Near-surface Geologic Environments

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

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