PRECIPITATION AND SOIL ZONE CONTROLS THE HYDROCHEMICAL
EVOLUTION OF SOUTHLAND’S GROUND AND SURFACE WATERS
Rissmann C,1,2 Rodway, E,1 Daughney, C,2 Horton, T,3 Killick, M,1 Pearson, L,1, Beyer,
M,1 Hodson R1, Dare, J1, Akbaripasand, A1, Ellis, T,1 Ward, N,1 May, D, Kees, L,1 Millar,
R,1 Lawton, M,1 McMecking, J,1 Moreau M,2 Leybourne, M,4 Baisden, T,2 Friedel M,2
Morgenstern U,2.
1
Environment Southland
GNS Science
3
University of Canterbury
4
Laurentain University, Canada
2
Aims (11 pt. Arial; Bold)
The Southland region of New Zealand has an area of ca. 34,000 km 2 with complex geology and
pedology. Agricultural intensification over recent decades has led to degradation of water quality
in streams and aquifers, particularly in terms of nutrients, sediment and microbial pathogens
(Snelder et al., 2014). Improved understanding of Southland’s coupled groundwater-surface
water bodies is required to meet the outcomes sought in the New Zealand’s National Policy
Statement for Freshwater Management (2014). Accordingly, the aim of this study was to
determine the drivers of hydrochemical evolution of Southland’s surface and groundwaters with a
view to understanding spatial and temporal variability in water quality. This study complements
the hydrochemical assessment of Southland’s water (Daughney et al., 2015) by adding additional
information on the drivers of hydrochemical evolution.
Method (11 pt. Arial; Bold)
A biogeochemical assessment of Southland precipitation, soil, soil water, surface and ground
water was undertaken using a range of approaches including standard hydrochemical, soil
biogeochemical and multivariate statistical methods. For this, we employed a chemical dataset
comprised of ca. 28,000 ground, surface, precipitation, soil water and soil analyses. Some of the
waters were analysed for up to 50 parameters including stable isotopes of boron (δ11B-B), carbon
(δ13C-DIC), water (δ18O and δ2H) and nitrate (δ15N and δ18O). Soil chemical data for 600
individual soil profiles was obtained from TopoClimate South.
Results (11 pt. Arial; Bold)
The results of the chemical assessment reveal that marine aerosolic loadings and soil chemistry
are the main determinants over the hydrochemical variation of ground and surface waters
regionally. Sodium and Cl are overwhelmingly (90 - 100%) derived from precipitation with
significant concentration occurring within the soil zone in response to evapotranspiration.
Endogenous or epigenic sources of Cl are negligible and the majority of regional waters show no
significant enrichment in Na above a marine aerosolic source. Ca and soil water DIC are
controlled primarily by soil base saturation (BS%) that is a factor of the degree of weathering of
the soil and its parent materials.
There is little evidence for the evolution of major ion signatures for Southland ground and surface
waters after leaving the soil zone with the exception of areas of reactive carbonate rock and
strongly reducing aquifers. Carbon isotope equilibria suggest that 60% of the alkalinity within
calcite saturated aquifers hosted by marine carbonates is associated with soil zone recharge with
the remainder due to water-rock interaction between soil zone carbonic acid and calcite. The
other source of post-infiltration evolution occurs in response to heterotrophic oxidation of terminal
electron acceptors that generate additional DIC and liberate dissolved Fe and Mn. Both
carbonate and reducing aquifers are well defined and subsequent evolution of soil zone recharge
is readily accounted for.
In conclusion, robust prediction of the temporal and spatial variation in the hydrochemistry of
Southland is possible through combining a strong understanding of the spatial variance in marine
aerosolic loadings, soil chemistry and water-rock interaction. Through this integrated
understanding we are able to better predict and understand the spatial and temporal controls over
water quality outcomes for both N and P species.