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GROUNDWATER SCOPING REPORT FOR RAROTONGA, COOK ISLANDS
NIWA Client Report: WLG2010-57 October 2010 NIWA Project: GCI11301
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GROUNDWATER SCOPING REPORT FOR RAROTONGA, COOK ISLANDS Author Dr. Mandy Meriano
NIWA contact/Corresponding author
E W Maas
Prepared for
Ministry of Marine Resources NIWA Client Report: WLG2010-57 October 2010 NIWA Project: GCI11301
National Institute of Water & Atmospheric Research Ltd 301 Evans Bay Parade, Greta Point, Wellington Private Bag 14901, Kilbirnie, Wellington, New Zealand Phone +64-4-386 0300, Fax +64-4-386 0574 www.niwa.co.nz
Contents Executive Summary iv
1. Introduction 1
2. Background 1
3. Climate 3
4. Geological and Hydrogeological Setting 4 4.1 Volcanic Rocks (Old Caldera complex and phonolitic
eruptive rocks) 5 4.2 Sedimentary Deposits (Nikao Gravels, Aroa Sands and
stream alluvium) 5
5. Assessment Of Groundwater Resources 6 5.1 Groundwater Quantity (Estimate of Water Balance) 6 5.2 Groundwater Quality 7
6. Proposed Groundwater Monitoring For The Eu Muri Project 8 6.1 First Phase: Data Collection and Analysis (Conceptual
Model Development) 9
6.1.1 Monitoring Well Location and Installation 10
6.2 Slug Tests 14
6.2.1 Groundwater Surface and Flow Maps 15
6.2.2 Groundwater Travel Times 15
6.2.3 Groundwater Discharge (baseflow) Estimate 16
6.3 Groundwater Sampling 17
6.3.1 Sample Parameters and Load Calculation 19
6.4 Recommendations for a groundwater sampling programme (summary): 20
7. Second Phase 20
8. References 22
Reviewed by: Approved for release by:
Dr E Maas Dr Andrew Laing
Groundwater Scoping Report For Rarotonga, Cook Islands iv
Executive Summary
Rarotonga’s water cycle has been modified by human activities, such as diverting mountain stream
water for drinking water supply, irrigation, groundwater extraction, alteration of run off patterns for
agriculture, infilling of swamplands for urban development, and disposal of sewage effluent from
septic tanks into the subsurface and the lagoon that surrounds the island.
The European Union (EU) Muri project has been funded to investigate ways to improve water quality
in the Muri area by upgrading septic tanks. Currently, surface water, streams and the lagoon are
monitored for nutrients and bacteria, but the quality or flow of groundwater has not been assessed.
Therefore, the aim of this study was to review what is known about groundwater in the Muri
catchment and to provide information and methods for monitoring groundwater quantity/quality.
Once this monitoring has been put in place and completed, it will allow the EU Muri project to
evaluate the potential effect of wastewater treatment changes on water quality in the Muri area.
A review of reports held in Rarotonga revealed that the geology and climate of Rarotonga were well
described, but that very little, if any, information existed about groundwater flow, quantity and quality
in the Muri/Avana area.
Therefore, two phases of groundwater investigations are described. The first phase aims to establish a
groundwater monitoring network (primary focus of the scoping project) to gather representative data
to:
• Provide baseline data to map the spatial and temporal distribution of groundwater levels and
quality
• Identify short and long-term changes to groundwater levels and flow patterns from natural
recharge and discharge
• Isolate the impact to groundwater from septic tanks and contaminant releases due to agricultural
activities
• Present early warning of potential risks and need for mitigation
• Identify and evaluate the success of mitigating measures
• Provide real-time information for compliance with guidelines
• Support collaborating efforts with surface water resource management
• Support calibration efforts when conducting computer modelling.
