Detailed characterisation of ground water nitrate/leachate flow in gravelly deposits using EM and GPR methods with particular reference to temporal flow changes

by Matt James

Institution: University of Canterbury
Department: Geological Science
Year: 2015
Keywords: Geophysics; GPR; EM; DUAL-EM; Agriculture; Southland; New Zealand; Dairy; Contamination
Record ID: 1313972
Full text PDF: http://hdl.handle.net/10092/10270


Irthing Road is situated 20 kilometres north of the small town of Lumsden in Northern Southland, New Zealand. Irthing Road is accessed from State Highway 97 and leads north-west for 7 kilometres up the Irthing Creek Valley. The research site is situated 4.4 kilometres from the Irthing Road - State Highway 97 intersection and the area is at 300 metres elevation above sea level on gently south sloping Quaternary alluvial deposits. The study was initiated by Environment Southland and Southern Geophysical Ltd with the intention of investigating the potential uses of near surface geophysics in the mapping of shallow groundwater contamination, specifically agriculturally sourced nitrates and leachates. The changes in land use and the introduction of high density grazing of dairy cattle on free draining soils in Southland has created cause for concern around the ease at which large volumes of contaminants could potentially gain access to the shallow groundwater system. The investigation of the Irthing Road field site included: (1) background research into historical land use changes that may have affected the area 2) a study of the Lumsden area geological and hydrogeological setting 3) six trips to the field site throughout the year to collect near surface geophysical data using a Geonics Ltd EM31-MK2, Dualem Inc. DUAL-EM 421s, and Sensors & Software pulseEKKO Pro GPR system; 4) ground- water testing conducted by Environment Southland; 5) an evaluation of the geophysical and groundwater data sets to identify whether leachate concentrations were high enough to register an anomalous response 6) the identification of how the groundwater system at the Irthing Road field site behaves 7) a conclusion as to the effectiveness of all three near surface geophysical techniques in this application. The major conclusions that emerged from this study are: (1) the groundwater system is transporting a large volume of water beneath the site and this leads to such effi- cient removal of contaminants that the concentrations are not high enough to register a response in the geophysical data 2) the groundwater system is highly sensitive to rain- fall and this is a contributing factor to the variation within the geophysical data 3) the Geonics Ltd EM31-MK2 and Sensors & Software pulseEKKO Pro GPR system returned highly consistent results and have great potential in further contaminated groundwater applications 4) Environment Southlands' DUAL-EM 421s needs more consistency, how- ever the device has a lot of potential once reliability can be ensured 5) further research is needed to determine the contamination ow paths and destinations at a larger, regional scale.