DWEE Nitrate Hot Spot

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本研究基于多距离全局莫兰I检验（Global Moran’s I test）的结果调整测试临界值与距离带宽，以此缓解空间相关数据（spatially correlated data）中常见的错误发现率（false discovery rate）问题。分析采用ArcGIS专业版（ArcGIS Professional，Version 3.1，ESRI，2023）完成。《2023-2024年内布拉斯加州饮用水硝酸盐研究》中的图13展示了基于2003-2019年数据得到的硝酸盐相对热点与冷点区域。该样本结果可从内布拉斯加州地下水质量信息交换所（Nebraska Groundwater Quality Clearinghouse）获取。 对于研究周期内多次采样的井位，采用其浓度的中位数作为代表值。红色斑块代表具有统计显著性的热点区域，蓝色斑块代表具有统计显著性的冷点区域。热点区域同时具备空间聚集性与较高的异常浓度值。受内布拉斯加州硝酸盐浓度分布特征影响，全省范围内热点与冷点的相对浓度存在小幅差异。 本分析识别的硝酸盐浓度升高区域，与斯波尔丁（Spalding，1993）、麦克马洪（McMahon，2007）以及埃克斯纳等（Exner et al.，2014）针对内布拉斯加州硝酸盐开展的其他研究结论大体一致。上述区域的土地利用类型，与惠勒等（Wheeler et al.，2015）及加西亚（Garcia，2017）等建模研究中识别的贡献性土地利用模式相符。 热点区域的平均浓度为14.42 mg/L，冷点区域的平均浓度为3.71 mg/L，所有样本的平均浓度为7.05 mg/L。灰色圆点代表未被识别为具有统计显著性的热点或冷点的采样点位。 《2023-2024年内布拉斯加州饮用水硝酸盐研究》中的图14与图15展示了被识别为热点或冷点的点位的硝酸盐浓度分布情况。该报告的表7汇总了热点或冷点的平均采样浓度。 需要特别说明的是，本分析并非健康风险评估手段，部分冷点区域的硝酸盐浓度所在井位的浓度值可能超过《安全饮用水法》（Safe Drinking Water Act，SDWA）标准，这是由于其点位位置及相对于周边区域的浓度特征所导致。在整体浓度偏高的区域（如巴齐尔地下水管理区（Bazile Groundwater Management Area）），也可能存在统计意义上显著的低浓度异常值。 完整描述请参见：https://dee.nebraska.gov/water/nitrate-drinking-water-study