**PDM4 MODELING OF DIDECYL DIMETHYL AMMONIUM CHLORIDE (DDAC) IN
ONCE-THROUGH**

**INDUSTRIAL WATER SYSTEMS**

**David C. Bays, Microbiologist**

**Kathryn V. Montague, Biologist**

**Rick Petrie, Team Leader**

**Risk Assessment and Science Support Branch**

**Antimicrobials Division**

**Office of Pesticide Programs**

**U.S. EPA**

**[INTRODUCTION]{.underline}**

This document provides estimated environmental concentrations (EECs) in
the receiving body of water from the use of didecyl dimethyl ammonium
chloride (DDAC) in industrial cooling water systems. [DDAC]{.smallcaps}
has been shown to be hydrolytically stable under abiotic and buffered
conditions. Metabolic transformation is unlikely to occur; therefore,
degradation products of DDAC are not expected. The Probabilistic
Distribution Model version 4 (PDM4) was used to evaluate the DDAC
concentration in stream water resulting from the use of DDAC products in
once-through cooling water systems. An average sized plant was modeled
as being located on small, average, and large rivers. The chances of
exceedance on any given day were determined, as well as percent chances
of exceedance over extended durations.

**[Overview of PDM4]{.underline}**

PDM4 calculates downstream chemical concentrations from a chemical
discharge using the probabilistic distribution model. The algorithm used
is taken from DiToro (1984). A simple mass balance approach forms the
basis of PDM. However, the input variables are not single point
estimates, because, in reality, these variables are not constant.
Streams follow a highly variable seasonal flow pattern and numerous
variables in a manufacturing process can affect the chemical
concentration and flow rate of the effluent. PDM4 models the stream flow
as constantly changing and calculates the probability that the
concentration in a given target stream will exceed some target value.
Specifically, it calculates the percent of days per year that the
concentration of concern is exceeded. PDM4 uses probability
distributions as inputs and calculates the resulting probability
distribution of the concentration in the stream.

**[ONCE-THROUGH COOLING WATER]{.underline}**

**[Inputs]{.underline}**

[Application Rate]{.underline}

[]{.mark}

The dose rates for DDAC from product labels ranged from 32 to 63 ppm
a.i. (i.e., 1 part DDAC per million parts of water flowing through the
system) for continuous feed and from 1000 to1800 ppm a.i. for
intermittent treatment. For this assessment, four rates were modeled: 32
ppm a.i. (low dose -- continuous feed), 63 ppm a.i. (high dose --
continuous feed), 1000 ppm a.i. (low dose -- intermittent treatment) and
1800 ppm a.i. (high dose -- intermittent treatment).

[Power Plant Information]{.underline}

In modeling the fate of DDAC in the receiving body of water, it was
assumed that the 7Q10 flow rate (a flow rate that, once every ten years,
a stream is expected to be below for seven consecutive days) was assumed
to be the normal cooling water flow rate. This value was chosen because
it is assumed that electric plants would need to have a steady supply of
cooling water, and the 7Q10 flow reflects a rate that could be
maintained continuously by the power plant. This is not a conservative
assumption, since electric plants may use more cooling water under
normal conditions, though at a greater risk of running out of usable
water. For lack of better data, however, these values were used.

[PDM4 Model Inputs]{.underline}

A summary of the input parameters is described below.

1.  [Release Days]{.underline} - It has been assumed that the product is
    released 365 days per year.

2.  [Pretreatment Release]{.underline} -- The following equations were
    used to calculate the loading (kg/site/day):

> *[Continuous Feed Scenario-Calculation for Loading
> (kg/site/day)]{.underline}*:
>
> System Flow (MLD) \* 10^6^ (Conversion factor for million liters/day
> to liters/day) (Equation #1)
>
> \* 1 (kg water/liter of water) \* dose rate (kg/kg)
>
> where,
>
> Dose rate (kg of chemical/ kg of water) = dose rate (ppm) \* 10^-6^
> (Equation #2)
>
> The cooling system flow rate was assumed to be identical to the 7Q10
> flow rate (a flow rate that, once every ten years, a stream is
> expected to be below for seven consecutive days).
>
> *[Intermittent Feed Scenario-Calculation for Loading
> (kg/site/day)]{.underline}*: As no information was provided regarding
> the application time associated with intermittent treatment, it was
> assumed that one application is made per day for 15 minutes:
>
> System Flow (MLD) \* 10^6^ (Conversion factor for million liters/day
> to liters/day) (Equation #3)
>
> \* 1 (kg water/liter of water) \* dose rate (kg/kg) \* (0.25/24
> (hrs/day))
>
> where,
>
> Dose rate (kg of chemical/ kg of water) = dose rate (ppm) \* 10^-6^
> (Equation #4)

1.  [Concentration of Concern]{.underline} - PDM4 is designed to report
    the number of days that the concentration in the water body exceeds
    a concentration of concern that has been specified by the user. For
    this study, the model was run multiple times using different COCs to
    determine a curve. The COCs considered for each flow regime are as
    follows: 14, 18, 69, 73, 95, 320, 1000 ppb a.i. These COC values
    were derived from studies of the sensitivity of various species to
    DDAC.

The input parameters are summarized in Table 1 below.

+-------------------+------------------+------------------------------+
| **Table 1. PDM4   |                  |                              |
| Model Inputs**    |                  |                              |
+-------------------+------------------+------------------------------+
| Parameter         | Value            | Rationale                    |
+-------------------+------------------+------------------------------+
| Industry Type     | Steam Electric   | EPA assumption as being the  |
|                   | Power Plants     | representative facility for  |
|                   | (SIC #4911)      | once-through industrial      |
|                   |                  | water systems using DDAC.    |
|                   | Various NPDES    |                              |
+-------------------+------------------+------------------------------+
| Release Days      | 365              | EPA assumption.              |
+-------------------+------------------+------------------------------+
| Pretreatment      | Determined for   | See discussion regarding Eq. |
| Release           | each site using  | 1-4.                         |
|                   | Eq. 1-4          |                              |
+-------------------+------------------+------------------------------+
| Concentration of  | 14, 18, 69, 73,  | A range was used to          |
| Concern           | 95, 320, 1000    | determine a curve. The range |
|                   | ppb a.i          | is based on the sensitivity  |
|                   |                  | of different species to      |
|                   |                  | DDAC.                        |
+-------------------+------------------+------------------------------+

The three different flow regimes consisted of power plants with stream
flow rates of 100 ± 10 MGD (million gallons per day), 500 ± 50 MGD, and
1000 ± 50 MGD. The low, medium, and high stream flow rates correspond to
378.5 ± 37.85, 1982.5 ± 189.25, and 3785 ± 189.25 million liters per
day. These plants were pulled from a database of NPDES plant codes based
on the above criterion. Tables 2, 3, and 4 below show details regarding
the power plants and their cooling streams.

