Runoff

During October 2000–November 2001, 10 runoff-event samples were collected among the three stations and analyzed for the constituents listed in the table below. The EMCs by site and event are listed in table 4 (at end of report).

Constituents analyzed in runoff samples, 2000–2001

[Y, sample collected and analyzed; --, not collected or analyzed]

Nutrients, Major Inorganic Ions, and Trace Elements

Concentrations of selected nutrients, chloride, sulfate, and lead in runoff samples from the mixed agricultural watershed were compared to concentrations of the same constituents in runoff samples from the rangeland watersheds (fig. 5). Concentrations in samples from the mixed agricultural watershed generally were greater than concentrations in samples from the rangeland watersheds. The small number of samples from the two types of watersheds precluded the application of a statistical test (for example, Wilcoxon rank-sum) that could indicate whether concentrations differ significantly between the two types of watersheds.

Summary statistics of EMCs for nutrients, inorganic ions, and trace elements are listed in table 5 (at end of report). Because of the differences in land use and apparent differences in concentrations in samples from watersheds 1 and 2 compared with those from the Moody Creek watershed, statistics are presented for two groups of data. Data from watersheds 1 and 2 combined are listed as rangeland, and data from Moody Creek are listed as mixed agricultural. The median EMCs for most constituents were larger in the mixed agricultural samples than in the rangeland samples.

Pesticides

Runoff samples were analyzed for a suite of 50 pesticides (Zaugg and others, 1995), many of which are not applied in the study area. Only three pesticides were detected in the samples—atrazine, deethylatrazine (a breakdown product of atrazine), and trifluralin. These three pesticides are not used on the wildlife refuge (watersheds 1 and 2) but are used on cropland in the upper Moody Creek watershed (Leroy Wolff, U.S. Department of Agriculture, Natural Resources Conservation Service, oral commun., 2000). Pesticides were detected in two of eight samples analyzed (Moody Creek and watershed 1 samples collected Nov. 16, 2001, were not analyzed for pesticides). Three detections of pesticides were at watershed 1, and two detections were at Moody Creek. No pesticides were detected at watershed 2. Four of the five detections were in the Jan. 11, 2001, runoff samples. All concentrations of pesticides detected are listed in the following table:

Concentrations of pesticides detected in runoff samples, 2000–2001

¹ Some reported concentrations were less than the laboratory minimum reporting level. In these instances, analytical results confirm identification of the compound, and the reported concentrations are considered estimates.

The largest reported pesticide concentration was 0.01 microgram per liter of deethylatrazine from the Moody Creek sample in January 2001. No guideline for protection of aquatic life has been established for deethylatrazine in the Texas State Water Quality Standards (TSWQS) (Texas Natural Resource Conservation Commission, 2002).

Bacteria

Results of bacteria analyses in runoff samples are listed in table 6 (at end of report). The runoff samples were collected as discrete grab samples and do not represent EMCs. Because the number of samples is small and the concentrations are not EMCs, statistical comparisons of concentrations among watersheds are not considered appropriate. Concentrations from all three watersheds were grouped together, and summary statistics were computed (table 7, at end of report). Table 7 also lists TSWQS for the tidal reach of the Aransas River, which is the receiving water body for runoff from the study area watersheds.

All fecal coliform densities and most E. coli densities were greater than the recommended TSWQS for freshwater and saltwater receiving waters. The receiving waters (tidal segment of the Aransas River and Copano Bay) for the study area watersheds are saltwater. The primary indicator bacteria for saltwater is enterococcus, which was not analyzed in any of the samples.

TSWQS for bacteria are appropriate for actual receiving waters and do not apply strictly to samples collected at locations above the point where runoff enters the receiving water body (as in this study). Also, because runoff and associated large bacteria densities represent a very brief and infrequent condition, the effect on downstream waters is not known. However, the relatively large bacteria densities (compared to TSWQS) indicate that runoff from these watersheds is a potential source of bacteria to receiving streams and bays.

