Radionuclides are radioactive forms of elements that can occur naturally in the environment or are man-made, either deliverability or as byproducts of nuclear reactions.   

Attenuation Processes for Metals and Radionuclides (APMR-1)
Section 2 (Physical Processes, Site Characterization, and Modeling Approaches)
Identifies the geochemical processes that govern the environmental behavior and fate of metals and radionuclides at sites where monitored natural attenuation (MNA) is being considered as a remedial alternative.

Radionuclides (RAD-2)
Section 2.2 (Categories of Radioactive Materials), pages 4-5
Describes the major categories of radioactive materials as defined in DOE’s Radioactive Waste Management Manual (DOE, 2001).

Attenuation Processes for Metals and Radionuclides (APMR-1)
Section 2.1.2 (Aqueous Speciation) through 2.1.7 (Geochemical Gradients), pages 12-22
Good General description of pH, ORP, eH and how that impacts groundwater plumes.