«Matt Dodson Philippa Aitchison-Earl Lisa Scott June 2012 Report No. R12/69 ISBN 978-1-927222-03-4 (print) 978-1-927222-04-1 (web) 24 Edward Street, ...»
Report No. R12/69
ISBN 978-1-927222-03-4 (print)
Report No. R12/69
ISBN 978-1-927222-03-4 (print)
24 Edward Street, Lincoln
PO Box 345
Phone (03) 365 3828
Fax (03) 365 3194
75 Church Street
PO Box 550
Phone (03) 687 7800
Fax (03) 687 7808
Customer Services Phone 0800 324 636 Ashley-Waimakariri groundwater resources investigation Community summary Environment Canterbury began a study of the water resources in the Ashley-Waimakariri plains (the area between the Waimakariri and the Ashley River/Rakahuri) in 2009. The study area is in the Waimakariri zone of the Canterbury Water Management Strategy (CWMS). The purpose of the investigation was to examine how the groundwater is connected to the rivers and streams and to review the groundwater allocation regimes.
We investigated the interactions between surface water and groundwater by constructing a water balance. A water balance accounts for all the major inputs and outputs of water in a hydrological system. By calculating the volume of water in each component of the system; understanding the connections between them and the time it takes to move from one to another; we can improve our management of the available water.
One of the water balance components is recharge. Recharge is the water that infiltrates through the soil or through the riverbed and down into groundwater. To estimate the amount of recharge, we had
to identify where the water was coming from. Recharge sources on the Ashley-Waimakariri plains are:
• land surface recharge (LSR): drainage from rain and irrigation water that falls on land overlying the region’s aquifers
• river and other recharge: water from rivers, streams, stock and irrigation water races and runoff from surrounding hills that ends up in the region’s aquifers Our estimate of LSR to the study area is 361 × 10 m /year (361 million cubic metres per year). The Waimakariri Irrigation Limited (WIL) scheme has increased the amount of LSR since it started in 1999.
The Waimakariri, Eyre and Cust rivers and the Ashley River/Rakahuri recharge various parts of the groundwater system. We estimate that these rivers recharge the aquifers at a rate of around 121 × 10 m /year. In addition, there is about 53 × 10 m /year recharge from stock water and irrigation races.
For our water balance, we also estimated the volume of water that discharges from the groundwater system. Major discharge sources are springs (223 × 10 m /year), groundwater abstraction (52 ×10 m /year) and offshore flow. Springs occur near Rangiora, Kaiapoi and near the Eyre and Cust rivers. Offshore flow is groundwater that flows out into the ocean. It is not possible to measure the volume of offshore flow, but we have estimated its volume from the difference between total recharge and discharge volumes.
The quality of groundwater in the Ashley-Waimakariri plains is generally very good. In particular, quality is good in areas recharged by the Ashley River/Rakahuri and the deep coastal confined aquifers. Water recharged through the land surface tends to have higher concentrations of nitrate. A few wells near Cust and Horrellville have sometimes had levels of nitrate that are not suitable for drinking water (above 11.3 mg/L when measured as nitrate nitrogen). Our long-term monitoring data show that nitrate concentrations have increased in three wells on the inland plains near Cust, Eyrewell and Swannanoa. We found no obvious point sources of contamination, so we think the increase is a result of more intensive farming. However, most of our monitoring wells show no long-term changes in nitrate. We agree with previous reports that iron and manganese and arsenic, which cause water quality problems in places, are associated with buried swamp deposits on the coastal plains and are a natural phenomenon.
Based on our recharge estimates, we recommend increasing the groundwater allocation limit for the Eyre River groundwater allocation zone from 81.0 to 99.07 × 10 m /year. We also recommend building a computer flow model of the study area to provide technical support for implementing the recommendations in the Waimakariri CWMS zone implementation programme (ZIP). The flow model should be designed to assist in managing water quantity and quality, including the setting of nutrient allocation limits.
