A PRELIMINARY MODEL TO ACCOUNT FOR HYDROGEOLOGIC
INFLUENCES ON SPATIAL NITROGEN ATTENUATION CAPACITY AND
LAND-BASED NITROGEN LOADS TO RIVERS IN THE MANAWATU RIVER
CATCHMENT
Elwan, A.,1 Singh, R.,1 Horne, D.,1 Roygard, J.,2 Clothier, B.3
1
Institute of Agriculture and Environment (IAE), Massey University
Horizons Regional Council
3
Plant and Food Research
2
Aims
Land-based nitrogen loads to surface water bodies are influenced by hydrogeochemical conditions
affecting the leached nitrogen’s flow pathways and potential attenuation in the subsurface
environment. Few studies available investigating the hydrogeological effects on land-based
nitrogen loads stresses the need for an improved understanding of the influence of hydrogeology
on nitrogen flow and attenuation in the subsurface. We defined the nitrogen attenuation factor 𝐴𝐹𝑁 ,
as the reduction ‘attenuation’ of nitrogen leached from the root zone, through different processes
such as denitrification in the subsurface environment, to the river nitrogen load measured at the
outlet of a catchment (Singh et al., 2014; Elwan et al., 2015). As different catchments often have
contrasting characteristics, the 𝐴𝐹𝑁 is expected to vary spatially as a function of these
characteristics. Thus, the estimations of the magnitude and variations of 𝐴𝐹𝑁 is of utmost
importance. Policy makers need to take account of the magnitude and variation in the 𝐴𝐹𝑁 , across
catchments, when formulating policies related to nutrient loss from agriculture to waterways.
The main aims of this study were (1) to quantify the magnitude and spatial variation of the 𝐴𝐹𝑁
throughout the Tararua Groundwater Management Zone (TGWMZ), of the Manawatu River
Catchment, (2) to evaluate the relationship between the 𝐴𝐹𝑁 and catchment characteristics (e.g.
Soil texture, rock type and land use), and (3) to use the estimated spatial nitrogen attenuation
‘reduction’ capacity of different rocks in a hydrogeologic based model to predict river nitrogen loads.
Methods
Fifteen surface water quality monitoring sites in the TGWMZ were used in ArcMap to delineate the
sub-catchments upstream of these sites. The resulting shapefiles from the delineation process
were used to extract physical characteristics of each sub-catchment (e.g. Soil types, rock types,
and land use). The 𝐴𝐹𝑁 was quantified using estimates of the root zone nitrogen leaching (based
on Overseer® estimates of the average nitrogen leaching from the root zone for different land uses)
and the river nitrogen load (based on measured monthly soluble inorganic nitrogen concentrations
and daily river flow). The potential relationship between the 𝐴𝐹𝑁 and catchment characteristics (e.g.
Soil textures, rock types, and Base Flow Index “BFI”) was investigated through regression analysis.
Finally, a preliminary hydrogeological based model to predict river nitrogen loads using the
approximate estimates of nitrogen attenuation capacities of combinations of different landuse area
and underlying rock types in the study area is presented.
Results and Discussion
Our results showed that the 𝐴𝐹𝑁 varied from 0.29 to 0.75 across the 15 sub-catchments with an
overall value of 0.58 for the whole TGWMZ. The regression analysis between the 𝐴𝐹𝑁 and subcatchment characteristics showed that the 𝐴𝐹𝑁 has a positive relationship with some catchment
characteristics (e.g. fine textured soils “clay loam”, R2 = 0.37, p <0.05) whereas it has a negative
relationship with other catchment characteristics (e.g. the well-drained soils; drainage class 5 in the
Fundamental Soil Layer “FSL”, R2 = -0.35, p <0.05, and the Base Flow Index “BFI”, R2 = -0.31, p
<0.05).
Regardless of the uncertainty in the estimation of the 𝐴𝐹𝑁 , it showed a strong spatial correlation
with the redox status of the groundwater in the TGWMZ (Figure 1A). Results showed that the subcatchments with higher 𝐴𝐹𝑁 (i.e. higher capacity to attenuate nitrogen) are dominated by
groundwater wells in reducing conditions. In contrast, sub-catchments with lower 𝐴𝐹𝑁 (i.e. lower
capacity to attenuate nitrogen) are dominated by groundwater wells in oxidizing conditions.
Our preliminary hydrogeological based model accounts for only the influence of rock type. The
dominant rock types were grouped into three main classes with low (<0.30), medium (0.30-0.60)
and high (>0.60) nitrogen attenuation capacities. The river nitrogen loads predicted using this
simple model are a strong match with the measured river nitrogen loads in the sub-catchments of
TGWMZ (Figure 1B). This new approach in predicting river nitrogen loads highlights the importance
of spatial attenuation capacities of different rocks in different sub-catchments. This model will be
refined by assessing effects of other hydrogeological factors, such as soil texture, shallow
groundwater conditions, and/or estimates of travel time, on the attenuation of the nitrogen leached
from farms in the study area.
A
B
Figure 1: (A) Spatial variation of estimated 𝐴𝐹𝑁 values for all sub-catchments in the TGWMZ showing a
significant match with the redox status of the groundwater (Rivas et al., 2014). Wells in the reducing
conditions are mainly located in sub-catchments with the high 𝐴𝐹𝑁 values (high capacity to attenuate
nitrogen), whereas wells in the oxidising conditions are located in sub-catchments with the low 𝐴𝐹𝑁 (low
capacity to attenuate nitrogen) values; and (B) Modelled river N loads showing a good match with the
measured river N loads for different sub-catchments in the TGWMZ. The hydrogeologic based model used
to estimate River N load for each sub-catchment is shown where 𝐴𝑖 = Area of each combination of main land
use and underlying rock types (L2); 𝑁𝑖 = Average annual nitrogen loss rate for land use type (M/L 2/T); and
𝐴𝐹𝑁𝑅𝑇 is the attenuation capacity for each rock type.
References
Elwan, A., Singh, R., Horne, D., Roygard, J., et al. (2015) Nitrogen attenuation factor: can it tell a story about
nutrients in different subsurface environments? In: L.D. Currie & L.L. Burkitt (eds.). Moving farm
systems to improved nutrient attenuation. Palmerston North, New Zealand, Occasional Report No. 28.
Fertilizer and Lime Research Centre, Massey University. p. 12. [Online] Available from:
http://flrc.massey.ac.nz/publications.html.
Rivas, A., Singh, R., Horne, D., Roygard, J., et al. (2014) Denitrification in the Subsurface Environment of
the Manawatu River catchment, New Zealand. In: 21st Century Watershed Technology Conference
and Workshop Improving Water Quality and the Environment Conference Proceedings, 3-6 November
2014, University of Waikato, New Zealand.
Singh, R., Rivas, A., Espanto, P., Elwan, A., et al. (2014) Assessment of transport and transformation of
nitrogen in the subsurface environment of Manawatu River catchment – work in progress. In: L.D.
Currie & C.L. Christensen (eds.). Nutrient management for the farm, catchment and community.
Palmerston North, New Zealand, Occasional Report No. 27. Fertilizer and Lime Research Centre,
Massey University. p. 11. [Online] Available from: http://flrc.massey.ac.nz/publications.html.