Comparison of Simulated to Measured Groundwater Nitrate-N Concentrations from SBR Discharges in the Taupo Catchment

TR 2006/32
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Report: TR 2006/32

Author: Greg Barkle (Aqualinc Research Ltd) and Fuli Wang (Lincoln Environmental)

Abstract

The propagation of a groundwater nitrate plume resulting from the discharge of treated effluent from sequential batch reactors at Acacia Bay site and Kinloch are modelled using a three-dimensional flow and contaminant transport numerical model (FEMWATER-N). The models simulate the transport and transformation of ammonium-N, nitrate-N and dissolved organic carbon in the vadose zone and aquifer. Nitrification and denitrification are described by first-order and multiple Monod kinetics models, respectively.

The models have been calibrated by comparing simulated nitrate-N results with measured concentrations at five monitoring wells for the Acacia Bay site and nine wells for the Kinloch site. The models were able to adequately describe the nitrate concentrations in most monitoring wells. However, the verification of the dynamics of the groundwater nitrate-N was hampered by the time between samplings in the monitoring wells.

Sensitivity analyses showed that nitrate-N concentrations at sampling wells were sensitive to model parameters in the order of hydraulic conductivity, rate of denitrification, rate of nitrification, and dispersivity.

Preliminary Monte Carlo numerical simulations at the Acacia Bay site demonstrated that the heterogeneity of aquifer hydraulic conductivity may be the major factor contributing to the discrepancy between simulated and measured concentrations at the sampling wells.

Comparison of Simulated to Measured Groundwater Nitrate-N Concentrations from SBR Discharges in the Taupo Catchment [PDF, 1.6 MB]

Contents
  Executive Summary 1
1 Introduction 2
2 FEMWATER-N Mathematical Descriptions 3
3 Monitoring Data and Site Layout of Acacia Bay SBR Discharge 4
4 Acacia Bay SBR Model Setup and Input Data 4
5 Analyses of Measured Nitrate-N Concentrations at Acacia Bay SBR Monitoring Wells 6
6 Acacia Bay SBR Model Calibration 10
7 Acacia Bay SBR Model Sensitivity Analysis 11
7.1 Hydraulic Conductivity 11
7.2 Rate of Denitrification 12
7.3 Rate of Nitrification 14
7.4 Dispersion Coefficient 17
7.5 Spatial Variation in Hydraulic Conductivity 20
8 Monitoring Data and Site Layout of Kinloch SBR Discharge 21
9 Kinloch SBR Model Setup and Input Data 22
10 Analyses of Measured Nitrate-N Concentrations at Kinloch SBR Monitoring Wells 24
11 Kinloch SBR Model Calibration 26
12 Kinloch SBR Model Sensitivity Analysis 31
12.1 Hydraulic Conductivity 31
12.2 Rate of Denitrification 31
12.3 Rate of Nitrification 35
12.4 Dispersion Coefficient 39
13 Summary 45
14 References 46