| (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.
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