DWR-Overview-HRL-chla-criteria

1

Overview of High Rock Lake (HRL) Chlorophyll a Site-Specific Standard
Proposal and Assessment Methodology Recommendation

N.C. Division of Water Resources

Introduction
After a multiyear evaluation process conducted in accordance with North Carolina’s Nutrient Criteria
Development Plan (NCDP), the N.C. Division of Water Resources (DWR) is proposing a site-specific
chlorophyll a standard for High Rock Lake (HRL) for adoption by the Environmental Management
Commission (EMC).
The NCDP was mutually agreed upon by North Carolina and the U.S. Environmental Protection Agency
(EPA) in 2014 and was renewed in 2019 with minor revisions. The plan commits North Carolina to
evaluate site-specific nutrient-related criteria for three pilot water bodies, each representing a distinct
water body type. Those pilot water bodies include High Rock Lake (lake), Albemarle Sound (estuary),
and the Middle Cape Fear river system (river and streams). Based upon lessons learned from these site-
specific evaluations, North Carolina will be better positioned to reevaluate nutrient-related criteria
statewide.
The NCDP also established two advisory bodies to assist with criteria development. The Scientific
Advisory Council (SAC) comprises experts in the fields of water quality, water quality engineering,
nutrient biogeochemistry, nutrient response variables, nutrient management and point and non-point
source nutrient abatement. The Criteria Implementation Committee (CIC) advises on the social and
economic implications of implementing proposed nutrient criteria to inform and assist DWR with fiscal
note preparation.
The SAC reviewed several nutrient-related parameters to determine if changes were warranted or if
criteria for new parameters should be adopted into standards. For High Rock Lake the parameters
reviewed included dissolved oxygen, clarity, algal assemblages, pH, cyanotoxins, chlorophyll a, nitrogen,
and phosphorus. No new criterion parameters were recommended, and the only standard
recommended for amendment was chlorophyll a.
The chlorophyll a site-specific standard proposal and DWR’s recommendations for an associated
assessment methodology, contained herein, reflect a combination of the SAC’s recommendations to
DWR, CIC input, and the expertise of DWR staff.
High Rock Lake Chlorophyll a Criterion Proposal
Overview
The Scientific Advisory Council began its work to evaluate site-specific criteria for High Rock Lake in
2015, ultimately concluding its recommendations in a report published in May 2020 (Appendix I). DWR
staff reviewed the SAC’s recommendation, considered all components brought forward by the SAC, and
has proposed a scientifically-based site-specific chlorophyll a standard for High Rock Lake. The SAC’s
report provides detailed justification for the necessary components of a water quality standard.

2

DWR has carried forward the SAC’s chlorophyll a standard proposal with a magnitude of 35 ug/L and a
seasonal duration, calculated as a geometric mean (or geomean). The DWR recommendation includes
the spatial extent and depth to which the site-specific standard would apply within a waterbody and
identifies the waterbodies to which the proposed site-specific standard would be applicable, as is
required. The proposed language, to amend 15A NCAC 02B .0211(4), is as follows:
(4) Chlorophyll a (corrected): except as specified in Sub-Item (a) of this Item, not greater than 40
ug/l for lakes, reservoirs, and other waters subject to growths of macroscopic or microscopic
vegetation not designated as trout waters, and not greater than 15 ug/l for lakes, reservoirs, and
other waters subject to growths of macroscopic or microscopic vegetation designated as trout
waters (not applicable to lakes or reservoirs less than 10 acres in surface area). The Commission or
its designee may prohibit or limit any discharge of waste into surface waters if the surface waters
experience or the discharge would result in growths of microscopic or macroscopic vegetation such
that the standards established pursuant to this Rule would be violated or the intended best usage of
the waters would be impaired;
(a) Site-specific High Rock Lake Reservoir [Index Numbers 12-(108.5), 12-(114), 12-117-(1), 12-
117-(3), and 12-118.5] Chlorophyll a (corrected): not greater than a growing season geometric
mean of 35 ug/L in the photic zone based on samples collected in a minimum of five different
months during the growing season. For the purpose of this Sub-Item, the growing season is April
1 through October 31 and the photic zone is represented by a composite sample taken from the
water surface down to twice the measured Secchi depth. Chlorophyll a shall not occur in
amounts that result in an adverse impact as defined in 15A NCAC 02H .1002.1
DWR’s recommendations adopt the SAC’s recommendation with some pragmatic adjustments based on
CIC and DWR input. Specifically, DWR has recommended refinements regarding three issues:
supplemental narrative standard language, applicability of the proposed standard in shallow waters, and
an exceedance frequency. Table 1 below provides a comparison of SAC and DWR recommendations.

