Identifying changes in groundwater quantity and quality resulting from border-dyke to spray irrigation conversion
Thesis DisciplineWater Resource Management
Degree GrantorUniversity of Canterbury
Degree NameMaster of Water Resource Management
Over the past decade, the groundwater quantity and quality in the Hinds-Rangitata Plain (HRP) has continuously declined posing a serious risk to drinking water sources and waterways ecology. The aim of this project is to investigate and analyse the change in HRP groundwater hydrology following the conversion from border-dyke to spray irrigation.
An extensive groundwater sampling program was carried out over the period of August 2016 to June 2017. The results were compared to data from a similar sampling program carried out by Dommisse (2006) over the period of August 2005 to September 2006, prior to the conversion from border-dyke to spray irrigation. Results of the sampling program show nitrate-N concentration has increased substantially in groundwater, spring-fed streams and the Hinds River over the past decade. Over the course of this study, the average nitrate-N concentration in shallow groundwater (<30 m depth) ranged from 9.1 to 12.9 mg/L. Notably, in February 2017 average nitrate-N concentration in shallow groundwater exceeded the drinking water standard.
From a δ18O, δ2H and δ13C analysis, HRP groundwater appears to be a system principally sourced from local precipitation with little evidence of recharge from alpine derived irrigation water (i.e., Mayfield-Hinds Irrigation Scheme water). The HRP aquifer and flow in the spring-fed drains are therefore highly dependent on the levels of local rainfall.
A trend analysis of historic groundwater levels was conducted using Eigen-modelling. The model uses a built in soil moisture deficit model that requires daily rainfall and ET data. The modelled groundwater levels were then compared with the observed groundwater levels, the difference indicating the recharge component from irrigation. Border dyke irrigation was shown to cause artificially high groundwater levels between the early 1980’s and mid 2000’s. The decline in groundwater levels that follows this artificial high coincides with major improvements to HRP irrigation efficiencies.
A major piezometric survey was carried out during the 2016/2017 irrigation season, and compared with data from a piezometric survey undertaken during the 2005/2006 irrigation season by Dommisse (2006). The comparison shows an annual volumetric decrease in groundwater of 23.87 GL between 2006 and 2017 and illustrates part of the downward trend in HRP groundwater levels.
The findings from this study suggest three possible reasons that either individually or collectively explain the elevation of nitrate-N levels in HRP groundwater: the improvements in irrigation efficiencies resulted in a decline in groundwater recharge and dilution, the increased use of land for dairy and dairy support has resulted in increased nitrate leaching, and a volumetric decline in aquifer storage has resulted in reduced dilution and therefore greater nitrate-N concentrations.