The source of radiation on Surferʻs Beach in Half Moon Bay has been determined to be primarily Thorium 232 and Radium 226, as reported at the Geiger Counter Bulletin. These are in a class of radioactive materials designated as NORM (Naturally Occurring Radioactive Materials). More on NORM here.
A subcategory of NORM is TENORM, or technologically enhanced naturally occurring radioactive materials. They have been concentrated through human activities, such as mining, oil exploration. More on TENORM here
NORM class materials are regulated in industrial settings, and there are even EPA guidelines on safety for NORM materials in the home. See EPA on radon gas.
The EPA has also issued information on Thorium 232, one of the radionuclides found on Surferʻs Beach. See EPA Fact Sheet.
Radium 226 is also covered by an EPA Fact Sheet here.
The best instrument for detecting Radium 226 and/or Thorium 232 is an instrument with a pancake GM tube. Optionally, but more expensive, a scintillation detector can be used. A geiger counter or standard scintillator will not identify the radionuclide, only show its presence. An MCA, multi channel analyzer, is needed to identify the radionuclides.