1. (a) Assessment monitoring is required whenever a statistically significant change from background has been detected for one or more of the constituents listed in §330.241 of this title (relating to Constituents for Detection Monitoring), or in the alternative list established pursuant to §330.234(a)(2) of this title (relating to Detection Monitoring Program), and this constitutes triggering.

2. (b) Within 90 days of triggering an assessment monitoring program in accordance with §330.234(d) of this title (relating Detection Monitoring Program), and not less than annually thereafter, the owner or operator shall sample and analyze the groundwater monitoring system for all constituents identified in paragraph (1) of this subsection.

   1. (1) The constituents to be analyzed in samples collected pursuant to subsection (b) of this section shall be those listed in Appendix II to 40 Code of Federal Regulation Part 258 and those in the alternative list established pursuant to §330.234(a)(2) of this title (relating to Detection Monitoring Program). All of these constituents are hereinafter referred as "assessment constituents." Appendix II to 40 Code of Federal Regulation Part 258, effective October 9, 1993, is herein adopted by reference.
   2. (2) A minimum of one sample shall be collected from each well and analyzed for the assessment constituents during each new sampling event. For any constituent(s) detected in the downgradient wells as a result of the analysis of the assessment constituents, a minimum of four statistically independent samples from each background and downgradient well shall be collected and analyzed to establish background levels for the constituent(s). The executive director may specify an appropriate subset of wells to be sampled and analyzed for the assessment constituents during assessment monitoring and may delete any of the assessment constituents for a municipal solid waste landfill (MSWLF) unit if it can be documented that the removed constituents are not reasonably expected to be in or derived from the waste contained in the unit.
3. (c) The executive director may specify an appropriate alternative frequency for repeated sampling and analysis for the assessment constituents required by subsection (b) of this section during the active life and the closure and post-closure care period of the unit. The alternative frequency shall be no less than annual and shall be based on factors such as lithology and hydraulic conductivity of the aquifer and unsaturated zone, groundwater flow rates, minimum distance of travel from the waste nearest to any downgradient monitoring well, resource value of the uppermost aquifer, and nature (fate and transport) of any constituents detected in response to this section.

4. (d) Not later than 45 days after each sampling event, the owner or operator shall submit to the executive director the results from the initial and subsequent sampling events required in subsection (b) of this section and also place them in the operating record. The owner or operator shall also:

   1. (1) within 90 days of submittal of the results from a sampling event and on at least a semiannual basis thereafter, resample all wells specified by §330.231(a) of this title (relating to Groundwater Monitoring Systems) and conduct analyses for all constituents in §330.241 of this title (relating to Constituents for Detection Monitoring) or in the alternative list established pursuant to §330.234(a)(2) of this title (relating to Detection Monitoring Program) and for those constituents in Appendix II of 40 CFR Part 258 that are detected in response to subsection (b) of this section. The results shall be submitted to the executive director not later than 45 days after the sampling event and shall also be placed in the operating record. At least one sample shall be collected and analyzed from each background and downgradient well at each sampling event. The executive director may specify an alternative monitoring frequency during the active life and the closure and post-closure care period for the constituents referred to in this paragraph. The alternative frequency for constituents in §330.241 of this title (relating to Constituents for Detection Monitoring), or the alternative list established pursuant to §330.234(a)(2) of this title (relating to Detection Monitoring Program), during the active life and the closure and post-closure care period shall be not less than annual. The alternative frequency shall be based on consideration of the factors described in subsection (c) of this section;
   2. (2) establish background concentrations for any constituents detected pursuant to subsection (b) of this section or paragraph (1) of this subsection;
   3. (3) establish groundwater protection standards for all constituents in downgradient wells detected pursuant to subsection (b) of this section or paragraph (1) of this subsection. The groundwater protection standards shall be established in accordance with subsection (h) or (i) of this section.
5. (e) If the concentrations of all assessment constituents are shown to be at or below background values, using the statistical procedures in §330.233(g) of this title (relating to Groundwater Sampling and Analysis Requirements) for two consecutive sampling events, the owner or operator must notify the executive director in writing and return to detection monitoring if approved.
6. (f) If the concentrations of any assessment constituents are above background values, but all concentrations are below the groundwater protection standard established under subsection (h) or (i) of this section, using the statistical procedures in §330.233(g) of this title (relating to Groundwater Sampling and Analysis Requirements), the owner or operator shall continue assessment monitoring in accordance with this section.

