This dataset contains information from groundwater monitoring wells at the National Crude Oil Spill Fate and Natural Attenuation Research Site near Bemidji, Minnesota, USA. The information includes field and laboratory methods, site locations, and inorganic and organic chemistry data. Samples were collected between 2009 and 2023, and analyzed for inorganic anions: F (fluoride), Cl (chloride), Br (bromide), NO3 (nitrate), PO4 (phosphate), SO4 (sulfate), and cations: Ca (calcium), Na, (sodium), Mg (magnesium), K (potassium), Si (silicon), Sr (strontium), Al (aluminum), Fe (iron), Mn (manganese), Ba (barium), B (boron), Li (lithium), Ag (silver), As (arsenic), Be (beryllium), Bi (bismuth), Cd (cadmium), Ce (cerium), Co (cobalt), Cs (cesium), Cr (chromium), Cu (copper), La (lanthanum), Mo (molybdenum), Ni (nickel), Pb (lead), Rb (rubidium), Sb (antimony), Se (selenium), Sn (tin), Th (thorium), Tl (thallium), U (uranium), V (vanadium), W (tungsten), and Zn (zinc). Additionally, samples were analyzed for benzene, toluene, ethylbenzene, o, m, p-xylene, total VHC (30 volatile hydrocarbons), NVDOC (non-volatile dissolved organic carbon), methane, ammonia as nitrogen, alkalinity as HCO3 (bicarbonate), and LMWOA (low molecular weight organic acids; lactate, acetate, propionate, formate, butyrate, pyruvate, and benzoate). The following analyses were performed during a select number of years: delta 13C of DIC (dissolved inorganic carbon) delta 13C of DOC, (dissolved organic carbon) and delta 2H in H2O, delta 18O in H2O. Field measurements for specific conductance, pH, and dissolved oxygen were measured daily. Water levels were measured during the sampling events. The supporting metadata files contain site information, field and laboratory methods, water chemistry, and quality-control results. There are three tables.