This section summarizes the compounds detected in water samples using nontargeted chemical screening and provides more explanation about nontarget screening. This method screens for tens of thousands of organic compounds that may be present in water samples. Many of these compounds may be naturally occurring organic substances present in surface or groundwater samples because of natural processes while others may be contaminants released by human activities. Some of the detected compounds do not even have a tentative identification and are listed as "unknown" in the tables. Because the compounds that are tentatively identified in these tables are typically less well studied than the targeted compounds, many of the suspected compounds have little or no human toxicity information available. The values provided here represent instrument responses and not concentrations.