Groundwater Scoping Report For Rarotonga, Cook Islands v
The Ministry of Infrastructure and Planning have installed bores in the Muri/Avana area. These bores
can form part of the groundwater monitoring program, however these bores need to be widened and
loggers need to be installed. The report provides information about how to evaluate the current
position of the bores and extent this to cover the different land uses, population densities and soil types
in the Muri/Avan area. Methods for conducting slug tests, calculating groundwater surface and flow
maps, groundwater travel times and groundwater discharge are explained. A groundwater sampling
plan is provided, including which parameters need to be measured to calculate nutrient and bacterial
loads in groundwater.
Successful implementation of a groundwater monitoring programme is highly dependent on the
availability of trained individuals. Hence, suitable training and knowledge transfer opportunities need
to be provided to technicians and students.
A summary of a second phase groundwater monitoring programme is provided. However this is only a
brief description and highlights some future objectives of the groundwater monitoring programme for
water resource managers.
Groundwater Scoping Report For Rarotonga, Cook Islands 1
1. Introduction
The aim of the groundwater scoping project was to provide the water resource
managers with a suitable groundwater monitoring plan that will assist them with
the assessment and evaluation of groundwater impact on water quality and lagoon
health. The scoping report was compiled following a field visit to Muri catchment
in Rarotonga, Cook Islands, carried out between 06-10 September 2010. Dr. Els
Mass and Dr. Julie Hall of New Zealand’s National Institute of Water and
Atmospheric Research (NIWA) coordinated the visit to Muri catchment.
The following individuals are acknowledged and thanked for making available
relevant data/reports and their assistance in the field: Ms. Dorothy M. Solomona,
Acting Director, Pearl Support Division, Ministry of Marine Resources; Mr. Paul
Teariki Maoate, Integrated Water Resource Management (IWRM) Project
Manager, Ministry of Infrastructure and Planning; and Mr. Tekao Herrmann, Civil
Engineering consultant.
The objectives of the groundwater scoping project were to:
• Establish contact with EU Project staff and discuss information on sources
of contamination in the Muri/Avana catchment.
• Collate historical, current and relative data on groundwater levels and
quality.
• Identify regional estimates of water balance.
• Develop a groundwater monitoring plan for various groundwater zones
within the catchment.
2. Background
The volcanic island of Rarotonga (Figure 1) covers an area of 67 km2, and
receives an annual rainfall between two and four meters (Clement and Bourguet,
1992). The island is oval in shape and is mountainous with numerous steep ridges
and peaks of up to 600 m above sea level (a. s. l.). It is bordered by a narrow
coastal lowland alluvium and raised coral with a fringing reef (Wood and Hay,
1970). It has a north-south width of about 6.5 km and an east-west length of some
11 km and has a circumferential road (the Ara Tapu) about 28 km in length,
around the edge of coastal plain. The interior is rugged and bush-clad.
The island’s population of about 9,000 is concentrated in the coastal plain, which
is approximately 1 km wide.
Groundwater Scoping Report For Rarotonga, Cook Islands 2
Figure 1. Map showing physiographic regions of Rarotonga. Provided by the Ministry of Infrastructure and Planning.
The Muri catchment is approximately 850 ha and includes the subcatchments of
Avana, Aroko, Nukupure, Areiti, Aremango, Vaii and Maii (Figure 2).
Avana (perennial) and Paringaru (intermittent) streams define the northern and
southern boundaries of the catchment, respectively. Streams drain radially
outwards from the steep interior. In their losing segments, stream water may seep
into the streambed alluvium and recharge the shallow aquifer system where it
crosses the coastal plain. In their gaining segments, groundwater is discharged
into the stream maintaining streamflow (baseflow), particularly during drier
periods (no precipitation). This has important implications regarding water
resources. It indicates that groundwater is a contributing component of streamflow
in the catchment.
Groundwater Scoping Report For Rarotonga, Cook Islands 3
Figure 2. Map showing Muri catchment, subcatchment boundaries, and location of water intake on Avana stream. Provided by the Ministry of Infrastructure and Planning.
