As part of the project: NPS Prevention in the Arroyo Colorado Watershed, the Texas Agricultural Extension Service assembled a database of available water quality data on the Arroyo Colorado, which is included on this Web Site. This report summarizes our analysis of the data base to determine its usefulness in defining water quality problems and trends in the Arroyo. Here we report on the long-term trends of 7 water quality indicators:
While approximately 48 monitoring stations were used on the Arroyo during the period of record (1982-1994), only a few were used consistently and have complete sets of data. For this first analysis, we chose the three stations that had the most complete and longest periods of record. One station is located in the non-tidal reach, one in the tidal reach, and the third on the North Floodway, a tributary to the Arroyo.
We first compared the detected levels to the Surface Water Quality Criteria (SWQC). Since SWQC do not exist for phosphorous and nitrate, we used the Texas Natural Resource Conservation Commission's screening levels for total and dissolved phosphorous, nitrogen, and the safe drinking water standards for nitrate. The results were that all 7 parameters may be potential water quality problems in the Arroyo based on this criteria.
For the toxic substance data base, we found that most of the data is
useless for determining water quality due to the sensitivity of the testing
methods used; i.e., the lowest limit of the testing method is above the
concentration found and above the standards established for aquatic life
and human health protection. While the presence of about 55 substances
were detected in the Arroyo during approximately 10 years of sampling and
analysis, there were only two substances that exceeded standards, lead
and cadmium, which occurred once in 1986 at one location. Numerous analysis
results from sediment and fish tissue samples are included in the database
for which no standards exists. However, concentrations of 13 toxic substances
appear elevated in the sediment and tissue samples. Sampling and analysis
for toxic substances were performed erratically during the 10 year period
considered here; and no trends can be determined.
INTRODUCTION
This study was conducted as part of the project NPS Prevention in the Arroyo Colorado Watershed, funded by the U.S. Environmental Protection Agency (EPA) through the Texas Natural Resource Conservation Commission (TNRCC) and the Texas State Soil and Water Conservation Board (TSSWCB). In addition to the Texas Agricultural Extension Service (TAEX), other participating agencies were the Texas Institute for Applied Environmental Research (TIAER), the Natural Resources Conservation Service (NRCS) and the Southmost Soil and Water Conservation District.
The project was better known as the "Section 319 Arroyo Colorado Project." Task 3.1 of the project work plan directed TAEX to organize available water quality data on the Arroyo Colorado into a database. This task was completed and the database is provided on this Web Site. The database is divided into two sections: routine water quality parameters and toxic substances.
In this report, we examine 7 parameters which are often used as water
quality indicators and for which sufficient analysis results were completed:
WATER QUALITY DATA BASE
In assembling the Arroyo Colorado Database, we first contacted a number of state and federal agencies and requested any and all available water quality data collected for the Arroyo Colorado including data in electronic format and any written reports or publications. These contacts then lead to others. However, we limited our search to data files and publications that contain actual data that were not duplicated elsewhere.
The most extensive bibliography on water resources in the Lower Rio Grande Valley was put together by Judd (1994). A number of publications were found that contain actual water quality data and are included in Bibliography section of this progress report. Additional data requested but not provided to us: the Coastal Monitoring Impact Study conducted by the General Land Office, and 1993-94 Shrimp Farm Impacts Study and Coastal Fisheries Database from the Texas Parks and Wildlife Department.
We found that most of the water quality data that has been collected since 1982 is already contained in the Surface Water Quality Monitoring (SWQM) Paradox database. This database is a consolidation of water quality data collected by the TNRCC, USGS and International Boundary and Water Commission (IBWC), which is maintained by the TNRCC.
From SWQM, we extracted the water quality data for the Arroyo Colorado
and its tributaries and reorganized it into our database which is also
in Paradox format which is located on this web site. In doing so,
we simplified the database structure in order to facilitate the analysis
of the information and developed a series of maps and tables to aid in
its use. Similarly the TNRCC's database on the Rio Grande Toxic Substances
Study for the Arroyo Colorado was simplified and included in our database.