+------------+----------------------------+---------------------------+
| **Table 2. |                            |                           |
| Low Flow   |                            |                           |
| (100       |                            |                           |
| MGD^1^)    |                            |                           |
| Stream     |                            |                           |
| Specif     |                            |                           |
| ications** |                            |                           |
+------------+----------------------------+---------------------------+
| NPDES      | Mean Stream                | Mean7Q10^3^               |
|            |                            |                           |
|            | Flow (MLD^2^)              | Stream Flow (MLD)         |
+------------+----------------------------+---------------------------+
| IA0033235  | 401                        | 2.84                      |
+------------+----------------------------+---------------------------+
| PA0002062  | 391                        | 4.09                      |
+------------+----------------------------+---------------------------+
| LA0003042  | 384                        | 44.6                      |
+------------+----------------------------+---------------------------+
| MI0038172  | 379                        | 14.0                      |
+------------+----------------------------+---------------------------+
| OK0002682  | 363                        | 3.37                      |
+------------+----------------------------+---------------------------+
| WV0005525  | 359                        | 14.6                      |
+------------+----------------------------+---------------------------+
| IL0036919  | 356                        | 3.89                      |
+------------+----------------------------+---------------------------+
| LA0036145  | 354                        | 13.6                      |
+------------+----------------------------+---------------------------+
| UT0000116  | 352                        | 32.4                      |
+------------+----------------------------+---------------------------+
| TX0054500  | 352                        | 26.8                      |
+------------+----------------------------+---------------------------+
| PA0008443  | 337                        | 12.6                      |
+------------+----------------------------+---------------------------+
| IL0048321  | 336                        | 97.0                      |
+------------+----------------------------+---------------------------+

1\. MGD = million gallons per day

2\. MLD = million liters per day

3\. Mean 7Q10 = Seven consecutive days of lowest stream flows over a ten
year period

+------------+----------------------------+---------------------------+
| **Table 3. |                            |                           |
| Medium     |                            |                           |
| Flow (500  |                            |                           |
| MGD^1^)    |                            |                           |
| Stream     |                            |                           |
| Specif     |                            |                           |
| ications** |                            |                           |
+------------+----------------------------+---------------------------+
| NPDES      | Mean Stream                | Mean7Q10^3^               |
|            |                            |                           |
|            | Flow (MLD^2^)              | Stream Flow (MLD)         |
+------------+----------------------------+---------------------------+
| MA0004367  | 2,030                      | 227                       |
+------------+----------------------------+---------------------------+
| IA0000108  | 1,970                      | 61.3                      |
+------------+----------------------------+---------------------------+
| NM0000108  | 1,970                      | 5.92                      |
+------------+----------------------------+---------------------------+
| FL0025526  | 1,960                      | 653                       |
+------------+----------------------------+---------------------------+
| IN0032948  | 1,950                      | 153                       |
+------------+----------------------------+---------------------------+
| TX0001163  | 1,870                      | 11.4                      |
+------------+----------------------------+---------------------------+
| OH0010421  | 1,840                      | 59.8                      |
+------------+----------------------------+---------------------------+
| IN0041246  | 1,820                      | 167                       |
+------------+----------------------------+---------------------------+
| PA0002054  | 1,790                      | 115                       |
+------------+----------------------------+---------------------------+
| MN0000906  | 1,750                      | 28.9                      |
+------------+----------------------------+---------------------------+
| NH0001431  | 1,730                      | 75.7                      |
+------------+----------------------------+---------------------------+
| IN0038806  | 1,720                      | 58.2                      |
+------------+----------------------------+---------------------------+

1\. MGD = million gallons per day

2\. MLD = million liters per day

3\. Mean 7Q10 = Seven consecutive days of lowest stream flows over a ten
year period

+------------+----------------------------+---------------------------+
| **Table 4. |                            |                           |
| High Flow  |                            |                           |
| (1000      |                            |                           |
| MGD^1^)    |                            |                           |
| Stream     |                            |                           |
| Specif     |                            |                           |
| ications** |                            |                           |
+------------+----------------------------+---------------------------+
| NPDES      | Mean Stream                | Mean7Q10^3^               |
|            |                            |                           |
|            | Flow (MLD^2^)              | Stream Flow (MLD)         |
+------------+----------------------------+---------------------------+
| GA0004341  | 3,960                      | 838                       |
+------------+----------------------------+---------------------------+
| WA0003280  | 3,830                      | 592                       |
+------------+----------------------------+---------------------------+
| KS0079057  | 3,760                      | 20.4                      |
+------------+----------------------------+---------------------------+
| NC0005088  | 3,690                      | 812                       |
+------------+----------------------------+---------------------------+
| SC0001104  | 3,650                      | 50.3                      |
+------------+----------------------------+---------------------------+
| IL0002186  | 3,640                      | 1,170                     |
+------------+----------------------------+---------------------------+

1\. MGD = million gallons per day

2\. MLD = million liters per day

3\. Mean 7Q10 = Seven consecutive days of lowest stream flows over a ten
year period

Twelve different sites throughout the United States were tested in the
model from the low and medium stream regimes, and six were tested from
the high flow regime.

**[RESULTS]{.underline}**

For each reach, PDM4 calculates the percent of days per year that a
reach would have concentrations above a particular COC. These values
were averaged by Versar within each of the three different flow regimes
(i.e., low, medium, and high), giving average percentages of exceedance.
Versar also looked at the worst-case scenarios for the low, medium, and
high stream flows considered. These calculations were performed for four
dose rates: intermittent low- and high-end doses, and the continuous
low- and high-end doses. Versar also calculated the chance of exceeding
a particular COC over time periods of 48 hours (2 days) and 96 hours (4
days). Average results for each of the three flow regimes are presented
in Tables 5, 6, and 7.

