Water-Quality Assessment of South-Central Texas—

Comparison of Water Quality in Surface-Water Samples 

Collected Manually and by Automated Samplers

Surface-water sampling protocols of the U.S. Geological Survey National Water-Quality Assessment (NAWQA) 

Program specify samples for most properties and constituents to be collected manually in equal-width increments 

across a stream channel and composited for analysis. 

Single-point sampling with an automated sampler 

(autosampler) during storms was proposed in the upper 

part of the South-Central Texas NAWQA study unit, raising the question of whether property and constituent concentrations from automatically collected samples differ 

significantly from those in samples collected manually. 

Statistical (Wilcoxon signed-rank test) analyses of 3 to 16 

paired concentrations for each of 26 properties and constituents from water samples collected using both methods at 

eight sites in the upper part of the study unit indicated that 

there were no significant differences in concentrations 

for dissolved constituents, other than calcium and organic 

carbon.

In 1991, the U.S. Geological Survey (USGS) implemented the National Water-Quality Assessment (NAWQA) 

Program to describe the status and trends in water quality of 

a large, representative part of the Nation’s surface- and 

ground-water resources. The program is based on a multidisciplinary approach using standardized protocols to collect data in more than 50 study units on a rotational schedule 

(Hirsch and others, 1988; Leahy and others, 1990). The 

South-Central Texas (SCTX) study began in 1994.

The NAWQA design (Gilliom and others, 1995) and the 

surface-water sampling protocols (Shelton, 1994) specify 

that surface-water samples for most properties and constituents should be collected manually in equal-width increments 

(EWI method) across a stream channel to ensure collection 

of a representative sample. The surface-water component of 

the NAWQA program requires collection of water-quality 

samples during extreme flow conditions as well as during 

more normal flow conditions. Because streams in the upper 

part of the SCTX study unit are “flashy” (water levels rise 

and fall in a short period of time) during storms, sampling 

the streams during storms with an autosampler was proposed. Autosamplers allow collection of water-quality 

samples without endangering field personnel during hazardous conditions, and autosamplers can collect samples 

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet FS–172–99

November 1999

Environmental Setting

The upper part of the SCTX study unit comprises the 

Edwards aquifer in the San Antonio region and its catchment area. The upper part of the study unit is divided into 

two distinct landforms by the Balcones escarpment 

(fig. 1). North and west of the escarpment is a rugged, 

steep terrain of upland plateaus and rolling hills that are 

dissected by many small streams that flow into the deeply 

incised valleys of several major streams. This area of 

rocky soils is known locally as the Texas Hill Country. 

South and east of the escarpment, the terrain smooths to 

rolling prairies and broad stream valleys with clayey soils. 

Primarily steep terrain coupled with intense rainfall 

account for the rapid rise and fall of streams during 

storms.

throughout the duration of a storm. The use of autosamplers 

raises the question of whether property and constituent 

concentrations in automatically collected samples differ significantly from those in samples collected using the EWI or 

other standard manual methods.

Cross-sectional variations in stream water quality can 

occur because of incomplete mixing of upstream tributary 

inflows, ground-water seepage, and point-source discharges. 

Previous water-quality studies have reported cross-sectional 

variations in concentrations of suspended sediment and 

constituents associated with sediment such as phosphorus, 

trace metals, and hydrophobic pesticides (Martin and others, 

1992). Unlike EWI and other manually collected samples 

that comprise composite subsamples, automatically collected samples are obtained from a single point in the 

stream. Therefore, differences in property and constituent 

concentrations between samples collected manually and 

those collected automatically could occur.

The purpose of this report is to compare property and 

constituent concentrations in water-quality samples collected manually using the EWI method (or other standard 

manual method) with concentrations of samples collected 

by autosamplers. Statistical comparisons are used to analyze 

3 to 16 paired concentrations for each of 26 properties and 

constituents from water samples collected at eight sites in 

the upper part of the SCTX study unit. 
2

Figure 1. Upper part of study unit and location of sampling sites.

