The current state of groundwater quality in the upper Waitaki Report No. R15/42 ISBN 978-0-478-15176-3 (print) 978-0-478-15177-0 (web) 978-0-478-15178-7 (cd) Report prepared by Marta Scott Environment Canterbury August 2015 Report No. R15/42 ISBN 978-0-478-15176-3 (print) 978-0-478-15177-0 (web) 978-0-478-15178-7 (cd) PO Box 345 Christchurch 8140 Phone (03) 365 3828 Fax (03) 365 3194 75 Church Street PO Box 550 Timaru 7940 Phone (03) 687 7800 Fax (03) 687 7808 Website: www.ecan.govt.nz Customer Services Phone 0800 324 636 The current state of groundwater quality in the upper Waitaki Summary Environment Canterbury is working with the zone committee and the local community to set nutrient load and flow limits for the lower Waitaki catchment. The nutrient limits are a way of managing diffuse sources of nitrogen loss in the catchment. This report describes the current state of groundwater quality in the lower Waitaki catchment to inform that process. The technical team has used test scenarios to explore groundwater quality under different management actions and nitrogen loads. This is discussed in a separate report (see Etheridge and Scott 2015). The upper Waitaki catchment has two major depressions, the Mackenzie and Omarama basins. The area may also be further divided into smaller sub-basins, based on geological features and glacial movements. The surrounding hills have provided the deposits which form the aquifer material, but various glacial and interglacial periods have resulted in substantial heterogeneity. Quaternary deposits host the main aquifers in the area. There are also deeper, semi-confined aquifers which are likely to be located within old fans deposited by rivers, but may not be laterally continuous. The groundwater recharge sources include: rainfall, rivers, lakes and canals. The interaction between groundwater and surface water is evident throughout the area. Distinguishing between the different recharge sources using water chemistry can be difficult, however, due to low concentrations of dissolved ions. There are three large natural lakes (Lake Tekapo, Lake Pukaki and Lake Ohau), which are lined with thick silt deposits and therefore unlikely to recharge the local groundwater. Other major Lakes (Lake Benmore, Lake Aviemore and Lake Waitaki) are man-made and therefore lack the thick silt deposits of the natural lakes. Lake Benmore provides the only exit point for groundwater in the catchment, with groundwater flow generally following surface topography. In general groundwater from the Mackenzie basin will flow towards the Haldon arm of Lake Benmore whereas groundwater in the Omarama basin will mainly drain to the Ahuriri arm of Lake Benmore. The groundwater divide is positioned somewhere near Wairepo Creek. In general, the electrical conductivity and average concentrations of major ions in upper Waitaki groundwater are lower than average concentrations in the whole of Canterbury. This is likely to be due to the large component of alpine water in the groundwater recharge, greater distance from the coast and a lower intensity of land use. Most of the wells have high dissolved oxygen concentrations and therefore denitrification in groundwater is not likely to be significant. Concentrations of iron and manganese are generally below their Guideline Values, which further supports the inference that denitrification is not prevalent. The average nitrate-N concentrations in upper Waitaki are significantly lower than the average for the whole of the Canterbury region. Concentrations are elevated in areas where land use is more intensive and some wells show an increasing trend. There may be a time lag between land use changes and increasing nitrate concentrations. Dissolved reactive phosphorus concentrations are the highest either in deeper wells, where they are likely to be natural, or in shallow wells, where they may be due to land use intensification. E. coli have been detected in those areas of more intense land use where significant numbers of animals are present. E. coli are normally detected in shallow wells but deeper wells with poor wellhead security or soils prone to bypass flow can provide pathways for E. coli to travel deeper into groundwater. The main uses of wells are for domestic and/or stock supply but significant numbers of wells are also used for irrigation water. Groundwater is used as a source of drinking-water supply in most of the major population centres (Twizel, Tekapo, Omarama, Otematata, Mt Cook and Parsons Rock). Other public supplies rely on surface waters. I recommend that community drinking water supply wells should have their protection zones reviewed, and protection zones should be delineated for wells with no protections zones currently set. Environment Canterbury Technical Report i The current state of groundwater quality in the upper Waitaki ii Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki Table of contents Summary ..................................................................................................................... i 1 Project area ..................................................................................................... 1 1.1 Upper Waitaki ................................................................................................................. 1 1.1.1 General topography and geology....................................................................... 1 1.1.2 Groundwater occurrence ................................................................................... 2 1.1.3 Groundwater recharge ....................................................................................... 3 1.1.4 Groundwater flow direction ................................................................................ 3 1.1.5 Groundwater age ............................................................................................... 4 1.1.6 Groundwater chemistry ...................................................................................... 4 1.1.7 Groundwater use................................................................................................ 7 2 Contaminant sources ..................................................................................... 9 2.1 Nutrients in groundwater ...............................................................................................10 2.1.1 Nitrate ...............................................................................................................10 2.1.2 Phosphate ........................................................................................................16 2.2 Bacterial contamination in groundwater........................................................................19 2.3 Drinking water wells ......................................................................................................20 3 Summary of current state of groundwater quality ..................................... 22 4 Acknowledgements ...................................................................................... 22 5 References ..................................................................................................... 23 Environment Canterbury Technical Report iii The current state of groundwater quality in the upper Waitaki List of Figures Figure 1-1: Location of the project area showing the upper Waitaki and Lower Waitaki sub- catchments ........................................................................................................................ 1 Figure 1-2: Location of upper Waitaki sub-catchment and major lakes and towns ............................. 2 Figure 1-3: Comparison of screen depth within the well and groundwater age .................................. 4 Figure 1-4: Minimum conductivity recorded in wells from Environment Canterbury sampling or from consent monitoring data ............................................................................................ 6 Figure 1-5: Minimum DO recorded in wells from Environment Canterbury sampling or from consent monitoring data .................................................................................................... 7 Figure 1-6: Locations of wells used for the abstraction of water ......................................................... 8 Figure 2-1: Locations of discharge sources which contribute nutrients to groundwater ..................... 9 Figure 2-2: Maximum nitrate-N concentrations ever recorded in wells .............................................11 Figure 2-3: Locations of long-term monitoring wells. .........................................................................12 Figure 2-4: Trends in Environment Canterbury long-term monitoring wells ......................................13 Figure 2-5: Trends in Environment Canterbury long-term monitoring wells ......................................14 Figure 2-6: Locations of long-term monitoring wells H39/0229 and CA15/5007 and aerial view indicating areas of more intense land use where irrigation occurs .................................15 Figure 2-7: Nitrate leaching risk map .................................................................................................16 Figure 2-8: Maximum DRP concentrations ever recorded in wells....................................................18 Figure 2-9: Phosphate leaching risk map ..........................................................................................19 Figure 2-10: Locations of E. coli detections in wells ............................................................................20