INVESTIGATING HOW SOIL DRAINAGE CLASS AFFECTS THE REDOX
STATUS OF SHALLOW GROUNDWATER
Juliet Clague,1 Roland Stenger,1 Brian Moorhead,1
1
Lincoln Agritech Ltd
Introduction
We have recently extended our research on the assimilative capacity of groundwater systems for
nitrate to the Reporoa Basin in Waikato. The particular focus of the presented study is to
investigate to which extent the drainage class of the soil zone (approx. 1 m depth) affects the
redox status of the underlying shallow groundwater. The Reporoa Basin offers good opportunities
to address this question as soils ranging in their drainage class from well-drained to very poorly
drained occur in close proximity and at locations with reasonably shallow depth to the
groundwater table.
Method
Potential well locations were identified using S-map to cover the variety of soil types and drainage
classes found in the area (Figure 1). Preliminary site investigations were then carried out to
ascertain how accurate the S-map predictions were, and to document the aquifer material
beneath the soil zone. The Childs test was used to indicate where possible reducing zones might
be located (Childs, 1981). Six monitoring wells (50 mm diameter PVC) were then installed at
selected locations using a novel coring method which eliminates the problem of annular gap
bypass flow. An attempt was made to cover the well-drained, imperfectly drained, poorly drained
and very poorly drained drainage classes found in the area; however, the supposedly very poorly
drained site (GW05) turned out to be imperfectly drained. A mini bladder pump (150 mm long)
located between two inflatable packers was used to sample the wells at 3 - 5 discrete intervals
and characterise the groundwater chemistry. Field measurements of dissolved oxygen, pH and
electrical conductivity were monitored until stable and then samples were collected. An aliquot
was field-filtered (0.45 µm) and acidified (nitric acid) for the analysis of dissolved iron and
manganese. A 500 mL sample was also taken and analysed for NNN (nitrate + nitrite), dissolved
reactive phosphorus, total phosphorus, sulphate and silica. Future samplings will entail a
comprehensive suite of analytes including carbon species, cations, anions and dissolved gases.
Figure 1: Location of the six wells installed in the Reporoa Basin, and the corresponding S-map soil and
drainage classification.
Results
The information found in S-map provides a starting block for field investigations since such
information is rarely accurate at the paddock scale. Finding appropriate profiles, with shallow
groundwater required comprehensive field investigations before wells could be installed at
suitable sites. Table 1 shows the classification, Childs test response and initial groundwater redox
status of the six wells installed so far. More wells will be installed to provide another profile
example of both the well-drained and impeded Pumice soils found in the area.
Table 1: Soil type, drainage class, Childs test response and initial redox status of groundwater for the six
wells installed in the Reporoa Basin.
Redox status of
Well ID Soil
Drainage class
Childs test Response
groundwater
GW04 Immature Orthic Pumice Well-drained
Negative
Oxidised
GW05 Mottled Impeded Pumice Imperfectly drained Positive 1.0 – 1.5 m
Reduced
TW09 Mottled Orthic Pumice
Imperfectly drained Positive 1.5 – 4.0 m
Reduced
TW12 Mottled Orthic Pumice
Imperfectly drained Positive 2.0 – 3.0 m
Oxidised over reduced
WW04 Mellow Humic Organic
Poorly drained
Positive 2.0 – 4.5 m
Reduced
WW05 Mellow Humic Organic
Very poorly drained Positive 1.5 – 4.0 m
Reduced
Results from the initial round of sampling indicate that denitrification is occurring in the shallow
groundwater of several locations in the Reporoa area. Denitrification substantially changes the
chemical composition and environmental impact of the water entering Waiotapu stream and the
Waikato River by reducing nitrate to dinitrogen gas.
The well-drained, oxidised profile found in GW04, has high NNN concentrations throughout the
profile (Figure 2). This well represents the impact high intensity dairying can have on the
underlying groundwater, and potential effect on nearby surface water.
Figure 2: Profiles of the dissolved oxygen and nitrate + nitrite concentrations measured in the six wells
sampled in the Reporoa Basin.
In contrast, the imperfectly drained TW12 profile shows a redox gradient with depth and
concomitantly declining NNN concentrations (Figure 2). This is likely due to active denitrification,
although the electron donor involved remains unknown at this stage since analysis of the
predominantly sand and pumice core samples revealed low total carbon concentrations
throughout (<1%).
The two poorly drained profiles (WW04 and WW05) have peat in the upper part of the profile, and
very shallow groundwater (<0.4 m bgs) that is already reduced and nitrate-free (Figure 2). It is
highly likely that denitrification has occurred at these sites since dissolved iron and manganese
concentrations are elevated (data not shown) and the peat layer would provide a suitable electron
donor for denitrification.
Future samplings will reveal how temporally stable the geochemistry of the shallow groundwater
in the Reporoa Basin is, and whether the denitrification process is dynamic or constant.
This research was conducted under the ‘Groundwater Assimilative Capacity’ programme funded
by MBIE.
References
CHILDS, C. W. (1981) Field test for ferrous iron and ferric-organic complexes (on exchange sites or in
water-soluble forms) in soils. Australian Journal of Soil Research, 19, 175-180.