Evaluation of Septic Tank and Subsurface Wetland for

Appendix 3 - Determination of 90% Confidence Intervals for Water Quality Data
Means
Arithmetic means for BOD, TSS, Total N, NO3-N, NH3-N, Total P and pH.
Geometric means for Total Coliform and Fecal Coliform.
Sample BOD TSS Tot. N NO3-N NH3-N Tot. P Tot. Coli. Fecal Coli. pH
mg/l mg/l mg/l mg/l mg/l mg/l MPN MPN
PS1 58 92 33.3 0.2 76.1 12.8 3,498,060 3,199,644 7.3
PS2 27 57 29.8 0.5 57.6 9.6 422,127 378,758 7.4
PS3 13 13 1.6 0.7 0.4 0.3 234 140 7.7
RT1 40 41 20.0 0.4 13.0 3.3 2,044,760 3,362,645 7.2
RT2 15 6 6.8 0.4 5.8 1.5 94,405 84,952 7.2
RT3 20 98 7.8 0.5 0.4 1.2 386 73 7.5
Standard Deviations
Sample BOD TSS Tot. N NO3-N NH3-N Tot. P Tot. Coli. Fecal Coli. pH
mg/l mg/l mg/l mg/l mg/l mg/l MPN MPN
PS1 20.741 45.632 19.299 0.114 51.684 7.959 2,912,779 2,787,609 0.41
PS2 7.890 59.676 16.722 0.274 40.568 6.877 1,520,822 1,644,040 0.58
PS3 3.215 7.659 0.819 0.340 0.435 0.175 17,181 950 0.05
RT1 28.856 25.211 18.410 0.302 13.642 1.962 13,695,793 14,649,187 0.31
RT2 10.134 3.912 3.008 0.378 2.614 0.653 71,415 78,771 0.32
RT3 9.966 90.812 7.094 0.378 0.624 0.637 1,555 872 0.19
Statistical t Table for Determining Confidence Interval
df \ p 0.4000 0.2500 0.1000 0.0500 0.0250 0.0100 0.0050 0.0005
1 0.3249 1.0000 3.0777 6.3138 12.7062 31.8205 63.6567 636.6192
2 0.2887 0.8165 1.8856 2.9200 4.3027 6.9646 9.9248 31.5991
3 0.2767 0.7649 1.6377 2.3534 3.1825 4.5407 5.8409 12.9240
4 0.2707 0.7407 1.5332 2.1318 2.7765 3.7470 4.6041 8.6103
5 0.2672 0.7267 1.4759 2.0150 2.5706 3.3649 4.0321 6.8688
6 0.2648 0.7176 1.4398 1.9432 2.4469 3.1427 3.7074 5.9588
7 0.2632 0.7111 1.4149 1.8946 2.3646 2.9980 3.4995 5.4079
8 0.2619 0.7064 1.3968 1.8595 2.3060 2.8965 3.3554 5.0413
9 0.2610 0.7027 1.3830 1.8331 2.2622 2.8214 3.2498 4.7809
10 0.2602 0.6998 1.3722 1.8125 2.2281 2.7638 3.1693 4.5869
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