Water supply in the Muri catchment is provided by a distribution network, fed by
the capture of surface water at the Avana Water Intake. Figure 2 shows the
location of the water intake in the catchment. Currently, water is distributed free
of charge. The distribution system is affected by losses through pipe leakage and
difficulties are encountered during drier periods to cover demand. Presently, water
requirements include domestic, agricultural, gardening, piggeries, and cattle.
3. Climate
There are two climate seasons in Rarotonga, a rainy season (“summer” October to
April) and a dry season (“winter” May to September). The centre of the island
receives the highest mean annual rainfall (4000mm), whereas the coastal margins
receive between 2000mm and 3000mm in the north-west and south of the island,
respectively (Ricci and Scott, 1998). Mean annual rainfall is about 2100mm and
the mean annual temperature is about 24 deg C (Parakoti and Davie, 2007).
Groundwater Scoping Report For Rarotonga, Cook Islands 4
Rainfall is the only source of water on the island providing surface runoff to
streams and recharge to aquifer systems through infiltration. Evapotranspiration
removes water from the system and during drier periods impedes the water flow
into streams. Excess water from rainfall, after removal of evapotranspiration and
runoff, infiltrates and passes through lavas and breccias to reach the older
weathered volcanic formation. General groundwater flow within the aquifer
system is towards the lagoon and the sea.
4. Geological and Hydrogeological Setting
Rarotonga is a non-active volcano of late tertiary age (8 to 2 million years ago)
surrounded by quaternary sediments, gravel fan deposits, coastal terraces, mud
swamps, coral sands and reefs (15,000 years old) (Figure 1). Figure 3 depicts a
geologic cross-section showing geological relationships and groundwater flow
paths in the coastal plain. Several studies summarize the geology and water
resources of Rarotonga (see Wood and Hay, 1970; Clement and Bourguet, 1992;
Burke and Ricci, 1997; Ricci and Scott, 1998). A summary is presented below.
Figure 3. Geologic cross-section showing geologic materials and groundwater flow directions. Modified from Burke and Ricci (1997).
Groundwater Scoping Report For Rarotonga, Cook Islands 5
4.1 Volcanic Rocks (Old Caldera complex and phonolitic eruptive rocks)
The Old Caldera Complex (Late Pliocene) comprises a mixture of breccias,
pyroclastic flows, scoria and ashes. Due to its age, the formation has been highly
weathered and the volcanic rocks have been transformed into clay minerals.
Hydrogeologically, this is of great importance as these altered formations may act
as an aquitard (less permeable formation incapable of transmitting significant
quantities of water). Conversely, the more recent less weathered breccias, ashes
and fractured volcanic rocks (Late Pliocene-Early Pleistocene) act as a complex
aquifer (permeable formation capable of transmitting significant quantities of
water) system. The position of the aquifer system in the subsurface is dependent
on the local sequence and superposition of various volcanic layers. It has been
suggested that a significant portion of the groundwater in Rarotonga could be
contained within alluvium or scoria deposits in old buried stream valleys that
extend down to the impervious ancient basement rocks (Clement and Bourguet,
1992).
4.2 Sedimentary Deposits (Nikao Gravels, Aroa Sands and stream alluvium)
Nikao Gravels form the weathered volcanic gravels and sands of coastal terraces
and fans. The Aroa Sands comprise coral sand and gravel beach deposits forming
a low bench around the island between the foot of the Nikao Gravels and the inner
edge of the lagoon (Burke and Ricci, 1997). The highly permeable stream
alluvium consists of fans of volcanic gravels and debris deposited by streams.
These fans have been deposited at the foothill of the volcano below an elevation
of 30 to 40 m. a.s.l.
Hydrogeologically, the fan deposits are highly permeable and constitute the
alluvial aquifer system. Local variability in the permeability of the fan deposits
has been observed where older more weathered Nikao Gravels form less
permeable lenses (Clement and Bourguet, 1992; Burke and Ricci, 1996).