Bacteria densities in samples from the agricultural and rangeland watersheds also were compared to bacteria densities from an area of urban land uses. As part of the Corpus Christi National Pollutant Discharge Elimination System permit application (City of Corpus Christi, 1993), 30 stormwater-runoff samples were collected from five locations (residential, commercial, and industrial watersheds) during November 1992–April 1993. Grab samples were collected and analyzed for fecal coliform and fecal streptococcus bacteria. Mean and median bacteria densities were 40,100 and 30,500 colonies per 100 milliliters for fecal coliforms and 4,200,000 and 125,000 colonies per 100 milliliters for fecal streptococci. Mean and median bacteria densities for the urban watersheds were substantially larger than those for the agricultural and rangeland watersheds.

Loads and Yields

The load of a constituent in runoff is the mass of the given constituent transported past a site during a specified time. Daily runoff loads were computed for selected constituents for each watershed from runoff and concentration data. For runoff events that were sampled and for which EMCs were determined, the daily constituent load at a particular site is

                  L_n = EMC x RV x Cf,             (3)

where

   L_n = constituent load, in pounds per day at site _n;

EMC = event-mean concentration during runoff event, in milligrams per liter;

    RV = runoff volume, in acre-feet per day; and

      Cf  = conversion factor, 2.719 for EMCs in milligrams per liter.

For unsampled events, median EMCs for samples collected during the study were used in equation 3 to compute daily loads. Daily loads were summed to compute monthly and annual loads. Monthly and annual loads, by watershed, for selected constituents are shown in table 8 (at end of report).

Constituent yield, a measure of the load-producing characteristics of a watershed, is computed by dividing the load by the drainage area of the watershed:

                          Y = L/DA,             (4)

where

         Y  = constituent yield, in pounds per acre per month (or year);

          L  = constituent load exiting the watershed in pounds per month (or year); and

       DA  = contributing area of the watershed, in acres.

The average annual yields of selected constituents, by watershed, are listed in the following table.

Average annual yields of selected constituents in runoff
from study area watersheds, 2000–2001

[In pounds per acre per year]

Runoff yields generally were greater at watershed 1 than at watershed 2. Runoff yields from Moody Creek (agricultural watershed) were greater than the total yields from the rangeland watersheds. The 2000–2001 average annual yields of total nitrogen in runoff exiting the agricultural watershed (0.57 pound per acre) and exiting the rangeland watersheds (0.21 pound per acre) were less than the 2000–2001 average annual rainfall deposition of total nitrogen (1.3 pounds per acre) listed in the table in the section “Rainfall Deposition of Nitrogen.”

Loads and yields were not computed for bacteria because bacteria densities do not represent EMCs. Loads and yields also were not computed for pesticides because pesticides were not detected in most samples, and those pesticides detected were at very small concentrations.

SUMMARY

During 2000–2001, rainfall and runoff were monitored at a NOAA weather station and at three streamflow-gaging and water-quality sampling stations in agricultural and rangeland areas in San Patricio County in the Coastal Bend area of South Texas. Five rainfall samples were collected and analyzed for selected nutrients, and 10 runoff-event composited samples were collected at the streamflow stations during runoff events and analyzed for selected nutrients, major ions, trace elements, and pesticides. Grab samples also were collected during runoff events and analyzed for bacteria.

Rainfall and runoff data and water-quality analyses were used primarily to estimate rainfall total nitrogen loads to the study area watersheds, to compare runoff concentrations from one mixed agricultural watershed and two rangeland watersheds, and to compute runoff loads and yields of selected constituents entering the receiving bays and estuaries from these watersheds.

Study area rainfall during 2000 and 2001 was 33.27 and 28.20 inches, respectively, less than the long-term average annual of 36.31 inches. Rainfall in the study area was below average for 16 of the 24 months. Because of the combination of soils, vegetation, mild land surface slopes, and frequent dry periods, runoff from the study area occurred only after heavy rains. Nine runoff events occurred, producing relatively small runoff volumes. To illustrate how the hydrology of the area is dominated by extreme events, the three largest runoff events resulted in 29 percent of the total rainfall and produced 86 percent of the total runoff for 2000–2001. Total runoff from the study area watersheds during 2000–2001 was 2.46 inches (2,800 acre-feet); the regional average is about 2 inches per year.