+---------+---------+--------+---------+---------+---------+---------+
| **Table |         |        |         |         |         |         |
| 5.      |         |        |         |         |         |         |
| Average |         |        |         |         |         |         |
| Percent |         |        |         |         |         |         |
| of Days |         |        |         |         |         |         |
| per     |         |        |         |         |         |         |
| Year    |         |        |         |         |         |         |
| COC     |         |        |         |         |         |         |
| Exc     |         |        |         |         |         |         |
| eeded** |         |        |         |         |         |         |
|         |         |        |         |         |         |         |
| **(Dose |         |        |         |         |         |         |
| = 32    |         |        |         |         |         |         |
| ppm     |         |        |         |         |         |         |
| a       |         |        |         |         |         |         |
| .i.-Con |         |        |         |         |         |         |
| tinuous |         |        |         |         |         |         |
| Low-End |         |        |         |         |         |         |
| Dose)** |         |        |         |         |         |         |
+---------+---------+--------+---------+---------+---------+---------+
| COC^1^  | LF^2^   | LF-St  | MF^3^-  | MF-S    | HF^4^-  | HF-S    |
| (ppb)   | Percent | andard | Percent | tandard | Percent | tandard |
|         | Days    |        | Days    | De      | Days    |         |
|         | COC     | Dev    | COC     | viation | COC     | De      |
|         | E       | iation | E       | (%)     | E       | viation |
|         | xceeded | (%)    | xceeded |         | xceeded | (%)     |
|         | (%)     |        |         |         |         |         |
|         |         |        | (%)     |         | (%)     |         |
+---------+---------+--------+---------+---------+---------+---------+
| 14      | 96.4    | 9.59   | 99.4    | 1.34    | 100     | 0.0744  |
+---------+---------+--------+---------+---------+---------+---------+
| 18      | 94.6    | 13.4   | 99.2    | 1.95    | 99.9    | 0.177   |
+---------+---------+--------+---------+---------+---------+---------+
| 69      | 80.1    | 32.8   | 94.9    | 10.3    | 95.9    | 7.82    |
+---------+---------+--------+---------+---------+---------+---------+
| 73      | 79.4    | 33.4   | 94.6    | 10.9    | 95.5    | 8.69    |
+---------+---------+--------+---------+---------+---------+---------+
| 95      | 76.3    | 35.9   | 92.9    | 14.1    | 93      | 13.4    |
+---------+---------+--------+---------+---------+---------+---------+
| 320     | 62.7    | 40.4   | 81.2    | 27.1    | 80      | 35.7    |
+---------+---------+--------+---------+---------+---------+---------+
| 1000    | 47      | 37.7   | 63.3    | 27.9    | 70.6    | 41.6    |
+---------+---------+--------+---------+---------+---------+---------+

1\. COC = Concentration of Concern

2\. LF = Low Flow Regime (100 MGD)

3\. MF = Medium Flow Regime (500 MGD)

4\. HF = High Flow Regime (1000 MGD)

+---------+---------+--------+---------+---------+---------+---------+
| **Table |         |        |         |         |         |         |
| 6.      |         |        |         |         |         |         |
| Average |         |        |         |         |         |         |
| Percent |         |        |         |         |         |         |
| of Days |         |        |         |         |         |         |
| per     |         |        |         |         |         |         |
| Year    |         |        |         |         |         |         |
| COC     |         |        |         |         |         |         |
| Exc     |         |        |         |         |         |         |
| eeded** |         |        |         |         |         |         |
|         |         |        |         |         |         |         |
| **(Dose |         |        |         |         |         |         |
| = 63    |         |        |         |         |         |         |
| ppm     |         |        |         |         |         |         |
| a.i.-   |         |        |         |         |         |         |
| Con     |         |        |         |         |         |         |
| tinuous |         |        |         |         |         |         |
| H       |         |        |         |         |         |         |
| igh-End |         |        |         |         |         |         |
| Dose)** |         |        |         |         |         |         |
+=========+=========+========+=========+=========+=========+=========+
| COC^1^  | LF^2^   | LF-St  | MF^3^-  | MF-S    | HF^4^-  | HF-S    |
| (ppb)   | Percent | andard | Percent | tandard | Percent | tandard |
|         | Days    |        | Days    | De      | Days    |         |
|         | COC     | Dev    | COC     | viation | COC     | De      |
|         | E       | iation | E       | (%)     | E       | viation |
|         | xceeded | (%)    | xceeded |         | xceeded | (%)     |
|         | (%)     |        |         |         |         |         |
|         |         |        | (%)     |         | (%)     |         |
+---------+---------+--------+---------+---------+---------+---------+
| 14      | 99.1    | 2.59   | 99.8    | 0.439   | 100     | 0.00802 |
+---------+---------+--------+---------+---------+---------+---------+
| 18      | 98.4    | 4.53   | 99.7    | 0.671   | 100     | 0.0184  |
+---------+---------+--------+---------+---------+---------+---------+
| 69      | 88.2    | 24.1   | 97.8    | 4.72    | 99.1    | 1.55    |
+---------+---------+--------+---------+---------+---------+---------+
| 73      | 87.6    | 25     | 97.6    | 5.05    | 99      | 1.82    |
+---------+---------+--------+---------+---------+---------+---------+
| 95      | 84.5    | 28.6   | 96.7    | 6.91    | 98.1    | 3.62    |
+---------+---------+--------+---------+---------+---------+---------+
| 320     | 70.2    | 39.3   | 88.3    | 21.1    | 87      | 24.7    |
+---------+---------+--------+---------+---------+---------+---------+
| 1000    | 56.9    | 39.9   | 75.2    | 28.5    | 76.2    | 39.4    |
+---------+---------+--------+---------+---------+---------+---------+

1\. COC = Concentration of Concern

2\. LF = Low Flow Regime (100 MGD)

3\. MF = Medium Flow Regime (500 MGD)

4\. HF = High Flow Regime (1000 MGD)

