Preliminary assessment of the ability of detainment bunds to attenuate sediment and phosphorus transported by surface runoff in the Lake Rotorua catchment

Water-quality impairment due to excess nutrients entering Lake Rotorua has been recognised since the 1960s. So as to improve water-quality, the 2012 Lake Rotorua Nutrient Management Plan has set a target to reduce phosphorus (P) loads delivered to the lake by 10 t/year from a baseline of similar to 40 t P/year. Dissolved P and sediment-bound P loss from agriculture have been identified as significant sources of P entering the lake. Storm periods present significant opportunities to mitigate these losses. Detainment bunds (DBs) are a mitigation strategy that could potentially decrease P losses in storm events. A detainment bund is a low earthen dam constructed on productive pasture, which is capable of temporarily ponding large quantities of overland flow delivered by storm-generated ephemeral streams. Prior research on DBs and sedimentation basins serves as a proof of concept for the technology; however, there is no quantitative data available on the capacity of DBs to attenuate contaminants under New Zealand conditions. Quantification of DB performance is essential to the potential implementation of the technology at regional or national scales, and to the modelling of treatment efficacy in nutrient-management software such as OVERSEER (R). The present study reports on preliminary data from a DB receiving surface runoff from 19.7 ha within the Lake Rotorua catchment during three contrasting storm events. The results indicated that retaining water in DBs resulted in discharges with decreased volumes of water and decreased concentrations of suspended sediments, dissolved reactive P and total P. Load attenuation related to event size and resulted in decreased loads of sediment (14-91%), dissolved reactive P (19-69%) and total P loads (18-67%) discharged downstream. These preliminary results indicated the potential of this mitigation strategy to decrease loads of sediment and P delivered to Lake Rotorua by surface runoff from pastoral agriculture.