Water quality in the Waikato River is much better than it was in the 1950s. Urban and industrial wastewater treatment has improved considerably. But over the past twenty years, levels of nitrogen in the river have risen, probably as a result of land use changes over recent decades.
The Waikato River's water quality has improved a great deal. The country's longest river was, 60 years ago, one of the dirtiest. Today, while not pristine, it's much cleaner than in the 1950s and 60s. A description from the Pollution Advisory Council in 1956 sums up the way the river was once used:
"the daily flow of sewage effluent and trade wastes from Hamilton City is three million gallons… in effect, partly digested sludge and raw sewage is being disposed of into the Waikato River".
Water quality records from upstream of Hamilton city, dating back to 1949, show that levels of sewage-related contaminants have stayed roughly the same. However, as the city grew, measurements taken downstream of Hamilton city reveal contaminants increased tenfold between the 1950s and the early 1970s.
In those days, Hamilton's wastewater was discharged to the river, after holding in 14 large septic tanks. The tanks themselves were also emptied into the river up to three times each year. The river also took sewage from other towns, along with effluent from the Horotiu freezing works, Kinleith paper mill, power projects and dairy farms.
Since the 1970s, considerable work has been done to improve the quality of effluent from industrial and urban sources. The 1970s saw major improvements to urban wastewater treatment. The longest water quality record for the Waikato River (at Mercer) shows that this was when river water quality began to improve.
The high levels of contaminants evident in the early 1970s fell to levels ten times lower during the late 1970s and early 1980s.
Limited monthly monitoring of water quality in the Waikato River began in 1980, with a more comprehensive programme beginning in 1987. We now sample at 10 sites along its length (view a map of monitoring sites (external link) ). Analysis of these records indicates how water quality has changed (table 1).
Table 1 shows the changes in several water quality measures during the 25-year period between 1991 and 2015. Overall, 23 per cent of water quality measures improved at individual sites, and 14 per cent deteriorated.
The records of temperature and dissolved oxygen have generally remained broadly stable.
Trends show an improvement in total phosphorus and chlorophyll a overall, with less algae in the river, particularly its lower reaches. But turbidity has shown some deterioration.
Levels of total nitrogen increased at several sites along the river, probably as a result of land use changes over recent decades. The overall rate of increase over the past 25 years was about 1 per cent per year. Pressures from wastewaters have generally decreased over recent years, but agricultural land use has continued to intensify.
As the region continues to grow and develop, putting pressure on the river's catchment, careful management is needed to maintain and improve the quality of the Waikato River.
Table 1: Water quality trends in the Waikato River between 1991 and 2015 (based on methods from Waikato Regional Council's Technical Report 2013/20.)