Description: The layer was developed to identify outliers in the data and look for spatially significant hot spots in nitrate concentrations using a Getis-Ord Gi* test (Ord, 1995). This test identifies outliers in the dataset based on the concentration of nitrate and the spatial grouping of sample data. The test critical values and distance band are adjusted based on the results of multi-distance Global Moran’s I test, which helps alleviate the false discovery rate issue common in spatially correlated data. Analysis was performed using ArcGIS Professional (Version 3.1, ESRI, 2023). Figure 13 of the 2023-2024 NE Nitrate in Drinking Water Study shows the relative hot and cold spots for nitrate based on data from 2003-2019. These sample results are accessible from the Nebraska Groundwater Quality Clearinghouse. At wells sampled more than once during the study period, the median concentration was used. Red spots indicate statistically significant hot spots, and blue spots indicate statistically significant cold spots. A hot spot indicates both spatial clustering and high outlier concentrations. Because of the distribution of nitrate concentrations across Nebraska, the relative concentrations of hot and cold spots differ slightly across the state. This analysis shows areas of elevated nitrate concentrations are largely consistent with other studies of nitrate in Nebraska such as those conducted by Spalding (1993), McMahon (2007), and Exner et al. (2014). Land use in these areas is similar to contributing land use patterns identified in modeling efforts such as Wheeler et al. (2015) and Garcia (2017). The average concentration at a hot spot was 14.42 mg/L. The average concentration at a cold spot was 3.71 mg/L. The average for all data was 7.05 mg/L. Grey dots represent sample locations that are not identified as statistically significant hot or cold spots. Figures 14 and 15 in the 2023-2024 NE Nitrate in Drinking Water Study show the distribution of nitrate concentration at points identified as hot spots or cold spots. Table 7 of the same report summarizes the average sample concentration for hot or cold spots. It is important to note that this is not a health measure, and some of the cold spot nitrate concentrations are at wells that exceed the SDWA standard because of their location and concentration relative to the area around them. It is possible to be a statistically significant outlier on the low end, in an area where the overall distribution is higher concentrations, such as in the Bazile Groundwater Management Area.For a complete description see: https://dee.nebraska.gov/water/nitrate-drinking-water-study
Service Item Id: b3ec2af28f614f459cb794bffd1161cc
Copyright Text: Department of Water, Energy and Environment (DWEE), Nebraska groundwater quality clearinghouse