The ability of detainment bunds to mitigate the impact of pastoral agriculture on surface water quality in the Lake Rotorua catchment : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, Palmerston North, New Zealand

Abstract

Identifying and implementing cost-effective mitigation strategies are necessary to achieve reductions in the anthropogenic phosphorus (P) and nitrogen (N) loads that contribute to eutrophication and toxic algal blooms in Lake Rotorua, in the Bay of Plenty Region on the North Island of New Zealand. Storm generated surface runoff from grazed pastures, that cover ~48% of Lake Rotorua’s catchment, contribute 67% of the total N (TN) and 43% of the total P (TP) loads delivered from the catchment to the lake. Detainment bunds (DBs) are a novel mitigation strategy targeted at decreasing nutrient and sediment losses by impeding and temporarily ponding stormflows for up to 3 days. A DB is an earthen, stormwater retention structure, approximately 1.5-2 m high and 20-80 m long, constructed on pastures across the flow path of targeted low-order ephemeral streams. Two DBs on pastures in the Lake Rotorua catchment, with 20 and 55 ha catchments, were monitored over 12 months. Nearly 20 storm events resulted in ponding at each site. Detailed hydrological analyses were conducted for each storm in order to establish water balances, as well as to analyse contaminate loads delivered to, and discharged from the DBs. Surface runoff flows were measured, and samples were collected, to determine the DB mitigation performance and to identify the processes affecting the outcomes. The DBs prevented an estimated 51-59% of the annual suspended sediment loads, 47-68% of the annual TP loads, and 57-72% of the annual TN loads delivered to the DBs in runoff, from reaching the lake. An estimated 43-63% of the annual surface runoff delivered to the DBs infiltrated the soil, as a result of increased residence times of surface runoff on well-drained pasture soils. Soil infiltration was mainly responsible for decreased contaminant loads delivered to surface waters downstream of the bunds, while sorption and sedimentation also contributed to some load reductions. The inability to impound only portions of the runoff generated during rare, high magnitude storm events limited the performance of DBs. Furthermore, declining soil infiltration rates and increasing soil P concentrations in the ponding areas could affect the longer-term performance of DBs. A cost: benefit analysis of the DB strategy was conducted in order to compare the cost-effectiveness of DBs to other nutrient migration strategies, with results demonstrating that the DB strategy is a highly cost-effective edge of field mitigation option available to pastoral farmers in the Lake Rotorua catchment.