Conclusion
The results of this current study found that DBs located on pastures in
the Lake Rotorua catchment attenuated 789 kg SS at the Hauraki site, and
1280 kg SS at the Awahou site, accounting for 51% and 59% of the
annual inflow SS loads, respectively. These findings demonstrate that
DBs are an effective strategy available to pastoral farmers attempting
to mitigate their contribution to Lake Rotorua eutrophication, since and
the majority of P transported from the catchment is sediment-bound and
has the potential to become biologically available.
Large portions of the annual SS yields attenuated by the DBs occurred
during rare, high runoff magnitude events, which delivered the greatest
surface runoff and SS yields to the bunds during this study. Identifying
the ability of DBs to effectively decrease SS yields during these rare,
high magnitude storm events is important since large but less frequent
runoff events have been observed to contribute significant sediment
loads to streams in the northwest portion of the Lake Rotorua catchment
where this study was conducted.
The annual SS yield treatment efficiencies observed in this present
study were related to changes in SS concentrations through
sedimentation, and the portion of runoff infiltrating the soil in the
ponding area. Greater SS yields were discharged from the DBs with
greater discharged runoff volumes, which emphasises the importance of
optimising DB design to maximise the amount of runoff infiltrating the
soil. Still, the impoundment of runoff generally decreased event SS
concentrations, suggesting that DBs may effectively decrease SS loads
where soil infiltration rates, and pond storage to catchment area
ratios, are not as high as those this present study.
While this study found DBs consistently decreased SS yield discharged
from the DBs, identifying methods to improve concentration treatment
efficiencies, such as integrating flocculants, or allowing the
bottommost layer of the pond to infiltrate the soil rather than be
released, would improve yield treatment efficiencies. Also, cost:benefit
analyses should be conducted to determine whether removing the pond
drainage mechanism (i.e.. riser/outlet valve/discharge pipe unit) would
be beneficial, keeping in mind this might affect pasture productivity.
Longer-term studies should also be conducted on more DBs in the Lake
Rotorua catchment to further understand the potential of the strategy to
effectively mitigate pastoral farming’s impact to surface water quality.
Studies should also investigate the cause of declining soil infiltration
rates in the ponding area and methods for maintaining or rehabilitating
infiltration rates in order to maintain SS yield treatment efficiencies
over the life of the DB.