Validation of the InQuest nitrate screen

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Purpose

To determine if SDI’s InQuest Nitrate FS test kit produced results that were comparable to laboratory analysis of N03-N at levels below 10 ppm. Procedure

The University of Georgia Cooperative Extension Services operates an Agricultural Services Laboratory that does analytical analysis of water samples for anyone in the State. One service this lab offers is a standard Nitrate test for $8. As part of a funded water quality project, the University was offering free Nitrate tests to any county Extension agents in Georgia. Many of these county agents had farmers or rural residents in their counties that took advantage this offer and had their water tested. For this test, we simply selected fifty of these samples and tested these samples using the Nitrate FS test kit before sending them to the lab. Neither the Agricultural Services Lab nor the county agents were aware that two tests were being run.

The samples came from a variety of sources throughout the State. Fifteen different counties submitted samples during this period. While most of the samples were from wells, a few springs, ponds, and stream samples were included as well. Most of the samples had the potential to be impacted by nitrogen as they were collected in agricultural areas. Many of the samples were specifically taken because of their proximity to nitrate sources such as poultry and livestock operations, septic tanks, or agricultural cropland. Often the end use of the water was either human or animal consumption.

Since the InQuest Nitrate FS kit is a visual, colorimetric test for nitrate, determination of nitrate levels is somewhat subjective. The kits measure N03-N from 0. 5 to 10 ppm with color reference blocks at 0. 5, 2.5, 5, and 10 ppm. For this test, we attempted to choose the reference block that was closest to the color of the sample. Occasionally, when it was difficult to determine exactly which block was closest, the midpoint was selected. In addition, if no color change at all occurred, the sample was recorded at zero ppm. Initially, most of the samples were read and placed into categories by two individuals, however, both individuals always obtained the same results. The Nitrate FS samples were very easy to run and generally took less than four minutes per sample.

Results

The results are shown on Worksheet I and Chart 1. Overall, the Nitrate FS test kit did very well especially at the low levels on N03-N. Since the Nitrate FS test kit represents a discrete population that only ranged from zero to 10 ppm, comparing it with a continuous population using standard statistics is difficult. However, we can make some general observations to show the strengths and weaknesses of this test kit.

In six cases, the Nitrate FS did not change colors at all and the laboratory analysis confirmed that all of these samples had less than 0.2 ppm N03-N. Eighteen of the samples had Nitrate FS readings of 0.5 ppm. Lab analysis indicated that these sample actually had between 0 and 2.3 ppm N03-N with a mean of 0.6 ppm N03- N. Only one of these eighteen was not between 0.25 and 1.5 ppm.

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