1. Near-surface Geologic Environments
Bruce Herbert
Geology & Geophysics

2. Hierarchy model of Landscapes
Descriptions start at small scales
Describe flow between these systems
Describe spatial and temporal variation at higher scale

3. Hierarchy model of contaminated near-surface environments

4. Components of Near-surface Environments
Important Components
Hydrology
Sediment & rock mineralogy
Organic matter
Gaseous composition
Aqueous chemistry
Morphology & Structure
There is a strong correlation between hydrology and composition & structure. WHY?

5. Hydrologic Flow Regimes
The hydrology of the subsurface environment in part determines the fate of pollutants.
The rate of water flow affects the attainment of chemical equilibrium by the subsurface geochemical processes.
Hydrologic flow paths also determine the type of solid surfaces that can interact with dissolved compounds.
The most fundamental distinction between hydrologic regimes is the differences between the unsaturated and saturated zones

6. Components of Landscape Prisms
For a landscape prism, we are typically interested in:
Mineralogy
Soil & ground water chemistry
Organic matter composition
Composition of soil gases
Microbial ecology

7. Soils and Regoliths
Unsaturated zone: pore spaces are filled with gases and water
Water is under negative hydraulic pressure.
Recharge water has high concentrations of O2 and CO2
Solid phase is dominated by unconsolidated sediment and organic matter
Soil are biologically active because of the large inputs of plant debris to these systems.
Flow of contaminants to groundwater usually passes through unsaturated zone first.
Upward movement of water by capillarity in glass tubes of different diameter and capillarity in soils.

8. Saturated Zones Saturated zone: pore spaces are filled with water.
Boundary between the saturated and unsaturated zone is the capillary fringe.
Groundwater, lake and estuary, wetlands.
Porosity can vary between 0 to 60%
Organic carbon is usually less than 2% except for shales, lignites or other C rich zones.
Much lower biological activity in this zone compared to the unsaturated zone.
Solid phase can be consolidated rock or unconsolidated sediment

9. Saturated Zones: Flow Systems
A hydrologic flow system is a subsurface region where hydrologic flow lines remain adjacent throughout the region
Local flow systems
Intermediate flow systems
Regional flow systems

10. Saturated Zones: Local Flow Systems
The recharge area at a topographic high and the discharge area at a topographic low are adjacent.
Hydrologically active: residence times of water are on the order of days to years.
These systems are relatively open in respect to nutrients and O2. This makes them biologically active.
Roughly 80 to 90% of all ground water contamination events are in local flow systems.

11. Saturated Zones: Intermediate Flow Systems
Recharge and discharge areas are separated by one or more topographic highs.
These aquifers produce a high percentage of GW used for human consumption.
Much lower biological activity due to lower amounts of O2 and nutrients.

12. Saturated Zones: Regional Flow Systems
Recharge area occupies the water divide and the discharge area occurs at the bottom of the basin.
Ground water has a long residence time. Typically the water has a high salt content.
Some anaerobic microbial activity

13. Watersheds as Geologic Systems
For a drainage basin, we are typically interested in:
Texas Watersheds
Soil type & bedrock geology
Surface & ground water chemistry
Basin hydrology
Sediment composition
Fluvial landforms
Channel morphology