7. (g) If one or more assessment constituents are detected at statistically significant levels above the groundwater protection standard established under subsection (h) or (i) of this section in any sampling event, the owner or operator shall notify the executive director and appropriate local government officials in writing and place a notice in the operating record within 60 days of the sampling event identifying the assessment constituents that have exceeded the groundwater protection standard.

   1. (1) The owner or operator shall also:

      1. (A) characterize the nature and extent of the release by installing additional monitoring wells as necessary;
      2. (B) install at least one additional monitoring well at the facility boundary in the direction of contaminant migration and sample this well in accordance with §330.235(d)(1) of this title (relating to Assessment Monitoring Program);
      3. (C) notify in writing all persons who own or occupy the land that directly overlies any part of the plume of contamination if contaminants have migrated off site as indicated by sampling of wells in accordance with subsection (d)(1) of this section; and
      4. (D) initiate an assessment of corrective measures as required by §330.236 of this title (relating to Assessment of Corrective Measures) all within 90 days of the notice to the executive director.
   2. (2) The owner or operator may demonstrate that a source other than an MSWLF unit caused the contamination or that the statistically significant change resulted from error in sampling, analysis, statistical evaluation, or natural variation in groundwater quality. A report documenting this demonstration shall be prepared and certified by a qualified groundwater scientist and submitted to the executive director for review and approval, and shall be placed in the operating record. If a successful demonstration is made, the owner or operator shall continue monitoring in accordance with the assessment monitoring program pursuant to this section and may return to detection monitoring if the assessment constituents are at or below background as specified in subsection (e) of this section. Until a successful demonstration is made, the owner or operator shall comply with paragraph (1) of this subsection including initiating an assessment of corrective measures.

8. (h) The owner or operator shall establish a groundwater protection standard for each assessment constituent detected in the down-gradient monitoring wells. The groundwater protection standard shall be:

   1. (1) for constituents for which a maximum contaminant level (MCL) has been promulgated under §1412 of the Safe Drinking Water Act (codified) under 40 Code of Federal Regulations Part 141, the MCL for that constituent;
   2. (2) for constituents for which MCLs have not been promulgated, the background concentration for the constituent established from wells in accordance with §330.231(a)(1) of this title (relating to Groundwater Monitoring Systems); or
   3. (3) for constituents for which the background level is higher than the MCL identified under paragraph (1) of this subsection or health-based levels identified under subsection (i) of this section, the background concentration.

9. (i) The executive director may establish an alternative groundwater protection standard for assessment constituents for which MCLs have not been established. These groundwater protection standards shall be appropriate health-based levels that satisfy the following criteria:

   1. (1) the level is derived in a manner consistent with Environmental Protection Agency guidelines for assessing the health risks of environmental pollutants (51 FedReg 33992, 34006, 34014, 34028, September 24, 1986);
   2. (2) the level is based on scientifically valid studies conducted in accordance with the Toxic Substances Control Act Good Laboratory Practice Standards (40 Code of Federal Regulation Part 792) or equivalent;
   3. (3) for carcinogens, the level represents a concentration associated with an excess lifetime cancer risk level (due to continuous lifetime exposure) with the 1 x 10[sup]-4[/sup] to 1 x 10[sup]-6[/sup] range; and
   4. (4) for systemic toxicants, the level represents a concentration to which the human population (including sensitive subgroups) could be exposed to on a daily basis that is likely to be without appreciable risk of deleterious effects during a lifetime. For purposes of this subchapter, systemic toxicants include toxic chemicals that cause effects other than cancer or mutation.
10. (j) In establishing groundwater protection standards under subsection (i) of this section, the executive director may consider multiple contaminants in the groundwater, exposure threats to sensitive environmental receptors, and other site-specific exposure or potential exposure to groundwater.

Source Note:The provisions of this §330.235 adopted to be effective October 9, 1993, 18 TexReg 4023.