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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
detections1

Minimum
concen-
tration

Maximum
concen-
tration

Mean
difference

Percent
difference

2-sided p-value2

Specific conductance (µS/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 (µg/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 (µg/L)

6 of 6

.01     

.56     

-.0080

 0-90

.266
Benfluralin (µg/L)

1 of 6

.0035

.0036

.0001

 3 --
Carbaryl (µg/L)

1 of 6

.009   

.01     

-.0011

 12 --
Chlorpyrifos (µg/L)

3 of 6

.0021

.0052

.0002

 0-22 --
Desethyl atrazine (µg/L)

6 of 6

.0044

.02     

-.0004

 3-28

.688
Diazinon (µg/L)

5 of 6

.001   

.063   

-.0008

 0-300

.5     
Diuron (µg/L)

1 of 6

.03     

.03     

0         

 0 --
Lindane (µg/L)

2 of 6

.009   

.013   

.001   

 11-23 --
Malathion (µg/L)

1 of 6

.019   

.02     

.001   

 5 --
Metolachlor (µg/L)

3 of 6

.004   

.005   

.0003

 0-20 --
p,p'-DDE (µg/L)

1 of 6

.003   

.0033

-.0003

 10 --
Prometon (µg/L)

5 of 6

.0037

.04     

0          

 3-36

1        
Simazine (µg/L)

3 of 6

.0051

.018   

.0009

 6-15

.25   
Tebuthiuron (µg/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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