment loads,
These data requirements are summarized below.
4.3.1 Precipitation
Precipitation data is necessary on a daily basis (if not more often)
from a minimum of three stations for the period to be modelled. The
specification of three stations is based on the HSPF Application Guide
(Donigian et al., 1984) recommendations for any moderately large (40
sq. mi.) basin. In the Puget Sound area, however, the precipitation
varies considerably over short distances, such as the normal annual
totals of 39 inches in Seattle and 93 inches at Snoqualmie Pass. It
may be necessary to use more than 3 stations for a model (perhaps 7 or
more for larger basins), but this should be easy to do since there are
more than 60 weather stations in the Puget Sound region (NOAA, 1985).
This data can be obtained in computer-readable form from the National
Weather Service (NWS). Additional information on general climatic
conditions is available from the State Extension Service and could be
used to divide a watershed into subregions on isohyetal lines.
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4.3.2 Potential Evapotranspiration
Potential evapotranspiration (PET) refers to the maximal amount of
moisture which evaporates or is transpired by plants on a given day
according to climatological factors; actual evapotranspiration depends
on the availability of moisture in the soil. This data will be
somewhat more difficult to obtain. There are relationships established
for calculating PET from pan evaporation, however there are only two
Puget Sound area NWS stations which measure pan evaporation (Bellingham
2 N and Puyallup 2 W Experimental Station). It should also be possible
to derive estimates for PET from temperature data which is available
for many of the NWS stations.
4.3.3 Land Use
Land use information appears to be somewhat problematic to obtain.
Some counties do have data, if not what the land use is then at least
what it is planned to be at some future date. When available, this
data can generally be obtained from the County Planning Departments.
Some information, especially about acreages in the various crops and
the management practices used, may also be available from the County
Extension Service offices or the Soil Conservation Service or, for land
use in Incorporated areas, City Planning Departments. Upland areas of
the Puget Sound region are in National Forest land and information
about these areas should be available from the National Forest
Service. In some cases it may be necessary to obtain aerial or
satellite photography and interpret it to provide a data base.
Compilation of required data may require a considerable effort, even if
some information is available in a map display format since it must be
converted into numerical values of total areas in each land use type.
4.3.4 Soil Properties
Soil properties are readily available for most counties in the Puget
Sound region in the form of Soil Surveys compiled by the Soil
Conservation Service. Some of these are rather old (eg., the most
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current one for Thurston County dates back to 1958) and may not have
all required information such as erosion parameters which have been
developed since their publication. New Surveys are being compiled for
some of these areas and may be released fairly soon. The National
Forest Service has apparently also produced soil surveys for their
lands which would be valuable for some of the upland areas of Puget
Sound drainage basins. Some values of soil properties can also be
estimated with information presented in the manuals for HSPF
application. Again, as with land use, compilation of the data into
tabular numerical form will be the most time-consuming part of data
collection. This task will be particularly complex because the soil
data and land use must be combined, and their diversity reduced to a
limited number of categories appropriate to modelling.
4.3.5 Contaminant Loadings from Land Uses
Contaminant loadings due to different land uses will have to be
generated from a variety of sources. For nutrients, fertilizer
application rates and dates, plant uptake and scheduling of planting
and harvesting, herd sizes and productivities, and various management
practices will have to be obtained from Extension Service or Soil
Conservation offices, and interpreted according to data from previous
studies and information in manuals applicable to HSPF. In urban areas,
nutrient loadings from point sources (sewage treatment plant outfalls
and combined sewer outfalls, CSO's) will have to be obtained from
permit data or monitoring agencies. Pesticides will be have to be
estimated in a similar way in rural areas. Heavy metals and organics
loadings to street surfaces will have to be estimated from studies of
urban runoff, and frequency of street-sweeping will be obtained from
city and county public works departments.
4.3.6 Chemical Properties of Contaminants
Chemical properties for the various contaminants can be obtained from
the chemical literature and the HSPF manuals. Examples include
partition coefficients (measurements of how strongly a chemical adheres
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to the soil) and decay rates. In some previous studies it has been
found that laboratory values of some coefficients do not agree with the
values under field conditions. When this occurs, the values will have
to be adjusted by calibration procedures.
4.3.7 Historical Measurements of Material Discharges
Historical measurements of streamflows, contaminant concentrations, and
sediment transport are extremely important for the use of HSPF, since
they allow the calibration of the model. This process involves
adjusting parameters in the input data to make the output approximate
reality more closely. Step-by-step procedures to make the operation
simpler are given in the HSPF manuals. Given the minimal nature of the
modelling which is assumed at this point, monthly or annual mean values
of the measurements will probably be sufficient, even though daily
model timesteps are used because channel transport processes are not
included in this modelling effort.
