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Surface Water Quality Bureau
Monitoring & Assessment
Probable Sources
of Impairment

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                                                                                                    Probable Sources contact:
                                                                                                    Heidi Henderson


                                                                                                    Probable Source Form

TMDLs include a list of “probable sources” in the contributing watershed.  These are defined as activities that may contribute pollutants or stressors to a waterbody (EPA 1997).  The probable source list included with any cause of impairment includes any and all activities occurring or likely to occur in the watershed that have the potential to contribute to the identified impairment.  It is not intended to single out any particular land owner or single land management activity, and has therefore been labeled “probable” and generally includes several possible items.  Probable sources listed for any particular waterbody have not been proven to be a source or the only sources of the identified impairment.  It is based on qualitative field observations made by field staff for AUs sampled during rotational watershed surveys and watershed restoration projects. This is combined with knowledge of known land management activities that have the potential to contribute to the identified impairment.

Specifically, Probable Source Sheets are first drafted during rotational watershed surveys and watershed restoration activities by SWQB staff.  Information gathered from the Probable Source Sheets are used to generate a draft Probable Source list in consequent draft TMDL planning documents.  These draft Probable Source lists are finalized with watershed group/stakeholder input received during any one of the following: pre-survey public meeting, TMDL public meeting, watershed-based planning activities, and various public comment periods.  The SWQB maintains a standard operating procedure for this topic.

As part of the ATTAINS re-design, there were several discussions between EPA and states regarding the reporting of probable sources since most states do not have dedicated funding for source identification.  EPA Office of Water staff confirmed that probable sources for impaired AUs (i.e., IR Category 4 and 5) are an optional data element and not required in the new ATTAINs system.  Therefore, New Mexico is no longer reporting “Source Unknown” for AU_cause pairs without approved TMDLs.  As stated above, documenting probable sources is part of the TMDL process in New Mexico as opposed to the listing process.  Accordingly, probable sources have also been removed from the Integrated List (Appendix A). However, the SWQB does maintain probable sources documented in approved TMDLs in SQUID in order to provide a summary discussion of the primary sources of impairment in New Mexico.  This fulfills the CWA 305(b)(1)(E) requirement to provide “a description of the nature and extent of nonpoint sources of pollutants.” 

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USEPA. 1997. Guidelines for preparation of the comprehensive state water quality assessments (305(b) reports) and electronic uptakes. EPA-841-B-97-002A. Washington, D.C.

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