CHALLENGES IN NATIONAL SCALE GROUNDWATER QUALITY AND
QUANTITY MODELLING
Beyer, M.1 Jackson, B.1 Trodahl, M.1 Maxwell, D. 1
1
Victoria University of Wellington
Introduction
Information on groundwater quality and quantity are important to allow for protection of fresh water
resources. Currently, there is no comprehensive national coverage of groundwater quality and
quantity data in New Zealand. Groundwater quality i.e. chemistry data are available at ca. 100
groundwater monitoring sites across New Zealand within the National Groundwater Monitoring
Programme (NGMP). Further groundwater quality monitoring is carried out by individual regional
authorities through State of the Environment (SoE) reporting. These data give useful information at
a point, but do not provide complete spatial coverage. To assess water quality in an aquifer,
estimate groundwater quantity, and identify pressures on aquifers some regional authorities have
developed small to regional scale groundwater models. National water quality modelling in New
Zealand is attempted by only two frameworks, namely CLUES (Catchment Land Use for
Environmental Sustainability model) [Harris et al., 2009 and references therein] and LUCI (Land
Utilisation and Capability Indicator) [Jackson et al., 2013]. At this point, both GIS based frameworks
have an extremely simplified representation of groundwater. This is in part due to the difficulties in
representing and parameterising complex groundwater processes with the very limited available
data. More detailed groundwater representations are necessary to more robustly identify and locate
natural and anthropogenic pressures, such as land use practices and climate change, on water
quality and quantity.
Aims
In this study, we aim to integrate more detailed groundwater quality and quantity modelling into
LUCI (for New Zealand). Since surface water and groundwater interact, e.g. surface water can
recharge groundwater and groundwater can discharge into surface water, a more detailed
groundwater and unsaturated zone model is needed. This can help understand the dynamics and
vulnerability of both groundwater and surface water and will ultimately help decision making in
protecting fresh water resources. To achieve national groundwater and unsaturated zone modelling
some simplifications and assumptions need to be made to reflect limitations in available data,
computational limitations and the capacity of the modelling framework. Ultimately, we need to feed
our model with complete spatial coverage of parameters such as hydraulic conductivity and/or
residence time of the water in the saturated and unsaturated zone. However, complete spatial
coverage of the required data does not exist. We present a first attempt to relate and interpolate
nationally available data to parameters suitable to go into a national groundwater model and the
challenges faced while doing this. Our work will also be useful in other modelling efforts that require
better representation of national groundwater (and surface water) processes.
Methods
In this study, we identified nationally and regionally (publically) available data that can be related
to infer parameters for a national groundwater and unsaturated zone model. We discuss choices
for interpolating and extrapolating point measurements, such as groundwater quality data, and the
use of mapped information, such as QMAP (the geological map of New Zealand) and S-MAP (the
New Zealand soil database) that have been established through extrapolation of point
measurements and expert knowledge.
Results
Examples of the identified data sources that can be used to infer parameters required for a national
groundwater quality and quantity model include groundwater chemistry/quality information gained
from the NGMP and SoE networks. These are expected to aid in the estimation of pristine
groundwater chemistry (as demonstrated in Daughney and Reeves (2003)), may also help locate
groundwater systems (simply through their location) and identify recharge areas (e.g. through
recharge related changes in hydrochemistry). We present approaches to extrapolate these point
measurements to achieve complete national coverage.
The Fundamental Soils Layer (FSL) and the more detailed soil map (S-MAP) contain information
on e.g. permeability and nutrient retention of soil. These information is expected to help identify
potential groundwater recharge areas and nutrient transport into groundwater (including nutrient
degradation and adsorption in the soil), respectively. Although S-MAP contains more detailed
information on soil properties than the FSL it covers less than 30% of New Zealand as of date (as
illustrated in Fig. 1). We present ways to extrapolate detailed soil properties from S-MAP to areas
with less detailed data.
We highlight the benefits of more recent, more detailed data sets, such as S-MAP, and assess
where and what data/information are most critical to allow for national groundwater modelling to
guide ongoing and future work. Such work includes the extension of S-MAP and the development
of a Groundwater Portal (within the SAC project) [Klug and Kmoch, 2014] which aims to become a
web-based data sharing, processing and visualisation framework for hydro(geo)logical datasets
(e.g. residence time information, groundwater models, satellite inferred recharge and water table
data) in New Zealand. It is important to support these efforts as these developments aid in more
robust modelling of groundwater and soil water processes and ultimately better protection of fresh
water resources.
Figure 1: coverage of S-MAP (grey areas) in New Zealand, white areas are only covered by the FSL
References:
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