ASBOG ASBOG - Association of State Boards of Geology Geochemistry and Soil Science Questions and Answers 2 — Questions and Answers
Question 1: In geochemical exploration, what is the 'threshold' concentration?
- The maximum concentration found anywhere on a site
- The detection limit of the analytical instrument
- The upper limit of background values above which concentrations are considered anomalous (Correct answer)
- The average of all samples collected
Correct answer: The upper limit of background values above which concentrations are considered anomalous
The geochemical threshold is the statistically determined upper boundary of background values; sample concentrations exceeding this level are considered anomalies potentially indicating mineralization.
Question 2: The Unified Soil Classification System (USCS) uses two-letter symbols such as SP, ML, and CH. What does the first letter indicate?
- Plasticity class
- Primary grain size group (Correct answer)
- Soil color
- Permeability class
Correct answer: Primary grain size group
In USCS, the first letter denotes the primary grain size group: G (gravel), S (sand), M (silt), C (clay), O (organic), or Pt (peat).
Question 3: Sandy soils have higher hydraulic conductivity than clay soils primarily because:
- Sand particles are more chemically reactive
- Sand has larger pore spaces allowing faster water flow (Correct answer)
- Clay contains more organic matter
- Sand particles have higher surface area per unit mass
Correct answer: Sand has larger pore spaces allowing faster water flow
Larger inter-particle pore spaces in sandy soils offer less resistance to water movement, resulting in hydraulic conductivity orders of magnitude higher than fine-grained clay soils.
Question 4: Carbon-14 isotopic dating is effective for determining the age of groundwater up to approximately how many years?
- 5,000 years
- 500 years
- 500,000 years
- 40,000 years (Correct answer)
Correct answer: 40,000 years
With a half-life of approximately 5,730 years, carbon-14 is effective for dating groundwater and organic materials up to roughly 40,000 years before the present.
Question 5: What is diagenesis in the context of sediment geochemistry?
- High-temperature metamorphic recrystallization
- Physical and chemical changes in sediments after deposition but before metamorphism (Correct answer)
- Erosion of pre-existing sedimentary rocks
- Formation of igneous intrusions within sedimentary sequences
Correct answer: Physical and chemical changes in sediments after deposition but before metamorphism
Diagenesis encompasses compaction, cementation, dissolution, and recrystallization processes that convert loose sediment into sedimentary rock under relatively low temperature and pressure conditions.
Question 6: Which laboratory technique bombards a rock sample with X-rays to determine major and trace element concentrations?
- Mass spectrometry (ICP-MS)
- Atomic absorption spectroscopy (AAS)
- X-ray fluorescence (XRF) (Correct answer)
- Neutron activation analysis (NAA)
Correct answer: X-ray fluorescence (XRF)
XRF analysis excites atoms in a sample with primary X-rays, causing them to emit secondary X-rays at energies characteristic of each element, enabling non-destructive bulk geochemical analysis.
ASBOG ASBOG - Association of State Boards of Geology Geochemistry and Soil Science Questions and Answers 2
In geochemical exploration, what is the 'threshold' concentration?