The Nature and Origin of Saline Groundwater in the Wairau Valley, Marlborough, New Zealand.
Thesis DisciplineEngineering Geology
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
In the Wairau Valley 40 km southwest of Blenheim, elevated salinities are present in the groundwater below a depth of approximately 15 m, to the north of the Wairau Fault. Saline water is present very close to the surface between the Southern Hills and the Wairau Fault. Highest concentrations are located in well O28/w/0219 with total dissolved solids concentrations approximately 31,000 mg/L. Only a few wells in the study area have intercepted the saline groundwater. A report by Taylor (2003) has identified the groundwater below the Holocene terrace surface is recharged from Southern Hills runoff, however the Wairau Fault has a significant impact on the groundwater flow on the south bank acting as a semi-permeable barrier to groundwater flow from the southern Hills streams identified by several spring which emerge on the fault trace. The scope of this investigation was to identify the extent of the saline groundwater in the Homelands area and to attempt to define the origin of the highly saline groundwater. Furthermore, to define the groundwater flow path below the upper terrace surface to recharge the Wairau Valley Aquifer. The Multi-Electrode Resistivity technique was used to define the extent of the saline groundwater. This shows the saline groundwater is ubiquitous at depth in the study area. The depth to the freshwater/saline water interface varies laterally in the resistivity profiles. A major control on the presence of the groundwater salinity is considered to be the permeability of the gravel. Gravels with a higher permeability are probably washed of any residual salinity that may have been present in the past. Investigations into the origins of the saline groundwater were completed using stable isotope analysis (¹⁸O, ²H, and ¹³C), hydrochemistry and age dating techniques (³H and ¹⁴C). Due to the complex chemistry a single source could not be identified, however two methods were identified as the most likely. This was evaporative concentration of fresh water in the Wairau Valley, or the upward migration from the Wairau Fault of formation water probably of seawater origin. The stable isotope data fits best with an evaporative concentration of freshwater within the Wairau Valley, however, ratios of chemical constituents are very similar to other formation waters found in other parts of the world. Stream gauging of streams on the south bank show no significant water loss in the reaches north of the Wairau Fault. Therefore, recharge must be crossing the Fault trace as groundwater. Boundary Creek looses all of its surface flow for most of the year upon reaching the valley floor. Bounday Creek has washed out sections of the Wairau Fault and Major terrace riser between Wr 1 and Wr 2 terrace surfaces. It is proposed that groundwater flowing in the gravels reworked by Boundary Creek is the major recharge source for the Wairau Valley Aquifer.