Ohau and Waikawa catchments of the Horowhenua Groundwater Management Zone: groundwater dynamics, source, and hydrochemical processes as inferred from the groundwater tracer data

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Morgenstern, U.; van der Raaij, R.W.; Baisden, W.T.; Stewart, M.K.; Martindale, H.; Matthews, A.; Collins, S. 2019 Ohau and Waikawa catchments of the Horowhenua Groundwater Management Zone : groundwater dynamics, source, and hydrochemical processes as inferred from the groundwater tracer data. Lower Hutt, N.Z.: GNS Science. GNS Science report 2018/06. 52 p.; doi: 10.21420/G2434W


Groundwater age, chemistry, gas, and isotope tracers were applied in the Ohau and Waikawa catchments of the Horowhenua Groundwater Management Zone to understand groundwater dynamics, source, and hydrochemical processes.
The measurement of river and stream water age provided a detailed understanding of the ability of geologic formations to let rainwater enter into large groundwater systems with large groundwater stores. Groundwater age tracer results allowed for the identification of: groundwater recharge rates; areas of recharge and discharge; and waters that were recharged during a previous climatic period and therefore not relevant for studies of anthropogenic impacts on groundwater resources.
The delineation of recharge sources using this tracer data set, including radon, provided insight on groundwater and surface water interactions and associated nutrient contamination pathways. Hydrochemistry (particularly nitrate), stable isotope, and gas data so far indicate groundwater recharge is dominated by local rain. Indication of river-recharged groundwater was not prevalent but is indicated to occur at sites near the Ohau River and the Waikawa Stream.
The origin of two groundwater pollutants that contribute to lake water eutrophication, phosphorous (P) and nitrogen (N), can be identified using groundwater age: P from geologic sources is indicated by elevated P only in old groundwater (>100 years), and N from land-use intensification is indicated by elevated N concentrations only in waters younger than 60 years.
Radon data provided detailed understanding of groundwater discharge areas in the Ohau River and Waikawa Stream. Significant stretches of groundwater discharge into the river/stream are indicated by high radon concentrations in the lower reaches of the Ohau River and Waikawa Stream just upstream from where they intercept and flow across the Quaternary sands. It is inferred that the permeability of these sands is too low to allow groundwater discharge to the sea. Instead, at the end of the transmissive quaternary gravels, their groundwater flow discharges into the surface waterways.
High groundwater nitrate concentrations in the Ohau and Waikawa catchments were identified as the result of groundwater recharge by local rain which carries land surface derived nitrate into the groundwater systems. Nitrate concentrations in the groundwater of the Ohau catchment are lower than in the southern Waikawa catchment. The tracer signatures indicate partial groundwater recharge from the river in the Ohau catchment, implying that the lower nitrate concentrations in the Ohau catchment are likely to be a result of dilution of the nitrateladen rainfall recharge derived water by near-pristine river water. In the Manakau catchment, absence of stream or river recharge signature implies that nitrate laden groundwater is not diluted by low-nitrate stream water, leading to the observed higher groundwater nitrate concentrations.
The gravels in the Ohau catchment are highly transmissive and as a result the water flowing through these gravels is relatively young as it nears the discharge area of the gravel system and returns into the river. Groundwater is older in the less-transmissive gravels of the Manakau catchment where it discharges into the Manakau Stream at the interception between the Quaternary gravel and sand near the coast.
Groundwaters in the Ohau and Waikawa catchments are typically not highly reduced and therefore not expected to be associated with complete denitrification. The estimated oxygen reduction rates for these gravel aquifers suggest that the groundwater requires decades in the groundwater system to reach anoxic conditions favourable for denitrification reactions. The longer groundwater transit time through the Manakau catchment may allow for greater denitrification potential in the gravels, before the water returns into the stream, than in the Ohau catchment.
In the lower Waikawa Stream, downstream from the main groundwater discharge area, dual nitrate isotope signature indicates occurrence of denitrification in the flow system. Decreasing nitrate concentrations downstream indicate that at least part of this denitrification occurs in the stream. However, higher denitrification potential in the groundwater system of the Manakau Stream catchment may also provide this environmental service of denitrification in the groundwater system, so part of the immediate denitrification signature in the groundwater discharge area of the stream may originate from the groundwater flow system. The Ohau groundwater system, with faster transit, only shows normal N retention isotopic signature (typical nitrate isotope signature from pastoral soils) in the lower reaches of the river. (auth)