Iowa’s streams, rivers, lakes, and wetlands are required to be protected for beneficial uses such as swimming, fishing, and livestock watering, etc. All facilities that discharge pollutants from any point source into surface waters are required to obtain an NPDES (National Pollutant Discharge Elimination System) or operation permit from DNR. The permits require compliance with all federal standards, state rules, and state administrative code. A wasteload allocation (WLA) is the portion of a receiving water’s assimilative capacity that is allocated to one of its existing or future point sources of pollution. WLAs ensure that the water quality-based effluent limits for permitted point source discharges will be protective of the designated uses of the waterbodies.
Why is a WLA needed?
A WLA serves several purposes. A WLA is needed to ensure that NPDES permit limits will be protective of the water quality criteria that support the designated uses of the receiving waterbody. WLAs can also be used to establish design requirements for new or upgraded wastewater treatment plants. Water quality criteria are updated from time to time as new scientific information becomes available. As a result, water quality-based effluent limits also need to be updated from time to time. Additionally, an antidegradation alternatives analysis includes the WLA for the selected alternative.
How to Request a WLA:
To request a WLA, please fill in the WLA request form and send the WLA requests electronically to WLArequest@dnr.iowa.gov.
WLA Request Form and Instructions
WLA Request Form and Instructions
If multiple treatment and/or outfall location scenarios are being considered for a facility, please use the Ammonia Tool to calculate preliminary ammonia limits and minimize the number of formal WLAs requested*:
To calculate how a mixing zone study and/or a site-specific chemistry study would impact a facility's preliminary chloride, sulfate, and copper limits, please use the Chloride, Sulfate, and Copper (CSC) Tool*:
CSC Tool and Instructions
How is a WLA calculated?
The establishment of a WLA for a particular discharge is dependent on the outfall location of the facility, stream critical low flows, the available dilution or mixing zone, water quality criteria, discharge flows and background conditions of the receiving water. Default mixing zones and water chemistry are used to calculate WLAs. Facilities have the option to collect site-specific mixing zone data and/or water chemistry data to obtain site-specific water quality based effluent limits. The mixing zone and water chemistry study procedures can be found in the Wasteload Allocation Procedure Document shown below. If a dye is used in the site-specific mixing zone study, an authorization from DNR is required to use the dye if the dye has the potential to reach a Water of the United States. Please contact Ian Willard at 515-954-6450 or ian.willard@dnr.iowa.gov for more information.
The current rule-reference document that outlines WLA methodology is as follows:
Wasteload Allocation Procedure Document (November 2020)
Please note: The only change from the Wasteload Allocation Procedure Document (February 2018) was the removal of the sentence “Iowa’s numerical chemical criteria are expressed in total recoverable concentrations” on page 77. This change ensured consistency with the metals aquatic life water quality criteria rulemaking (effective November 11, 2020).
DNR Contact:
Ian Willard
515-954-6450
ian.willard@dnr.iowa.gov
For WLA request submission: WLArequest@dnr.iowa.gov