4.4 APPLICATION OF NONPOINT SOURCE MODELLING
A decision to carry out detailed modelling of one or more drainage
basins contributing nonpoint source (NPS) pollution into an embayment
of Puget Sound will have to be based on a number of considerations,
including:
o Whether NPS pollution appears to be significant in relation to
other point source loadings.
o Whether the contaminants emitted by NPS appear to be damaging
to the Puget Sound ecosystem at the loadings anticipated.
o Whether detailed modelling would clarify the sources, total
loadings, or other factors which would distinguish NPS from
other pollution sources.
o What would modelling at the appropriate detail cost, in time
and money.
o What further use would the model have beyond the initial
application.
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The relative significance of NPS pollution can be estimated using flow
and concentration data or the WQA method and comparing the fluxes to
known contaminant loadings from point sources which can be calculated
from NPDES monitoring report data. Even if NPS pollution is
significant relative to point sources, the question still remains
whether it is actually damaging. To some extent this can be answered
by considering the chemicals which are emitted by the nonpoint sources
and their toxicology.
The chemical characteristics of the runoff of an area may be judged on
the basis of measurements taken in rivers draining into the embayments
or known chemicals associated with the land uses of the area. The
diffuse nature of the NPS phenomenon makes it difficult to identify all
chemicals present especially when they may be released at trace levels,
however, an obvious case where nonpoint pollution would be indicated to
be a problem is where there are few known point sources and yet signs
of distress in the biota of the embayment are evident.
It is obvious that a detailed modelling effort would provide greater
Insight into the sources of NPS pollution. If done properly, modelling
could point out geographic areas of greater and lesser emissions.
Where preliminary screening has identified significant sources of NPS
pollution that are contributing to environmental degradation, detailed
modelling may be warranted to evaluate control alternatives. Detailed
nonpoint source modelling would also be recommended for evaluation of
different land use alternatives in regions where NPS pollution is known
to be significant or proposed actions are likely to enhance/alter NPS
pollution.
4.5 MODEL LIMITATIONS
The main limitations of the HSPF model, and any study predicated on it,
are caused by the limitations of available data. This emphasizes the
importance of the proper and complete research which must be done to
get suitable parameters for the model. Calibration of the model to
monitoring data which are accurate and appropriate to the assumptions
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used is similarly very important. For some embayments the data may be
readily available, for others it may exist but be very difficult to
find, while for many there is likely to be a scarcity of the kind of
information required for adequate modelling. Some of this information
can be obtained by field studies or remote sensing, some (such as
historical contaminant concentrations) will never be available if it
has not been collected. There may be a few Puget Sound areas where a
considerable amount of the required data have already been gathered for
a previous study; examples include the study of the Snohomish River
system which was carried out by the Snohomish Country Planning
Department and Systems Control, Inc. (1974) which appears to have
accurate hydraulics data which would be very expensive to obtain again,
or a study, using the HSPF precessor HSP, which modelled the Lower,
Middle, and Upper Green River and is mentioned in Donigian and Crawford
(1976). It should be noted that, even if such data are found, careful
checking is necessary before it is incorporated into a new model.
4.6 ESTIMATES OF EFFORT AND COST REQUIRED FOR MODELLING
Costs associated with a detailed hydro!ogic modelling of Elliott and
Skagit bays would range between $60,000-$!00,000 depending upon the
quality of input data obtained and whether channel hydraulic processes
were to be modeled. These estimates are based in part on previous
studies carried out with HSPF discussed in Donigian et al. (1984) and
on the preliminary data collection carried out for the simpler
modelling exercises discussed in this report.
For the lowest estimate ($60,000) a number of assumptions have been
made; these assumptions are itemized briefly below:
o the two prototype embayments previously discussed, Skagit and
Elliott Bays, would be used
o the preliminary estimates of data availability and adequacy
are accurate for initial modelling
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o the contaminants to be studied are limited to those whose
levels have been monitored historically in order that
calibration would be possible
o no in-stream hydraulics or transformations need be considered
o four best management practices (BMPs) would be developed and
their effects on the NPS loading rates evaluated
o steps in the model development are those discussed in section
4.2 of this report
With these assumptions, it is estimated that such HSPF modelling of the
Skagit and Elliott Bay embayments would require approximately 6-8
months.