It contains toxic substance data collected by the TNRCC from ten monitoring
stations. The database contains some data from 1983, and data from 1986
to through 1994.
THE ARROYO COLORADO
The Arroyo Colorado flows through Hidalgo, Cameron and Willacy County in the Lower Rio Grande Valley of Texas into the Laguna Madre. The Arroyo Colorado waters include possible base flow from the Rio Grande River, urban runoff, agricultural runoff, irrigation return flow, municipal and industrial wastewater and effluent. Perennial flow is supported by municipal discharges from the cities of Mission, McAllen, Pharr, Donna, Harlingen, and San Benito (TDWR, 1981). During flood events, water is diverted from the Rio Grande into the Arroyo and North Floodway
Segment Numbers
See the Maps section of this Web Site for the location of the Arroyo and its three segments. The segment descriptions are as follows:
Segment 2200 identifies the tributaries to the Arroyo along segments 2201 and 2201, including the North Floodway. The North Floodway forks from the Arroyo in Hidalgo County below Weslaco, flows through the northwest portion of Cameron County into Willacy County, and joins the Arroyo near the Laguna Madre.
Segment 2201 (tidal segment) is east of FM. 510 and runs from a point 100 m downstream of Cemetery Road, south of Port Harlingen to the confluence with the Laguna Madre.
Segment 2202 (non-tidal segment) is west of F.M. 510. It runs from F.M. 2602 in Hidalgo County to a point 100 m downstream of Cemetery Road, south of Port Harlingen Monitoring Stations
For routine monitoring, some 48 locations have been used for sampling of the Arroyo Colorado and tributaries over the period of record (see Appendix A and Table E-1). However, most of these locations were used for only for short periods of time and, in some cases, for single sampling events. Currently, 4 stations are being used for routine sampling and analysis: stations 13071, 13074, 13081 and 13782. For toxic analysis, a total of 10 stations have been used for various durations, although only 2 have been used consistently.
For this analysis, we chose the 3 stations that had the longest and
most complete data sets of routine (i.e., not toxic) water quality data.
These are:
Routine Water Quality Parameters
Texas has established Surface Water Quality Criteria (SWQC) for many water bodies in the state based on designated uses (see Title 30, Chapter 307 of the Texas Administrative Code). The Arroyo Colorado is designated as "contact recreational," and SWQC have been established for 5 parameters on segment 2201 and for 7 parameters on 2202 (Table 1). Segment 2200, a tributary, has no SWQC. On the charts for Segment 2200 (see Charts section of this Web Site), we show the SWQC of segment 2202 to facilitate comparison of levels with other two segments of the Arroyo.
For a number of other parameters, the TNRCC has established "screening levels" which are used as a general indicator of potential water quality concerns. These are based on best professional judgement. For the parameters considered here, these are 0.1 mg/l for dissolved phosphorus, 0.01 for total phosphorus, and 1 mg/l for total nitrogen. For nitrates, we also used the EPA Safe Drinking Water Standard of 10 mg/l in our analysis..
Analysis Results by Individual Stations
In the Charts Section of this Web Site are figures which show the levels
of each of the 7 parameters by station and sampling event during the period
of record between 1982-1994. The actual sampling dates at each station
varied from year to year. Location of the bars on these figures correspond
to the dates the samples were taken. The results of this analysis are summarized
below and in Table 2 and 3.
