www.pca.state.mn.us
Streamlined variance approach to chloride toxicity
levels in water quality standards
New chloride water quality standards may require some municipalities to
consider a variance as a strategy in National Pollutant Discharge
Elimination System permit compliance
Since 2013 the Minnesota Pollution Control Agency (MPCA) has received requests from a number of entities to
revise the existing chloride water quality standard (WQS) to protect aquatic life. To be responsive to requests for
a revised chloride WQS that is based on up-to-date science, the MPCA requested assistance from U.S.
Environmental Protection Agency (EPA) in evaluating data for toxicity tests with chloride. In a related effort
MPCA has developed a streamlined approach to considering variance requests from National Pollutant
Discharge Elimination System/State Disposal System permittees based on chloride water quality standards. This
document describes the proposed path forward for revising the existing chloride standard, the streamlined
variance process as well plans to communicate the work with interested parties.
Approach for developing a revised chloride WQS
Ongoing work to revise Minnesota’s WQS for chloride is ongoing with consideration of the data supporting the
2009 Iowa chloride standard and any new information about the toxicity of chloride to aquatic organisms. Since
2009, additional toxicity tests have been completed that are important to Minnesota’s current evaluation of the
chloride WQS including those that help our understanding of the relationship of chloride toxicity and water
hardness.
Water hardness is an important water chemistry parameter and is strongly correlated to the toxicity of chloride
to aquatic organisms. In efforts to model this
effect, toxicity tests using exposures from a
range of hardness and chloride concentrations
are examined using regression techniques to
develop an equation to calculate aquatic life
chloride criteria values. Sulfate toxicity, another
prevalent ion in water, is also being considered
for inclusion in this revised approach.
For this summary, the MPCA is working with all
available toxicity data to provide a snapshot of
the approach that will be used to revise the
existing chloride WQS. Examples of the range of
chronic and acute criteria values that may result
using the toxicity model being considered are
shown in Figures 1 and 2 and Table 1. These
examples show two scenarios for considering
water hardness using toxicity models to calculate
prospective acute (Figure 1) and chronic
(Figure 2) criteria values. The MPCA anticipates
drafting a technical support document by early
summer 2016, with a Request for Comments published summer 2016.
Minnesota Pollution Control Agency
651-296-6300 | 800-657-3864 or use your preferred relay service | [email protected]
March 2016 | wq-wwprm7-65
Available in alternative formats
Acute Chloride Criterion (mg/L)
900
860 mg/L Acute
criterion
800
700
600
Example 1
500
Example 2
400
860 mg/L Acute
Criterion
300
0
100
200
300
400
Hardness
Figure 1. Estimated acute chloride criteria values for differing hardness levels. Both examples use the most current
dataset for chloride, but the model for factoring in the hardness relationship to chloride toxicity differs.
Chronic Chloride Criterion (mg/L)
250
230
210
190
170
150
Example 1
130
Example 2
110
230 mg/L
Chronic Criterion
90
70
50
0
100
200
300
400
Hardness
Figure 2. Estimated chronic chloride criteria values for differing hardness levels. Both examples use the most current
toxicity dataset for chloride, but the model for factoring in the hardness relationship to chloride toxicity differs.
Table 1. Range of estimated chloride acute and chronic criteria values for differing hardness levels, using two models for
factoring in the hardness relationship to chloride toxicity.
Hardness
Corresponding estimated
Corresponding estimated
Acute Criteria Values (mg Chloride/L)
Chronic Criteria Values (mg Chloride/L)
50
407 – 430
119 – 126
100
496
145
200
572 – 605
167 – 177
300
622 – 679
182 – 199
400
660 – 737
193 – 216
Page 2 of 3
March 2016 | wq-wwprm7-65
Streamlined approach to variances from chloride WQS
Traditionally, it is the role of the permittee to develop a variance application and produce information justifying
the need for a variance, which typically requires hiring an outside party to prepare the necessary documents and
navigate the regulatory process. For some permittees, the variance application process can be burdensome and
confusing. Due to the particular demands of the chloride water quality standard, the MPCA would like to provide
as much assistance as possible to the permittees affected by the enforcement of the chloride water quality
standard by using a combination of compliance schedules and a streamlined variance process. This approach will
advance the goals of achieving the highest attainable conditions by reducing chloride toxicity.
The streamlined variance process will allow a permittee to use a calculator created by the MPCA to easily
determine whether they are eligible for a variance and simplify the application process so that a consultant is
not needed. The streamlined variance process is designed to comply with all MPCA and EPA variance
regulations; using the streamlined process does not make a variance “easier” to receive, the process simply
makes the variance application and justification more user-friendly. For example, some publicly available data,
such as median income in the municipality, are already collected and incorporated into the tool. The MPCA is
committed to assisting permittees applying for a variance from the chloride water quality standard using this
process. All water quality variances are temporary, subject to public review, and must receive final approval by
the MPCA and EPA.
Page 3 of 3
March 2016 | wq-wwprm7-65