Northland Regional Council : Mangawhai Groundwater Resource ( 2005 )
Mangawhai Village and Mangawhai Heads are coastal resort towns located approximately 100 km
north of Auckland. The towns, in particular Mangawhai Heads, are currently experiencing
subdivision, which is increasing the demand on groundwater. Groundwater in the area is used
mainly as a supplementary domestic supply for many of the residences, and to a lesser extent for
horticultural irrigation and stock drinking water. A number of bores have poor water quality,
believed to be due to seawater intrusion, hence the classification of the area as being an “at-risk”
aquifer.
The total study area is 14 km2
encompassing Mangawhai Village, Mangawhai Heads and the
sandspit bordering the estuary to the east (see Figure 1). Most of the study area is low-lying around
the estuary, extending to 53 m above mean sea level (mAMSL) at Mangawhai Heads and
approximately 100 mAMSL in the rolling hills behind Mangawhai Village.
A volcanic intrusion just to the north of the Mangawhai Heads peninsula reaches 107 mAMSL.
Figure 1 shows the main points of interest of the study area, including the aquifer boundary. The
aquifer boundary has been delineated based on the lithological change from coastal sands to
consolidated hardrock of various types, and is as defined in the Northland Regional Water & Soil
Plan (NRC, 2004). The NRC holds borelogs, hydraulic information, groundwater quality and
groundwater level data for bores located outside of, but in close proximity to the aquifer boundary.
Information from these bores has been included and discussed in this report for comprehensiveness.
Regional Geology
The geology of the Mangawhai Area is described on the 1:250,000 Geological Map Sheet 2A for
Whangarei, and is reproduced in Figure 2.
Greywacke and argillite basement rocks of Permian to Jurassic age (295 to 145 million years
before present) underlie the study area at depths of between 100 and 690 m below mean sea level
(Massey, 1987). The rocks are dark grey or green, intensely deformed, jointed or sheared with
secondary silica or calcite along the joint planes (Thompson, 1961). They are upfaulted to the
north of the study area along the E-NE trending Waipu Fault.
Overlying the downfaulted basement rocks within the study area are Cretaceous to Oligocene age
(145 to 25 million years) Northland Allochthon sediments. The Northland Allochthon represents a
SINCLAIR KNIGHT MERZ
D:\Projects\AE02189\Deliverables\Mangawhai Report_Final.doc PAGE 2 Mangawhai Groundwater Resource
series of discrete lithological units that were emplaced as part of a large gravity slide affecting most
of Northland. As a result of this mode of deposition, the Northland Allochthon rocks have been
substantially faulted, fractured and sheared, and rocks of differing age are commonly found
together or within close proximity to each other. The rocks that outcrop within the study area
include Mangakahia Group sandstones of Cretaceous age, and Opahi Formation greensands,
argillaceous limestone and shale of Eocene age. The Mangakahia Group sediments outcrop in the
centre of the study area near Tara Creek, and are grey, sulphurous and micaceous. The Opahi
Formation sediments outcrop to the north of the study area, with closely fractured grey-white
argillaceous limestone being most commonly exposed.
Overlying the Northland Allochthon sediments are Waitemata Group sandstones and mudstone of
Miocene age (25 to 6 million years old). The Waitemata sediments are widespread beneath and
around the study area and comprise dark grey non-calcareous sandstones thinly or thickly bedded
with mudstone. There are occasional thin coal seams, fossils (shell fragments, limestone) and
volcanic breccia beds (Thompson, 1961). The rocks are moderately hard and can be weakly to
moderately fractured. They typically weather to soft yellow brown silty clays to depths of 30 m
(NRC, 1989).
There are intrusions of Late Miocene age Parahaki Volcanics (dacite lava and tuff) on the northern
headland near Mangawhai Heads and to the west of the study area. The dacites are light grey, and
weather to soft white or brown clays (NRC, 1989).
There are small isolated outcrops of Pliocene age (6 to 2 million years) Ti Point olivine basalts near
the centre of the study area (Thompson, 1961).
