Groundwater and surface water conceptual flow from environmental tracer signatures in the Pukekohe and Bombay area

Summary

Better understanding of the following is needed to inform improved nutrient-management tools and water-take limits in the Pukekohe–Bombay area:

• Conceptual groundwater flow
• Connection with surface water
• Nitrate pathways
• Potential risk of nitrate contamination of deeper aquifers
• Future nitrate loads to come’
• Connections within the aquifer system.

Refined groundwater ages, together with hydrochemistry, water isotopes and atmospheric gas concentrations, enabled detailed understanding of groundwater flow processes.

Groundwaters in the aquifers underlaying the Pleistocene deposits and in the deep basalt aquifers are all old, implying very little active circulation through these deeper systems. Rain onto the Pleistocene deposits therefore drains, mainly during the wet season, through shallow, fast-flow paths, without sufficient storage reservoir to sustain significant stream flow during baseflow in the dry season.

Active subsurface drainage through large groundwater reservoirs only occurs through the basalt lava of the Pukekohe and Bombay volcanoes and, to a lesser extent, through the unconsolidated volcanic materials, including tuff and tephra. The active groundwater flow systems in the basalt lava have sufficient storage to maintain significant stream baseflow over the course of years.

Three main streams drain the basalt lava of the Pukekohe and Bombay volcanoes on their northern perimeters: Whangamaire Stream, Whangapouri Creek and Hingaia Stream. These streams have a combined groundwater storage of approximately 350 Mm3. On the south-western flanks of these volcanoes, the upper reaches of the Mauku and Ngakoroa streams also receive discharge from the basalt lava. They have a combined groundwater storage of approximately 37 Mm³.

Three main spring systems drain the basalt lava groundwater systems: Hickey, Hillview and Patumahoe springs. The average groundwater flow rates to these springs are approximately 190, 160, and 65 m/y, respectively.

Whangamaire Stream and Whangapouri Creek drain the Pukekohe basalt lava at the northern perimeter of the plateau, where they receive most of their flow. Groundwater age, isotopes and hydrochemistry match those of the water in these streams, including high nitrate concentrations. There is no significant flow gain north of the basalt lava within the Pleistocene deposits, indicating absence or insignificance of deep groundwater flux to these streams. This implies that these areas drain only via shallow flow paths that are depleted during summer baseflow conditions.

Hingaia Stream receives water in its upper reaches partially from impermeable greywacke basement rock catchments and partially from the north-eastern perimeter of the basalt lava. The mixing of water from the essentially pristine greywacke catchment with water recharged in areas of high nitrate leaching activities results in medium nitrate concentrations. At the northern perimeter of the volcanic cone, where Hillview Spring discharges into Hingaia Stream, flow more than doubles, as does nitrate concentration. Groundwater ages, isotopes and hydrochemistry match those of the water discharging from Hillview Spring, including high nitrate concentrations.

In the upper reaches of Ngakoroa Stream, isotopes and nitrate indicate flow contribution from the south-western, low-altitude flanks of the Bombay basalt lava. North of the basalt, the stream does not gain any significant flow. However, nitrate concentration decreases significantly, probably due to in-stream processes.

Mauku Stream gains flow in its upper reaches from the Pukekohe basalt lava along its southwestern perimeter. Groundwater ages, isotopes and hydrochemistry match those of the water in the upper reaches of the stream. From the north-western flanks of the Pukekohe basalt lava, the stream receives water from a groundwater system recharged at lower altitudes and likely to be anoxic. Stream flow more than doubles, while nitrate concentrations become diluted.

Oira Creek, having very low flow, discharges locally recharged, relatively old, groundwater with low nitrate concentrations. Similarly, Waitangi Stream discharges locally recharged groundwater with low nitrate concentrations. Both streams are likely to drain anoxic groundwater systems.

The most active groundwater drainage flow with the youngest water feeding the springs occurs near the surface of the water tables. Slightly deeper groundwater is older than expected for the upstream position in the active flow path towards the springs. Recharge to the deeper groundwater system is estimated to be 280 mm/y. As indicated by calculated recharge temperatures, recharge to the deeper groundwater system appears to be preferential flow through fractures, allowing fast flow to beneath the water table. This contrasts with the groundwater providing the spring discharges, which is recharged through matrix flow through the bulk material of the unsaturated zone.

Fracture flow in the shallow basalt lava appears to be unstable. In all four basalt lava wells in the Pukekohe and upper Waikato area with long-term monitoring data, hydrochemistry parameters (indicative of land use and geologic sources) have changed drastically over recent decades. These changes were permanent, usually from older to younger water, with raising water tables. This indicates changing capture zones for these wells, with each well changing at a different time.

Weathered basalts form rich horticultural soils. Since the 1950s, high-intensity market gardening, associated with high nitrate leaching, has resulted in high nitrate loads into the transmissive basalt lava of the Pukekohe and Bombay volcanoes. These basalt lavas discharge oxic groundwater into streams, indicating absence of organic matter and inorganic electron donors in the aquifer, which would be required for facilitation of microbial reactions, including denitrification. Without denitrification occurring, the nitrate load is expected to discharge from this basalt lava without any nitrate attenuation, with a lag time equal to the travel time (age) of the water through the aquifer.

On average, it takes 18 years for the nitrate load to travel through the Pukekohe basalt lava and 36 years to travel through the Bombay basalt lava. Assuming approximately constant nitrate load for circa 65 years since the onset of industrial agriculture in around 1955, these groundwater discharges from the basalt have essentially adjusted to the high land-use nitrate loads in the recharge areas. However, nitrate concentrations in the deeper groundwater are still increasing and adjusting to the high nitrate loads in the recharge areas. Due to the relatively long lag times noted above, reducing land-use nitrate loads will take decades to manifest in the baseflow spring discharges of the basalt in the Pukekohe–Bombay area.

In contrast to the oxic water discharging from the basalt lava, significant amounts of water (approximately 60 L/s) discharge from the north-western flanks of the Pukekohe basalt lava that appear to be anoxic and low in nitrate, despite discharging from areas with land use associated with high nitrate leaching. There is potential for significant nitrate attenuation in this groundwater system. Denitrification may also be possible in the shallow drainage system of the Pleistocene deposits, near the redox zone.

GNS Science report, 2022/63

March 2023

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See also

Groundwater and surface water conceptual flow from environmental tracer signatures in the Pukekohe and Bombay area. Summary