Data from Decadal Change in Groundwater Quality Web Site, 1988-2024

Evaluating Decadal Changes in Groundwater Quality: Groundwater-quality data were collected from 5,000 wells between 1988-2001 (first decadal sampling event) by the National Water-Quality Assessment Project. Samples are collected in groups of 20-30 wells with similar characteristics called networks. About 1,500 of these wells in 67 networks were sampled again approximately 10 years later between 2002-2012 (second sampling event) to evaluate decadal changes in groundwater quality. Between 2012 and 2022 (third sampling event), a subset of these networks were sampled again. A 4th round of sampling started in 2023, allowing additional results to be displayed on the web page: Decadal changes in groundwater quality (https://nawqatrends.wim.usgs.gov/decadal/). This is the eighth iteration of data added to the website. With the additional data, it is possible to evaluate changes in water quality between the 3rd and 4th sampling events for 15 networks and changes across all 4 sampling events for 11 networks. Samples were obtained from monitoring wells, domestic-supply wells, and some public-supply wells before any treatment on the system.

Groundwater samples used to evaluate decadal change were collected from networks of wells with similar characteristics. Some networks, consisting of domestic or public-supply wells, were used to assess changes in the quality of groundwater used for drinking water supply. Other networks, consisting of monitoring wells, assessed changes in the quality of shallow groundwater underlying key land-use types such as agricultural or urban lands. Networks were chosen based on geographic distribution across the Nation and to represent the most important water-supply aquifers and specific land-use types.

Decadal changes in concentrations of nutrients, metals, and pesticides and other organic contaminants in groundwater were evaluated in a total of 91 networks across the Nation by comparing changes between selected sampling events.

Decadal changes in median concentrations for a network are classified as large, small, or no change in comparison to a benchmark concentration. For example, a large change in chloride concentrations indicates that the probability of the test is less than or equal to 0.10 and the median of all differences in concentrations in a network is greater than 5 percent of the chloride benchmark per decade. For chloride, which has a Secondary Maximum Contaminant level of 250 milligrams per liter, this would mean the change in concentration exceeded 12.5 milligrams per liter (mg/L), or 5 percent of the benchmark.

Since the first decade of sampling from 1988 to 2001 to assess the status of the Nation's groundwater quality, 91 networks have been sampled at least one more time to evaluate trends. Each dot on the map on the "About-Learn more" tab of the Decadal mapper website, (https://nawqatrends.wim.usgs.gov/decadal/) represents the center point of a network of about 20 to 30 wells. Networks are visited once per decade, with the second round of sampling being 2002-2012 and the third round being 2013-2022. The fourth decade of sampling began in 2023. Networks that have been sampled in at least 2 decades are shown in green. Networks sampled in 2023 and 2024 and at least one previous sampling event are shown in orange.

地下水水质十年变化评估：美国国家水质评估计划（National Water-Quality Assessment Project）于1988年至2001年间（首次十年采样周期）从5000口井中采集了地下水水质数据。采样以20至30口特征相似的井为一组，组成采样网络（networks）。其中67个采样网络内的约1500口井，在约10年后的2002年至2012年间再次接受采样（第二次采样周期），以评估地下水水质的十年变化。2012年至2022年（第三次采样周期）期间，对其中部分采样网络再次开展采样。第四轮采样于2023年启动，相关新增结果可在“地下水水质十年变化”专题网页（https://nawqatrends.wim.usgs.gov/decadal/）查看。本次为该网站新增的第八批数据集。依托新增数据，可针对15个采样网络评估第三次与第四次采样周期间的水质变化，同时可针对11个采样网络分析四次采样周期的全周期水质变化。本次采样对象为系统处理前的监测井、生活供水井及部分公共供水井。

用于十年变化评估的地下水样品采自特征相似的井组采样网络。部分由生活供水井或公共供水井组成的采样网络，用于评估饮用水供水水源地下水的水质变化；其余由监测井组成的采样网络，则用于评估农业、城市等关键土地利用类型下覆浅层地下水的水质变化。采样网络的选取以全美地理分布覆盖度为依据，同时需代表性覆盖全美重要供水含水层与特定土地利用类型。

通过对比选定采样周期间的变化，全美总计91个采样网络的地下水营养盐、金属、农药及其他有机污染物浓度的十年变化均得到了评估。

采样网络的水质浓度中位数十年变化，可依据基准浓度划分为大幅变化、小幅变化及无变化三类。例如，氯化物浓度大幅变化的判定标准为：检验显著性概率≤0.10，且采样网络内所有浓度差值的中位数每十年超过氯化物基准浓度的5%。对于次级最大污染物限值（Secondary Maximum Contaminant level）为250毫克/升的氯化物而言，这意味着浓度变化量超过12.5毫克/升（mg/L），即基准浓度的5%。

自1988年至2001年首次开展采样以评估全美地下水水质状况以来，已有91个采样网络至少额外开展过一次采样以评估变化趋势。在十年变化制图网站（https://nawqatrends.wim.usgs.gov/decadal/）的“关于-了解更多”标签页的地图中，每个圆点均代表一个由20至30口井组成的采样网络的中心点。采样网络每十年开展一次采样：第二轮采样为2002年至2012年，第三轮采样为2013年至2022年；第四轮采样于2023年启动。至少在两个十年周期内完成采样的网络以绿色标注；2023年、2024年开展过采样且至少完成过一次前期采样的网络以橙色标注。