Groundwater 2009/10
Key points:
• A drier than average winter in 2009 resulted in average to below average groundwater levels in the region’s aquifers. However, a wet January delayed the irrigation season in the Wairarapa until early autumn, allowing some recovery in groundwater levels during summer.
• Twenty nine of the 44 groundwater bores routinely monitored for E. coli bacteria and all but one of the 71 bores monitored for nitrate nitrogen during 2009/10 complied with national drinking water standards.
• Shallow groundwater in the Mangatarere catchment has elevated levels of nitrate as a result of pig and dairy farming activities.
What happened in 2009/10?
Groundwater levels
Groundwater levels during 2009/10 tended to follow historic trends and mirrored patterns in rainfall and river flows for the same period. Groundwater levels were generally higher during the months of June to late December, and lower from February onwards.
Groundwater levels were generally above average on the Kapiti Coast, average in the Hutt Valley and around average to below average in the Wairarapa. In the Wairarapa, January 2010 had exceptionally high rainfall and the groundwater levels during late January and early February were generally above average. The wet January meant that farmers were irrigating less, and so groundwater abstraction contributed to less seasonal decline in groundwater levels than normal. However, irrigation later in the season and lower rainfall in autumn saw groundwater levels drop slightly.
The drier than usual 2009 winter in the Wairarapa was reflected in below average groundwater levels seen in the Oliver Deep bore in Te Ore Ore, Masterton (left). Some recovery in groundwater level was seen in late January due to high rainfall and reduced irrigation. However, due to a dry autumn and increased irrigation, groundwater levels fell to historic minimums. Groundwater levels at Rangihiroa Street, Waikanae (right) were above average (and occasionally above historic maximums) during September to December. Groundwater levels declined during the months January to April due to low rainfall.
Wairarapa Valley groundwater investigation
The Mangatarere Stream at Anderson’s Line in late summer. The stream in this reach loses flow to groundwater during parts of the year and is often dry in summer. Downstream of this point the stream receives flow from groundwater, highlighting that streams and groundwater need to be managed together when assessing the effects of water takes.
The 2009/10 year saw the completion of three computer models that simulate the groundwater and surface water environment in the lower, middle and upper parts of the Wairarapa Valley. The third and final phase of the groundwater investigation also began, which involves using the models to simulate groundwater levels and groundwater flow into streams under a range of climatic and water abstraction scenarios. The results of the groundwater investigation to date have highlighted the importance of taking into account linkages between groundwater and the surface water environment when reviewing the water allocation provisions in the Regional Freshwater Plan.
Mangatarere catchment investigation
Between October 2008 and October 2009, 13 groundwater bores in the Mangatarere catchment, Carterton, were sampled on seven occasions as part of Greater Wellington’s investigation into poor water quality in the catchment (see Mangatarere Stream catchment water quality investigation report card). Sampling groundwater was important because the flow of water between surface water and groundwater within the catchment is strongly interconnected.
Median (red circles) and range of nitrate nitrogen levels in groundwater samples collected from 13 bores in the Mangatarere catchment between October 2008 and October 2009. The median values for eight bores were above the ‘elevated’ threshold for groundwater and two bores recorded maximum values above the Ministry of Health drinking water standard. Almost all bores sampled recorded median nitrate levels above what is typically acceptable for healthy streams – a concern given groundwater feeds stream flow in the lower catchment.
The groundwater monitoring results revealed elevated nitrate nitrogen levels (>3 mg/L) in many bores, particularly shallow bores located downgradient of intensive land uses that incorporate the application of piggery and dairy effluent discharges to land. The highest nitrate nitrogen levels were recorded in the winter months when groundwater levels were closer to ground level. This increases the likelihood of groundwater intercepting effluent that has been applied to land and leached through the soil. Nutrients are transported from the soil to the groundwater and subsequently to the rivers and streams.
Groundwater quality
Three-monthy testing of groundwater quality across the region during 2009/10 showed that E. coli bacteria counts met the Ministry of Health drinking water standard (<1 cfu/100 mL) in two thirds of the bores monitored. However, samples from 15 bores located in Kapiti and the Wairarapa Valley exceeded the standard on at least one sampling occasion. The highest E. coli count recorded was 800 cfu/100mL in a non-potable bore at Te Horo Beach. Median nitrate nitrogen concentrations were high (greater than 7 mg/L) in six bores and one individual sample result for a bore in the upper Wairarapa Valley exceeded the drinking water standard.
Collecting a water sample from one of the waterways entering Lake Wairarapa. This is one of 15 waterways and four groundwater bores that we sampled in December 2009 to get an idea of the water quality of streams, drains and shallow groundwater entering Lake Wairarapa. The results indicated that elevated levels of nutrients are entering the lake from drainage and stream networks located on its northern and eastern margins where intensive farming occurs.
What is Greater Wellington doing?
• Monitoring groundwater levels at 140 sites across the region.
• Monitoring groundwater quality at 71 sites across the region to check long-term changes in water quality.
• Investigating hydrological influences on ten significant wetlands in the Wellington region considered to be vulnerable to stress from groundwater abstraction.
• Reviewing existing groundwater allocation in the Wairarapa Valley, taking into account the interactions between surface water and groundwater.
What can you do?
• Apply for a resource consent before drilling a bore. You will need an additional consent if you plan to take more than 20,000 litres of water per day from your bore.
• If you have a consented groundwater take, read your meter regularly – this will aid any future consent renewal and assist with modelling and management of the groundwater resource.
• If you have your own bore for a domestic water supply, it’s essential to have good well head protection, and to get the water tested regularly – we suggest annually. Greater Wellington staff can advise you on how to get the water tested.
• Manage animal effluent disposal systems and fertiliser use to ensure that application rates are appropriate for the soil type and soil moisture conditions.
More information
The information on this card is a summary of the 2009/10 annual groundwater monitoring report, which is available at www.gw.govt.nz/envreports
If you would like to know more about groundwater, visit our website or contact us:
Phone: 04 384 5708 (Wellington office) or 06 378 2484 (Masterton office)