+---------+---------+--------+---------+---------+---------+---------+
| **Table |         |        |         |         |         |         |
| 7.      |         |        |         |         |         |         |
| Average |         |        |         |         |         |         |
| Percent |         |        |         |         |         |         |
| of Days |         |        |         |         |         |         |
| per     |         |        |         |         |         |         |
| Year    |         |        |         |         |         |         |
| COC     |         |        |         |         |         |         |
| Exc     |         |        |         |         |         |         |
| eeded** |         |        |         |         |         |         |
|         |         |        |         |         |         |         |
| **(Dose |         |        |         |         |         |         |
| = 1000  |         |        |         |         |         |         |
| ppm     |         |        |         |         |         |         |
| a.i     |         |        |         |         |         |         |
| .-Inter |         |        |         |         |         |         |
| mittent |         |        |         |         |         |         |
| Low-End |         |        |         |         |         |         |
| Dose)** |         |        |         |         |         |         |
+---------+---------+--------+---------+---------+---------+---------+
| COC^1^  | LF^2^   | LF-St  | MF^3^-  | MF-S    | HF^4^-  | HF-S    |
| (ppb)   | Percent | andard | Percent | tandard | Percent | tandard |
|         | Days    |        | Days    | De      | Days    |         |
|         | COC     | Dev    | COC     | viation | COC     | De      |
|         | E       | iation | E       | (%)     | E       | viation |
|         | xceeded | (%)    | xceeded |         | xceeded | (%)     |
|         | (%)     |        |         |         |         |         |
|         |         |        | (%)     |         | (%)     |         |
+---------+---------+--------+---------+---------+---------+---------+
| 14      | 85.8    | 27.1   | 97.1    | 6.04    | 98.5    | 2.71    |
+---------+---------+--------+---------+---------+---------+---------+
| 18      | 82.8    | 30.3   | 96.1    | 8.08    | 97.4    | 4.96    |
+---------+---------+--------+---------+---------+---------+---------+
| 69      | 67.4    | 40.1   | 85.7    | 24      | 84      | 29.8    |
+---------+---------+--------+---------+---------+---------+---------+
| 73      | 66.7    | 40.2   | 85.1    | 24.5    | 83.4    | 30.8    |
+---------+---------+--------+---------+---------+---------+---------+
| 95      | 63.8    | 40.5   | 82.3    | 26.6    | 80.8    | 34.7    |
+---------+---------+--------+---------+---------+---------+---------+
| 320     | 47.2    | 37.7   | 63.7    | 28      | 70.8    | 41.5    |
+---------+---------+--------+---------+---------+---------+---------+
| 1000    | 28.2    | 29.3   | 37.4    | 24.5    | 54.3    | 37.6    |
+---------+---------+--------+---------+---------+---------+---------+

1\. COC = Concentration of Concern

2\. LF = Low Flow Regime (100 MGD)

3\. MF = Medium Flow Regime (500 MGD)

4\. HF = High Flow Regime (1000 MGD)

+---------+---------+--------+---------+---------+---------+---------+
| **Table |         |        |         |         |         |         |
| 8.      |         |        |         |         |         |         |
| Average |         |        |         |         |         |         |
| Percent |         |        |         |         |         |         |
| of Days |         |        |         |         |         |         |
| per     |         |        |         |         |         |         |
| Year    |         |        |         |         |         |         |
| COC     |         |        |         |         |         |         |
| Exc     |         |        |         |         |         |         |
| eeded** |         |        |         |         |         |         |
|         |         |        |         |         |         |         |
| **(Dose |         |        |         |         |         |         |
| = 1800  |         |        |         |         |         |         |
| ppm     |         |        |         |         |         |         |
| a.i     |         |        |         |         |         |         |
| .-Inter |         |        |         |         |         |         |
| mittent |         |        |         |         |         |         |
| H       |         |        |         |         |         |         |
| igh-End |         |        |         |         |         |         |
| Dose)** |         |        |         |         |         |         |
+---------+---------+--------+---------+---------+---------+---------+
| COC^1^  | LF^2^   | LF-St  | MF^3^-  | MF-S    | HF^4^-  | HF-S    |
| (ppb)   | Percent | andard | Percent | tandard | Percent | tandard |
|         | Days    |        | Days    | De      | Days    |         |
|         | COC     | Dev    | COC     | viation | COC     | De      |
|         | E       | iation | E       | (%)     | E       | viation |
|         | xceeded | (%)    | xceeded |         | xceeded | (%)     |
|         | (%)     |        |         |         |         |         |
|         |         |        | (%)     |         | (%)     |         |
+---------+---------+--------+---------+---------+---------+---------+
| 14      | 92.2    | 18     | 98.7    | 2.89    | 99.7    | 0.469   |
+---------+---------+--------+---------+---------+---------+---------+
| 18      | 89.6    | 22.1   | 98.2    | 4.01    | 99.4    | 1.05    |
+---------+---------+--------+---------+---------+---------+---------+
| 69      | 73.8    | 37.5   | 91.2    | 17      | 90.7    | 17.8    |
+---------+---------+--------+---------+---------+---------+---------+
| 73      | 73.2    | 37.9   | 90.7    | 17.7    | 90.1    | 19      |
+---------+---------+--------+---------+---------+---------+---------+
| 95      | 70.3    | 39.3   | 88.4    | 21.1    | 87      | 24.6    |
+---------+---------+--------+---------+---------+---------+---------+
| 320     | 55.9    | 39.8   | 74.1    | 28.6    | 75.7    | 39.8    |
+---------+---------+--------+---------+---------+---------+---------+
| 1000    | 38.2    | 34.4   | 51.4    | 26.6    | 64.4    | 41.2    |
+---------+---------+--------+---------+---------+---------+---------+

1\. COC = Concentration of Concern

2\. LF = Low Flow Regime (100 MGD)

3\. MF = Medium Flow Regime (500 MGD)

4\. HF = High Flow Regime (1000 MGD)

Tables 9, 10, 11, and 12 list the calculations for those power plants
with the highest exceedance rates for each of the three stream flow
sizes considered.