Environment Canterbury Technical Report i Ashley-Waimakariri groundwater resources investigation Executive summary This report summarises our understanding of the groundwater resources of the Ashley-Waimakariri plains. It is one of the products from two years of water resources investigation in the area. The main purpose of the work was to investigate groundwater-surface water interactions and to review the appropriateness of the groundwater allocation regimes. We used groundwater levels, geochemistry, stable isotopes and age determination to study recharge and discharge sources in the AshleyWaimakariri plains.
Groundwater and surface water systems are highly connected, with surface water recharging groundwater below the inland plains, and groundwater feeding the lowland streams. Major recharge sources in the inland plains are land surface recharge (LSR, including rainfall and irrigation-induced recharge); losses from the Eyre and Cust rivers and water race losses from the stock water and irrigation schemes. In the coastal plains, recharge comes from LSR, losses from the Ashley River/Rakahuri and the Waimakariri River and water race losses. Springs, abstraction and offshore flow are the major discharges from the system.
Average annual rainfall increases from the coast to the inland plains. Average monthly rainfall is relatively similar all year round, but average monthly potential evapotranspiration (PET) is seasonal, increasing from September to March. This leads to greater volumes of LSR over the winter months.
We have quantified recharge and discharge to the groundwater systems where possible to produce a water balance. We have also separated out the water balance components for the Ashley, Cust and Eyre River Groundwater Allocation Zones (GAZs) to assist with the allocation review.
Our estimates for the major recharge sources are;
LSR: 361×10 m /year (million cubic metres per year) surface water losses: 121× 10 m /year stock water and irrigation race losses: 53 ×10 m /year.
Our estimates for the major discharge sources are:
spring discharge: 223 ×10 m /year groundwater abstraction: 52 ×10 m /year Offshore discharge cannot be measured, but is estimated from the difference between inputs and outputs to the groundwater system to be less than 259.7×10 m /year.
The quality of groundwater in the Ashley-Waimakariri plains is generally very good. The best quality groundwater occurs in areas recharged by the Ashley River/Rakahuri and the deep coastal confined aquifers. Reduced groundwater conditions are encountered in the swamp deposits associated with the coastal confined system. These cause water quality problems such as elevated concentrations of manganese, iron and arsenic.
Groundwater recharged from the land surface is susceptible to contamination by nitrate and bacteria.
We identified increasing nitrate trends in 5 out of 23 wells with long-term data. The three wells with most significant increases are in LSR-dominated areas near Cust, Swannanoa and Eyrewell. We suspect that land use intensification may be responsible. Most wells show no trend in nitrate levels over the past 10 years. Groundwater-fed lowland streams are also vulnerable to nitrate enrichment.
Nitrate concentrations are lowest in the lowland streams arising near the Ashley River/Rakahuri (e.g.
Waikuku Stream has less than 2 mg/L). But nitrate levels in the streams increase with increasing proportions of LSR in the groundwater (e.g. the Kaiapoi River and Ohoka Stream have nitrate concentrations of 3 to 5 mg/L).
As a result of our revised recharge estimates, we recommend increasing the allocation in the Eyre River GAZ from 81.0 to 99.07 × 10 m /year. We also recommend constructing a flow model of the study area to provide technical support for implementing the recommendations made in the Waimakariri CWMS zone implementation programme (ZIP). The flow model should be designed to assist in managing water quantity and quality, including better constraining parts of the water balance (e.g. offshore discharge) and setting of nutrient allocation limits.
