SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Mr. Kirk Williams • - 4 - • 25 April 2008 Low-level analyses (SRL 524M) of 1,2,3-TCP are removed from the schedule for upgradient well MW-10. Concentrations of 1,2,3-TCP on-site are several orders of magnitude greater than those found in MW-10, and at the present time, knowledge of low levels of 1,2,3-TCP coming from upgradient sources will not affect remediation or site assessment. As remediation becomes more complete, low-level analyses should be reinstated in the upgradient monitoring well. The monitoring well network also includes redundancy around the center of mass. The monitoring frequency for some of the redundant wells has been reduced, such as MW-13A and MW-13B. Immediately downgradient of the center of mass, MW-8 is added to the monitoring network. In MW-8, 1,2,3-TCP will be measured on a biennial basis. This identifies if the center of mass is approaching a residential neighborhood. Further downgradient of the center of mass, MW-6 shall continue to be monitored for herbicides and pesticides. This well serves as a sentry well for these compounds. In this draft MRP, two wells will not be regularly sampled for constituents (MW-3 and E-2). These two wells shall be included in the annual water level monitoring network to ensure that they are located and inspected at least once a year. One of the C-zone wells is situated near the center of mass of the constituents (MW-7). This well serves as a sentry well in the vertical plane and shall continue to be sampled for constituents observed in the B-zone. Analytical Methods and Detection Limits GeoSyntec Consultants identified that in Spring 2002, Western Farm Service sampled monitoring wells for triazine pesticides, organochlorine compounds, and organophosphate compounds. Since these analytes were not detected, they are not proposed in the draft MRP. There are three methods for identifying 1,2,3-TCP in groundwater, each method has a very different detection limit. The most expensive method has the lowest detection limit. This is SRL 524M, also referred to as low-level analysis, and can achieve a detection limit of 0.005 ug/L for 1,2,3-TCP. EPA Method 504.1, the fumigant analysis, can achieve a detection limit of 0.02 ug/L for 1,2,3-TCP, and EPA Method 8260B (for volatile organic compounds) achieves 5 ug/L. When concentrations of 1,2,3-TCP are higher than 0.02 ug/L in groundwater, Western Farm Service may use less expensive methods that have higher detection limits than the low level method, provided that they are appropriate for the concentrations typically observed. The draft MRP identifies which of the monitoring points contain 1,2,3-TCP at concentrations great enough to warrant analyses with higher detection limits. In 2004, Western Farm Service changed analytical laboratories, which brought about the following changes: • the new laboratory cannot meet the detection limits for chlorinated herbicides that was achievable by the previous laboratory; • the new laboratory conducts carbamate/urea analyses using EPA Method 632, which does not include bromacil, whereas the previous laboratory analyzed by EPA Method 8321, which did include bromacil;