As one of a number of studies supporting an ICMP, we were commissioned by the sponsors of a proposed Greenfields urban development at Rotokauri near Hamilton City to: (i) provide defensible water quality targets for a range of receiving waters potentially impacted by the development, (ii) identify the contaminants that are most limiting (i.e. will require the greatest treatment to ensure that water quality targets are not breached), and (iii) quantify the stormwater treatment required both during the development phase and when the urbanized area is ‘mature’. Here we focus on Lake Rotokauri, since this shallow hypertrophic lake is closest to the bulk of the development area.
There were relatively abundant nutrient data available for the lake from previous studies but data on potential urban contaminants, e.g. metals, PAHs, and emerging contaminants, were sparse. We obtained baseline data on these contaminants by deploying passive sampling devices on inflow and outflow streams, and from lake sediment samples. From these data, we could propose water quality targets for lake water and sediments based on USEPA and ANZECC guidelines.
Nutrient limits for the development were based on the policy (Vision & Strategy) that some improvement in lake water quality is required. We constructed a spreadsheet model to estimate the annual nutrient load during both construction and mature phases, which provided input to a calibrated SLAM (Simplified Lake Analysis Model) for the lake. The SLAM model predicted the effects of different nutrient loads to the lake on phytoplankton concentrations (as chlorophyll a) in the water column. We showed that phosphorus is the key nutrient to target with stormwater treatment, and that provided at least 70% removal can be achieved during both the development phase and when the urban catchment is mature, water quality of Lake Rotokauri should not deteriorate and may even improve.