1 Adverse impact, as defined in 15A NCAC 02H .1002, means a detrimental effect upon water quality or best
usages, including a violation of water quality standards, caused by or contributed to by a discharge or loading of a
pollutant or pollutants.

3

Table 1: SAC and DWR chlorophyll a criterion and assessment recommendations.
Component

SAC
Recommendation
SAC Notes on Selection DWR
Recommendation
DWR Notes on Selection
Magnitude 35 ug/L Selected from a range
of chlorophyll a
concentrations deemed
to be protective of HRL
designated uses.
Same Selection of 35 ug/L derived
by SAC from a station by
station analysis deemed to be
protective of HRL designated
uses.
Period/
Duration
Seasonal
Geomean
Calculated geomean
based on all data from
growing season.
Same None
Growing
Season/
Duration
April-October Include samples
collected in at least five
different growing
season months for
each year of data
included in the
analysis.
Same None
Frequency Maximum
Exceedance
Frequency of one-
in-three years
Compute the geometric
mean for each year of
individual data and
apply a frequency
component of not
more than one
exceedance out of
three years of data.
The frequency
component is a
“shall not exceed”
value. A
minimum dataset
of two seasonal
geometric means
is needed for
assessment
purposes.
Acknowledges year-to-year
variability in chlorophyll a
concentrations and the need
for more than one year of
exceedance before making an
assessment decision.
Requiring consistent results
based on data from two or
more growing seasons for
listing or delisting will provide
greater certainty that the
“shall not exceed” standard is
indeed impaired or no longer
impaired, respectively.
Spatial
Considerations
Open Waters Photic zone composite
based on twice the
Secchi depth; shallow
waters and isolated
coves exempt from
numeric criteria; all
data within each
assessment unit would
be incorporated into
the calculated
geomean.
All waters within
the associated
index numbers
(12-(108.5), 12-
(114), 12-117-(1),
12-117-(3), and
12-118.5).
Same as SAC
recommendation except no
shallow water/isolated coves
exemption. Station by station
assessment consistent with
methodology used to develop
the 35 ug/L geometric mean.

