This data release contains a netCDF file containing decadal estimates of nitrate leached from septic systems (kilograms per hectare per year, or kg/ha) in the state of Wisconsin from 1850 to 2010, as well as the python code and supporting files used to create the netCDF file. The netCDF file is used as an input to a Nitrate Decision Support Tool for the State of Wisconsin (GW-NDST; Juckem and others, 2024). The dataset was constructed starting with 1990 census records, which included responses about households using septic systems for waste disposal. The fraction of population using septic systems in 1990 was aggregated at the county scale and applied backward in time for each decade from 1850 to 1980. For decades from 1990 to 2010, the fraction of population using septic systems was computed on the finer resolution census block-group scale. Each decadal estimate of the fraction of population using septic systems was then multiplied by 4.13 kilograms per person per year of leached nitrate to estimate the per-area load of nitrate below the root zone. The data release includes a python notebook used to process the input datasets included in the data release, shapefiles created (or modified) using the python notebook, and the final netCDF file.