| TABLE 1: Surface Water Quality Criteria (SWQC) for
the Arroyo Colorado on Segment 2201 (Tidal) and
Segment 2202 (Non-Tidal). |
||
|
|
Segment 2201 | Segment 2202 |
| Dissolved Oxygen (mg/L) | 4.0 | 4.0 |
| Temperature (F) | 95.0 | 95.0 |
| PH | 6.5-9.0 | 6.5-9.0 |
| Chloride (mg/L) | ------ | 1200 |
| Sulfate (mg/L) | ------ | 1000 |
| Total Dissolved Solids (mg/L) | ------ | 4000 |
| Fecal Coliform (#/100 mL) | 200 | 200 |
| TABLE 2: Potential water quality problems in the
Arroyo Colorado by segment number for 7 parameters
considered in this report. |
||
|
|
SEGMENT 2201
(TIDAL) |
SEGMENT 2202
(NON-TIDAL) |
| Nitrate | Nitrate | Nitrate |
| Dissolved Phosphorous | Dissolved Phosphorous | Dissolved Phosphorous |
| Total Phosphorous | Total Phosphorous | Total Phosphorous |
| Sulfate | Sulfate | Sulfate |
| Chloride | Chloride | ------ |
| ------ | Dissolved Oxygen | Dissolved Oxygen |
| Fecal Coliform | ------ | Fecal Coliform |
| Table 3: Comparison of 7 water Quality indicators to established screening criteria and standards in the Arroyo Colorado. |
| Sulfate levels at station 13036 (Floodway) exceeded the SWQC (non-tidal) in 8 of the 26 samples taken over the 10 years of record. Station 13074 had lower sulfate levels which exceeded the SWQC in only 3 of the 50 samples taken from 1982-1994. Sulfate levels for Station 13071 (tidal segment) had very high peeks, wide fluctuations in levels, and exceeded the SWQC 1000 mg/l (non-tidal criteria) in 37 of the 53 samples over the 13 years of record. |
| Chloride followed a similar pattern as sulfate. The non-tidal station 13074 only exceeded the SWQC two times during the 13 years of record. High chloride levels wide side fluctuations occurred at station 13071 (tidal), where only 5 of the samples were below the 1200 mg/l SWQC. Station 13036 (Floodway) had 12 out of 27 samples above the non-tidal SWQC. |
| Fecal coliform levels at station 13036 have fallen from peak levels (2000 to 3000 counts/100 ml) in 1985 and 1986 to well below the SWQC 1991 and 1992, but then rose again in 1993. At the tidal station 13071, only three samples were higher than the SWQC, the last occurring in early 1987. However, station 13074 (non-tidal) continues to show very high spikes of above 5000 counts/100 ml and large fluctuations. |
| Total nitrogen levels (ammonia + nitrate) nearly always exceed the TNRCC screening level 0.1 mg/l. However, when compared to the EPA Safe Drinking Water Standards, nitrate values were are almost always well below the 10 mg/l standard. Station 13036 recorded the highest nitrate levels and greatest fluctuations when compared to the other two stations. |
| Dissolved oxygen values never fell below the SWQC of 4 mg/l at station 13036 (Floodway) or station 13074 (non-tidal). However, at station 13071 (tidal), low dissolved oxygen levels occurred in 35 of the 132 samples taken over the 13 year period, and levels fluctuated from almost 0 to over 17 mg/l. A high frequency of low oxygen levels occurred most recently in 1992. |
| Dissolved phosphorous levels are compared to TNRCC screening levels since no SWQC or drinking waters standards exist for phosphorous. Relative low levels of dissolved phosphorous occurred at stations 13036 and 13071 (Floodway and tidal), although most samples exceeded this screening level. Station 13074 (non-tidal) saw consistently higher dissolved phosphorous levels, whose peaks have remained fairly constant since 1986. |
| Total phosphorous followed the same pattern as with dissolved phosphorous, with the lowest levels occurring at stations 13036 and 13071 (Floodway and tidal). Station 13074 (non-tidal) had the highest levels, with a single very high spike occurring in 1985, 1990, and 1994. |
We also examined the correlation of flow in the Arroyo to the detection levels of each parameter. We obtained flow measurement data from the International Boundary and Water Commission (IBWC) for stations 13071 (tidal) and 13074 (non-tidal). As no exact flow data is available for station 13036 (Floodway), we used the scale reported with the samples: 0 (no flow) to 5 (high flow).
This analysis was run for dissolved oxygen, fecal coliform, nitrate, sulfate, dissolved phosphorous and total phosphorous. The results are given in Table 3. No correlation existed (R2 < 0.1, correlation > +0.4) between flow and nitrate, dissolved and total phosphate, and fecal coliform.