4

Narrative Criterion
The SAC recommended the use of a narrative criterion for shallow waters and isolated coves but did not
offer specific narrative language. DWR agrees that a narrative component of the criterion is appropriate
and proposes to supplement the quantitative site-specific, seasonal standard with the following
narrative language: “Chlorophyll a shall not occur in amounts that result in an adverse impact as defined
in 15A NCAC 02H .1002.” The DWR recommendation makes reference to “adverse impact” because this
term has been previously adopted by the EMC in its rules and has been approved by the Rules Review
Commission. As noted above, adverse impact is defined in rule as a detrimental effect upon water
quality or best usages, including a violation of water quality standards, caused by or contributed to by a
discharge or loading of a pollutant or pollutants.
DWR’s supplemental narrative component recommendation provides protection to any shallow waters
and isolated coves, seasons, or instances not covered by the site-specific, seasonal geomean. This is
consistent with the SAC’s recommendation for shallow waters to be addressed by narrative criteria.
Shallow Water Areas
The SAC recommended the categorical exclusion of shallow waters and isolated coves from the
proposed numeric criterion, with a parenthetical suggestion of all waters less than ten feet deep. This
recommendation was included in the final SAC report but was not discussed as part of the 2018 meeting
during which the SAC voted upon its criteria recommendation. DWR does not recommend incorporating
this exclusion for several reasons.
First, reducing coverage of numeric nutrient-related standards in state waters is not the NCDP’s goal.
The NCDP’s purpose is to refine and expand the use of numeric standards to address nutrient issues, not
to reduce Clean Water Act (CWA) protections. If the recommended site-specific criterion including the
narrative component were not to apply to all waters under consideration, then the existing standard in
15A NCAC 02B .0211(4) would apply to any waters not subject to a site-specific standard. Shallow
waters are often the very places in need of numeric standards, particularly for recreational and fishing
uses.
Second, the definition of shallow waters as being less than 10 feet deep (3.0 m) raises substantial
pragmatic and operational issues. Water levels in High Rock Lake fluctuate 10 feet or more, making the
application of this limitation uncertain in relation to many fixed sampling locations.
Third, the Monte Carlo analysis used to derive this site-specific magnitude recommendation did not
exclude data based on depth. Of four monitoring stations chosen for that analysis, station HRL051 was
included to represent riverine waters despite being well below the recommended ten-foot depth
threshold. The SAC noted that “waters at HRL051 reflect turbid river conditions, and the average
chlorophyll a is lower than in downstream waters.”2 Thus, the analysis, at least in part, supports applying
the derived site-specific criterion in shallower waters.
While not incorporating the exclusion of shallow waters from the application of this criterion, DWR
understands the underlying concern that segments may be CWA Section 303(d) listed as impaired based

2 N.C. Nutrient Criteria Scientific Advisory Council, page 65. A Chlorophyll a Criterion for High Rock Lake. May 26,
2020.

5

on nonrepresentative sampling. Current monitoring, quality assurance and transparency protocols are
sufficient to ensure representative sampling in High Rock Lake.
DWR recommends that any new monitoring efforts in High Rock Lake, whether by DWR or by third
parties, comply with the following existing protocols:
• Photic zone composite and boat- or bridge-based sampling, which provide natural access and
depth limitations
• DWR or third-party compliance with the DWR Ambient Lakes Monitoring Program Quality
Assurance Project Plan, which provides:
“Actual sampling points are generally located within the center or main-stem of the lake, or
as determined by field staff as representative of the lake or specific areas of concern within
the lake.”
• Submission of third-party data for public review during biannual integrated reports and
associated Quality Assurance Protocol Procedures requirements
These safeguards ensure nonrepresentative shallow water samples are not used for assessment
purposes in High Rock Lake or statewide.
Assessment Frequency
DWR’s proposed 303(d) assessment methodology implements the SAC’s premise that data included in
the assessment be collected in two or more years to incorporate year-to-year variability in chlorophyll a
concentrations. The SAC noted that “Limited available data with which to assess compliance with a
seasonal geomean criterion for chlorophyll a presents an obvious challenge to considering a frequency
component to the standard. The most common frequencies used by states are instantaneous or a
frequency based on some limited number of exceedances...”3
The SAC recommended a “greater than 1 in 3”4 approach for assessment purposes to implement the
recommendation of a seasonal geomean for chlorophyll a in High Rock Lake. However, the SAC
provided no justification to support the recommendation other than stating that some other states
apply the “greater than 1 in 3” approach to non-toxic parameters including seasonally averaged
chlorophyll a. Also, the SAC did not address delisting recommendations, which is a required component
of an assessment methodology.
DWR’s proposed assessment methodology represents the SAC’s proposal in a practical and
implementable format. The intent behind the SAC’s “greater than 1 in 3” proposal for impairment
determination is to verify that there is the persistence of a problem by capturing variability. DWR
incorporated this concept into the proposed assessment methodology, but also ensured the assessment
approach balanced impairment listing as well as delisting decisions.

3 N.C. Nutrient Criteria Scientific Advisory Council, page 61. A Chlorophyll a Criterion for High Rock Lake. May 26,
2020.