The higher cost estimate is based upon extension of scope to include
channel hydraulics and processes, and refined NPS loadings for critical
acute conditions and upon a more extensive data collection effort. The
more extensive data collection effort would include aquisition and
interpretation of aerial photographs or satellite photographs to obtain
more comprehensive land use data. Costs to apply the HSPF model to
other embayments would depend on the size and complexity of the
drainage area and the availability of data. If field collection of
data were required, costs could be substantially higher than the
estimates presented above.
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5.0 RECOMMENDATIONS
Three procedures, of varying complexity have been described to evaluate
nonpoint source pollution loadings to Puget Sound. The simplest method
is based on measured flows and concentrations to estimate what loadings
have historically entered an embayment via a river. The second method
is the Water Quality Assessment Method developed by the EPA. The third
method is simulation of a watershed using computer-based models; the
EPA model HSPF was highlighted as a suitable candidate program.
The simplest method can be applied only if there are several years of
good data available; it cannot be used if a river is not the main
conduit of runoff to the embayment. This method does give a relatively
accurate assessment of contaminant loadings, however, the contributions
of upstream point and nonpoint source loadings cannot be delineated.
Where there are few known upstream point sources and direct runoff is
known to be minimal, this would be the preferred method for estimating
nonpoint source contaminant loading.
The WQA procedure as applied here appeared to greatly underestimate the
contaminant loadings. The explanation for this discrepancy appears to
be that the Water Quality Assessment (WQA) method requires adjustments
of its parameters to account for local conditions which are different
from conditions elsewhere in the country where the method was
developed. Use of historic flow and concentration measurements to
calibrate the WQA procedure could result in an inexpensive method that
would be applicable to Puget Sound conditions. However, limitations of
available water quality data appears to make this approach somewhat
doubtful in its potential benefits.
Further work, the objective of which is accurate quantitative NPS
loading estimates should be based on an established methodology such as
the HSPF model. The HSPF model should be able to give an accurate
prediction of contaminant loadings to any embayment of Puget Sound,
however, it has been the experience in other parts of the country where
the method has been attempted, that model predictions can vary widely
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from known values even after an extensive data collection effort.
Prior to implementation of a detailed modelling study, the simple
screening methods discussed in this report and evaluation of available
environmental data should be employed to assess the probable benefits
of a more detailed NFS analysis.
Other steps which should be undertaken as part of future work in the
effort to quantify nonpoint source pollutant loading must include
continued support of data collection programs. The National Weather
Service program to gather precipitation and temperature data, appears
to be adequate at the present time (however, the variation of
precipitation across the Puget Sound lowland is great enough that more
data is always welcome); the U.S. Geological Survey flow monitoring
system is also very good although there are many small streams and
segments of major streams which do not have the density of gaging
stations to determine the hydrology adequately for short-time (single
event) modelling purposes.
Water quality data appears to be the greatest shortcoming which may
impede future modelling and estimation efforts. For many of the rivers
draining to Puget Sound, the number of stations and chemical
constituents monitored are insufficient for detailed investigation of
runoff water quality.
Finally, further studies to investigate runoff quality and transport
mechanisms at the point of origin should be supported. The Soil
.Conservation Service is putting some effort in this direction, but
their mandate is more to developing best management practices (BMPs)
rather that to develop data indicative of the magnitude of the
problem. Individual cities (particularly METRO in the Seattle area)
have conducted studies of urban runoff, but few programs of a general
nature appear to have been conducted in this respect. The NURP data
are available as a first approximation of loading from urban runoff if
land use data evaluations are available to develop estimates of runoff
volume. Similar efforts to characterize agricultural and logging
runoff volumes and chemistry should also be implemented.
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6.0 REFERENCES
Dendrou, Sterios A., "Overview of Urban Stormwater Models", Chapter 8
(pp. 219-247) in Kibler fed.), 1982.
Dexter, R.N., D.E. Anderson, E.A. Quinlan, L.S. Goldstein, R.M.
Strickland, S.P. Pavlou, J.R. Clayton, Jr., R.M. Kocan, and M.
Landolt, A Summary of Knowledge of Puget Sound Related to Chemical
Contaminants, Technical Memorandum OMPA-13, National Oceanic and
Atmospheric Administration (NOAA), Boulder, CO, December 1981.
Donigian, Anthony S. Jr. and Norman H. Crawford, Modeling Nonpoint
Pollution From the Land Surface, EPA, EPA-600/3-76-083, July 1976.
Donigian, Anthony S., Jr., John C. Imhoff, Brian R. Bicknell, John L.
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