Groundwater monitoring is ongoing and a number of bores throughout the Waituna catchment have been monitored on a monthly basis since 2012. High frequency monitoring also takes place with extra samples collected prior to, during, and directly after artificial lagoon openings, and in response to floods.
A comprehensive scientific study of the Waituna catchment’s dynamic groundwater system has identified three distinct zones. Each zone is distinguished by clear differences in geology, soil types, hydrological behaviour and water quality, which means they are affected differently by contaminants.
Also identified was a significant amount of direct groundwater seepage from Southern Zone aquifers into the lagoon via the lagoon bed. The study was completed in 2012 by Environment Southland and its findings are contained in the Waituna Catchment Groundwater Resource Technical Report. The following is a brief summary of the report’s key findings.
Northern Waituna Zone
Waituna Creek catchment north of Mokotua
The Northern Waituna Zone has relatively good groundwater quality compared to Southland-wide norms. This is due to the presence of thick, stoneless, mineral-brown soils (types: Woodlands, Mokotua, Waikiwi and minor Tisbury) and the long delay (several months) between rainfall and its deep infiltration to the underlying aquifer.
Thick mineral soils and slow infiltration rates play an important role in the removal of soil contaminants associated with intensive farming. Excluding tile drainage, which is elevated in nutrients, shallow aquifers across this zone show little impact from intensive land use.
Mokotua Infiltration Zone
Waituna Creek catchment between Mokotua and Caesar Road
The Mokotua Infiltration Zone is characterised by rapid infiltration, which is likely to be associated with the reworking of soil and aquifer materials during a former high sea level, approximately 70,000–100,000 years ago.
Across this zone, groundwater quality is poor. This is due to the rapid infiltration of soil water, high in nutrients from intensive land use, direct to the aquifer. The movement of water through the soil and into the underlying aquifer takes 1–2 weeks and appears to contribute to the deterioration in the water quality of Waituna Creek, south of Mokotua.
Southern Waituna Zone
South of Caesar Road to the Waituna Lagoon
This southern, predominately wetland area, includes both the Moffat and Carran Creek catchments.
The Southern Waituna Zone is dominated by oxygen-poor/ iron-rich groundwater due to the abundance of wetland peat deposits, and to a lesser extent, lignite measures.
The shallow groundwater systems fill rapidly following rainfall (within 3– 5 days) via the soil. The low oxygen in the groundwaters across this zone result in the active removal of leached nitrate, which keeps concentrations very low.
However, these low oxygen groundwaters and wetland soil types favour phosphorus loss. Concentrations are up to 50 times higher here, than in the high-oxygen groundwaters of the Northern and Mokotua groundwater zones.
The elevated phosphate concentrations in the groundwaters reflect both the leakiness of phosphate from organic soils, and the naturally higher solubility and mobility of phosphate in low oxygen conditions. The origin of the majority of the phosphate is from intensive land use, especially the application of super phosphate and losses via winter grazing.
Direct Groundwater Seepage Into Waituna Lagoon
In addition to the three distinct groundwater zones, a considerable volume of groundwater discharges directly into the lagoon. Measurements indicate between 340 to 460 litres per second seep through via the bed of the lagoon. This is enough groundwater to completely fill an Olympic sized swimming pool every 1½ to 2 hours.
Direct seepage of groundwater to the lagoon is likely to be even higher during the winter months when water tables are high. Direct groundwater seepage to the lagoon also transports nitrogen and phosphorus derived from intensive land use directly to the lagoon.