Sample Collection and Analysis

Two sets of paired water samples were collected at each of eight sites in the upper part of the study unit. One of each 

pair of samples was collected manually using the EWI or other standard manual method following NAWQA protocols; 

the other sample was pumped from a single point in the stream into glass bottles by an autosampler. Identical sample 

processing and procedures following NAWQA protocols were used for both samples. Each set of paired samples was 

collected simultaneously to reduce potential temporal variation. Paired samples were not collected during storms because 

of potential physical danger to field personnel. Although samples were not collected during storms, most samples were 

collected when discharge at the sites was greater than the 50th percentile of discharge for the month of collection (for 

the respective periods of record) (table 1). Discharge at each of the eight sites when the first set of paired samples was 

collected was larger (1.07 to 10.6 times) than when the second set of paired samples was collected. All paired samples 

were analyzed for major ions, nutrients, dissolved and suspended organic carbon (DOC and SOC), and suspended sediment. Paired samples for pesticide analysis were collected only at the three sites where pesticide samples were collected 

routinely (08178800, 08180640, 08181800). All samples were analyzed by the USGS National Water Quality Laboratory.

RIVER

GUAD ALU PE

MEDINA

RIVER

WEST

NUECES

RIVER

RIVER

NUECES

FRIO

RIVER

SABINAL

RIVER

HONDO

CREEK

MEDINA

 LAKE

98o

99o

100o

30o

29o

San Antonio

TEXAS

Upper part of

 study unit

South-Central Texas

 NAWQA study unit

LOCATION MAP

EXPLANATION

08181800

Balcones escarpment (modified 

 from Abbott and Woodruff, 

 1986)

Surface-water site and 

 number

08195000 08198000

08160640

08181800

08178800

08169000

08167500

08171000

SAN ANTONIO

 RIVER

CANYON

 LAKE

0 10 20 30 40 MILES
3

Table 1. Discharges for paired water-quality samples collected in the upper part of the South-Central Texas study unit 

[ft3

/s, cubic feet per second] 

Station

number

Sample

date

Discharge

when sample

collected

(ft3/s)

Mean monthly

discharge for

station

(ft3/s)

Mean annual

discharge for

station

(ft3/s)

Relative magnitude

of sampled discharge

08167500 03–27–98 756 362.2 353.3 Greater than 75th percentile for month

08167500 04–20–98 492 391.5 353.3 Greater than 75th percentile for month

08169000 03–25–98 342 301.8 293 At 75th percentile for month

08169000 04–23–98 321 305.3 293 Greater than 50th percentile for month

08171000 04–03–98 366 181.4 133.9 Greater than 75th percentile for month

08171000 04–22–98 176 181.4 133.9 Greater than 50th percentile for month

08178800 03–23–98 24 34.6 44.5 Greater than 25th percentile for month

08178800 04–27–98 9.8 42.5 44.5 Less than 25th percentile for month

08180640 03–24–98 665 261 140.1 Greater than 75th percentile for month

08180640 04–24–98 63 153.8 140.1 Greater than 50th percentile for month

08181800 03–26–98 794 499.1 550.7 Greater than 75th percentile for month

08181800 04–28–98 201 518.7 550.7 Less than 25th percentile for month

08195000 04–02–98 126 107.6 123.1 Greater than 75th percentile for month

08195000 04–29–98 92 107.6 123.1 Greater than 50th percentile for month

08198000 04–06–98 80 58.4 63.5 Greater than 75th percentile for month

08198000 04–30–98 54 58.4 63.5 Greater than 50th percentile for month

Figure 2. Typically, an autosampler (white object in center of 

shelter) is installed on relatively high ground with a sampling tube 

running underground to the stream. Power is supplied by batteries 

recharged by a solar panel (at left). (Photograph by C.L. Otero, 

U.S. Geological Survey.)