Conversely, the Aroa Sands have more uniform permeability. The alluvial aquifer
system is directly recharged by runoff and is considered to be an important source
of groundwater.
Groundwater Scoping Report For Rarotonga, Cook Islands 6
5. Assessment of Groundwater Resources
Currently little is known of the aquifers underlying Rarotonga. The Ministry of
Infrastructure and Planning has installed 10 shallow monitoring wells in the Muri
catchment area with plans to monitor groundwater levels and quality. To date,
interaction between groundwater and surface water along losing and gaining
stream segments has not been investigated. The stresses imposed on surface and
groundwater resources and the effect on water quality and quantity may not be
isolated from one another. For example, heavily polluted groundwater can
discharge into streams and lagoons, thus negatively impacting surface waters and
their dependent ecosystems.
5.1 Groundwater Quantity (Estimate of Water Balance)
The less permeable nature of the volcanic rocks in Rarotonga results in reduced
infiltration of rainfall and the emergence of streams in the high interior part of the
island. Streams can persist as far as the coast, particularly in the northern and
eastern parts of the island where large streams maintain direct channels to the sea.
The proportion of the rainfall that reaches the water table aquifer is greatest where
the rocks are more permeable (i.e., weathered volcanic alluvium of the coastal
terraces and streams as well as beach deposits in the coastal fringe). Ricci and
Scott (1998) calculated average annual recharge from rainfall to be 640 mm – or
about 30% of the average annual rainfall. The water that reaches the water table
aquifer becomes part of the groundwater reservoir (recharge) and moves through
the fractures and pores of the saturated subsurface material and reappears at the
surface at lower elevations in the form of springs and seeps (discharge) which in
turn feed the lowland streams. In addition to rainfall recharge, the aquifer also
receives recharge from streams as observed by the disappearance of streamflow
into the alluvium on emerging from the volcanic interior (Waterhouse and Petty,
1986).
Drinking water is distributed through a piped network that is known to leak
(Binnie & Partners, 1984; WMI & BURGEAP, 1992). Burke and Ricci (1997)
report unaccounted losses from the system in the order of 40-60%.
While approximately 70% of the corroded and leaking pipelines in Rarotonga has
been replaced, the effectiveness of the new distribution network remains
unquantifiable due to lack of metering (ADB Report, 2009). High water losses
from the distribution network to the subsurface provides additional source of
recharge to the shallow aquifer system and may lead to rising groundwater levels
and baseflow quantities in the catchment.
Groundwater Scoping Report For Rarotonga, Cook Islands 7
5.2 Groundwater Quality
Agricultural practices and poor sewage treatment mechanisms have been
identified as main sources of groundwater contamination in Rarotonga. Piggeries,
poultry and cattle were spotted on small family farms on the terrace at the foot of
the volcanic hills in the catchment. Septic tank systems are widely used
throughout Rarotonga and are in various state of repair (sample photos in
Appendix I). They are not connected to a sewer system and are specifically
designed for subsurface drainage of human waste at slow rates. An improperly
designed, located, constructed, or maintained septic system can leak contaminants
(i.e., bacteria, viruses, chemicals) into the ground causing groundwater
contamination. Therefore, they present a serious source of contamination to the
aquifer system.
Although, there were no groundwater quality data available during this visit, it is
generally believed that nitrate and bacteriological pollution have deteriorated
groundwater quality (Burke and Ricci, 1997). Bacterial contamination from
human and animal wastes and nitrate from intensive agriculture may be the most
likely sources of groundwater contamination. Leaching of contaminants can
occur more readily on sandy soils due to their low nutrient and water holding
capacity (i.e., Nikao Gravels, Aroa Sands). Any increase in the concentration of
these contaminants can be alarming as both contaminants present health concerns.
