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# Hierarchy of Loading Calculations

The Loading Tool calculates DMR pollutant loadings using a quantity or concentration value and a wastewater flow, reported in the DMR data. There are a total of two quantity and three concentration data fields that can be populated in the DMR data (i.e., five measurement value fields):

- Quantity 1 (kg/day)
- Quantity 2 (kg/day)
- Concentration 1 (mg/L)
- Concentration 2 (mg/L)
- Concentration 3 (mg/L)

The quantity measurement fields may represent average, maximum, or total values, while the concentration measurement fields may represent minimum, average, or maximum values. Facilities may use a variety of measurements to populate the above five measurement value fields. For example, a facility can use a monthly average, daily average, 30 day geometric average, etc. to represent the average quantity. The DMR data contain statistical base codes to indicate the types of measurements reported.

EPA's goal for calculating pollutant loads is to characterize the average pollutant mass discharges to receiving streams; therefore, the Loading Tool selects the appropriate DMR measurement data field using a hierarchy that gives priority to the average values. Calculating annual pollutant loadings using concentrations requires the use of wastewater flow data, which increases the uncertainty of the calculation. Therefore, the Loading tool measurement selection hierarchy generally prioritizes average quantities over average concentrations.

The only exception to this rule is if the permit limit is on a concentration basis and there is no corresponding mass-based quantity limit. In this case, the loading tool will prioritize concentrations over quantities. In order to calculate the load over limit estimates, the Loading Tool selects the same DMR measurement as the permit limit, which could be either a concentration or quantity. Although the pollutant loadings are calculated by prioritizing measurements that are the same measurement as the permit limit, average measurements are still prioritized over minimum, total, and/or maximum measurements.

Pollutants in facility wastewater discharges are typically measured at regular intervals called monitoring periods (e.g., monthly, quarterly). After the Loading Tool selects the appropriate measurement values, the following values are calculated for each pollutant at individual outfalls (discharge points) at the facility over a monitoring period:

- Average Daily Load (kg/day);
- Average Concentration (mg/L); and
- Average Wastewater Flow (Million Gallons/day or MGD).

The Loading Tool then calculates the following values for each monitoring period, assuming that the discharge occurs continuously for the duration of the monitoring period. Note that for intermittent discharges reported in DMRs, the Loading Tool adjusts the number of days in the monitoring period using the duration of discharge parameter. The following general equations are used to calculate the load discharged during a monitoring period:

**Monitoring Period Load (kg/monitoring period)**

or

**Monitoring Period Wastewater Flow (MG/monitoring period)**

## Load Over Limit Calculations

Where permit limits are available, the Loading Tool calculates the Load Over Limit (LOL) for the monitoring period for each pollutant. Load over Limits are provided in the results of Custom Search.

Key considerations:

- Load over Limit is calculated for each monitoring period (e.g., month, quarter). The difference between the average load and permitted load for each monitoring period are then summed to generate an Annual Load over Limit (kg/year).
- Load over Limit is blank for parameters that have requirements to monitor (i.e., measure) discharges, but do not have numeric permit limits.
- Load over Limit represents reported data. The Load over Limit is not estimated for monitoring periods where no data is reported.

This calculation is done by summing the monthly differences between the average load and the permit limit in two ways, as illustrated by Figures 1 and 2 below. Both options subtract the DMR measurement value from the corresponding permit limit value. Option 1 (Figure 1) sets any negative differences equal to zero, while Option 2 (Figure 2) retains the calculated negative value.

The following examples demonstrate how the Load over Limit is calculated, using loadings data from the Facility Loading Calculations monitoring period table. See Section 3.2 of the Technical Users Background Document for the Water Pollutant Loading Tool for more specific information on the calculations.

- Indicates there was one or more exceedances of permit effluent limits for this pollutant during the reporting year.

Monitoring Period | Discharge Information | Below Detection Limit? | Measurement Type | Avg Daily Value | Limit | Wastewater Flow (MGD) | Number of Days | Monitoring Period Load (lb/period) | Load Equation | Daily Load Over Limit (lb/day)^{1} | LOL Option 1 (lb)^{2} | LOL Option 2 (lb)^{2} |
---|---|---|---|---|---|---|---|---|---|---|---|---|

1/31/2014 | Occurred? Yes | No | Quantity (kg/day) | 2.54 | 21.2 | 0.16 | 31 | 174 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -18.66 | 0 | -578.46 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

2/28/2014 | Occurred? Yes | No | Quantity (kg/day) | 2.408 | 21.2 | 0.15 | 28 | 149 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -18.792 | 0 | -526.176 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

3/31/2014 | Occurred? Yes | No | Quantity (kg/day) | 3.047 | 21.2 | 0.14 | 31 | 208 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -18.153 | 0 | -562.743 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

4/30/2014 | Occurred? Yes | No | Quantity (kg/day) | 3.68 | 7.074 | 0.14 | 30 | 244 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -3.394 | 0 | -101.82 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

5/31/2014 | Occurred? Yes | No | Quantity (kg/day) | 3.011 | 7.074 | 0.13 | 31 | 206 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -4.063 | 0 | -125.953 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

6/30/2014 | Occurred? Yes | No | Quantity (kg/day) | 2.33 | 7.074 | 0.11 | 30 | 155 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -4.744 | 0 | -142.32 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

7/31/2014 | Occurred? No | No | Quantity (kg/day) | 7.074 | 0 | 31 | No Load Calculated | |||||

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

8/31/2014 | Occurred? No | No | Quantity (kg/day) | 7.074 | 0 | 31 | No Load Calculated | |||||

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

9/30/2014 | Occurred? No | No | Quantity (kg/day) | 7.074 | 0 | 30 | No Load Calculated | |||||