+---------------+-----------------+---------------+-------------------+
| **Table 9.    |                 |               |                   |
| Worst Case    |                 |               |                   |
| Scenarios:    |                 |               |                   |
| Percent of    |                 |               |                   |
| Days per Year |                 |               |                   |
| COC           |                 |               |                   |
| Exceeded**    |                 |               |                   |
|               |                 |               |                   |
| **(Dose = 32  |                 |               |                   |
| ppm           |                 |               |                   |
| a.            |                 |               |                   |
| i.-Continuous |                 |               |                   |
| Low-End       |                 |               |                   |
| Dose)**       |                 |               |                   |
+---------------+-----------------+---------------+-------------------+
| COC^1^ (ppb)  | LF^2^-Percent   | MF^3^-Percent | HF^4^-Percent     |
|               | Days COC        | Days COC      | Days COC Exceeded |
|               | Exceeded (%)    | Exceeded      |                   |
|               |                 |               | (%)               |
|               |                 | (%)           |                   |
+---------------+-----------------+---------------+-------------------+
| 14            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 18            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 69            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 73            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 95            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 320           | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 1000          | 98.8            | 99.2          | 99.6              |
+---------------+-----------------+---------------+-------------------+

1\. COC = Concentration of Concern

2\. LF = Low Flow Regime (100 MGD)

3\. MF = Medium Flow Regime (500 MGD)

4\. HF = High Flow Regime (1000 MGD)

+---------------+-----------------+---------------+-------------------+
| **Table 10.   |                 |               |                   |
| Worst Case    |                 |               |                   |
| Scenarios:    |                 |               |                   |
| Percent of    |                 |               |                   |
| Days per Year |                 |               |                   |
| COC           |                 |               |                   |
| Exceeded**    |                 |               |                   |
|               |                 |               |                   |
| **(Dose = 63  |                 |               |                   |
| ppm a.i.-     |                 |               |                   |
| Continuous    |                 |               |                   |
| High-End      |                 |               |                   |
| Dose)**       |                 |               |                   |
+---------------+-----------------+---------------+-------------------+
| COC^1^ (ppb)  | LF^2^-Percent   | MF^3^-Percent | HF^4^-Percent     |
|               | Days COC        | Days COC      | Days COC Exceeded |
|               | Exceeded (%)    | Exceeded      |                   |
|               |                 |               | (%)               |
|               |                 | (%)           |                   |
+---------------+-----------------+---------------+-------------------+
| 14            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 18            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 69            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 73            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 95            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 320           | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 1000          | 99.9            | 99.9          | 100               |
+---------------+-----------------+---------------+-------------------+

1\. COC = Concentration of Concern

2\. LF = Low Flow Regime (100 MGD)

3\. MF = Medium Flow Regime (500 MGD)

4\. HF = High Flow Regime (1000 MGD)

+---------------+-----------------+---------------+-------------------+
| **Table 11.   |                 |               |                   |
| Worst Case    |                 |               |                   |
| Scenarios:    |                 |               |                   |
| Percent of    |                 |               |                   |
| Days per Year |                 |               |                   |
| COC           |                 |               |                   |
| Exceeded**    |                 |               |                   |
|               |                 |               |                   |
| **(Dose =     |                 |               |                   |
| 1000 ppm      |                 |               |                   |
| a.i.          |                 |               |                   |
| -Intermittent |                 |               |                   |
| Low-End       |                 |               |                   |
| Dose)**       |                 |               |                   |
+---------------+-----------------+---------------+-------------------+
| COC^1^ (ppb)  | LF^2^-Percent   | MF^3^-Percent | HF^4^-Percent     |
|               | Days COC        | Days COC      | Days COC Exceeded |
|               | Exceeded (%)    | Exceeded      |                   |
|               |                 |               | (%)               |
|               |                 | (%)           |                   |
+---------------+-----------------+---------------+-------------------+
| 14            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 18            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 69            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 73            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 95            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 320           | 98.8            | 99.2          | 99.6              |
+---------------+-----------------+---------------+-------------------+
| 1000          | 84.8            | 88.5          | 88.6              |
+---------------+-----------------+---------------+-------------------+

1\. COC = Concentration of Concern

2\. LF = Low Flow Regime (100 MGD)

3\. MF = Medium Flow Regime (500 MGD)

4\. HF = High Flow Regime (1000 MGD)

+---------------+-----------------+---------------+-------------------+
| **Table 12.   |                 |               |                   |
| Worst Case    |                 |               |                   |
| Scenarios:    |                 |               |                   |
| Percent of    |                 |               |                   |
| Days per Year |                 |               |                   |
| COC           |                 |               |                   |
| Exceeded**    |                 |               |                   |
|               |                 |               |                   |
| **(Dose =     |                 |               |                   |
| 1800 ppm      |                 |               |                   |
| a.i.          |                 |               |                   |
| -Intermittent |                 |               |                   |
| High-End      |                 |               |                   |
| Dose)**       |                 |               |                   |
+---------------+-----------------+---------------+-------------------+
| COC^1^ (ppb)  | LF^2^-Percent   | MF^3^-Percent | HF^4^-Percent     |
|               | Days COC        | Days COC      | Days COC Exceeded |
|               | Exceeded (%)    | Exceeded      |                   |
|               |                 |               | (%)               |
|               |                 | (%)           |                   |
+---------------+-----------------+---------------+-------------------+
| 14            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 18            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 69            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 73            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 95            | 100             | 100           | 100               |
+---------------+-----------------+---------------+-------------------+
| 320           | 99.8            | 99.9          | 100               |
+---------------+-----------------+---------------+-------------------+
| 1000          | 95.2            | 96.7          | 97.5              |
+---------------+-----------------+---------------+-------------------+

1\. COC = Concentration of Concern

2\. LF = Low Flow Regime (100 MGD)

3\. MF = Medium Flow Regime (500 MGD)

4\. HF = High Flow Regime (1000 MGD)

Tables 13, 14, 15, and 16 show the average of all power plants modeled,
and compares these values to the average of the three worst-case
scenarios that were presented in Tables 9, 10, 11, and 12, respectively.

