Current through Reg. 49, No. 12; March 22, 2024
(a) Flow
conditions.
(1) The following standards do
not apply below critical low-flows:
(A)
site-specific criteria for dissolved oxygen, pH, temperature, and numerical
chronic criteria for toxic materials, as listed in Appendices A, D, and E of
§
307.10 of this
title (relating to Appendices A - G);
(B) numerical chronic criteria for toxic
materials as established in §
307.6
of this title (relating to Toxic Materials);
(C) total chronic toxicity restrictions as
established in §
307.6
of this title;
(D) maximum
temperature differentials as established in §
307.4(f)
of this title (relating to General Criteria); and
(E) dissolved oxygen criteria for
unclassified waters, as established in §
307.4(h)
of this title and §
307.7(b)(3)
of this title (relating to Site-Specific Uses and Criteria).
(2) Critical low-flows for streams
or rivers that are dominated by springflow are listed in the standards
implementation procedures as amended and are calculated as follows:
(A) for springflow-dominated streams or
rivers that contain federally listed endangered or threatened aquatic or
aquatic dependent species, the critical low-flow value is the 0.1 percentile
value derived from a lognormal distribution for the period of record at the
nearest United States Geological Survey (USGS) or International Boundary and
Water Commission (IBWC) gauging station;
(B) for springflow-dominated streams or
rivers that do not contain federally listed endangered or threatened species,
the critical low-flow value is the 5th percentile value of the flow data for
the period of record at the nearest USGS or IBWC gauging station.
(3) Numerical acute criteria for
toxic materials and preclusion of total acute toxicity as established in §
307.6
of this title are applicable at stream flows that are equal to or greater than
one-fourth of critical low-flows.
(4) Harmonic mean flow is the applicable
upstream flow when calculating wastewater permit limits for criteria that are
assessed as long-term means, such as criteria for total dissolved solids,
chloride, sulfate in Appendix A of §
307.10 of this
title, and human health toxic criteria in Table 2 of §
307.6(d)(1)
of this title. These criteria are applicable at all flow conditions except as
specified for the applicability of assessment data in §
307.9
of this title (relating to Determination of Standards and
Attainment).
(5) Critical low-flows
and harmonic mean flows for some classified segments are listed in the
standards implementation procedures as amended. These critical low-flows are
not for the purpose of regulating flows in water bodies in any manner or
requiring that minimum flows be maintained in classified segments.
(6) Critical low-flows and harmonic mean
flows listed in the standards implementation procedures as amended apply only
to river basin and coastal basin waters. They do not apply to bay waters, gulf
waters, reservoirs, or estuaries.
(7) Critical low-flows and harmonic mean
flows in the standards implementation procedures as amended were calculated
from historical USGS or IBWC daily streamflow records. If the calculated
critical low-flow or harmonic mean flow value was equal to or less than 0.1
cubic foot per second (cfs), it was rounded up to 0.1 cfs.
(8) Flow values are periodically recomputed
to reflect alterations in the hydrologic characteristics of a segment,
including reservoir construction, climatological trends, and other
phenomena.
(9) The general criteria
are applicable at all flow conditions except as specified in this section or in
§
307.4
of this title.
(b)
Mixing zones. A reasonable mixing zone is allowed at the discharge point of
permitted discharges into surface water in the state, in accordance with the
following provisions.
(1) The following
portions of the standards do not apply within mixing zones:
(A) site-specific criteria, as defined in
§
307.7
of this title and listed in Appendices A, D, E, F, and G of §
307.10 of this
title;
(B) numerical chronic
aquatic life criteria for toxic materials as established in §
307.6
of this title;
(C) total chronic
toxicity restrictions as established in §
307.6
of this title;
(D) maximum
temperature differentials as established in §
307.4(f)
of this title;
(E) dissolved oxygen
criteria for unclassified waters, as established in §
307.4(h)
of this title;
(F) dissolved oxygen
criteria for intermittent streams, as established in §
307.4(h)(4)
of this title;
(G) aquatic
recreation criteria for unclassified waters, as established in §
307.4(j)
of this title and in §
307.7(b)(1)
of this title;
(H) specific human
health criteria for concentrations in water to prevent contamination of
drinking water, fish and shellfish so as to ensure safety for human
consumption, as established in §
307.6
of this title.