Dissolved oxygen showed some correlation to flow at station 13036
with R2 =0.19 and correlation coefficient of 0.44. Flow vs.
sulfate regression statistics give an R2 =0.55 and correlation
coefficient of -0.75 suggesting that sulfate levels are consistent, thus
concentrations increase with decreasing flow and vise-versa. However, to
fully investigate the effect of flow on substance concentrations, a more
rigorous sampling protocol should be implemented.
| Table 4: Summary Table of Regression and Correlation
Analysis for the Arroyo Colorado (flow may be correlated to
level detected for R2 > 0.1 and correlation > + 0.4). |
||||||
|
|
||||||
| 13036 | 13071 | 13074 | ||||
| R square | Correlation | R square | Correlation | R square | Correlation | |
| Flow vs Dissolved Oxygen | 0.1948 | 0.4414 | 0.0001 | -0.0086 | 0.0868 | -0.2946 |
| Flow vs Fecal Coliform | 0.0521 | 0.2283 | 0.0006 | 0.0257 | 0.0208 | 0.1444 |
| Flow vs Nitrate | 0.0417 | 0.2041 | 0.0172 | 0.1313 | 0.0013 | -0.036 |
| Flow vs Sulfate | 0.0054 | 0.0732 | 0.3656 | -0.6046 | 0.5602 | -0.7485 |
| Flow vs Dissolved Phosphorous | 0.0009 | 0.0311 | 0.0001 | -0.0032 | 0.0008 | 0.0286 |
| Flow vs Total Phosphorous | 0.1176 | -0.3429 | 0.0004 | -0.0206 | 0.0072 | 0.0849 |
TOXIC SUBSTANCES DATA BASE
The Toxic Substances Data Base (included on this Web Site) contains data collected by the TNRCC's Rio Grande Toxic Substances Study for the Arroyo Colorado. The data base contains data collected from tem monitoring stations in 1983 and 1986 through 1994. Here, we have restructured the data base to facilitate it's use. Tables 5, 6 and 7 summarized our review of this data base.
For toxic substances, Texas has established standards for aquatic life protection (criteria in water for 34 substances) and for human health protection (criteria in water for water and fish for 60 substances). These can be found in Title 30, chapter 307 of the Texas Administrative Code (these regulations may be accessed through the INTERNET at TNRCC's Web Site).
Use of this data for evaluating the water quality status of the Arroyo is limited by the same factors discussed above for the surface water data. These include improper analytic method (i.e., lowest detection limit being above the standard) and erratic sampling frequency. For example, only 8 of the 37 substances analyzed for in Segment 2200 exceeded the test detection lower limit, 18 out of 49 for Segment 2201, and about 20 out of 55 for segment 2202. Of these, only 2 substances exceeded established standards: lead and cadmium, during one sampling event in 1986.
Numerous sediment and tissue samples were also taken and are reported
in the data base. While no standards exist, the following substances appear
elevated: for sediment, chlordane, DDE, PCB, oil and grease, Nickel, lead,
DDT, dieldrin, zinc, silver; for tissue, chlordane, DDE, DDT.
| Table 5. Toxic Substance Standards and Number of Substances Analyzed in the Arroyo Colorado Toxic Substance Data Base. |
| STANDARDS EXIST FOR:
Aquatic life protection - criteria in water 34 substances Human health protection - criteria in water for water and fish 60 substances |
| SUBSTANCES TESTED FOR:
Segment 2202 - non-tidal segment 55 substances Segment 2201 - tidal segment 49 substances Segment 2200 - tributaries 37 substances |
| Table 6. Analysis Results of the Arroyo Colorado Toxic Substance Data Base. | ||
| Substances exceeding test detection "lower" limits | ||
| Segment 2202
Segment 2201 Segment 2200 |
Sediment | Tissue |
| 18
18 8 |
20
6 0 |
|
| Toxic concentrations, in water, that exceed standards | ||
| Lead (07/14/1986
Cadmium (07/14/1986) |
||
| Table 7. Toxic Substances in Sediment and Tissue Samples that Appear Elevated. | |
| Sediment
Chlordane Lead DDE DDT PCB's Dieldrin Oil & Grease Zinc Nickel Silver |
Tissue
Chlordane DDE DDT |
ACKNOWLEDGMENTS
The following TAEX personnel contributed to this report:
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