Groundwater Age and Isotope Tracers to Understand Sources, Flowpaths, and Sinks of Nitrate

High-intensity land use – mainly market gardening on the weathered basalts that form rich horticultural soils – has caused high nitrate concentrations in groundwater and streams discharging from the Pukekohe and Bombay volcanoes, New Zealand, exceeding the national bottom line for nitrate toxicity in rivers and maximum acceptable value in drinking water.

The main nitrate load, from market-gardening activity, infiltrates into large groundwater stores in the basalt formation, discharging mainly through three large springs with a Mean Transit Time (MTT) of 17 and 36 years.

Without denitrification in this groundwater system (no electron donors to facilitate microbial denitrification reactions), the entire nitrate load into the basalt eventually returns to the surface with a lag time of 17 and 36 years at the springs, with high nitrate-N concentrations up to 25 mg/L, causing high nitrate-N concentrations in the receiving streams. Because of the long MTTs of nitrate loads through the basalt, results of potential source-mitigation actions will be delayed.

Tritium data showed that all sampled streams contained younger water in winter compared to summer, indicating activation of shallower flow paths into the streams during the wet season. However, even at winter baseflow, the stream waters were still relatively old, with MTTs 6-12.5 years.

While the passage of the high nitrate load through the basalt formation and the seasonal flushing of nitrate from the pastoral grazing land in the Pleistocene is reasonably well understood, little is known about nitrate sinks within this catchment. Nitrate loads significantly decreased along the course of most of the sampled streams. Some streams, mainly those
in the Pleistocene formation, had near-zero nitrate concentrations despite pastural farming being the predominant land use in their catchments. This implies significant nitrate sinks in these catchments. Better understanding of these nitrate sinks may enable enhancement of natural attenuation of high nitrate loads in these waterways.

Anoxic groundwater discharges, presence of excess nitrogen, and dilution of stream nitrate loads by anoxic groundwater discharges indicate that denitrification in groundwater systems occurs in some formations.

The largest nitrate sinks within the catchment were found within the surface waterways. Nitrate stable isotopes indicate that the main process of nitrate removal from these waterways is through natural denitrification.

In smaller streams and a pond, 80–100% of nitrate from land-use activities was found to have been removed. In two of the largest streams, nitrate-load reductions of c. 30% were observed between sampled sites.

In the Pleistocene formation, with land use mostly pastural farming, nitrate is flushed
out seasonally. This area discharges water and nitrate loads only via shallow flow paths, which are not active in summer. In one stream, nitrate had been completely removed from the water during the low summer flow. But even in winter, when discharges are active,
around 80% of the nitrate is being removed from the water. This nitrate removal may partially occur within the stream bed but could also have significant