Runoff from the larger mixed agricultural watershed generally had larger concentrations of selected nutrients, major ions, and trace elements, compared with runoff from the rangeland watersheds. Also, the 2000–2001 average annual yields (pounds per acre) of selected constituents in runoff exiting the mixed agricultural watershed were larger than yields exiting the rangeland watersheds. For example, total phosphorus yield from the mixed agricultural watershed was 0.072 pound per acre per year compared with 0.023 pound per acre per year from the rangeland watersheds. The suspended solids yield from the rangeland watersheds averaged 28 pounds per acre per year, which was much smaller than the 76 pounds per acre per year from the mixed agricultural watershed.

Rainfall deposition is a major source of nitrogen delivered to the study area. Rainfall nitrogen (mostly ammonia and nitrate) exceeded the runoff yield. The average annual rainfall deposition of total nitrogen on the study area watersheds was 1.3 pounds per acre. In contrast, an average annual yield of 0.57 and 0.21 pound per acre of total nitrogen in runoff exited the mixed agricultural watershed and the rangeland watersheds, respectively.

Pesticides were detected in two of eight runoff samples (two runoff samples were not analyzed for pesticides). All of the detections occurred at Moody Creek and watershed 1. Three pesticides (atrazine, deethylatrazine, and trifluralin) were detected in very small concentrations. All but one pesticide detection was reported at or below the laboratory minimum reporting level. The largest measured pesticide concentration was 0.01 microgram per liter of deethylatrazine (a breakdown product of atrazine) at Moody Creek during the January 2001 runoff event.

Bacteria in runoff is a potential water-quality concern. Although bacteria densities in the study area watersheds are much smaller than concentrations in urban runoff, all densities of fecal coliform and E. coli from the agricultural watershed and the rangeland watersheds exceeded recommended TSWQS for receiving waters. Also, because runoff and associated large bacteria densities represent a very brief and infrequent condition, the effect on downstream waters is not known. However, the relatively large bacteria densities (compared to TSWQS), indicate that runoff from these watersheds is a potential source of bacteria to receiving streams and bays.

REFERENCES

Baird, F.C., Dybala, T.J., Jennings, Marshall, and Ockerman, D.J., 1996, Characterization of nonpoint sources and loadings to the Corpus Christi Bay National Estuary Program study area: Texas Natural Resource Conservation Commission, Corpus Christi Bay National Estuary Program CCBNEP–05, 226 p.

Buchanan, T.J., and Somers, W.P., 1969, Discharge measurements at gaging stations: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. A8, 65 p.

City of Corpus Christi, 1993, City of Corpus Christi part II NPDES permit application, wet weather sampling program—Executive summary: Corpus Christi, Tex., 16 p.

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Rantz, S.E., and others, 1982, Measurement and computation of streamflow—Volume 1. Measurement of stage and discharge: U.S. Geological Survey Water-Supply Paper 2175, 284 p.

Soil Conservation Service, 1979, Soil survey, San Patricio County, Texas: U.S. Department of Agriculture, 122 p.

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Texas Natural Resource Conservation Commission, 1996, 1994 Regional assessment of water quality in the Nueces Coastal Basins: Texas Natural Resource Conservation Commission, Agency Studies AS–035, 567 p.

______2002, Texas Surface Water Quality Standards: Accessed May 6, 2002, at URL http://www.tnrcc.state.tx.us/permitting/waterperm/wqstand/

Zaugg, S.D., Sandstrom, M.W., Smith, S.G., and Fehlberg, K.M., 1995, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—Determination of pesticides in water by C–18 solid-phase extraction and capillary-column gas chromatography/mass spectrometry with selected-ion monitoring: U.S. Geological Survey Open-File Report 95–181, 60 p.  

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