Watershed Protection Plan - Lower Rio Grande Valley Development ...

STATE OF THE WATERSHED
in the Arroyo Colorado (APAI 2006).
Figure 20 shows TSS concentrations at stations
located upstream and downstream of the tidal/non-tidal
boundary. Based on samples collected between 1990
and 2006, the average concentration of TSS at the
downstream end of the non-tidal portion of the Arroyo
Colorado (just upstream of the Zone of Impairment)
is 147 mg/l. The average concentration of TSS at
a station located in the tidal portion of the Arroyo
Colorado (at the Zone of Impairment) is 27 mg/l (Figure
20). Although occasionally high (80-90 mg/l), TSS in
the Tidal Segment of the Arroyo Colorado is generally
much lower than in the Above Tidal Segment due to
the change in stream velocity that occurs at the tidal
boundary. As the freshwater from the Arroyo Colorado
Above Tidal enters the Port of Harlingen, it slows down,
causing sediment to fall out of suspension. Most of the
sediment lost at the beginning of the tidal segment is
thought to be deposited at or near the Port of Harlingen.
Suspended sediment is thought to play an important
role in oxygen depletion and nutrient cycling in this
portion of the Arroyo Colorado. The TSS data shown in
Figure 20 shows no apparent trend with time or season
of the year.
Escherichia coli
Fecal Bacteria
Fecal coliform bacteria are biological indicators of
infectious disease. They are present in the intestines of
many animals including humans and, although many of
these bacteria do not typically cause illness in humans,
their presence in water indicates the possibility that
other disease-causing microbes could also be present.
Escherichia coli (E. coli) is a type of fecal coliform
that is thought to be more specifi cally linked to human
and animal waste. The presence and abundance of
E. coli in water is an important measure of the impact
human and animal waste may have on a water body. In
marine water bodies, Enterococcus bacteria are used to
measure the disease potential. A water quality criterion
for individual E.coli samples has been set by the State
of Texas at a maximum 394 colony-forming units per
one hundred milliliters (cfu/100ml) and 89 (#/100ml)
for Enterococcus to be protective of swimmers. In
preparing the 2004 Texas Water Quality Inventory, if the
screening level was exceeded by more than 25% of the
samples collected in the assessment period; the water
body was listed as impaired by the TCEQ.
Figure 21. shows E. coli and Enterococcus
concentrations at water quality stations located
upstream (13074) and downstream (13072) of the
Tidal Segment boundary of the Arroyo Colorado from
2001 to 2006. E. coli concentrations in the Above Tidal
Segment of the Arroyo Colorado exceed the criteria
more frequently than in the Tidal Segment. The Above
Tidal Segment of the Arroyo Colorado is currently listed
on the 2004 Texas 303(d) list for elevated levels of E.
coli.
Legacy Pollutants
The term legacy pollutant is used to describe toxic
chemicals that persist in the environment long after
their use has been banned or severely restricted.
Prior to 2004, the Arroyo Colorado appeared in the
State of Texas’ 303(d) Lists for failing to meet the fi sh
consumption use. Legacy pesticides such as DDD,
DDT, DDE, chlordane, dieldrin, endrin, heptachlor,
heptachlor epoxide, hexachlorobenzene, lindane
and toxaphene were found in concentrations that
exceeded the human health risk criteria in the Above
Tidal segment of the Arroyo Colorado, causing the
Texas Department of Health to issue fi sh consumption
advisories in 1980 and 1993.
In 2000, the TCEQ completed TMDLs for the legacy
pollutant impairments in the
Arroyo Colorado. The USEPA
approved TMDLs for chlordane,
DDE and toxaphene in fi sh tissue
on June 14, 2001, and for DDD,
DDT, dieldrin, endrin, heptachlor
epoxide, hexachlorobenzene and
lindane in fi sh tissue on May 15,
2004. A TMDL implementation
plan designed to reduce legacy
pollutant concentrations in fi sh
tissue in the Arroyo Colorado
Fish consumption
warnings
Arroyo Colorado Watershed Protection Plan 46 January 2007