Landuse associated groundwater pollution in the Wairarapa region

Abstract

The Wairarapa Valley has been experiencing rapid intensification of landuse and as a result restrictions of surface water for irrigation. The utilisation of available groundwater in the area has therefore been increasing and this is likely to continue. Availability and quality are two critical parameters which determine the viability of groundwater use. Consequently this study examines aquifer characteristics and dynamics, and landuse associated groundwater pollution in the Wairarapa Valley. Extensive field work has shown that: 1. Within the Wairarapa Valley groundwater movement occurs by way of an aquifer system that can be located and characterised by field tests. Flow occurs preferentially in old river channels which have higher permeability and flow velocities. 2. Contamination of groundwater within the Wairarapa Valley can be traced and related to specific sources and landuse activities, such as the Waingawa Meat Works and Masterton Borough Tip. 3. The dynamics and characteristics of the shallow aquifer can be used to predict potential pollution within the area. 4. An appreciation of the relationship between shallow aquifer characteristics and pollution risk is essential for sound groundwater management. In the area surrounding the Waingawa Meat Works the shallow unconfined aquifer is bounded by the Masterton and Carterton faults and the Waiohine and Waingawa rivers, delineating an aquifer 1.5 km wide. Borelogs and resistivity surveys indicate an aquifer 4-7 m deep. The aquifer medium is derived from Quaternary gravels, predominantly the Waiohine gravels, and has a high silt and clay content. Pump tests indicate a storativity of 0.1, while salt water tracer tests suggest a hydraulic conductivity of 189-300 m/day, and an average linear velocity of 6.5 m/day. Groundwater flow determined by potentiometric surveys and salt water tracer tests, is predominantly in a SW direction with an average hydraulic gradient of 0.009 m/m, although flow is apparent throughout the southerly quarter. Hydrographs indicate that groundwater is predominantly sourced from direct precipitation. The shallow aquifer in the vicinity of the Masterton Borough Tip is bounded by the Ruamahanga river to the north and east, the Waingawa river to the south, and the Tararua ranges to the west, delineating an aquifer 3.5 km wide and 4 km in length. Borelogs and resistivity soundings suggest that the aquifer is approximately 12-14m thick and unconfined. Again pump tests indicate a storativity of 0.1, a hydraulic conductivity of 150-190 m/day and an average linear velocity of 1.5-1.9 m/day. Potentiometric contours indicate groundwater flow to be in a SE direction, with a hydraulic gradient of 0.002 m/m. Isotope studies indicate that the aquifer is sourced by a river/rainfall mix. Aquifer contamination had been traced to two point sources; the Waingawa Meat Work's effluent treatment process, and Masterton Borough Tip leachate. Groundwater quality data at Waingawa shows a contaminant plume moving SE and SW. Significant correlations were found between high effective precipitation and associated groundwater level rises and pollution events. However, in bores adjacent to effluent treatment ponds low water levels resulted in higher chloride levels. Although leachate caused the contamination of a bore adjacent to the base of the landfill, significant groundwater pollution is not occurring in this area at present. The DRASTIC classification scheme suggests that the sites most appropriate for waste disposal and agriculture, with respect to restricting groundwater pollution, are found in the central plains where lower transmissivities are also found. These areas also coincide with a low concentration of groundwater users. Areas of higher pollution potential are located around the Ruamahanga and Waiohine floodplains, were high transmissivities are found, and thin soil cover make the aquifer more vulnerable to landuse associated groundwater pollution.