This U.S. Geological Survey (USGS) Data Release is focused on the geochemistry of wastewater (including flowback and produced water) samples, co-produced with natural gas, collected from the Marcellus Shale Energy and Environment Laboratory (MSEEL) site. MSEEL is a long-term field site and laboratory at the Northeast Natural Energy LLC (NNE) production facility, adjacent to the Monongahela River, located in western Monongalia County, West Virginia, USA. NNE began drilling two horizontal production wells, MIP (Morgantown Industrial Park) -5H and MIP-3H, in the Marcellus Shale in 2014. The wells were completed in December 2015. Large volumes of wastewater are generated with natural gas production. These wastewaters contain organic and inorganic chemical constituents from fracturing fluids used during drilling and stimulation of gas in host rocks/shale, as well as chemical compounds that are derived from formation water and the solid shale. Many of the organic and inorganic substances in the wastewater are potentially toxic and could pose an environmental risk if released due to spills, leaks, or unsafe disposal practices. Hydraulic fracturing fluid, field blanks, wastewater, and water from the Monongahela River stored in a lined holding pond adjacent to the MIP well pad, were collected from November 2015 through April 2019. The on-site storage tank was sampled from April 2017 through April 2019. Wastewater from the MIP-5H Separator Tank was collected daily at the beginning of the study to annually by the end of the study. One sample was collected from the MIP-3H Separator Tank in May 2018. This data release includes field measurements of temperature, specific conductance, total dissolved solids (TDS), and density; laboratory measurements of pH, non-volatile dissolved organic carbon (NVDOC), alkalinity, major ions, ammonia nitrogen, trace elements, low molecular weight organic acids (LMWOA), semi-volatile hydrocarbons, and radium isotopes. There are six files (*.xlsx and .csv) in this dataset: T1_DataDictionary, T2_RestonGeochemistry, T3_Mercury, T4_MenloGeochemistry, T5_pH_Buffers, and T6_QAQC.