Christchurch-West Melton groundwater quality : A review of groundwater quality monitoring data from January 1986 to March 2002 / repared by S A Hayward.

"July 2002."

Cover title.

On cover: Technical report Environmental Monitoring Group.

References: p. 108-110.

Groundwater quality data for the Christchurch-West Melton area were analysed to determine spatial, seasonal and long-term patterns, and were compared to New Zealand drinking-water standards. Inorganic and microbiological data were available for 3000 samples collected from 438 wells for the period January 1986 to March 2002. The quality of groundwater in the Christchurch-West Melton area generally reflects the quality of the recharge source, aquifer geology and land uses. The Waimakariri River is the dominant source of recharge of the unconfined groundwater immediately south of the river near Halket, and is the dominant recharge source for the confined groundwater system. The Waimakariri River is an alpinefed river and as such has a low ionic content resulting from little mineral weathering, low nutrient inputs and minimal influence from coastal sea spray. Consequently, groundwater recharged by the river had low concentrations of ions, as indicated by conductivity values less than 15 mS/m, chloride values less than 10 mg/L and nitrate-nitrogen concentrations less than 1 mg/L. Rainfall-derived infiltration is the dominant recharge source in the southern part of the unconfined zone, and extends eastwards into the confined zone. Rainfall recharge also appears to contribute to the deep groundwater in the northeast part of the confined aquifers via deep underflow beneath the Waimakariri River. Rainfall-derived groundwater generally contained higher concentrations of the major ions, with conductivity values typically within the range of 15-30 mS/m and concentrations of chloride ranged from 10-20 mg/L. Nitrate-nitrogen concentrations were highly variable, reflecting local land use patterns. The evolution of the groundwater chemistry from the recharge zone to coastal confined groundwater showed only subtle changes in groundwater quality. Despite estimated ages of groundwater in the deep confined aquifers to be hundreds to thousands of years, little modification of the groundwater chemistry had occurred. This reflects the relatively inert nature of the greywacke gravels that comprise the aquifer material. A distinction in groundwater quality between the confined and unconfined zones was observed. Groundwater in the unconfined zone was considerably more vulnerable to contamination from land use activities than in the confined zone. Microbial contamination was found to occur in groundwater from a number of shallow (less than 50 m deep) wells throughout the unconfined zone. This occurred in both river-recharged and rainfall-recharged areas. Of the 14 wells yielding groundwater with nitratenitrogen concentrations above the maximum acceptable value (MAV), 13 were located in the unconfined zone or at the confining boundary. The quality of the groundwater in the confined zone was generally very high. Only 2 samples from 47 wells collected from the confined zone of Aquifer 1 contained faecal coliforms. In both cases the detections occurred only once. No faecal coliforms have been detected in groundwater from the deeper confined aquifers. Elevated nitrate-nitrogen concentrations occurred in some areas of the southern confined zone of Aquifer 1. Groundwater from confined zones of Aquifer 2 and deeper generally contained very low concentrations of nitrate-nitrogen. Trace concentrations of arsenic were detected in groundwater from a number of wells, mostly in the confined zone. Generally, the concentrations were well below the MAV. In most cases, the arsenic was naturally derived and was associated with reducing conditions within the aquifer mobilising arsenic from sediment deposits. Groundwater from three wells contained arsenic at concentrations above the MAV. For two of these wells, the arsenic originated from former discharges from a timber treatment operation; the concentrations of arsenic in more recent samples from these wells were below the MAV. The arsenic in groundwater from the third well is likely to have originated from natural sources.

Index record, see series title.

Te Puna