Summary statistics and annual trends in chloride concentration in urban Minnesota lakes and streams

These data tables describe statistical summaries of chloride concentration, temporal trends in annual chloride, and projected risk of future chloride pollution in lakes and streams of the 17 most urban counties in Minnesota. Data were summarized separately for lakes and for streams, and include statistics (mean, median, standard deviation, upper and lower confidence intervals, and maximum) over the entire data record, over the warm season (May - October), and over the most recent 5 years (i.e., since 2018). Trends were computed on annual means and medians. For lakes, data were aggregated by lake basin (MN DNR Lake ID, or DOW) as well as by depth of sample (surface and deep). For streams, data were aggregated by individual site level as well as by stream reach (per MN Pollution Control Agency assessment units). Risk of chloride pollution was also determined for sites with longer records (10+ years) based on current concentration, number of exceedances of chronic standards, and projected chloride concentration based on current trends. Raw data were extracted from two sources: (1) the National Water Quality Portal (USGS & EPA) and (2) the Metropolitan Council Environmental Information Management System. The data were originally collected by a large number of entities, including watershed management authorities in the state of Minnesota, tribal groups, the Minnesota Pollution Control Agency, municipalities, university researchers, private consultants, and the Metropolitan Council. Some data records begin as early as the 1950's or 1960's, with many sites still including active data collection. A total of approximately 45,000 observations of chloride were included for lakes and wetlands, and approximately 70,000 observations for streams. Nearly 1600 stream sites and 700 lake/wetland sites were represented in the raw data, with 356 stream sites and 600 lakes represented in the summaries (after filtering out sites with less than 1 year of data collection). Primary data retrieval occurred during late 2023 and early 2024, and thus the data records will be incomplete for these latest years.