+-----------------+-------------------------+-------------------------+
| **Table 13.     |                         |                         |
| Total Average   |                         |                         |
| Exceedance      |                         |                         |
| Rates vs.       |                         |                         |
| Worst-Case      |                         |                         |
| Exceedance      |                         |                         |
| Rates**         |                         |                         |
|                 |                         |                         |
| **(Dose = 32    |                         |                         |
| ppm             |                         |                         |
| a.i.-Continuous |                         |                         |
| Low-End Dose)** |                         |                         |
+-----------------+-------------------------+-------------------------+
| COC^1^ (ppb)    | Percent of Days COC     |                         |
|                 | Exceeded (%)            |                         |
+-----------------+-------------------------+-------------------------+
|                 | Total Average           | Average of the Three    |
|                 |                         | Worst Case Scenarios    |
|                 |                         | (Table 9)               |
+-----------------+-------------------------+-------------------------+
| 14              | 98.3                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 18              | 97.5                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 69              | 89.2                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 73              | 88.7                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 95              | 86.3                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 320             | 73.6                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 1000            | 58.2                    | 99.2                    |
+-----------------+-------------------------+-------------------------+

1\. COC = Concentration of Concern

+-----------------+-------------------------+-------------------------+
| **Table 14.     |                         |                         |
| Total Average   |                         |                         |
| Exceedance      |                         |                         |
| Rates vs.       |                         |                         |
| Worst-Case      |                         |                         |
| Exceedance      |                         |                         |
| Rates**         |                         |                         |
|                 |                         |                         |
| **(Dose = 63    |                         |                         |
| ppm             |                         |                         |
| a.i.-Continuous |                         |                         |
| High-End        |                         |                         |
| Dose)**         |                         |                         |
+-----------------+-------------------------+-------------------------+
| COC^1^ (ppb)    | Percent of Days COC     |                         |
|                 | Exceeded (%)            |                         |
+-----------------+-------------------------+-------------------------+
|                 | Total Average           | Average of the Three    |
|                 |                         | Worst Case Scenarios    |
|                 |                         | (Table 10)              |
+-----------------+-------------------------+-------------------------+
| 14              | 99.6                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 18              | 99.3                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 69              | 94.2                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 73              | 93.9                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 95              | 92.1                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 320             | 80.8                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 1000            | 68.1                    | 99.9                    |
+-----------------+-------------------------+-------------------------+

1\. COC = Concentration of Concern

+-----------------+-------------------------+-------------------------+
| **Table 15.     |                         |                         |
| Total Average   |                         |                         |
| Exceedance      |                         |                         |
| Rates vs.       |                         |                         |
| Worst-Case      |                         |                         |
| Exceedance      |                         |                         |
| Rates**         |                         |                         |
|                 |                         |                         |
| **(Dose = 1000  |                         |                         |
| ppm             |                         |                         |
| a.              |                         |                         |
| i.-Intermittent |                         |                         |
| Low-End Dose)** |                         |                         |
+-----------------+-------------------------+-------------------------+
| COC^1^ (ppb)    | Percent of Days COC     |                         |
|                 | Exceeded (%)            |                         |
+-----------------+-------------------------+-------------------------+
|                 | Total Average           | Average of the Three    |
|                 |                         | Worst Case Scenarios    |
|                 |                         | (Table 11)              |
+-----------------+-------------------------+-------------------------+
| 14              | 92.9                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 18              | 91                      | 100                     |
+-----------------+-------------------------+-------------------------+
| 69              | 78                      | 100                     |
+-----------------+-------------------------+-------------------------+
| 73              | 77.4                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 95              | 74.6                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 320             | 58.5                    | 99.2                    |
+-----------------+-------------------------+-------------------------+
| 1000            | 37.1                    | 87.3                    |
+-----------------+-------------------------+-------------------------+

1\. COC = Concentration of Concern

+-----------------+-------------------------+-------------------------+
| **Table 16.     |                         |                         |
| Total Average   |                         |                         |
| Exceedance      |                         |                         |
| Rates vs.       |                         |                         |
| Worst-Case      |                         |                         |
| Exceedance      |                         |                         |
| Rates**         |                         |                         |
|                 |                         |                         |
| **(Dose = 1800  |                         |                         |
| ppm             |                         |                         |
| a.              |                         |                         |
| i.-Intermittent |                         |                         |
| High-End        |                         |                         |
| Dose)**         |                         |                         |
+-----------------+-------------------------+-------------------------+
| COC^1^ (ppb)    | Percent of Days COC     |                         |
|                 | Exceeded (%)            |                         |
+-----------------+-------------------------+-------------------------+
|                 | Total Average           | Average of the Three    |
|                 |                         | Worst Case Scenarios    |
|                 |                         | (Table 12)              |
+-----------------+-------------------------+-------------------------+
| 14              | 96.3                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 18              | 95                      | 100                     |
+-----------------+-------------------------+-------------------------+
| 69              | 84.1                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 73              | 83.6                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 95              | 80.9                    | 100                     |
+-----------------+-------------------------+-------------------------+
| 320             | 67.1                    | 99.9                    |
+-----------------+-------------------------+-------------------------+
| 1000            | 48.7                    | 96.5                    |
+-----------------+-------------------------+-------------------------+

1\. COC = Concentration of Concern

To calculate the chance of exceedance of a concentration of concern over
a consecutive number of days, the following assumptions were made:

-   The chance of exceedance on any given day at a site is independent
    of whether exceedance occurred on previous days.

-   Although the percentages calculated by PDM express the chance of
    exceedance during at least part of a day, it is assumed for this
    calculation that the percentages calculated by PDM imply exceedance
    over the total day.

Based on these assumptions, the following formula can be used to express
the probability of exceedance over N consecutive days:

\% probability = (Average Probability of Exceedance per Day/100%)^N^ \*
100%

Tables 17, 18, 19, and 20 show the calculation for these probabilities.
These tables can be used to determine an appropriate COC, based on the
level of risk that is deemed acceptable. The choice will depend on how
conservative a value is deemed necessary.

