near-surface geologic environments

ENVIRONMENTAL GEOCHEMISTRY AT TEXAS A&M UNIVERSITY

http://environmentalgeochemistry.pbworks.com//

Near-surface Geologic Environments

Bruce HerbertGeology & Geophysics



Hierarchy model of Landscapes

■ Descriptions start at small scales

■ Describe flow between these systems

■ Describe spatial and temporal variation at higher scale



Hierarchy model of contaminated near-surface environments



Components of Near-surface Environments

■ Hydrology■ Sediment & rock

mineralogy■ Organic matter■ Gaseous composition■ Aqueous chemistry■ Morphology & Structure

There is a strong correlation between hydrology and composition & structure.

WHY?

Important Components



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



Components of Landscape Prisms

Landscape Prism

For a landscape prism, we are typically interested in:

■ Mineralogy■ Soil & ground water

chemistry■ Organic matter

composition■ Composition of soil

gases■ Microbial ecology



Soils and Regoliths

■ 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.

Unsaturated zone: pore spaces are filled with gases and water



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



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



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.



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.



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



Watersheds as Geologic Systems

Texas WatershedsFor a drainage basin, we are typically interested in:

■ Soil type & bedrock geology

■ Surface & ground water chemistry

■ Basin hydrology■ Sediment composition■ Fluvial landforms■ Channel morphology

near-surface geologic environments

Documents

flow of contaminants

rate of water flow

hydrologic flow lines

hydrologic flow paths

unsaturated zone

saturated zonessaturated

recharge water

hydrologic regimes