WRC 'Water quality of Lake Taupō & its catchment to 2020' Technical Report, Bill Vant & John Hadfield

Water quality of Lake Taupō and its catchment to 2020 (waikatoregion.govt.nz)

Executive summary The water quality of Lake Taupō has been studied since the 1930s. The lakewater is generally clear and blue, reflecting the low concentrations of freely floating plant cells (“phytoplankton”) and the nutrients nitrogen (N) and phosphorus (P) which support their growth. During 2000–11 a statutory plan was developed to protect the lake’s excellent water quality. This involved managing the current inputs of N to the lake from the surrounding catchment; at the same time the inputs of P have been closely monitored.

Waikato Regional Council has routinely monitored the water quality of the lake at a deep-water site since 1994 (with the field and laboratory work being undertaken by NIWA). The survey results confirm the excellent quality of the lakewater, with low concentrations of N, P and chlorophyll. During 2016–20 most of the requirements of the National Policy Statement for Freshwater Management (NPS-FM) for “Band A” (=”oligotrophic”) lakes were met—although dissolved oxygen concentrations in the deeper waters only met the requirements for Band B lakes. The generally more stringent Waikato Regional Plan (WRP) requirements for the lake were also met—with the exception that an average total N of 70 mg/m3 was exceeded. (When the WRP standards were set it was recognized that the standards might not be met for several decades.)

There was an increase in total N concentrations in the lakewater from 2010, with the annual average peaking at 119 mg/m3 in 2013; this was followed by a decrease through to 2020 (annual average 71 mg/m3 ). No obvious explanation for these changes is apparent (and the changes are not seen in a contemporaneous, independent record of total N in the water flowing out of the lake). Apart from the changes in total N, lakewater quality has been largely stable since 1994. The concentrations of certain forms of N and P in the lakewater suggest that phytoplankton growth in the water is now dependent on the availability of both N and P; and while the concentrations of either or both nutrients remain low, phytoplankton levels are likely to do so as well.

The water quality of 14 rivers and streams flowing into the lake is also routinely monitored. In general, the water quality of these streams is good-to-excellent; often it is in Band A of the relevant NPS-FM attributes. Dissolved reactive P concentrations are an exception, however, probably reflecting the naturally high concentrations found in cold-water springs in the Taupō catchment (and elsewhere on the Central Volcanic Plateau).

Nitrogen concentrations in several streams in catchments where pasture is the dominant landcover have increased markedly since the 1970s. Phosphorus concentrations, however, have shown little change. On average, the combined load of river-borne nitrogen entering the lake from the catchment was estimated to have increased by about 5–7% over the past 20 years. Some of the increase has occurred in streams draining catchments where pine forest is currently the dominant landcover, but where records show that pasture was present in the past (1950s). This is likely to reflect the often slow movement of water through the land and into the inflowing streams, such that the load of N from historic farming has taken an appreciable time to reach the stream waters.

Mean residence time, MRT (or “water age”), during summer has been routinely determined in 11 streams draining areas in the northern and western part of the lake’s catchment where pasture is present. Four streams in the northern area have MRTs in the range 40–80 years; the other streams—in the western part of the catchment—have ages in the range 10–25 years. Nitrogen concentrations in these streams—particularly those with longer MRTs—have continued to increase, consistent with earlier predictions of a “nitrogen load to come”.

Groundwater is the primary link for the transport of nitrogen from land-use to the lake, either as direct seepage or indirectly via typically baseflow dominated streams. Most of the rain falling Doc # 21551312 Page vi in the Taupo catchment percolates through the soils and can take many years before reemerging and thus may reflect activities that occurred in the past. Much of the investigation of groundwater in the Taupō catchment has thus focused on the load of nitrogen that is still migrating to the lake, and the processes that can remove it from the water on the way. Research has progressively shown that conditions favourable to denitrification are widespread in catchment groundwater and described the mechanisms involved. This focus was signalled early on as important to estimate nitrogen load projections more confidently for management.

Groundwater quality is routinely monitored in a network of 34 wells in the Taupo catchment. Median nitrate-N concentrations are typically low (≈70% < 2 g/m3 ) and the two highest (9.75 and 23 g/m3 ) are impacted by point sources. Deeper and older groundwater has little nitrate, and it is not detected where conditions are anaerobic. Slight-to-moderate trends in nitrate concentration have been found at many wells, with decreases slightly outweighing increases.

Numerical modelling estimations of the nitrogen load to come have progressively considered the improved understanding of denitrification in the catchment. Despite remaining uncertainties, simulation of the potential eventual load to the lake indicates it is likely to be substantially less than early worst case predictions.

The water quality rules for the Taupō catchment in the Waikato Regional Plan (WRP,see chapter 3.10) aim to protect the water quality of the lake by (1) capping all sources of manageable nitrogen from the catchment at their 2001 levels, and (2) offsetting much of the load of nitrogen which is still in transit to the lake by reducing some of the manageable sources. A public fund has been used to purchase the rights to discharge about 20% of the nitrogen entering the lake from manageable sources. This has meant that some 6675 ha of pasture, or about 12% of the area of pasture in the Taupō catchment in the year 2000, has now been planted in forest. In due course the load of nitrogen delivered from this land is expected to fall. This will help offset the load from historic activities still travelling through the groundwater, which as noted, is now considered likely to be substantially smaller than initial conservative predictions.