Excess nitrate also causes eutrophic conditions in receiving waters which may
compromise ecosystem health.
The section 6 provides an overview of a proposed groundwater monitoring and
testing programme for the Muri catchment.
Groundwater Scoping Report For Rarotonga, Cook Islands 8
6. Proposed Groundwater Monitoring for the EU Muri Project
As an essential component of water resource management, groundwater
monitoring networks are designed to optimize the collection of vast amounts of
field data. Collection, analysis, and management of groundwater levels and
quality provide fundamental baseline information necessary for identifying
potential risks and managing groundwater as a sustainable resource.
The aim of the groundwater monitoring network is to:
• Provide baseline data to map the spatial and temporal distribution of
groundwater levels and quality
• Identify short and long-term changes to groundwater levels and flow
patterns from natural recharge and discharge
• Isolate the impact to groundwater from septic tanks and contaminant
releases due to agricultural activities
• Present early warning of potential risks and need for mitigation
• Identify and evaluate the success of mitigating measures
• Provide real-time information for compliance with guidelines
• Support collaborating efforts with surface water resource management
• Support calibration efforts when conducting computer modeling.
This data will be collected and analyzed in the first instance to support the EU
(Muri) project and provide information as to the effectiveness of the changes
made to the wastewater treatment systems. However, the basic design outlined
here could be duplicated around Rarotonga for other projects and to gain an
understanding of the groundwater in that specific location.
The groundwater monitoring is divided in two phases. The first phase details the
establishment of a groundwater monitoring programme (primary focus of the
scoping project). The second phase is preliminary in its scope and is only
suggested to assist with future planning of water resources. This phase can be
carried out after enough data have been collected in phase one and is not required
for the purpose of the EU project.
It must be noted that successful implementation of a groundwater monitoring
programme is highly dependent on the availability of trained individuals. Hence,
suitable training and knowledge transfer must be provided to the designated
groundwater team.
Groundwater Scoping Report For Rarotonga, Cook Islands 9
6.1 First Phase: Data Collection and Analysis (Conceptual Model Development)
Understanding the spatial aquifer heterogeneity and behaviour with some degree
of certainty in complex volcanic islands is a difficult task. With the lack of
existing data on aquifers, the data collection phase should provide information on
aquifer properties such as hydraulic conductivity (spatial), groundwater levels and
flow directions (spatial and temporal), and groundwater quality (spatial and
temporal). Table 1 summarizes the sequential steps to develop a conceptual model
as necessary for the quantification of the groundwater quantity and quality.
Table 1: Sequential steps necessary for the establishment of groundwater monitoring in support of the EU Muri Project
Action Purpose
1. Field survey To locate groundwater monitoring sites
2. Drilling, logging of sediment cores, and installation of monitoring wells
To access distinct parts of the aquifer system (i.e., volcanic, sand and gravel) with various land use intensity for groundwater level monitoring and sampling
3. Slug testing To measure in situ hydraulic conductivity of the aquifer material
4. Groundwater sampling To develop a groundwater quality baseline against which wastewater management methods can be evaluated and future changes in quality monitored
5. Groundwater flow mapping To develop contour maps of the water table elevation and groundwater flow direction
6. Estimating groundwater travel times To estimate groundwater velocity and time of travel in the aquifer
7. Estimating groundwater recharge To quantify groundwater recharge to the catchment and to calculate nutrient loading via groundwater to surface waters
During the field visit the need for cost-effective methods for obtaining
groundwater measurements was noted. Hence, some guidance is provided below
to assist water resource managers with the establishment of a groundwater
monitoring system and instrument acquisition. The following sections outline the
steps towards achieving the development of a groundwater monitoring
programme and a conceptual model for the EU Muri Project.