The predominant surface geology of the study area is Pleistocene to Recent (up to 2 million years
old) coastal sand dunes and alluvium, located at low elevations around the estuary and in the
stream valleys. The sands form active dunes around the ocean beach and fixed dunes and terraces
further inland (ie. Mangawhai Heads). The Pleistocene terrace deposits are poorly consolidated
silty sands, muds and gravels with minor vegetative remains (plant fragments and peat). A hard
iron pan often caps these deposits (Massey, 1987). In some locations where the deposits are
exposed at the surface the sediments have weathered to brown stained, soft clayey sand to depths of
15 m (NRC, 1989). Overlying the Pleistocene terraces are Recent fixed and active dunes, which
thicken around the estuary and coastline. These sediments are unconsolidated and unweathered
sands. The alluvium within the stream valleys is comprised of sands, muds, and gravels with minor
peat.
Figure 2. Regional Geology.
(see A3 attachment at rear).
SINCLAIR KNIGHT MERZ
D:\Projects\AE02189\Deliverables\Mangawhai Report_Final.doc PAGE 3 Mangawhai Groundwater Resource
2.3 Geological Structure
The greywacke basement rocks that outcrop to the north of the study site are upfaulted along the E-
NE trending Waipu Fault. Gravity data provided in Massey (1987) shows that the fault dips very
steeply on the down-thrown side reaching a maximum depth of 690 m just south of Sentinel Rock.
The greywacke basement gradually becomes shallower to the south, outcropping again near Te
Arai Point.
The fault abruptly terminates Northland Allochthon and Waitemata sediments, but is overlain with
Parahaki Volcanics. This indicates that the fault movement postdates Waitemata Group deposition
but predates Parahaki Volcanic emplacement.
The outcrop of Northland Allochthon (Mangakahia Group) sediments to the west of the estuary,
which is surrounded by approximately 200 m of Waitemata sediments (see log of bore 208246),
suggests that further faulting (block faulting) has occurred within the greywacke basement after
deposition of the Waitemata sediments.
2.4 Drillers Borelogs
There are 80 boreholes registered on the NRC bore database for Mangawhai. A summary of the
relevant bore construction and hydraulic test details are included in Appendix A, and Figure 1
shows the approximate location of the bores.
Eighteen bores are screened within the coastal dune sands to depths of between 4.5 and 33.5 m,
with diameters of 38 to 150 mm. The aquifer has yields ranging from 12 to 409 m3
/day. The
limited extent and depth of the sand aquifer beneath the water table and the presence of intercalated
silts, clays and thin iron pans restricts the volume of water that can be abstracted, as indicated by
the low number and distribution of bores drilled into the sand. Some logs have brief comments
relating to poor water quality (high iron, sulphur odours), which is typical of decomposition of
vegetative remains in recent alluvial sediments.
The majority of the bores tap into the Waitemata sediments. The bores are between 12.6 and 210
m depth, with an average depth of 65 m. The bores are 64 to 100 mm in diameter and are generally
open (unscreened) through the sandstone. Yields range from 9 to 454 m3
/day, with the greatest
yields from fractured zones with the sandstone. The fracture zones appear to be located at greater
depths. Artesian flows are reported in four bores (208249, 208276, 208344 and 208376), which are
relatively deep (>55 m) and situated at low elevations around the estuary and in the stream valleys.
Some bore logs have brief water quality comments relating to saltwater contamination, sulphur
odours and slight iron discolouration.
It should be noted that there may be inaccuracies in the drillers’ logs, in terms of approximate bore
locations, lithology and yield. This has implications on the conceptualisation of the aquifer
SINCLAIR KNIGHT MERZ
D:\Projects\AE02189\Deliverables\Mangawhai Report_Final.doc PAGE 4 Mangawhai Groundwater Resource
discussed later in this report, including aquifer lithology, aquifer properties and piezometric surface
distribution.
2.5 Hydrology
There are two main streams in the area, including Tara Creek that drains the Tara volcanic area
west of the study area and Bob Creek that drains the Waitemata sediments. Tara Creek is partly
spring fed from springs emerging from the basalt. No information is available on stream flows,
however monitoring of one spring flow near Tara indicates flow of up to 311 m3
/day (NRC,
unknown date). The stream flows are likely to be higher than this, due to the discharge of a
number of spring flows into the stream, and surface and groundwater inputs further down the
catchment.
