Report: TR 2008/33

Author: Bill Vant

Abstract

Long-term records of river water quality at 113 sites in the Waikato region were analysed using non-parametric statistical methods (seasonal Kendall slope estimator and trend test). At ten Waikato River sites, records of 19 water quality variables that began in 1987 or later, and ended in 2007 were analysed. At the 103 other river sites, records of 14 variables beginning in 1990 or later and ending in 2007 were analysed. The data were generally obtained at monthly intervals, but some records were based on quarterly sampling. Most of the records were adjusted to remove the effects of flow, and both raw and flow-adjusted records were analysed for trends.

A total of 188 Waikato River water quality records were considered. Significant trends (p < 5%) were found in 83 (44%) of these. Variables for which significant trends were found at five or more of the ten Waikato River sites were conductivity, dissolved colour, biochemical oxygen demand, arsenic, boron, nitrate, ammonia, total phosphorus and dissolved reactive phosphorus. The trends of decreased concentrations in dissolved colour, biochemical oxygen demand, arsenic, boron and ammonia all represented improvements in water quality, and mostly result from improved wastewater management over the past 20 years at known point source discharges (e.g. Hamilton and Taupo wastewater treatment plants, Kinleith mill, Wairakei power station). On the other hand, the increases observed in nitrate and total phosphorus concentrations represent deteriorations, and have probably resulted from the intensification of land use within the Waikato River catchment.

A total of 1373 water quality records from the other rivers and streams were considered. Significant trends were found in 590 (43%) of these. Across the region as a whole, the following overall patterns were apparent: (1) significant increases have occurred in pH, conductivity, total nitrogen, nitrate, total phosphorus, Escherichia coli and enterococci; and (2) significant decreases have occurred in dissolved oxygen, dissolved colour and ammonia. While the decreases in concentrations of ammonia represented improvements, many of the other trends were deteriorations.

In many parts of the region, relatively-rapid rates of deterioration in total nitrogen, nitrate and total phosphorus were found in these other rivers and streams. While ammonia concentrations improved in many areas, this was generally out-weighed by the much larger increases in the concentrations of other forms of nitrogen, so that the net result across the region was for a deterioration in total nitrogen. Trends in other water quality variables including water temperature and concentrations of E. coli and enterococci were less common and more localised.

In these other rivers and streams the observed increases in concentrations of total nitrogen, nitrate, total phosphorus, E. coli and enterococci, and the decrease in dissolved oxygen represent a disturbing pattern of insidious water quality degradation which in many cases is likely to be related to the widespread and intense use of land for pastoral farming in the Waikato region.

Trends in River Water Quality in the Waikato Region, 1987-2007 [PDF, 575 KB]