Groundwater-quality data in the Modesto-Turlock-Merced Domestic-Supply Aquifer Study Unit, 2020-2021: Results from the California GAMA Priority Basin Project

The U.S. Geological Survey collected groundwater samples from 71 wells used for domestic and small system drinking water supplies in Stanislaus and Merced counties of California during 2020-2021. The wells were sampled for the Modesto-Turlock-Merced Domestic-Supply Aquifer Study Unit of the State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic and small system drinking water supplies. The study unit was defined by the extent of the Modesto, Turlock, and Merced subbasins of the San Joaquin Valley groundwater basin. The study unit was divided into 62 grid cells and one or more domestic or small system well was sampled in 60 of the cells. Groundwater samples from a total of 71 sampled wells were analyzed for field water-quality parameters, volatile organic compounds, pesticides and pesticide degradants, stable isotopes of water, nutrients, major ions and trace elements, hexavalent chromium, perchlorate, per- and polyfluoroalkyl substances (PFAS), and microbial indicators. Groundwater levels were measured in 56 out 71 wells.

美国地质调查局（U.S. Geological Survey）于2020—2021年间，在加利福尼亚州斯坦尼斯劳斯县与默塞德县的71口用于家庭及小型系统饮用水供水的水井中采集了地下水样本。本次采样服务于加州水资源控制委员会（State Water Resources Control Board）地下水环境监测与评估（Groundwater Ambient Monitoring and Assessment, GAMA）计划优先流域项目，该项目针对用于家庭及小型系统饮用水供水的地下水资源质量开展评估，对应的研究单元为莫德斯托-特洛克-默塞德家庭供水含水层研究单元。该研究单元以圣华金河谷地下水盆地的莫德斯托、特洛克及默塞德次级流域范围划定。研究单元被划分为62个网格单元，其中60个网格单元内各布设了一口或多口家庭或小型系统水井并完成采样。本次共对71口采样井的地下水样本开展了多项指标分析，涵盖现场水质参数、挥发性有机化合物、农药及其降解物、水的稳定同位素、营养盐、主要离子与微量元素、六价铬、高氯酸盐、全和多氟烷基物质（per- and polyfluoroalkyl substances, PFAS）以及微生物指示物。在71口水井中，有56口完成了地下水位测量。