Groundwater Scoping Report For Rarotonga, Cook Islands 10
6.1.1 Monitoring Well Location and Installation
A groundwater monitoring network of 10 shallow wells, between 1.2 and 5.0 m
deep, was installed in June 2010. Figures 4 (in three view panels) and 5 (in two
view panels) show the monitoring locations in the Muri catchment within the
various surficial deposits. Groundwater levels were measured following well
installation and were recorded to be between 0.1 m to 4.6 m. a.s.l. (Figure 5).
These wells have narrow opening (20 mm diameter) limiting access and hindering
well development (pumping), groundwater sampling, and instrument housing. It is
recommended that these wells be made larger and in some cases deeper (very
shallow wells risk the chance of going dry during the dry season) for easier
monitoring and sampling. Assuming that an appropriate drill rig and skilled crew
is available, it is recommended that new monitoring wells be installed in the
coastal terraces and fans (Nikao gravels) and fan deposits and terraces bordering
the volcanic hills.
Figure 4. Distribution of monitoring wells within the various soils/surficial deposits in the Muri/Avana catchment (three views). Provided by the Ministry of Infrastructure and Planning (continued on next page).
Groundwater Scoping Report For Rarotonga, Cook Islands 11
Figure 4. Distribution of monitoring wells within the various soils/surficial deposits in the Muri/Avana catchment (three views). Provided by the Ministry of Infrastructure and Planning.
Groundwater Scoping Report For Rarotonga, Cook Islands 12
Figure 5. Distribution of monitoring wells and observed water level data (two views).
Provided by the Ministry of Infrastructure and Planning.
Groundwater Scoping Report For Rarotonga, Cook Islands 13
Well locations should target major land use and various population densities on
the volcanic terrace and coastal areas (areas most likely to detect contamination)
and presence or absence of sandy (more permeable) soils. This will allow for
comparison between groundwaters across the catchment with similar land use,
soil and sediment types. A field reconnaissance survey is therefore recommended
to identify specific locations (hotspots) that warrant detailed monitoring and
sampling (anecdotal evidence from farms/residents can also be considered in the
selection). It is noted that a survey of the septic tanks in the Muri/Avana
catchment has been completed by Mr. Tekao Herrmann (personal
communication). The results of this survey should be used in the monitoring well
site selection.
It is recommended that, at minimum, three wells be installed in each targeted
geographic location (i.e., Terraces, Fans, Flood Plains, Beach Ridges, and
Depressions (swamp deposits) – please refer to Figure 1 for the location of these
five physiographic regions in the catchment) to calculate the groundwater
direction and hydraulic gradients (see section Hydraulic Head and Gradients).
In smaller non-contiguous locations (i.e., depressions) fewer wells may be
necessary to characterize the groundwater. Therefore, one must exercise judgment
concerning the number of necessary wells in these smaller locations that fall
within the greater monitoring network. For example, a single well in a small
depression surrounded by an array of wells will fulfill the monitoring requirement.
Single monitoring wells must also be installed near streamflow gauging sites (see
Section: Groundwater Recharge Estimate). It is recommended that at minimum 10
– 15 wells to be installed in the Muri catchment. It is essential that the
groundwater and surface water monitoring programmes in the catchment are
coordinated so that groundwater level and quality data can be compared and
cross-referenced with streamflow and stream water quality data.
Hollow stem augers are suitable for drilling into shallow unconsolidated deposits.
However, rotary drilling may be appropriate for wells deeper than about 30 m or
into wells in bedrock. Core samples are to be collected from bored holes and
stratigraphically logged to delineate the site geology. Presently, staff at the
Ministry of Infrastructure and Planning has the appropriate geotechnical
background and experience to produce the lithologic logs. Monitoring wells are to
be installed in the completed boreholes. There must be a sufficient working
opening inside the augers or casing. For example for a 51 mm (2-in) monitoring
well, a minimum 108 mm (4.25-in) opening is required. Monitoring wells must be
screened in the bottom within the saturated zone. It is expected that the water
table in the upper catchment areas will be found at greater depths therefore
requiring deeper bore drilling.
