Radium in Groundwater

Question 1

Sources of Radium

Answer 1

Geogenic: Naturally occurring radioactive materials
anthropogenic: Oil and gas industry, uranium mining, nuclear waste, improperly disposed radioluminescent products

Question 2

Parent Nuclide occurrence

Answer 2

Uranium is mobile in ground water:
oxic conditions it occurs as U(vi)
anoxic conditions: U(IV) precipiates

thorium is immobile in groundwater
- th is insoluble

Question 3

Geologic sources of radium

Answer 3

Shales, bitumen slate, volcanic rocks

Question 4

Secular equilibrium

Answer 4

As parent isotope breaks down, the radioactivity of the daughter isotope increases
- after about 5 half lives, daughter and parent isotope activities will be equal

Question 5

alpha recoil

Answer 5

238U decaying into 234U will reach secular equilibrium

• Within the rock: the number of decays per second of 238U = the number of decays per second of 234U
• On the surface: the alpha particle emitted during decay will cause the daughter nuclide to move in the opposite direction

-May cause the 234U to be ejected into the aquifer
The aquifer will become enriched in daughter nuclides over time

Question 6

Cation Exchange

Answer 6

Ra ions stored on the surface of solid phases in exchange for another divalent cation or two monovalent cations

Question 7

adsorption

Answer 7

• radium readily sorbs to clays and iron/manganese (hydr)oxides minerals
• Changes in geochemical conditions can reduce Ra sorption efficiency to these surfaces, TDS increases and reducing conditions

Question 8

Co-precipitation

Answer 8

radium commonly co-precipitates with
barite
celestite
gypsum

Question 9

Confined aquifer

Answer 9

overlain by a unit with poor permeability
- often anoxic
-longer residence times
-higher TDS

Question 10

Unconfined aquifer

Answer 10

In contact with atm
Often more oxic
- shorter residence times
-lower tds

Question 11

Radium control: unconfined aquifer

Answer 11

• barite co-precipitation is the dominant control on Ra concentrations
• unusual in low-temp, freshwater aquifers
• sorption is usually dominant control

Question 12

Confined

Answer 12

• increased calcium: more strongly exchanged
• no low-redox zone: can dissolve solid phases releasing sorbed radium
• excess radium is present beyond barite control

Question 13

Possible sources for excess radium

Answer 13

1. the aquifer solids in shaly zones or rinds enriched in the parent isotopes
2. from the maquoketa shale confining layer leakage
3. transported into the aquifer by deep-seated brines originating from the michigan basin