4 The “greater than 1 in 3” approach means that a water body would be considered impaired if there were greater
than 1 in 3 exceedances of the seasonal geomean.

6

DWR’s proposed assessment methodology, as described below, requires at least two years of seasonal
geometric means for decision-making and is designed to ensure that the proposed site-specific standard
protects the lake’s designated uses. The requirement of two seasonal geometric means recognizes
concerns with data variability and allows for defensible delisting as well as listing decisions. Impairment
determinations will require at least two years exceeding the seasonal geometric mean within an
assessment period. Conversely, delisting or meeting criteria determinations will require at least two
years meeting the seasonal geometric mean within an assessment period.
An additional concern with the SAC’s recommendation of an impairment listing frequency of “greater
than 1 in 3” is that it is inconsistent with DWR’s current monitoring resources for High Rock Lake and
does not provide a pathway to assess water quality in High Rock Lake without additional resources, a
point that was also made by the CIC. DWR’s assessment window is five years. The SAC did not explicitly
address how “greater than 1 in 3” can apply to a five-year data window with existing resources. DWR’s
proposed assessment methodology is flexible enough to apply to the current monitoring schedule, but
also provides an assessment pathway when there is more frequent data collection.
In summary, DWR’s proposed 303(d) assessment methodology implements the SAC’s recommended
seasonal average geometric mean approach. DWR has drafted an assessment approach that balances
listing and delisting decisions and incorporates the SAC’s premise that more than one year of data is
needed for regulatory decision making for assessment purposes.
High Rock Lake Chlorophyll a Assessment Methodology Recommendation
DWR is not proposing a change to the assessment methodology as part of the site-specific standard
adoption process. After the adoption of a site-specific standard for High Rock Lake, DWR will
incorporate the complementary site-specific assessment methodology into the state’s comprehensive
303(d) listing and delisting assessment methodology for EMC approval.
However, there are several reasons to preview assessment recommendations in this document. First, it
clarifies assumptions necessary to conduct a Regulatory Impact Analysis and therefore to assess
potential fiscal impacts. Second, review of the assessment methodology ensures that the recommended
standard can be practically and functionally assessed, once adopted. Finally, this document will serve as
the reference framework to explain the necessity of the proposed methodology and the intended
implementation of the proposed High Rock Lake chlorophyll a site-specific standard.
DWR recommends the following assessment method to assess the proposed site-specific chlorophyll a
criterion:
Minimum Data Requirements
o Growing season geomean calculation requires a minimum of 5 samples per growing
season, collected during 5 separate months.
o At least 2 full growing seasons are needed to make listing or delisting decision. Data can
be augmented if there is only 1 growing season in current data window. To augment,
step year by year back until there are a total of 2 years of geomeans including the
current data window, only as far as previous 5 years.

7

List for Impairment – at least 2 years of data needed to confirm Exceedance of Criteria
o If there is 1 growing season geomean in current data window – both current and
augmented year exceed growing season geomean of 35 ug/L.
o If there are 2 or more growing season geomeans in current data window – more
than 1 growing season geomean exceeds 35 ug/L.
Delist for Impairment – at least 2 years of data needed to confirm Meeting Criteria
o If there is 1 growing season geomean in current data window – both current and
augmented year do not exceed growing season geomean of 35 ug/L.
o If there are 2 or more growing season geomeans in current data window – zero years
exceed growing season geomean of 35 ug/L. Unless there is a full 5 years of data – then
zero exceedances in most recent 2 years of data (and maximum of one exceedance of
geomean in 3 older years).
In order to be considered “meeting criteria,” there can be no exceedances in two out of two years (the
smallest dataset scenario) and no exceedances in the two most recent years, assuming no more than
one exceedance in the first three years (the largest dataset scenario). In summary, for assessment
purposes, when a minimum dataset of two growing seasons are available, two or more exceedances
result in an impairment decision, one exceedance is considered data inconclusive, and zero exceedances
means the standard is being met.