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

STATE OF THE WATERSHED
Figure 16. Nitrate plus Nitrite Nitrogen Concentrations at Water Quality
Monitoring Station 13072 located in the Tidal Segment of the
Arroyo Colorado at the Zone of Impairment
mg/L
6
5
4
3
2
1
0
Nitrate and Nitrite
13072 - Arroyo Colorado Tidal at FM 106 Bridge, Rio Hondo
Nov-96
May-97
Nov-97
May-98
Nov-98
May-99
Nov-99
May-00
Nov-00
May-01
Nov-01
May-02
Nov-02
May-03
Nov-03
May-04
Nov-04
May-05
Nov-05
May-06
Nov-06
Phosphorus concentrations continue to exceed the
screening criteria, indicating a concern for nutrient
enrichment in the Arroyo Colorado. Based on samples
collected between 1990 and 2006, the average
concentration of total phosphorus at the downstream
end of the non-tidal portion of the Arroyo Colorado
(just upstream of the Zone of Impairment) is 0.9 mg/l.
The screening criteria for total phosphorus (based on
the 85th percentile of tidal streams in Texas) is 0.8
mg/l (Figure 17). Phosphorus concentrations have
remained largely unchanged throughout the period of
record (1990-2006), and this trend
is similar for water quality monitoring
stations located in the tidal segment
of the Arroyo Colorado (Figure 18).
Phosphorus concentrations at stations
in the tidal segment also exceed
the screening criteria. However,
the average total phosphorus
concentration at station 13072, a
water quality monitoring station
located in the middle of the Zone of
Impairment at the Rio Hondo Bridge,
are below the screening criteria.
Chlorophyll-a
Chlorophyll-a is a pigment used
by plants during photosynthesis.
The concentration of this compound
in water, usually in micrograms
Field Filtered Lab Filtered Screening Criteria
per liter (μg/l), is an indicator of the
algal population (i.e., phytoplankton)
and hence a measure of the
photosynthetic activity occurring
in a water body (Figure 19).
High chlorophyll-a levels may
indicate excessive algal growth.
Consequently, chlorophyll-a is an
important measure of stream health.
Chlorophyll-a concentrations
continue to exceed the screening
criteria for the Tidal and Above Tidal
Segments of the Arroyo Colorado and
have reached very high levels within
recent years (2000-2006), displaying
a trend similar to that of nitrogencontaining
compounds. This is an
indication that primary productivity
overall is high in the Arroyo Colorado.
Algal blooms are common in the
Arroyo Colorado in the spring and summer months,
especially in the tidal segment. Wide daily swings
in the concentration of DO often accompany periods
of high productivity. Due to the high concentration
of phosphorus and nitrogen in the stream, efforts to
identify a limiting nutrient in the Arroyo Colorado have
not been successful. Research conducted by TAMU
in 1999 concluded that the Arroyo Colorado is neither
nitrogen-nor phosphorus-limited and that the factor
which most limits algal growth in the Arroyo Colorado is
light availability (Matlock and Demich 1999).
Figure 17. Total Phosphorus Concentrations at Water Quality Monitoring
Station 13074 Located Immediately Upstream of the Arroyo
Colorado Tidal Boundary
mg/L
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Feb-89
Feb-90
Feb-91
Total Phosphorus
13074 - Arroyo Colorado at Cemetary Road
Arroyo Colorado Watershed Protection Plan 44 January 2007
Feb-92
Feb-93
Feb-94
Feb-95
Feb-96
TP
Feb-97
Feb-98
Feb-99
Feb-00
Feb-01
Screening Criteria
Feb-02
Feb-03
Feb-04
Feb-05
Feb-06
Feb-07