+--------------+--------+--------+--------+--------+--------+--------+
| **Table 17.  |        |        |        |        |        |        |
| Percent      |        |        |        |        |        |        |
| Probability  |        |        |        |        |        |        |
| of           |        |        |        |        |        |        |
| Exceedance   |        |        |        |        |        |        |
| Over         |        |        |        |        |        |        |
| Consecutive  |        |        |        |        |        |        |
| Days**       |        |        |        |        |        |        |
|              |        |        |        |        |        |        |
| **(Dose = 32 |        |        |        |        |        |        |
| ppm          |        |        |        |        |        |        |
| a.i          |        |        |        |        |        |        |
| .-Continuous |        |        |        |        |        |        |
| Low-End      |        |        |        |        |        |        |
| Dose)**      |        |        |        |        |        |        |
+--------------+--------+--------+--------+--------+--------+--------+
| COC^1^ (ppb) | Number |        |        |        |        |        |
|              | of     |        |        |        |        |        |
|              | Conse  |        |        |        |        |        |
|              | cutive |        |        |        |        |        |
|              | Days   |        |        |        |        |        |
+--------------+--------+--------+--------+--------+--------+--------+
|              | Low    |        | Mid    |        | High   |        |
|              | Flow   |        | Flow   |        | Flow   |        |
|              | (100   |        | (500   |        | (1000  |        |
|              | M      |        | M      |        | M      |        |
|              | GD^2^) |        | GD^2^) |        | GD^2^) |        |
+--------------+--------+--------+--------+--------+--------+--------+
|              | 2 day  | 4 day  | 2 day  | 4 day  | 2 day  | 4 day  |
+--------------+--------+--------+--------+--------+--------+--------+
| 14           | 92.9   | 86.4   | 98.8   | 97.6   | 100    | 100    |
+--------------+--------+--------+--------+--------+--------+--------+
| 18           | 89.5   | 80.1   | 98.4   | 96.8   | 99.8   | 99.6   |
+--------------+--------+--------+--------+--------+--------+--------+
| 69           | 64.2   | 41.2   | 90.1   | 81.1   | 92     | 84.6   |
+--------------+--------+--------+--------+--------+--------+--------+
| 73           | 63     | 39.7   | 89.5   | 80.1   | 91.2   | 83.2   |
+--------------+--------+--------+--------+--------+--------+--------+
| 95           | 58.2   | 33.9   | 86.3   | 74.5   | 86.5   | 74.8   |
+--------------+--------+--------+--------+--------+--------+--------+
| 320          | 39.3   | 15.5   | 65.9   | 43.5   | 64     | 41     |
+--------------+--------+--------+--------+--------+--------+--------+
| 1000         | 22.1   | 4.88   | 40.1   | 16.1   | 49.8   | 24.8   |
+--------------+--------+--------+--------+--------+--------+--------+

> 1\. COC = Concentration of Concern
>
> 2\. MGD = million gallons per day

+--------------+--------+--------+--------+--------+--------+--------+
| **Table 18.  |        |        |        |        |        |        |
| Percent      |        |        |        |        |        |        |
| Probability  |        |        |        |        |        |        |
| of           |        |        |        |        |        |        |
| Exceedance   |        |        |        |        |        |        |
| Over         |        |        |        |        |        |        |
| Consecutive  |        |        |        |        |        |        |
| Days**       |        |        |        |        |        |        |
|              |        |        |        |        |        |        |
| **(Dose = 63 |        |        |        |        |        |        |
| ppm          |        |        |        |        |        |        |
| a.i          |        |        |        |        |        |        |
| .-Continuous |        |        |        |        |        |        |
| High-End     |        |        |        |        |        |        |
| Dose)**      |        |        |        |        |        |        |
+--------------+--------+--------+--------+--------+--------+--------+
| COC^1^ (ppb) | Number |        |        |        |        |        |
|              | of     |        |        |        |        |        |
|              | Conse  |        |        |        |        |        |
|              | cutive |        |        |        |        |        |
|              | Days   |        |        |        |        |        |
+--------------+--------+--------+--------+--------+--------+--------+
|              | Low    |        | Mid    |        | High   |        |
|              | Flow   |        | Flow   |        | Flow   |        |
|              | (100   |        | (500   |        | (1000  |        |
|              | M      |        | M      |        | M      |        |
|              | GD^2^) |        | GD^2^) |        | GD^2^) |        |
+--------------+--------+--------+--------+--------+--------+--------+
|              | 2 day  | 4 day  | 2 day  | 4 day  | 2 day  | 4 day  |
+--------------+--------+--------+--------+--------+--------+--------+
| 14           | 98.2   | 96.4   | 99.6   | 99.2   | 100    | 100    |
+--------------+--------+--------+--------+--------+--------+--------+
| 18           | 96.8   | 93.8   | 99.4   | 98.8   | 100    | 100    |
+--------------+--------+--------+--------+--------+--------+--------+
| 69           | 77.8   | 60.5   | 95.6   | 91.5   | 98.2   | 96.4   |
+--------------+--------+--------+--------+--------+--------+--------+
| 73           | 76.7   | 58.9   | 95.3   | 90.7   | 98     | 96.1   |
+--------------+--------+--------+--------+--------+--------+--------+
| 95           | 71.4   | 51     | 93.5   | 87.4   | 96.2   | 92.6   |
+--------------+--------+--------+--------+--------+--------+--------+
| 320          | 49.3   | 24.3   | 78     | 60.8   | 75.7   | 57.3   |
+--------------+--------+--------+--------+--------+--------+--------+
| 1000         | 32.4   | 10.5   | 56.6   | 32     | 58.1   | 33.7   |
+--------------+--------+--------+--------+--------+--------+--------+

> 1\. COC = Concentration of Concern
>
> 2\. MGD = million gallons per day

+--------------+--------+--------+--------+--------+--------+--------+
| **Table 19.  |        |        |        |        |        |        |
| Percent      |        |        |        |        |        |        |
| Probability  |        |        |        |        |        |        |
| of           |        |        |        |        |        |        |
| Exceedance   |        |        |        |        |        |        |
| Over         |        |        |        |        |        |        |
| Consecutive  |        |        |        |        |        |        |
| Days**       |        |        |        |        |        |        |
|              |        |        |        |        |        |        |
| **(Dose =    |        |        |        |        |        |        |
| 1000 ppm     |        |        |        |        |        |        |
| a.i.-        |        |        |        |        |        |        |
| Intermittent |        |        |        |        |        |        |
| Low-End      |        |        |        |        |        |        |
| Dose)**      |        |        |        |        |        |        |
+--------------+--------+--------+--------+--------+--------+--------+
| COC^1^ (ppb) | Number |        |        |        |        |        |
|              | of     |        |        |        |        |        |
|              | Conse  |        |        |        |        |        |
|              | cutive |        |        |        |        |        |
|              | Days   |        |        |        |        |        |
+--------------+--------+--------+--------+--------+--------+--------+
|              | Low    |        | Mid    |        | High   |        |
|              | Flow   |        | Flow   |        | Flow   |        |
|              | (100   |        | (500   |        | (1000  |        |
|              | M      |        | M      |        | M      |        |
|              | GD^2^) |        | GD^2^) |        | GD^2^) |        |
+--------------+--------+--------+--------+--------+--------+--------+
|              | 2 day  | 4 day  | 2 day  | 4 day  | 2 day  | 4 day  |
+--------------+--------+--------+--------+--------+--------+--------+
| 14           | 73.6   | 54.2   | 94.3   | 88.9   | 97     | 94.1   |
+--------------+--------+--------+--------+--------+--------+--------+
| 18           | 68.6   | 47     | 92.4   | 85.3   | 94.9   | 90     |
+--------------+--------+--------+--------+--------+--------+--------+
| 69           | 45.4   | 20.6   | 73.4   | 53.9   | 70.6   | 49.8   |
+--------------+--------+--------+--------+--------+--------+--------+
| 73           | 44.5   | 19.8   | 72.4   | 52.4   | 69.6   | 48.4   |
+--------------+--------+--------+--------+--------+--------+--------+
| 95           | 40.7   | 16.6   | 67.7   | 45.9   | 65.3   | 42.6   |
+--------------+--------+--------+--------+--------+--------+--------+
| 320          | 22.3   | 4.96   | 40.6   | 16.5   | 50.1   | 25.1   |
+--------------+--------+--------+--------+--------+--------+--------+
| 1000         | 7.95   | 0.632  | 14     | 1.96   | 29.5   | 8.69   |
+--------------+--------+--------+--------+--------+--------+--------+

