Spatial distribution of lag times and nitrate attenuation in Waikato groundwater
Hadfield, J1 Close, M2 Wilson, S3
1
Waikato Regional Council
2
ESR
3
Lincoln Agritech
Introduction
In the summer of 2014-15, short-term groundwater related investigations were undertaken in support
of the Waiora He Rautaki Whakapaipai project (Healthy Rivers - Plan for Change). This project will
establish limits and targets for nutrients (as well as sediment and E. coli) in waters of the Waikato and
Waipa catchments. A focus on time lags and losses inherent in nitrogen transport in groundwater was
part of the work undertaken and is described here. Brief reporting of radon surveys to characterise
groundwater input to streams is also reported.
Lag variation
Water was sampled for age dating throughout the Healthy Rivers study area and comprised 23
groundwater samples, three springs and 21 other surface water samples. The previous year 13 surface
water samples from the Upper Waikato were analysed for tritium with only two being available before
that. Groundwater age estimates, by contrast, were already available from 85 groundwater wells in the
study area (~160 regional-wide). The most recent groundwater samples included 10 from wells in the
Waipa catchment and all were analysed for CFCs, SF6 and tritium. Groundwater samples were also
taken from either side of the redox-cline from two sites adjacent to the Waikato River south of Karapiro
and one in the Hamilton Basin.
Apart from the Upper Waikato streams, the age of surface waters (expressed as mean residence time)
are generally less than 15 years and average about 10 years. The base-flow dominated Upper Waikato
sub-catchment streams are older with an average measured mean residence time of about 52 years
(median 35 years; flow weighted mean of about 47 years). The Upper Waikato main stem water age is
younger (about 12 years at Karapiro) due to the influence of Lake Taupo which provides two thirds of
the flow.
The age of groundwater throughout the study area is highly variable with mean residence times often
much older than surface waters (mean and median residence time estimated from the summer
monitoring results are about 114 and 95 years respectively). The mean and median of the previously
available groundwater age analyses was about 68 and 48 years respectively. Although the relationship
between groundwater age and depth is not significant, age is generally expected to increase with depth
in a recharging regime. Similarly there is no simple linear relationship between age and nitrate-N
concentration in wells. Typically, however, there is a wedge shaped distribution showing groundwater
older than the significant development of farming is low in nitrogen, whereas younger groundwaters
range in concentration dependent on land-use influence and attenuation.
Regional scale estimates were made of the lag time for groundwater flow through the vadose zone into
groundwater (Wilson and Shokri, 2015). The lag time estimates were compared with available
groundwater age dating information from samples taken close to the water table, and the two datasets
show a similar distribution. Estimated sub-catchment travel times ranged from 9 to >50 years, although
estimates in mountainous areas tended to over-predicted because of uncertainty of the water table
depth.
Loss variation and redox-cline occurrence
Nitrogen attenuation in groundwater is dependent on there being conducive, anaerobic conditions and
available electron donors. Experience to date with tracers in the Waikato Region, and particularly
Taupo, has shown that where dissolved oxygen concentrations are sufficiently low, nitrate-nitrogen will
almost invariably be attenuated. Very fast rates of nitrate-nitrogen reduction have been demonstrated
in peat formation at Rukuhia.
The depth to the redoxycline was investigated using the Childs’ test on core as an indicator of likely
denitrifying potential. Drilling and piezometer construction was undertaken at four selected sites
adjacent to the Upper Waikato hydro-lakes. At two of these sites (Little Waipa Reserve and Bulmer
Landing) the redox-cline was found to be within four metres of the water table. The redox-cline was not
encountered at the other two sites within 6 m and 11 m below the water table (Epworth Park and
HoraHora Domain respectively). The opportunity was also taken to test core from 18 holes drilled in the
Hamilton Basin for monitoring wells by Opus International Consultants Ltd for Transit New Zealand Ltd.
Although the depth to anaerobic conditions below the water table was highly variable (ranging up to 50
m), it occurred almost half the time within five metres. Groundwater quality was also tested at selected
sites used for Childs’ test investigation. Nitrate-nitrogen concentrations were either very low or nondetect at sites indicated to be anaerobic.
Regional and community monitoring network data can be used to indicate the wider occurrence of redox
conditions across the region. These may be divided into aerobic, anaerobic, mixed and indeterminate
categories based on nitrate, ammonia, dissolved iron and dissolved manganese concentrations
(dissolved oxygen being less reliable at many sites). Aerobic conditions are more prevalent in the
regional wells (~57%) than in the community water supplies (~30%). Anaerobic conditions are
represented in about 4% of regional wells and about 14% of community wells indicating some likely
bias. Although the spatial distribution of redox categories is complex, the prevalence of aerobic
conditions decreases with well depth.
Close (2015) used linear discriminant analysis and GIS to predict the spatial distribution of reduced
groundwater zones and hence where denitrification may occur in the Waikato. Similar percentages of
aerobic (56%) and anaerobic (22%) conditions were indicated in water from 435 wells (region-wide).
Radon surveys
Surveys of radon concentration were undertaken along the Pokaiwhenua and Little Waipa Streams,
comprising 10 sites in each, to identify and characterise areas of groundwater inflow. Radon is a
radioactive gas which can be used as a tracer of groundwater input to surface waters given it is present
in aquifers but readily dissipates in air. Flow and water quality were also measured during the surveys.
The radon results show considerable variation, ranging from non-detect (<0.01) to 5.4 Bq L-1 in the
Pokaiwhenua and from 0.2 to 28.1 Bq L-1 in the Little Waipa. The highest concentrations were
associated with springs. The results reflect the importance of fracture flow through these Upper Waikato
catchments which are dominated by ignimbrite geology. Water age dating also undertaken showed the
notable spring inflows to be of similar age to the streams consistent with their base-flow domination.
References
Wilson, S and Shokri, A, (2015) Estimation of lag time of water and nitrate flow through the vadose zone: Waikato
and Waipa River catchments, Lincoln Agritech Ltd client report to Waikato Regional Council.
Close, M (2015) Prediction of subsurface redox status for Waikato Healthy Rivers Plan for Change : Waiora he
Rautaki Whakapaipai Project. ESR client report to the Waikato Regional Council.