Figure 3. Equal-width increment sampling is done by wading 

where stream depths permit. (Photograph by C.A. Hartmann Jr., 

U.S. Geological Survey.)
4

Comparison of Water-Quality Data

The differences between paired concentrations (manual 

minus autosampler) of properties, major ions, nutrients, 

organic carbon, suspended sediment, and selected pesticides 

were computed and mean differences determined (table 2). 

In general, the mean differences do not show manual concentrations to be consistently larger or smaller than 

autosampler concentrations. Percentage differences were 

large for some constituents with small concentrations.

To assess the similarity of the paired concentrations, 

the Wilcoxon signed-rank test (Helsel and Hirsch, 1992; 

Hollander and Wolf, 1973) was used to indicate whether 

the paired concentrations can be considered to be from the 

same population; in other words, no significant difference 

exists between them. If the paired concentrations come 

from the same population, their differences should be symmetrically distributed about zero; accordingly, the test 

determines whether the median difference between paired 

concentrations is zero—that is, it tests the hypothesis (null 

hypothesis) that the median difference between paired concentrations is zero; the alternative hypothesis is that the 

median difference is either greater than or less than zero 

(thus, a “two-sided” hypothesis test). Because the ranks of 

the differences rather than the actual values of the differences are used in the test, the magnitudes of the differences 

do not influence the outcome of the test. The test result of 

interest is the p-value, which represents the probability of 

getting the particular test outcome when the null hypothesis 

is true—the smaller this probability, the stronger the evidence for rejecting the null hypothesis. P-values of about 

0.05 or less offer strong evidence for rejecting the null 

hypothesis and concluding that paired concentrations are 

significantly different.

The p-values from signed-rank tests for selected 

constituents and pesticides (table 2) indicate that for most 

of the 26 properties and constituents for which p-values 

could be computed, there are no significant differences 

between paired concentrations. However, the tests of dissolved calcium and DOC offer strong evidence (p-values 

0.028, 0.020) that the two sampling methods yielded significantly different concentrations; and total phosphorus and 

SOC offer fairly strong evidence (both p-values = 0.062) 

that the two sampling methods yielded significantly different concentrations. For each of the four constituents that 

signed-rank tests showed strong or fairly strong evidence of 

significant difference between manual and autosampler concentrations (dissolved calcium, DOC, SOC, and total phosphorus), at least 80 percent of the manual concentrations 

were less than or equal to the autosampler concentrations. 

Two cautions regarding the results of the signed-rank 

tests are appropriate. Because the number of paired concentrations in the tests is small (15 or fewer nonzero differences 

in all but one of the tests), the power of the tests to indicate 

significant differences is low. In other words, significant differences between concentrations of some constituents might 

not be detected by the tests. For example, the test of the pesticide simazine that involved only three data pairs did not 

yield strong enough evidence to reject the hypothesis that 

there is no significant difference between the manual and 

autosampler concentrations, although all three of the manual 

concentrations were less than the corresponding autosampler concentrations. In the case of tebuthiuron also, the 

small number of data pairs (four) might have contributed to 

the inability of the signed-rank test to indicate the significant difference; three of four manual concentrations were 

greater than the corresponding autosampler concentrations. 

Additionally, in cases where the median difference between 

paired concentrations is not zero (null hypothesis is false) 

but is close to zero, the power of the test to indicate rejection 

of the null hypothesis is low; thus, the probability of failing 

to reject the null hypothesis (which is false) and concluding 

that there is no significant difference in concentration is 

high.

NAWQA sampling protocols for DOC and SOC (unlike 

for all other properties and constituents sampled for in 

this study) call for samples to be collected using a singlevertical, midstream method (Shelton, 1994) rather than 

the EWI method. Whether the lack of similarity between 

manual and autosampler DOC and SOC concentrations 

indicated by the signed-rank tests is related to use of the 

single-vertical, midstream method rather than the EWI 

method is unknown; but the lack of similarity is enough to 

indicate caution in the use of autosamplers for DOC and 

SOC. 

