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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; mS/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; mg/L, micrograms per liter]


Property or constituent

Number of
pairs with
detections
1

Minimum
concen-
tration

Maximum
concen-
tration

Mean
difference

Percent
difference

2-sided p-value2


Specific conductance (S/cm)

16 of 16

404       

878       

0.5       

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 (g/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 (mg/L)

2 of 16

13        

16         

-.50     

0-7 --
Ammonia nitrogen, dissolved (mg/L)

14 of 16

.02    

.369  

.01     

0-61

.866

Nitrite nitrogen, dissolved (mg/L)

3 of 16

.012  

.108  

0          

0-20

.5     

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

.1      

1         

-.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

.01     

1.73    

0          

1-38

.156

Phosphorus, total (mg/L)

5 of 16

.016   

1.82    

-.01     

0-44

.062

Orthophosphate phosphorus, dissolved (mg/L)

9 of 16

.01     

1.8      

.01     

0-52

.875

Organic carbon, dissolved (mg/L)

16 of 16

.3       

3.5      

-.19     

0-93

.02   

Organic carbon, suspended (mg/L)

8 of 16

.2       

1.1      

-.16     

0-120

.062

Suspended sediment (mg/L)

16 of 16

6         

105         

-3.06     

11-105

.195

Atrazine (g/L)

6 of 6

.01     

.56     

-.0080

0-90

.266

Benfluralin (g/L)

1 of 6

.0035

.0036

.0001

3 --
Carbaryl (g/L)

1 of 6

.009   

.01     

-.0011

12 --
Chlorpyrifos (g/L)

3 of 6

.0021

.0052

.0002

0-22 --
Desethyl atrazine (g/L)

6 of 6

.0044

.02     

-.0004

3-28

.688

Diazinon (g/L)

5 of 6

.001   

.063   

-.0008

0-300

.5     

Diuron (g/L)

1 of 6

.03     

.03     

0         

0 --
Lindane (g/L)

2 of 6

.009   

.013   

.001   

11-23 --
Malathion (g/L)

1 of 6

.019   

.02     

.001   

5 --
Metolachlor (g/L)

3 of 6

.004   

.005   

.0003

0-20 --
p,p'-DDE (g/L)

1 of 6

.003   

.0033

-.0003

10 --
Prometon (g/L)

5 of 6

.0037

.04     

0          

3-36

1        

Simazine (g/L)

3 of 6

.0051

.018   

.0009

6-15

.25   

Tebuthiuron (g/L)

4 of 6

.007   

.15     

.006   

0-11

.25   


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 large-sample approximation by statistical software.

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