SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

.. .. Ms. Donna Heran, REHS September 3, 1993 San Joaquin County Public Health Services Page 2 Mr. Boggs indicated the Central Valley Region expects a 0.5 ppb detection limit because it is their opinion that the limit is commonly achievable by laboratories. He agreed that the Tri- Regional document is a set of recommendations and not regulations and further stated that the document recognizes that 0.5 ppb may not be achievable by all laboratories. He also stated that the document suggests that if the Practical Quantitation Limits (PQLs) are not achievable, an explanation should be submitted with the laboratory report. Mr. Boggs indicated that WESTON's position on PQL and Method Detection Limits (MDL) was consistent with laboratory practices and felt that should a higher detection limit be achieved, it would be sufficient to provide an explanation in the report. The practical result of these discussions as related to the San Joaquin project is that in future groundwater monitoring reports, WESTON will either meet the Tri-Regional recommended detection limit of 0.5 ppb for BTEX if possible or provide a statement of explanation to support reporting a higher detection limit. Concentrations Below Detection Limits Regarding the presence of constituents in groundwater at concentrations below the analytical detection limit, the analytical detection limit for TPH-G and TPH-D was reported as 50 ppb as shown in the analytical reports presented in Appendix B of the Quarterly Groundwater Monitoring Report, Second Quarter 1993. The detection limits are also shown at the top of the columns in Table 2 of that report. As noted by the EHD, the detection limit for TPH-G was incorrectly shown in the table. WESTON regrets this typographical error, however, this error in no way effects the data presented in the table, the body of the report, or the appendix. The EHD's letter, however, is incorrect in stating that in Table 2, ND indicates that no contaminants are detected. The nomenclature used in the table is clearly defined in four specific notes beneath the table which are keyed to superscripts in the table. As stated in the first note beneath the table, ND is defined as "non-detectable at the detection limit shown at top of column or in subscript". Hence, the symbol ND indicates that a chemical constituent is non-detectable at or above the limit of quantification. It is, of course, common that a constituent can be "seen" by the instrument below the detection limit and a concentration estimated. However, for site evaluation purposes, estimates at extremely low concentrations have relatively little practical value. In the recent site data, TPH-G and TPH-D may be present in wells MW-6 and MW-7 at extremely low estimated quantities, but they were reported in the summary table as ND due to the lack of quantification at the instrument detection limit. The WESTON analytical laboratory typically reports estimated chemical concentrations with a "J" symbol when chemicals are detected at concentrations below the detection limit. In such cases, it is WESTON's practice to report the concentration as ND, as defined above, in the body of the report, with the complete analytical report presented in an attached appendix. If in the event that for some practical reason the concentrations below the detection limits should be Sf MSMERA .WP