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

10/31/2014 | Occurred? Yes | No | Quantity (kg/day) | 0.71 | 21.2 | 0.17 | 31 | 48.67 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -20.49 | 0 | -635.19 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

Annual Load from Monitoring Data (lb/yr) | 1,184.67 | Annual Load over Limit (lb/yr) | 0 | -2,672.66 |

Monitoring Period | Discharge Information | Below Detection Limit? | Measurement Type | Avg Daily Value | Limit | Wastewater Flow (MGD) | Number of Days | Monitoring Period Load (lb/period) | Load Equation | Daily Load Over Limit (lb/day)^{1} | LOL Option 1 (lb)^{2} | LOL Option 2 (lb)^{2} |
---|---|---|---|---|---|---|---|---|---|---|---|---|

1/31/2014 | Occurred? Yes | No | Quantity (kg/day) | 0.72 | 2.22 | 0.076 | 31 | 49.6 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -1.5 | 0 | -46.5 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

2/28/2014 | Occurred? Yes | No | Quantity (kg/day) | 1.67 | 2.22 | 0.11 | 28 | 104 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -0.55 | 0 | -15.4 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

3/31/2014 | Occurred? Yes | No | Quantity (kg/day) | 0.72 | 2.22 | 0.11 | 31 | 49.6 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -1.5 | 0 | -46.5 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

4/30/2014 | Occurred? Yes | No | Quantity (kg/day) | 0.86 | 2.22 | 0.0906 | 30 | 57 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -1.36 | 0 | -40.8 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

5/31/2014 | Occurred? Yes | No | Quantity (kg/day) | 0.68 | 2.22 | 0.103 | 31 | 46.5 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -1.54 | 0 | -47.74 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

6/30/2014 | Occurred? Yes | No | Quantity (kg/day) | 0.72 | 2.22 | 0.101 | 30 | 48 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -1.5 | 0 | -45 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

7/31/2014 | Occurred? Yes | No | Quantity (kg/day) | 0.81 | 2.22 | 0.091 | 31 | 55.8 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -1.41 | 0 | -43.71 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

8/31/2014 | Occurred? Yes | No | Quantity (kg/day) | 1.088 | 2.22 | 0.102 | 31 | 74.4 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -1.132 | 0 | -35.092 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

9/30/2014 | Occurred? Yes | No | Quantity (kg/day) | 3.038 | 2.22 | 0.1004 | 30 | 201 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | 0.818 | 24.54 | 24.54 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

10/31/2014 | Occurred? Yes | No | Quantity (kg/day) | 0.77 | 2.22 | 0.078 | 31 | 52.7 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -1.45 | 0 | -44.95 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

11/30/2014 | Occurred? Yes | No | Quantity (kg/day) | 0.77 | 2.22 | 0.093 | 30 | 51 | Daily Load (kg/day) * Number of Days * 2.205 (lb/kg) | -1.45 | 0 | -43.5 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

Annual Load from Monitoring Data (lb/yr) | 789 | Annual Load over Limit (lb/yr) | 24.54 | -384.652 |

Monitoring Period | Discharge Information | Below Detection Limit? | Measurement Type | Avg Daily Value | Limit | Wastewater Flow (MGD) | Number of Days | Monitoring Period Load (lb/period) | Load Equation | Daily Load Over Limit (lb/day)^{1} | LOL Option 1 (lb)^{2} | LOL Option 2 (lb)^{2} |
---|---|---|---|---|---|---|---|---|---|---|---|---|

1/31/2014 | Occurred? No | No | Concentration (mg/L) | 6.2 | 0 | 31 | No Load Calculated | |||||

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

2/28/2014 | Occurred? No | No | Concentration (mg/L) | 6.2 | 0 | 28 | No Load Calculated | |||||

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

3/31/2014 | Occurred? Yes | No | Concentration (mg/L) | 6.3 | 6.2 | 30 | 31 | 48,899 | Concentration (mg/L) * Flow (MGD) * 3.785 (L/gal) * Number of Days * 2.205 (lb/kg) | 25 | 776 | 776 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

4/30/2014 | Occurred? No | No | Concentration (mg/L) | 6.2 | 0 | 30 | No Load Calculated | |||||

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

5/31/2014 | Occurred? Yes | No | Concentration (mg/L) | 6.8 | 6.2 | 32 | 31 | 56,298 | Concentration (mg/L) * Flow (MGD) * 3.785 (L/gal) * Number of Days * 2.205 (lb/kg) | 160 | 4,967 | 4,967 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

6/30/2014 | Occurred? Yes | No | Concentration (mg/L) | 7.2 | 6.2 | 29 | 30 | 52,279 | Concentration (mg/L) * Flow (MGD) * 3.785 (L/gal) * Number of Days * 2.205 (lb/kg) | 242 | 7,261 | 7,261 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

7/31/2014 | Occurred? No | No | Concentration (mg/L) | 6.2 | 0 | 31 | No Load Calculated | |||||

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

8/31/2014 | Occurred? No | No | Concentration (mg/L) | 6.2 | 0 | 31 | No Load Calculated | |||||

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

9/30/2014 | Occurred? Yes | No | Concentration (mg/L) | 8.3 | 6.2 | 25 | 30 | 51,953 | Concentration (mg/L) * Flow (MGD) * 3.785 (L/gal) * Number of Days * 2.205 (lb/kg) | 438 | 13,145 | 13,145 |

Reported? Yes | ||||||||||||

Estimated? No | ||||||||||||

Annual Load from Monitoring Data (lb/yr) | 209,429 | Annual Load over Limit (lb/yr) | 26,149 | 26,149 |

1 Daily Load Over Limit (lb/day) = Average Daily Value - Limit Value

2 Load Over Limit = Daily Load Over Limit * Number of Days