Table of contents Community summary
1.1 Study objectives
1.2 The Ashley-Waimakariri plains
1.2.1 Geographical setting
1.2.2 Water resource management
1.2.3 Groundwater allocation
1.3 Previous work
1.3.1 Groundwater quantity
1.3.2 Groundwater quality
1.3.3 Groundwater source and age
2.2.1 Pre-Quaternary stratigraphy
2.2.2 Quaternary stratigraphy
2.2.3 Quaternary stratigraphic nomenclature
2.2.4 Depth of Quaternary deposits
2.3 Geological model for the coastal plains
3 Climate and irrigation schemes
3.2.2 Potential evapotranspiration
3.3 Stock water and irrigation schemes
4 Surface water hydrology
4.2 Descriptions of the major rivers
4.2.1 Waimakariri River
4.2.2 Ashley River/Rakahuri
4.2.3 Cust River
4.2.4 Eyre River
4.3 Springs and lowland streams
4.3.1 Spring locations
4.3.2 Lowland streams
4.4 Stream-bed conductance
4.4.2 Estimates of stream-bed conductance in the Eyre GAZ
4.4.3 Estimates of stream-bed conductance in the Cust GAZ
4.5 Consented surface water takes
5.2.1 Piezometric surveys
5.2.2 Groundwater levels
5.2.3 Groundwater chemistry and isotopes
5.3 Groundwater flow directions
5.3.1 Eyre River GAZ
5.3.2 Cust GAZ
5.3.3 Ashley GAZ
5.4 Vertical hydraulic gradients
5.4.1 Eyre River GAZ
5.4.2 Cust GAZ
5.4.3 Ashley GAZ
5.5 Recharge sources
5.5.1 Land surface recharge
5.5.2 Waimakariri Irrigation Limited scheme recharge
5.5.3 Ashley River/Rakahuri recharge
5.5.4 Waimakariri River recharge
5.5.5 Eyre River recharge
5.6 Groundwater discharge
5.6.1 Ashley River/Rakahuri-fed streams and springs
5.6.2 Cust River
5.6.3 Waimakariri River tributaries
5.6.4 Groundwater abstractions
5.7 Summary by GAZ
5.7.1 Ashley GAZ
5.7.2 Cust GAZ
5.7.3 Eyre River GAZ
6 Groundwater quality
6.2 Land use impacts: nitrate and bacteria
6.3 Natural elements: iron, manganese and arsenic
6.3.1 Iron and manganese
7 Water balance
7.2 Hydrogeological conceptual model
iv Environment Canterbury Technical Report Ashley-Waimakariri groundwater resources investigation 7.3 Methods
7.3.1 Land surface recharge
7.3.2 Surface water losses and spring discharge
7.3.3 Stock water race losses
7.3.4 WIL race losses
7.3.6 Offshore flow
7.3.7 Interzone groundwater flow
7.4 Ashley-Waimakariri plains water balance
7.5.1 Water balance inputs
7.5.2 Water balance outputs
7.5.3 Ashley GAZ
7.5.4 Cust GAZ
7.5.5 Eyre River GAZ
8 Summary and conclusions
8.3 Implications for management
9.2 Groundwater allocation recommendations
9.2.1 Eyre River GAZ
9.3 Recommendations for further work
List of Figures Figure 1-1: Features of the Ashley-Waimakariri plains
Figure 1-2: Groundwater allocation zones in the Waimakariri CWMS zone
Figure 1-3: Hydrogeological zones mapped by Sanders (1997) in relation to current NRRP groundwater allocation zones
Figure 1-4: Conceptual model of the Ashley-Waimakariri plains from Sanders (1997)
Figure 2-1: Map showing the surface geology of the Waimakariri CWMS zone (simplified from Forsyth et al., 2008)
Figure 2-2: Schematic diagram through the Quaternary deposits beneath Christchurch area (after Brown and Weeber, 1992 and Browne and Naish, 2003).
Figure 2-3: Contours for the base of the Quaternary deposits (Jongens 2011)
Figure 2-4: Map showing the boundary of the geological model
Figure 2-5: Cross section through our geological model (Durney et al., 2011).)..
Figure 3-1: Climate stations and average annual rainfall isohyets (mm) for the AshleyWaimakariri plains
Figure 3-2: Average monthly rainfall at the Rangiora NIWA rainfall site (data averaged over the period 1918 to 2011).
Figure 3-3: Average monthly rainfall at the Oxford NIWA rainfall site (data averaged over the period 1951 to 1980).
Figure 3-4: Annual rainfall from Rangiora NIWA rainfall site.
Figure 3-5: Average monthly PET from Rangiroa NIWA EWS site.
Figure 3-6: Average monthly soil moisture deficit and surplus (yellow) from the Rangiora NIWA EWS site.
Figure 3-7: Extent of the Waimakariri Irrigation Limited (WIL) and Spencer-Bower and Pratley (SBP) irrigation schemes
Figure 4-1: Surface water systems, flow gauging site numbers and losing and gaining reaches in the Ashley-Waimakariri plains