Geol 230
GW/SW interaction
Week 3 Lecture
Role of the riparian environment
Reading: Ch. 3, Jones and Mulholland, pp. 83-107, by Harvey and Wagner
I) What is the riparian corridor?
- Zone influenced by stream processes
- Essentially the floodplain
- Mountainous areas: a narrow strip
- water table is lower than land surface
- stream is incised
- Lowland areas: a wide belt
- includes the transition from terrestrial to aquatic environments
- includes wetlands
- Our chapter: deals primarily with low order (1-3rd order) streams in humid
environments (excludes lowland floodplains, wetlands, etc)
II) Variables that affect riparian input:
- Magnitude of GW flows
- Seasonality of GW flows
- Retention efficiency
- Flowpaths (length, depth) see Fetter diagram: flowpath lengths
Note: GW input may take short, intermediate or long flowpaths
- Long flowpaths: result in direct GW input
- Short flowpaths: result in interaction of GW with riparian zone
before input to stream
- Overland flow vs. GW input
- Hydraulic gradient
- Water table position
- Water retention capacity of soils
- Soil thickness (depth to bedrock)
- Vegetation (and vegetative uptake)
- Aspect
- Substrate permeability
- Storm size (duration)
- Chemical and biological transformations:
redox reactions (anoxic):
- anaerobic bacteria use alternate electron acceptors:
- nitrate (NO3-) reduced to N2 or N2O
- Mn+4 → Mn+2
- Ferric iron (Fe+3) → ferrous iron (Fe+2)
oxidizing reactions (oxic):
- organic matter is oxidized by aerobic bacteria
- oxygen is used as an electron receptor
- Water table fluctuations
- May overprint a seasonal change in chemistry
- ex: declining water table exposes more soil to oxidizing conditions
III) Riparian influences on stream chemistry:
A) Nitrogen:
Compare: Nitrate (NO3-), Ammonium (NH4+) concentrations in: Uplands, riparian
zone, stream:
See J&M Figure 1, p. 88: Central Amazon watershed comparison of:
Upland:
Nitrate (NO3-)
Ammonium (NH4+)
Riparian
Nitrate (NO3-)
Ammonium (NH4+)
Stream
Nitrate (NO3-)
Ammonium (NH4+)
360 – 650 μg/l
LOW!
<50 μg/l
INCREASED! (probably from
denitrification in low O2 zone)
A mystery: very low!! You would expect
high ammonium from nearby riparian region
- A similar pattern was observed in a Puerto Rican Forest, where GW flowed
through a reduced zone.
- The process: denitrification
- But: this process isn’t completely understood
- ex: Why are streams so low in ammonium? They should have high
ammonium from nearby riparian zones
- A possible explanation: Hyporheic or in-stream interaction changes
ammonium to some other form of N (nitrification)
- Other examples in textbook:
tell of variable seasonal input to streams
tell of variable interaction between riparian zones and streams
- The bottom line:
- there isn’t a simple, repeatable pattern here
- there are complex interactions between riparian zone and stream
- nitrogen contributions (species, quantity) to streams vary!!
See Fig. 2: GW is high in nitrate (vs. stream)
See Fig. 3A: GW is low in nitrate (vs. stream)
- Some generalizations:
- Organic matter + nitrate (anoxic environment) → denitrification (nitrate
loss)
- Oxidizing environment → higher nitrate concentrations
B) DOC:
1) Wetlands:
- Anoxic or low oxygen environments
- Tend to have high DOC input to streams
2) DOC vs. discharge:
- DOC α discharge
3) DOC vs. storms:
- DOC increases during storms
- Possibly as stream gets input from organic-rich floodplain
C) Other elements:
1) Phosphorus:
- Riparian zone may be a sink our source
- May depend on oxygen levels
anoxic: iron and manganese oxides are reduced, releases
phosphorus
- Other times: patterns are less clear
2) Sulfate:
- Sulfate may decrease as subsurface water flows through the riparian zone
- Caused by: reduction of sulfate in low oxygen conditions of floodplain
and riparian zone
See Figure 6 from J&M, p. 97
- In contrast: Oxidation of peat-rich soils (rich in sulfur) may RELEASE
sulfate to nearby streams.
- May happen in dry summer? conditions when water table is low,
organic-rich soils are exposed to air
See Figure 7 from J&M, p. 98
- Note inverse relationship between water table in peaty soils and sulfate
concentration in nearby stream
3) Methyl mercury:
- Generally increases when oxygen content is low (organic-rich floodplain
sediments)
IV) Conceptual model:
See J&M Figure 9, p. 104
- Illustrates several options for position of water table relative to stream
- Includes some options for an impermeable layer
- The higher the water table, the more input from the shallow riparian zone
- Results in variable:
- Amounts of storage and contribution (of dissolved constituents) in
the riparian zone
- Connection to the stream
- Extreme case = Fige. 9E, where overland flow results