Caution in the use of autosamplers for suspended material in addition to SOC also might be prudent, on the basis 

of the apparent variability in the suspended sediment data 

(table 2). Additionally, total phosphorus, for which the 

signed-rank test indicated fairly strong evidence for a 

significant difference in paired concentrations, typically is 

associated with suspended material because phosphorus 

commonly attaches to soil particles. The amount of suspended material in streamflow often is related to discharge. 

Accordingly, the differences between manual and autosampler concentrations for SOC, suspended sediment, and 

total phosphorus were divided into two groups on the basis 

of discharge (larger or smaller) on the dates of sampling 

(table 1) and statistically tested to determine whether one 

group of differences was significantly larger than the other. 

The result of a Wilcoxon rank-sum test (also a two-sided 

hypothesis test) indicated no significant difference between 

the concentration differences of paired samples collected 

when discharges were larger and smaller (p-value = 0.333).
5

Table 2. Statistics of concentrations from paired water-quality samples collected at eight sites in the upper part of the 

South-Central Texas study unit 

[difference, manual concentration minus autosampler concentration; μS/cm, microsiemens per centimeter at 25 degrees Celsius; 

mg/L, milligrams per liter; --, Wilcoxon signed-rank test not done because fewer than three data pairs with nonzero differences; 

μg/L, micrograms per liter] 

1 Constituent concentrations less than reporting limits omitted from datasets before computation of statistics and Wilcoxon tests. 2 p-value from exact Wilcoxon signed-rank test for all properties and constituents except suspended sediment because all property and constituent 

pairs except suspended sediment had fewer than 16 nonzero differences (Helsel and Hirsch, 1992, p. 144). Suspended sediment p-value from largesample approximation by statistical software.

