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Seepage wetland protection review McKergow and Hughes (2016) (Aim…
Seepage wetland protection review
McKergow and Hughes (2016)
Aim
Determine whether protection of seepage wetlands
on pastoral lands is likely to be a useful WQ improvement
strategy
Review existing literature
Characterise seepage wetland
investigate challenges and methods in science
management options explored
information gaps ID'ed
DairyNZ brief
what protection do they offer
does failure to protect represent a threat to WQ
how much protection is needed
how does protecting seepage wetlands compare to those from other mitigation options
can existing SW's be modified to enhance nutrient removal
what gaps in knowledge need to be addressed
Characteristics
Origin
Mostly fed by subsurface flows
via springs
can emerge along line or at ground
without distinct origin
can emerge from single point
Saturation
permanent saturation
temp dryness
Vegetation
wetland grasses
rushes
sedges
Soils
dense mat of plant roots
up to 15cm deep
then unconsolidated saturated
organic soils
then Layer of less permeable soil
eg clay
Location
change of slope
where particulate solids, incl mineral sed
and organic matter, accumulate
Formation
remnant wetlands
more recent landscape features
Sediment deposited through disturbance events
infilled channels
mass wasting
blocking of channels
Science
challenging
inputs diffuse
therefore difficult to understand wetland-scale WQ improvements
soils unconsolidated
hotspots
time
#
eg denitrification
space
#
#
Solutions
wetland-scale experiments
Demonstrated
Burns and Nguyen 2002
rapid removal of nitrate from subsurface
minimal surface removal during rainfall events
Rutherford and Nguyen 2004
25% surface removal of nitrate during dry weather
Rutherford and Nguyen 2004
Rutherford et al 2000
seepage wetland soil properties vary with depth
Matheson et al 2002
Wetland plants promote denitrification
through rhizosphere oxidation
requires highly anoxic soils
discourages dissimilatory reduction of
nitrate to ammonium
Constructed wetlands
removal varies with season and hydraulic loading
230-280 mgN m2/d expected
Higher than inferred from tracer exps
on seepage wetlands (natural)
5-120 mgN m2/day
Reduced performance
Animal grazing and drainage affects
soil-water content
nutrient cycling
vegetation
soils
where animals have
direct access
faecal and urine inputs
soil disturbance
Collins 2004
Large inc in E coli
McKergow et al 2012
high total and organic nitrogen exports
Indirectly
altering soil (compaction)
damaging vegetation
protection
depth
more important to exclude cattle from shallow wetlands
cattle naturally weary
of deeper water
but not necessarily for wetlands with deep channel and
shallow margins
fencing
electric and permanent fencing used
Incentive for farmers when stock regularly entrapped
Shouldn't be done through creation
of "benches" on steep terrain
Can become preferential pathways
or resting areas for stock
Increases faecal inputs and erosional losses
Grass cover maintained through stock access to perennial channels
planting
larger plants not advised
flaxes, shrubs, trees
creates channels in mature wetlands
Recommend grasses at top or above wetland, mowed or grazed in summer
Drainage management
Only 1 study in NZ
soil redox potential and oxidation rapidly improved after drains were blocked
Key pollutants and attenuation
Nitrogen
Nitrogen transported by
water in several forms
dissolved inorganic N
nitrite
ammonium
nitrate
dissolved organic N
particulate associated N
particulate organic N
adsorbed ammonium
Nitrogen can be transformed
in several ways during transport
mineralisation of organic N
nitrification of ammonium
Phosphorus
Dissolved/filterable/soluble
(<0.45um)
reactive (DRP)
organic (DOP)
Can include particulate forms attached to colloidal material <0.45um)
particulate (PP)
organic
inorganic
can be sorbed by soil particles and organic matter
Suspended solids (SS)
organic
microbes
living organisms
organic particles
mineral
clay
silt
mostly <63um mineral and <1mm organic
Faecal microbes
Viruses
bacteria
protozoa
Attenuation
between where generated
and where lost
transformation
temporary storage
permanent loss
processes can be
physical
flow attenuation, deposition
chemical
sorption, precipitation
biological
denitrification
microbial production of nitric oxide (NO),
nitrous oxide (N2O) and nitrogen
gas (N2) from nitrate
assimilation,
Wetland science
Input and outflow monitoring
Nguyen et al 1999
Pukemanga wetland
Whatawhata
low flows
Sink for nitrate, DRP
high flows
source SS
no denitirfication
90% groundwater at low flows, 50% at high flows
seasonal differences
67% removal in summer/autumn
13% winter
30% spring
Kiwitahi
removed sediment from surface runoff
sub-wetland scale research
Key findings
rapid removal of nitrate from subsurface flows
but potential for N to be transported across surface in
high flows
SWs can remove nitrate at high rates from surface water
in dry conditions
SW soil properties vary with depth
wetland plants promote denitrification
through rhizosphere oxidation
discourages nitrate to ammonium transformation
denitrification rates
4-6 mg N kg-1 h-1 typical
~24-48 hours contact time for almost 100% removal
soils containing 0.5 g N m-3 of added Nitrate
Cooper 1990
High rates of removal from riparian wetland soils where nigh nitrate concentrations first came in contact with organic soils with denitrifying bacteria
Got less as concentrations decreased
Highest in top 10 cm (Rutherford & Nguyen 2004)
Probably 300-700 mg N m2 day
Wetland plants important for providing conditions suitable for denitrification
Matheson et al. 2002
Without plants49% nitrate to ammonium,
29% denitrified, 22% immobilised
with plants, <1% to ammonium, 60% denitrified, 24% to soil, 11-15% plant uptake
roots change soil conditions to favour bacteria for the denitirfication process
Using unrelated systems -
constructed wetlands
Performance differs with seasonal
differences
Better performance in warmer seasons
And with extended residence time
Toenepi
Nitrate
average aerial rate nitrate 280 mg N m2 d
TN
Typical TN = ~80-330 g m2 d = 7-36%
Seasonally, 50% decrease to 20% increase
Bogburn
nitrate
Long-term average = 230 mg N m2 d
winter = ~20-63%
Plant accumulation
uptake rates of 0.5-1 g N m2 d are possible
Conclusions
ID and management of seepage wetlands
will improve WQ outcomes
particularly livestock exclusion
Gaps
experimental trials of livestock exclusion
prevention of channelisation
seasonal grazing
importance of dissolved organic N
importance of stabilising seepage wetlands in extreme events
Improving drainage management
Science is challenging