> 1\. COC = Concentration of Concern
>
> 2\. MGD = million gallons per day

+--------------+--------+--------+--------+--------+--------+--------+
| **Table 20.  |        |        |        |        |        |        |
| Percent      |        |        |        |        |        |        |
| Probability  |        |        |        |        |        |        |
| of           |        |        |        |        |        |        |
| Exceedance   |        |        |        |        |        |        |
| Over         |        |        |        |        |        |        |
| Consecutive  |        |        |        |        |        |        |
| Days**       |        |        |        |        |        |        |
|              |        |        |        |        |        |        |
| **(Dose =    |        |        |        |        |        |        |
| 1800 ppm     |        |        |        |        |        |        |
| a.i.-        |        |        |        |        |        |        |
| Intermittent |        |        |        |        |        |        |
| High-End     |        |        |        |        |        |        |
| Dose)**      |        |        |        |        |        |        |
+--------------+--------+--------+--------+--------+--------+--------+
| COC^1^ (ppb) | Number |        |        |        |        |        |
|              | of     |        |        |        |        |        |
|              | Conse  |        |        |        |        |        |
|              | cutive |        |        |        |        |        |
|              | Days   |        |        |        |        |        |
+--------------+--------+--------+--------+--------+--------+--------+
|              | Low    |        | Mid    |        | High   |        |
|              | Flow   |        | Flow   |        | Flow   |        |
|              | (100   |        | (500   |        | (1000  |        |
|              | M      |        | M      |        | M      |        |
|              | GD^2^) |        | GD^2^) |        | GD^2^) |        |
+--------------+--------+--------+--------+--------+--------+--------+
|              | 2 day  | 4 day  | 2 day  | 4 day  | 2 day  | 4 day  |
+--------------+--------+--------+--------+--------+--------+--------+
| 14           | 85     | 72.3   | 97.4   | 94.9   | 99.4   | 98.8   |
+--------------+--------+--------+--------+--------+--------+--------+
| 18           | 80.3   | 64.5   | 96.4   | 93     | 98.8   | 97.6   |
+--------------+--------+--------+--------+--------+--------+--------+
| 69           | 54.5   | 29.7   | 83.2   | 69.2   | 82.3   | 67.7   |
+--------------+--------+--------+--------+--------+--------+--------+
| 73           | 53.6   | 28.7   | 82.3   | 67.7   | 81.2   | 65.9   |
+--------------+--------+--------+--------+--------+--------+--------+
| 95           | 49.4   | 24.4   | 78.1   | 61.1   | 75.7   | 57.3   |
+--------------+--------+--------+--------+--------+--------+--------+
| 320          | 31.2   | 9.76   | 54.9   | 30.1   | 57.3   | 32.8   |
+--------------+--------+--------+--------+--------+--------+--------+
| 1000         | 14.6   | 2.13   | 26.4   | 6.98   | 41.5   | 17.2   |
+--------------+--------+--------+--------+--------+--------+--------+

> 1\. COC = Concentration of Concern
>
> 2\. MGD = million gallons per day

**[UNCERTAINTIES/LIMITATIONS]{.underline}**

The following limitations apply to the results of this model:

-   The PDM4 database contains data on steam electric power plants in
    the United States. In the database, no differentiation was made
    between those power plants that used a recirculating cooling water
    system and those that did not. In the absence of better data, it was
    assumed that all power plants in the database used a once-through
    cooling water system, and that the amount of cooling water used in
    normal conditions was equal to the 7Q10 flow. These assumptions may
    not be conservative.

-   It is possible in all cases that more than one facility is located
    on a given reach. For the purposes of modeling, however, it is
    assumed that only one facility is located at each reach.

-   PDM4 is a screening-level model. Screening‑level models are rarely
    if ever used as the sole justification for regulatory
    decision‑making at EPA. Additional data and more rigorous tools are
    used to improve the estimates of exposures and risks for such
    decisions. Results may not accurately reflect all of the information
    and data used by EPA to make a regulatory decision on a chemical.

-   It has been assumed that a maintenance dose of the chemical is
    required daily, and that intermittent doses are applied for 15
    minutes a day.

**[REFERENCES]{.underline}**

Cloete TE, Smith Z, Saayman G. 1999. A Cooling Water System as a Biofilm
Reactor for the Treatment of Municipal Wastewater. Water SA Vol. 25 No.
3 July 1999. Available on website

DiToro, D. M. 1984. Probability Model of Stream Quality Due to Runoff.
ASCE. Journal of Environmental Engineering. 110(3):607‑628.

EFAST Help, beta version, 2004.

Genest, Dan, Dominion Power. Telephone interview. June 14, 2004.

Versar, 2005. Memorandum. Draft Environmental Fate Science Chapter for
DDAC RED. From Adria Diaz and Patricia Wood, Versar, Inc. to Srinivas
Gowda, USEPA. Date: August 30, 2005.

**Appendix: Input/Output Files**