Property or constituent

Number of

pairs with

detections1

Minimum

concentration

Maximum

concentration

Mean

difference

Percent

difference

2-sided

p-value2

Specific conductance (μS/cm) 16 of 16 404 878 0.50 0–4 0.648

Alkalinity (as CaCO3) (field) (mg/L) 16 of 16 160 280 –2.50 0–22 .633

Calcium, dissolved (mg/L) 16 of 16 59 91 –1.25 0–7 .028

Magnesium, dissolved (mg/L) 16 of 16 12 20 .06 0–7 --

Sodium, dissolved (mg/L) 16 of 16 6.1 66 0 0–11 .844

Potassium, dissolved (mg/L) 16 of 16 .77 7.3 .04 0–12 .523

Chloride, dissolved (mg/L) 16 of 16 8.7 84 .35 0–21 .275

Sulfate, dissolved (mg/L) 16 of 16 14 71 –.25 0–16 .524

Fluoride, dissolved (mg/L) 16 of 16 .12 .46 0 0–13 .865

Silica, dissolved (mg/L) 16 of 16 8.1 14 –.15 0–10 .124

Manganese, dissolved (μg/L) 2 of 16 13 16 –.50 0–7 --

Ammonia nitrogen, dissolved (mg/L) 14 of 16 .0200 .369 .01 0–61 .866

Nitrite nitrogen, dissolved (mg/L) 3 of 16 .0120 .108 0 0–20 .500

Nitrite plus nitrate nitrogen, dissolved (mg/L) 15 of 16 .292 11.3 .03 1–63 .868

Ammonia plus organic nitrogen, dissolved (mg/L) 7 of 16 .10 1.0 –.02 2–54 .376

Ammonia plus organic nitrogen, total (mg/L) 7 of 16 .11 1.1 –.02 0–19 .188

Phosphorus, dissolved (mg/L) 6 of 16 .0100 1.73 0 1–38 .156

Phosphorus, total (mg/L) 5 of 16 .0160 1.82 –.01 0–44 .062

Orthophosphate phosphorus, dissolved (mg/L) 9 of 16 .010 1.8 .01 0–52 .875

Organic carbon, dissolved (mg/L) 16 of 16 .30 3.5 –.19 0–93 .020

Organic carbon, suspended (mg/L) 8 of 16 .20 1.1 –.16 0–120 .062

Suspended sediment (mg/L) 16 of 16 6 105 –3.06 11–105 .195

Atrazine (μg/L) 6 of 6 .010 .56 –.0080 0–90 .266

Benfluralin (μg/L) 1 of 6 .0035 .0036 .0001 3 --

Carbaryl (μg/L) 1 of 6 .0090 .010 –.0011 12 --

Chlorpyrifos (μg/L) 3 of 6 .0021 .0052 .0002 0–22 --

Desethyl atrazine (μg/L) 6 of 6 .0044 .020 –.0004 3–28 .688

Diazinon (μg/L) 5 of 6 .0010 .063 –.0008 0–300 .500

Diuron (μg/L) 1 of 6 .030 .030 0 0 --

Lindane (μg/L) 2 of 6 .0090 .013 .0010 11–23 --

Malathion (μg/L) 1 of 6 .019 .020 .0010 5 --

Metolachlor (μg/L) 3 of 6 .0040 .0050 .0003 0–20 --

p,p’-DDE (μg/L) 1 of 6 .0030 .0033 –.0003 10 --

Prometon (μg/L) 5 of 6 .0037 .040 0 3–36 1.0

Simazine (μg/L) 3 of 6 .0051 .018 .0009 6–15 .250

Tebuthiuron (μg/L) 4 of 6 .0070 .15 .0060 0–11 .250
6

The results of the statistical tests indicate that autosamplers probably could be used in place of manual sampling 

methods for dissolved constituents. This conclusion is 

qualified because of the signed-rank-test results for dissolved calcium and organic carbon, which indicated 

significant differences in paired concentrations, and the fact 

that the dataset analyzed is small—3 to 16 paired concentrations for each of the 26 properties and constituents.

References

Abbott, P.L., and Woodruff, C.M., Jr., eds., 1986, The 

Balcones escarpment—Geology, hydrology, ecology, 

and social development in central Texas: Geological 

Society of America, Proceedings of Annual Meeting, 

San Antonio, Tex., November 1986, 200 p. 

Gilliom, R.J., Alley, W.M., and Gurtz, M.E., 1995, Design 

of the National Water-Quality Assessment Program—

Occurrence and distribution of water-quality conditions: 

U.S. Geological Survey Circular 1112, 33 p.

Helsel, D.R., and Hirsch, R.M., 1992, Studies in 

environmental science—Statistical methods in water 

resources: New York, Elsevier, 522 p.

Hirsch, R.M., Alley, W.M., and Wilber, W.G., 1988, 

Concepts for a National Water-Quality Assessment 

Program: U.S. Geological Survey Circular 1021, 42 p.

Hollander, Myles, and Wolfe, D.A., 1973, Nonparametric 

statistical methods: New York, John Wiley, 503 p.

Leahy, P.P., Rosenshein, J.S., and Knopman, D.S., 1990, 

Implementation plan for the National Water-Quality 

Assessment Program: U.S. Geological Survey 

Open-File Report 90–174, 10 p.

Martin, G.R., Smoot, J.L., and White, K.D., 1992, A 

comparison of surface-grab and cross sectionally 

integrated streamwater-quality sampling methods: 

Water Environment Research, v. 64, no. 7, p. 866–876.

Shelton, L.R., 1994, Field guide for collecting and 

processing stream-water samples for the National 

Water-Quality Assessment Program: U.S. Geological 

Survey Open-File Report 94–455, 42 p.

—Patricia B. Ging

Any use of trade, product, or firm names is for descriptive 

purposes only and does not imply endorsement by the 

U.S. Government.

Information on technical reports and hydrologic data 

related to this study can be obtained from:

District Chief

U.S. Geological Survey

8027 Exchange Dr. 

Austin, TX 78754–4733

Email: dc_tx@usgs.gov

Phone: (512) 927–3500

FAX: (512) 927–3590

World Wide Web: http://tx.usgs.gov