This data release provides analytical and other data in support of an analysis of nitrogen transport and transformation in groundwater and in a subterranean estuary in the Eel River and onshore locations on the Seacoast Shores peninsula, Falmouth, Massachusetts. The analysis is described in U.S. Geological Survey Scientific Investigations Report 2018-5095 by Colman and others (2018). This data release is structured as a set of comma-separated values (CSV) files, each of which contains data columns for laboratory (if applicable), USGS Site Name, date sampled, time sampled, and columns of specific analytical and(or) other data. The .csv data files have the same number of rows and each row in each .csv file corresponds to the same sample. Blank cells in a .csv file indicate that the sample was not analyzed for that constituent. The data release also provides a Data Dictionary (Data_Dictionary.csv) that provides the following information for each constituent (analyte): laboratory or data source, data type, description of units, method, minimum reporting limit, limit of quantitation if appropriate, method reference citations, minimum, maximum, median, and average values for each analyte. The data release also contains a file called Abbreviations in Data_Dictionary.pdf that contains all of the abbreviations in the Data Dictionary and in the well characteristics file in the companion report, Colman and others (2018).