Lowes Pit

Background 

Hastings District Council (HDC) hold a resource consent to divert and discharge stormwater from the Lowes Pit stormwater catchment area to water (Lowes Pit). Lowes Pit was established in the 1940s by Peter Lowe. At the time, the pit was surrounded by farmland and used to extract rock and gravel for building roads for the Hastings, Havelock North, and Napier councils. The pit eventually filled with water causing a small manmade lake. The pit is estimated to be about 6m deep with an area of 8,500m2. Over time, development in the area began to grow. Lowes Pit was used to drain stormwater from industrial properties that were too low to easily link into the council stormwater system. As a result, stormwater with nasty contaminants from industry yard runoff such as lead, copper, zinc, hydrocarbons, and E.coli entered the pit. The cost of diverting the stormwater into the wider council stormwater network was prohibitive.

Figure 1: Aerial image of Lowes Pit 1984 and 2023

The pit is situated over the Heretaunga Plains unconfined aquifer and is within the Frimley Bore source protection zone. This zone is a designated area around the Frimley borefields where strict regulations are in place to protect the quality of groundwater used for drinking water. In 2018, Lowes Pit was identified as a potential risk to the Heretaunga Plains unconfined aquifer. Concerns were raised around the potential for Lowes Pit to connect with groundwater beneath it, and in 2019 consultants Tonkin & Taylor completed an assessment of the contamination risks to the Frimley water supply fields posed by Lowes Pit. The investigation confirmed that;

• Water quality monitoring data collected showed that Lowes Pit and the stormwater discharges into the Pit are of typical urban and industrial stormwater quality – as compared to other similar catchments throughout New Zealand.
• Based on the travel times and distances, E.coli or viruses would be expected to die off well before reaching the Frimley Park borefield.
• Higher levels of contaminants than currently measured in Lowes Pit would not be expected to pose an unacceptable risk to the Frimley Park borefield.
• The Frimley borefield is not threatened by the E.coli levels measured in Lowes Pit to date. Overall, the pit is viewed as a very low contamination risk to the Frimley borefield.

Tonkin and Taylor reported that although the water quality of Lowes Pit presents a very low risk to drinking water, several management and engineering steps could still be considered to achieve improved water quality. This led Council to adopt a three stage barrier approach.

Three barrier approach

Figure 2: Schematic of the Three Barrier Approach strategy used to improve stormwater quality entering Lowes Pit

As outlined in the schematic, Barrier 1 consists of on-site pollution management controls (both quality and quantity) from individual industrial properties within the catchment area. Barrier 2 involves the capture and diversion of first flush and spill event contaminants. The term "first flush" refers to the initial phase of a rainfall event when the concentration of pollutants in the runoff is significantly higher than in the later stages. First flush contaminant removal is achieved using roadside sump gross litter capture devices (LittaTraps) and small pump stations. The LittaTraps capture floatable materials, sediment, and leaves that can block downstream pumps and bind to the bioscape media. The pump stations work further downstream to divert dry-weather flows (including spills and illegal discharges into the HDC network) as well as the first volume of rain from the catchment once rainfall begins. Each pump has a peak flow equivalent to a few homes. These flows are then diverted to the HDC wastewater network.

Figure 3: Barrier 2- LittaTrap installed in Hazelwood Street, Hastings

Barrier 3 is the two stormwater bioscapes installed on each side of Lowes Pit. These bioscapes are the final barrier prior to stormwater entering the environment. Any stormwater flows greater than the pump capacity, or those occurring after the pump has finished diverting the first volume of runoff will flow directly into the bioscape. The bioscape treats stormwater runoff through sedimentation, filtration, infiltration, absorption, and biological processes as flows pass through the specifically engineered soil media and plant roots. The treated stormwater is then captured by underlying drainage and directed to the bioscape outlet where the improved stormwater enters Lowes Pit.

Figure 4: Barrier 3- Western stormwater bioscape

Figure 5: Visual improvement of stormwater quality as it moves through the first flush and bioscape system (barrier 2 and 3)

Figure 5 shows the visual improvement of stormwater quality as it moves through the first flush and bioscape system (barrier 2 and 3). The left sample has been taken from the first flush pump station manhole which captures the initial phase of a rainfall event before being diverted to wastewater. This is the most contaminated stormwater. The middle sample has been taken from the bioscape inflow pipe within the stormwater diversion manhole, capturing the flows entering the bioscape. Lastly, the right sample shows the final condition of the stormwater prior to discharging into Lowes Pit after passing through the treatment media. This image is important as it captures the performance of the bioscape and its positive impact on stormwater quality.

What are the main sources of stormwater pollution in the Lowes Pit stormwater catchment? 

• Contaminants from public road surfaces e.g. vehicle related oils, hydrocarbons, sediments, heavy metals, etc.
• •Contaminants from private industrial sites e.g. chemical spills, truck washing, fuels/oils, and other process waste.

See the diagram below to understand how contaminants enter HDC’s public stormwater drains.  

What can industries do to improve the quality of stormwater coming from their site?

Industries create waste and need to minimise stormwater contamination on their sites. Industries can review their site practices and;

• Store equipment properly on-site. Any contaminants kept outdoors, such as drums, jerry cans, and containers, must be stored under cover. Large volumes of contaminants should also be placed in a bunded area or on spill pallets.
• Arrange for a professional to remove stormwater yard sump contents annually.
• Install LittaTraps in yard stormwater sumps to capture litter, sediment, and debris.
• Implement appropriate treatment devices based on site activities and contaminants generated, such as an oil and grit separator, swale, rain garden, or filter.
• Keep at least one spill kit on-site and ensure all staff are trained in spill response.
• Educate staff on the on-site stormwater network and how site activities can impact stormwater quality and downstream environments.
• Implement stormwater management procedures for the site.