The low-cost radioactive scan threshold for further analysis is normally the ______ for ______ or ______ but can be varied by the state agency.

• A. MCL, uranium, thorium
• B. detection limit, gross alpha, gross beta emitters
• C. MCL, rem, rad
• D. detection limit, uranium, thorium

Answer: (B)

Screening for radioactivity in drinking water relies on a simple, low-cost scan that uses the detection limit for gross alpha and gross beta emitters as the threshold for calling for further analysis. This approach is practical because gross alpha and gross beta tests are quick and inexpensive, serving as a first check to flag samples that may have elevated radioactivity. If the result meets or exceeds the detector’s limit, the sample is then subjected to more detailed, isotope-specific testing (such as looking for uranium, thorium, or other radionuclides) to determine exact levels. The state agency can adjust this threshold to reflect policy, risk tolerance, or laboratory capabilities. The other options don’t fit because MCLs are regulatory limits for compliance, not screening thresholds; rem and rad are dose and energy units, not screening criteria; and a detection limit for uranium or thorium would refer to targeted analysis rather than the broad, inexpensive screening using gross alpha and gross beta.