| (j) When determining concentrations of COCs in an environmental medium, the person shall collect and handle samples in accordance with sampling methodologies which will yield representative concentrations of COCs present in the sampled medium. (k) When determining concentrations of COCs in surface water and sediment, the person shall collect and handle samples in accordance with the requirements in the agency's Surface Water Quality Monitoring Procedures, Volume I, as amended, or shall use an alternative methodology approved by the executive director. (l) The person shall determine concentrations of COCs within the environmental media at the affected property. The executive director may approve the use of statistical or geostatistical methods to determine representative concentrations of COCs at the affected property or within areas representative of site-specific background conditions as long as the following conditions are satisfied. (1) The person shall ensure that all assumptions for the selected statistical or geostatistical method are met or critically examined and explained if the assumptions cannot be met (e.g., random sampling design, normal or log-normal distribution, etc.). Judgmental samples may be used, as long as it can be demonstrated that the resulting estimated representative concentration is not biased low. (2) An appropriate number of samples for the statistical method shall be used. If site-specific background is determined using the upper confidence limit or similar statistical method, then a minimum of eight samples shall be used. If the person uses an arithmetic average to determine the background concentration, then a minimum of five samples shall be used. (3) The soil exposure area for existing residential yards or platted residential properties shall not exceed 1/8th acre or the size of the front or back yard of the affected residential lot, unless it is demonstrated that a larger area, not to exceed 1/2 acre, is appropriate based upon the activity patterns of residents at a specific affected property. For other properties classified as residential (e.g., parks, hospitals), the executive director may approve a larger exposure area if justified based on site-specific conditions. If an area larger than 1/8th acre or the size of the front or back yard of the existing affected residential lot is approved by the executive director, then the person shall comply with the applicable institutional control in requirements §350.111(b), (b)(8) or (10) of this title (relating to Use of Institutional Controls). If COCs are relatively homogeneous over an area larger than the residential default size, the executive director may allow concentrations to be averaged over this larger area, in which case the institutional control would not be required. (4) The soil exposure area for commercial/industrial properties shall not exceed 1/2 acre, unless it is demonstrated that a larger area is appropriate based upon documented activity patterns for commercial/industrial workers at an active commercial/industrial facility (the assumed exposure area should represent the smallest area over which an individual can be expected to move randomly). In approving an exposure area for an active commercial/industrial facility, the executive director may consider any appropriate site-specific information which documents typical worker activity patterns. If an area larger than 1/2 acre is approved by the executive director, then the person shall comply with the institutional control requirements in §350.111(b), (b)(9) or (11) of this title (relating to Use of Institutional Controls), as applicable. If COCs are relatively homogeneous over an area larger than 1/2 acre, the executive director may allow concentrations to be averaged over this larger area, in which case the institutional control provision would not be required. (5) The executive director may require a separate assessment of smaller but notable areas of soil contamination (i.e., "hot spots") at sites where site-specific features are present such that there is likely to be preferential exposure to this smaller area (e.g., worker exposures around the physical infrastructure of a work space, soils within a child's play area). The presence of hot spots with respect to ecological risk shall be determined on a site-specific basis. (m) If a person does not desire to determine a site-specific soil background concentration, then they may use the Texas-specific median background concentrations for metals provided in the following figure. The Texas-specific background concentrations may be used to determine the critical PCL and then used in comparisons to individual measurements of COCs or representative concentrations of COCs in accordance with §350.79(1) or (2)(A) of this title (relating to Comparison of Chemical of Concern Concentrations to Protective Concentration Levels), respectively. Attached Graphic (n) Analytical results, including non-detected analytical results, should be considered whether doing direct comparisons of individual measurements or when using statistical or geostatistical approaches. In cases where there is reason to believe, based on available analytical data, that the COC could be present at that sampling location and that the concentration of the COC is suspected to be near but below the sample detection limit, the full value of the sample detection limit should be used as a proxy for the non-detected result. If there is reason to believe, based on available analytical data, that the COC could be present at that sampling location and that the concentration of the COC is suspected to be below, but not near to, the sample detection limit, then 1/2 the sample detection limit should be used as a proxy for the non-detected result. Other statistically-based approaches for handling non-detected results or assigning proxy values may be appropriate and approved if there is sufficient technical basis. If greater than 15 percent non-detected results are reported for a particular medium, and the exposure area cannot be definitively identified based on documented and verifiable site-specific information, the executive director may require persons to utilize alternative statistical methods for calculating the concentration term. (o) When required by the executive director, the person shall classify an affected property in accordance with a risk-based system established by the executive director. The classification shall consider all information collected during the affected property assessment, any historical knowledge concerning the conditions at the affected property, and the short-term or long-term potential for human or ecological receptors to be exposed to COCs. |