(2)
Numerical acute aquatic life criteria for toxic materials and preclusion of
total acute toxicity as established in §
307.6
of this title are applicable in mixing zones. Acute criteria and acute total
toxicity levels may be exceeded in small zones of initial dilution (ZIDs) at
discharge points of permitted discharges, but there must be no lethality to
aquatic organisms that move through a ZID. ZIDs must not exceed the following
sizes:
(A) 60 feet downstream and 20 feet
upstream from a discharge point in a stream and river. In addition, ZIDs in
streams and rivers must not encompass more than 25% of the volume of stream
flow at or above seven-day, two-year low-flow conditions;
(B) a 25-foot radius in all directions (or
equivalent volume or area for diffuser systems) from a discharge point in a
lake or reservoir; and
(C) a
50-foot radius in all directions (or equivalent volume or area for diffuser
systems) from a discharge point in a bay, a tidal river, an estuary, or the
Gulf of Mexico.
(3)
Provisions of the general criteria in §
307.4
of this title remain in effect in mixing zones unless specifically exempted in
this section.
(4) Water quality
standards do not apply to treated effluent at the immediate point of discharge
prior to any contact with either ambient waters or a dry streambed. However,
effluent total toxicity requirements may be specified to preclude acute
lethality near discharge points, or to preclude acute and chronic instream
toxicity.
(5) Where a mixing zone
is defined in a valid permit of the Texas Commission on Environmental Quality,
the Railroad Commission of Texas, or the United States Environmental Protection
Agency, the mixing zone defined in the permit must apply.
(6) Mixing zones must not preclude passage of
free-swimming or drifting aquatic organisms to the extent that aquatic life use
is significantly affected, in accordance with guidelines specified in the
standards implementation procedures as amended.
(7) Mixing zones must not overlap unless it
can be demonstrated that no applicable standards will be violated in the area
of overlap. Existing and designated uses must not be impaired by the combined
impact of a series of contiguous mixing zones.
(8) Mixing zones must not encompass an intake
for a domestic drinking water supply. Thermal mixing zones are excepted from
this provision unless elevated temperatures adversely affect drinking water
treatment.
(9) Mixing zones must be
individually specified for all permitted domestic discharges with a permitted
monthly average flow equal to or exceeding one million gallons per day and for
all permitted industrial discharges to water in the state (excepting discharges
that consist entirely of stormwater runoff). For domestic discharges with
permitted monthly average flows less than one million gallons per day, a small
mixing zone must be assumed in accordance with guidelines for mixing zone sizes
specified in the standards implementation procedures as amended; and the
commission may require specified mixing zones as appropriate.
(10) Different mixing zone sizes for specific
numeric criteria, such as for the protection of human health, aquatic life, and
temperature, may be specified in a wastewater permit.
(c) Minimum analytical levels. The specified
definition of permit compliance for a specific toxic material must not be lower
than established minimum analytical levels, unless that toxic material is of
particular concern in the receiving waters, or unless an effluent specific
method detection limit has been developed in accordance with 40 Code of Federal
Regulations Part 136. Minimum analytical levels are listed in the standards
implementation procedures as amended.
(d) Once-through cooling water discharges.
When a discharge of once-through cooling water does not measurably alter intake
concentrations of a pollutant, then water-quality based effluent limits for
that pollutant are not required. For facilities that intake and discharge
cooling water into different water bodies, this provision only applies if water
quality and applicable water quality standards in the receiving water are
maintained and protected.
(e)
Stormwater discharges. Pollution in stormwater must not impair existing or
designated uses. Controls on the quality of stormwater discharges must be based
on best management practices, technology-based limits, or both in combination
with instream monitoring to assess standards attainment and to determine if
additional controls on stormwater quality are needed. The standards
implementation procedures as amended describe how water quality standards are
applied to Texas Pollutant Discharge Elimination System stormwater discharges.
The evaluation of instream monitoring data for standards attainment includes
the effects of stormwater, as described in §
307.9
of this title.