Data from Decadal Change in Groundwater Quality Web Site, 1988-2022

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, allowing additional results to be displayed on the web page: Decadal changes in groundwater quality (https://nawqatrends.wim.usgs.gov/decadal/). This is the seventh iteration of data added to the website. With the additional data, it is possible to evaluate changes in water quality between the 2nd and 3rd sampling events for 76 networks, changes in water quality between the 1st and 3rd sampling events for 63 networks, and changes across all 3 sampling events for 60 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 89 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. 230 networks were sampled from 1988 to 2001 to assess the status of the Nation's groundwater quality. 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 (centroid) of a network of about 20 to 30 wells. Networks sampled in the first sampling event only are shown in green. There were 67 networks resampled from 2002 to 2012 to assess decadal changes in groundwater quality. Networks sampled from 2012 to 2022 and at least one previous sampling event are shown in orange and trend networks that have not yet been resampled in the third decadal sampling event are shown in blue. Networks sampled in the first and second sampling events but are no longer being sampled are shown in gray.

本数据集旨在评估地下水水质的十年际变化：美国国家水质评价计划（National Water-Quality Assessment Project）于1988年至2001年间（首次十年采样周期）从5000口水井中采集地下水水质数据。采样以具有相似特征的水井组（称为采样网络）为单位，每组包含20至30口水井。约67个采样网络中的1500口水井于2002年至2012年间（第二次采样周期）再次接受采样，以评估地下水水质的十年际变化。2012年至2022年（第三次采样周期）期间，研究人员对其中部分采样网络再次开展采样，相关新增结果可在以下网页查看：《地下水水质十年际变化》（https://nawqatrends.wim.usgs.gov/decadal/）。本次为该网站新增数据的第七次迭代。依托本次新增数据，可针对76个采样网络分析第二次与第三次采样周期间的水质变化，针对63个采样网络分析第一次与第三次采样周期间的水质变化，还可针对60个采样网络分析全部三次采样周期内的水质变化。本次采样对象涵盖监测井、家庭供水井，以及部分未经系统预处理的公共供水井取水样。用于十年际变化评估的地下水样本均采集自特征相似的水井网络。部分由家庭供水井或公共供水井组成的网络，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网络，则用于评估农业、城镇等关键土地利用类型下浅层地下水的水质变化。采样网络的选取基于全美地理分布特征，同时覆盖最重要的供水含水层及特定土地利用类型。研究通过对比选定采样周期间的浓度变化，对全美共计89个采样网络内地下水中营养盐、金属、农药及其他有机污染物的十年际变化开展评估。针对单个采样网络，其浓度中位数的十年际变化可与基准浓度对比，划分为大幅变化、小幅变化或无变化。例如，氯化物浓度的大幅变化判定标准为：统计检验显著性水平≤0.10，且该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%。氯化物的二次最大污染物限值（Secondary Maximum Contaminant Level）为250毫克每升（mg/L），据此计算，浓度变化幅度将超过12.5毫克每升，即基准浓度的5%。1988年至2001年间，研究人员共对230个采样网络开展采样，以评估全美地下水水质现状。在十年际变化绘图网站的"About-Learn more"标签页中，地图上的每个点均代表一个包含20至30口水井的采样网络的质心（centroid）。仅在首次采样周期接受采样的网络以绿色标注。2002年至2012年间，共有67个采样网络接受二次采样，用于评估地下水水质的十年际变化。2012年至2022年间接受采样且至少参与过一次前期采样周期的网络以橙色标注；尚未在第三次十年采样周期接受二次采样的趋势分析网络以蓝色标注。仅参与首次与第二次采样周期、后续未再采样的网络以灰色标注。