2.6 Rainfall and Evaporation
Figure 3 shows mean monthly rainfall for Tara (Station A641511) for the period 1946 to 2004
(missing data between 1983 to 1989), and mean monthly evaporation for Leigh (Station A64282)
for the period 1972 to 2004. The annual average rainfall is 1,600 mm. Rainfall exceeds
evaporation for all but the summer months, indicating the availability of water for groundwater and
surface water recharge for most of the year. ( see the link about to the .pdf file for graphics and more.... )
Mangawhai Village and Mangawhai Heads are coastal resort towns located approximately 100 km
north of Auckland. The towns, in particular Mangawhai Heads, are currently experiencing
subdivision, which is increasing the demand on groundwater. Groundwater in the area is used
mainly as a supplementary domestic supply for many of the residences, and to a lesser extent for
horticultural irrigation and stock drinking water. A number of bores have poor water quality,
believed to be due to seawater intrusion, hence the classification of the area as being an “at-risk”
aquifer.
The total study area is 14 km2
encompassing Mangawhai Village, Mangawhai Heads and the
sandspit bordering the estuary to the east (see Figure 1). Most of the study area is low-lying around
the estuary, extending to 53 m above mean sea level (mAMSL) at Mangawhai Heads and
approximately 100 mAMSL in the rolling hills behind Mangawhai Village.
A volcanic intrusion just to the north of the Mangawhai Heads peninsula reaches 107 mAMSL.
Figure 1 shows the main points of interest of the study area, including the aquifer boundary. The
aquifer boundary has been delineated based on the lithological change from coastal sands to
consolidated hardrock of various types, and is as defined in the Northland Regional Water & Soil
Plan (NRC, 2004). The NRC holds borelogs, hydraulic information, groundwater quality and
groundwater level data for bores located outside of, but in close proximity to the aquifer boundary.
Information from these bores has been included and discussed in this report for comprehensiveness.
Regional Geology
The geology of the Mangawhai Area is described on the 1:250,000 Geological Map Sheet 2A for
Whangarei, and is reproduced in Figure 2.
Greywacke and argillite basement rocks of Permian to Jurassic age (295 to 145 million years
before present) underlie the study area at depths of between 100 and 690 m below mean sea level
(Massey, 1987). The rocks are dark grey or green, intensely deformed, jointed or sheared with
secondary silica or calcite along the joint planes (Thompson, 1961). They are upfaulted to the
north of the study area along the E-NE trending Waipu Fault.
Overlying the downfaulted basement rocks within the study area are Cretaceous to Oligocene age
(145 to 25 million years) Northland Allochthon sediments. The Northland Allochthon represents a
SINCLAIR KNIGHT MERZ
D:\Projects\AE02189\Deliverables\Mangawhai Report_Final.doc PAGE 2 Mangawhai Groundwater Resource
series of discrete lithological units that were emplaced as part of a large gravity slide affecting most
of Northland. As a result of this mode of deposition, the Northland Allochthon rocks have been
substantially faulted, fractured and sheared, and rocks of differing age are commonly found
together or within close proximity to each other. The rocks that outcrop within the study area
include Mangakahia Group sandstones of Cretaceous age, and Opahi Formation greensands,
argillaceous limestone and shale of Eocene age. The Mangakahia Group sediments outcrop in the
centre of the study area near Tara Creek, and are grey, sulphurous and micaceous. The Opahi
Formation sediments outcrop to the north of the study area, with closely fractured grey-white
argillaceous limestone being most commonly exposed.
Overlying the Northland Allochthon sediments are Waitemata Group sandstones and mudstone of
Miocene age (25 to 6 million years old). The Waitemata sediments are widespread beneath and
around the study area and comprise dark grey non-calcareous sandstones thinly or thickly bedded
with mudstone. There are occasional thin coal seams, fossils (shell fragments, limestone) and
volcanic breccia beds (Thompson, 1961). The rocks are moderately hard and can be weakly to
moderately fractured. They typically weather to soft yellow brown silty clays to depths of 30 m
(NRC, 1989).
There are intrusions of Late Miocene age Parahaki Volcanics (dacite lava and tuff) on the northern
headland near Mangawhai Heads and to the west of the study area. The dacites are light grey, and
weather to soft white or brown clays (NRC, 1989).
There are small isolated outcrops of Pliocene age (6 to 2 million years) Ti Point olivine basalts near
the centre of the study area (Thompson, 1961).
The predominant surface geology of the study area is Pleistocene to Recent (up to 2 million years
old) coastal sand dunes and alluvium, located at low elevations around the estuary and in the
stream valleys. The sands form active dunes around the ocean beach and fixed dunes and terraces
further inland (ie. Mangawhai Heads). The Pleistocene terrace deposits are poorly consolidated
silty sands, muds and gravels with minor vegetative remains (plant fragments and peat). A hard
iron pan often caps these deposits (Massey, 1987). In some locations where the deposits are
exposed at the surface the sediments have weathered to brown stained, soft clayey sand to depths of
15 m (NRC, 1989). Overlying the Pleistocene terraces are Recent fixed and active dunes, which
thicken around the estuary and coastline. These sediments are unconsolidated and unweathered
sands. The alluvium within the stream valleys is comprised of sands, muds, and gravels with minor
peat.
