1,2,3,-Trichloropropane (1,2,3 - TCP)


On December 14, 2017, the California Water Resources Control Board Division of Drinking Water (DDW) adopted a regulation promulgating a Maximum Contaminant Level (MCL) for 1,2,3-trichloropropane (1,2,3-TCP) of 0.000005 milligrams per liter (mg/L, or 5 parts per trillion or 5 ppt). This regulation required Community Water Systems (CWS) and Nontransient Noncommunity (NTNC) Water Systems to begin initial sampling of 1,2,3-TCP from their drinking water sources. Initial sampling began January 1, 2018, with the requirement of four quarterly samples to be completed during the year. With the passing of the initial monitoring, the first three quarters of 1,2,3-TCP data that have exceeded the MCL have been compiled and evaluated as provided in the interactive graph shown below.

Graph 1 gives an indication of the areas across California that are most impacted by groundwater contamination from 1,2,3-TCP.  As shown, there are a total of 569 sources for CWS and NTNC water systems within 30 counties that have had 1,2,3-TCP MCL exceedances.  The graph shows a clear correlation between the location of the of drinking water sources that exceed the 1,2,3-TCP MCL and agricultural/industrial activities. The majority of these impacted drinking water sources are in the Central San Joaquin Valley, which is attributed to the past agricultural practice of using soil fumigants that contained 1,2,3-TCP. Much of the Southern California area impacts come from past industrial activities (defense/aerospace) that have resulted in Superfund cleanup projects.

By toggling between the first three quarters of 2018 (Q1, Q2 and Q3), the graph shows the impacts across different counties, the number of sources impacted and those that remain online without treatment as summarized in the table below.


Counties Impacted

Sources Impacted

Sources online without treatment

Quarter 1




Quarter 2




Quarter 3




The following bullets outline how to interact with the data presented in Graph 1:

  • The ‘Quarter’ dropdown feature allows you to choose any configuration of the first three quarters of 2018 to display drinking water sources that exceeded the 1,2,3-TCP MCL.
  • The ‘Drinking Water Source Status’ dropdown feature allows you to choose any configuration of the source status.
  • Click on the different 1,2,3-TCP ranges in the upper right corner to highlight sources that fall within the selected range.
  • Click on the different counties to see the sources impacted.
  • Hover over a source in the map to see details.
  • The color of a source is matched with the range of 1,2,3-TCP concentration colors shown in the upper right-hand corner of the graph.  A single source may have varying colors which indicates that it has had varying concentrations of 1,2,3-TCP detections.
  • To obtain the raw data go here.


  • A single exceedance of the MCL does not necessarily constitute a violation of the MCL. Regulations outline the requirement for increased monitoring following any exceedance of an MCL, from which compliance is then determined.
  • Some sources do not show a sample collected for each quarter.  This may be due to one of various reasons:
    • Water Systems may have applied to receive a waiver from 1,2,3-TCP monitoring.  If granted, the water system would not need to sample.
    • Water Systems may have requested for previous monitoring to be substituted for a quarter’s monitoring.  Pursuant to Section 64445(i), results obtained from groundwater sources not more than two calendar years prior to the effective date of the 1,2,3-TCP regulation may be substituted to satisfy the initial monitoring requirements.
    • Water Systems may have failed to conduct the required sampling.  Notices of Violation would have been issued to water systems that failed to sample.
    • Monitoring may not have been required for the drinking water source listed.  Some sources listed may have been entered into the Safe Drinking Water Information System database incorrectly and therefore appear on this list as needing to be monitored when in fact they do not.
  • The operational status of the source was at the time of data collection for the quarter being reported.
  • Not all sources are displayed, as some are missing latitude/longitude data.


Information for Drinking Water Systems

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