Figure 2. Regional Geology.
(see A3 attachment at rear).
SINCLAIR KNIGHT MERZ
D:\Projects\AE02189\Deliverables\Mangawhai Report_Final.doc PAGE 3 Mangawhai Groundwater Resource
2.3 Geological Structure
The greywacke basement rocks that outcrop to the north of the study site are upfaulted along the E-
NE trending Waipu Fault. Gravity data provided in Massey (1987) shows that the fault dips very
steeply on the down-thrown side reaching a maximum depth of 690 m just south of Sentinel Rock.
The greywacke basement gradually becomes shallower to the south, outcropping again near Te
Arai Point.
The fault abruptly terminates Northland Allochthon and Waitemata sediments, but is overlain with
Parahaki Volcanics. This indicates that the fault movement postdates Waitemata Group deposition
but predates Parahaki Volcanic emplacement.
The outcrop of Northland Allochthon (Mangakahia Group) sediments to the west of the estuary,
which is surrounded by approximately 200 m of Waitemata sediments (see log of bore 208246),
suggests that further faulting (block faulting) has occurred within the greywacke basement after
deposition of the Waitemata sediments.
2.4 Drillers Borelogs
There are 80 boreholes registered on the NRC bore database for Mangawhai. A summary of the
relevant bore construction and hydraulic test details are included in Appendix A, and Figure 1
shows the approximate location of the bores.
Eighteen bores are screened within the coastal dune sands to depths of between 4.5 and 33.5 m,
with diameters of 38 to 150 mm. The aquifer has yields ranging from 12 to 409 m3
/day. The
limited extent and depth of the sand aquifer beneath the water table and the presence of intercalated
silts, clays and thin iron pans restricts the volume of water that can be abstracted, as indicated by
the low number and distribution of bores drilled into the sand. Some logs have brief comments
relating to poor water quality (high iron, sulphur odours), which is typical of decomposition of
vegetative remains in recent alluvial sediments.
The majority of the bores tap into the Waitemata sediments. The bores are between 12.6 and 210
m depth, with an average depth of 65 m. The bores are 64 to 100 mm in diameter and are generally
open (unscreened) through the sandstone. Yields range from 9 to 454 m3
/day, with the greatest
yields from fractured zones with the sandstone. The fracture zones appear to be located at greater
depths. Artesian flows are reported in four bores (208249, 208276, 208344 and 208376), which are
relatively deep (>55 m) and situated at low elevations around the estuary and in the stream valleys.
Some bore logs have brief water quality comments relating to saltwater contamination, sulphur
odours and slight iron discolouration.
It should be noted that there may be inaccuracies in the drillers’ logs, in terms of approximate bore
locations, lithology and yield. This has implications on the conceptualisation of the aquifer
SINCLAIR KNIGHT MERZ
D:\Projects\AE02189\Deliverables\Mangawhai Report_Final.doc PAGE 4 Mangawhai Groundwater Resource
discussed later in this report, including aquifer lithology, aquifer properties and piezometric surface
distribution.
2.5 Hydrology
There are two main streams in the area, including Tara Creek that drains the Tara volcanic area
west of the study area and Bob Creek that drains the Waitemata sediments. Tara Creek is partly
spring fed from springs emerging from the basalt. No information is available on stream flows,
however monitoring of one spring flow near Tara indicates flow of up to 311 m3
/day (NRC,
unknown date). The stream flows are likely to be higher than this, due to the discharge of a
number of spring flows into the stream, and surface and groundwater inputs further down the
catchment.
2.6 Rainfall and Evaporation
Figure 3 shows mean monthly rainfall for Tara (Station A641511) for the period 1946 to 2004
(missing data between 1983 to 1989), and mean monthly evaporation for Leigh (Station A64282)
for the period 1972 to 2004. The annual average rainfall is 1,600 mm. Rainfall exceeds
evaporation for all but the summer months, indicating the availability of water for groundwater and
surface water recharge for most of the year. ( see the link about to the .pdf file for graphics and more.... )