Groundwater Scoping Report For Rarotonga, Cook Islands 14
Currently the monitoring wells in the catchment (Figure 4) appear to be in the
correct locations - targeting the Beach and some of the older and younger Fan
deposits - but a final walk thru is recommended by MOIP with the EU project
team, to make sure that the wells cover all the different land uses, population
densities identified in EU Muri project. It is recommended that the wells
identified for monitoring are widenend to 2.5-in (63.5 mm) and deepened into the
saturated aquifer to reduce the chance of wells going dry due to seasonal water
table fluctuations and to ensure groundwater sampling during dry season when
water quality may be at its worst.
Completed monitoring wells should be equipped with pressure transducers for
continuous measurement of groundwater levels. Temperature-compensated
conductivity probes equipped with data loggers should be used to record
continuous temperature and electrical conductivity at several locations in the
catchment. Ideally, the loggers must be spatially distributed in the catchment to
representatively sample groundwater from a variety of land use.
Hourly measurements of groundwater levels, temperature and electrical
conductivity can be recorded in the data logger and should be downloaded
regularly. Manual measurement of groundwater level in monitoring wells not
equipped with level loggers should be done weekly using a water level meter.
Temperature and electrical conductivity should be measured concurrently using a
handheld probe. At minimum, one well in each ground type should be installed
with pressure transducers (see Recommended Equipment in section 1.3.1) the
remaining wells can be monitored manually.
All groundwater activities described below are to be completed using the installed
monitoring wells.
6.2 Slug Tests
These tests are to be carried out in each well once after the completion of well
installation. In situ hydraulic conductivity (K) of the aquifer material should be
determined by means of slug tests. K represents a measure of the ability of flow
through porous media (i.e., volcanic gravels, beach sand and gravels, etc) and has
the dimensions of length/time. Slug tests are particular tests where the rate of
groundwater recovery is measured after a volume (slug) of water is suddenly
displaced. A wide variety of formulas are available for the analysis of slug test
data, based on aquifer type and geometry and underlying assumptions (e.g.,
Hvorslev, Bouwer and Rice, etc). For a comprehensive introduction to various
methods please refer to Freeze and Cherry (1979). As mentioned previously, it is
Groundwater Scoping Report For Rarotonga, Cook Islands 15
essential that the groundwater technicians receive appropriate training for data
processing and interpretation of results.
Groundwater flow paths and travel times between septic systems and/or farmlands
and streams are important factors that influence both water quantity and quality.
Determining flow paths and travel times is therefore a prerequisite for interpreting
water quality changes. The following outlines a hydraulic approach to assess
groundwater flow and travel times.
6.2.1 Groundwater Surface and Flow Maps
These maps can be calculated as detailed below from the information obtained in
1.2 and can be calculated for the Muri area, this will help us to understand the
direction and magnitude of groundwater flow in the catchment.
Water in the aquifer stands at a particular level (hydraulic head). In practice,
depth to groundwater measurement is obtained and is subtracted from the top of
the well casing elevation to measure total head (datum is calibrated to sea level).
The collected groundwater level measurements are to be used to generate a
groundwater surface map. This is achieved by contouring measured head values in
the shallow aquifer (water table map).
Groundwater flow direction can be understood in the fact that groundwater always
flows in the direction of decreasing head. The magnitude of the movement,
however, is dependent on the hydraulic gradient (i) which is defined as the
change in head (∆h) per unit distance (∆l) (i=∆h/∆l). A minimum of three wells
located in a triangular pattern is required to calculate the direction and hydraulic
gradient of a geographic location. Groundwater flow direction is perpendicular to
the groundwater level contour line.
6.2.2 Groundwater Travel Times
Groundwater travel times can be calculated using the data obtained. This will help
us to understand how long groundwater stays in the ground and will help us to
predict how long it might be before changes to water quality can be noticed, that
is if groundwater stays in the ground a long time (longer flow paths) it might be a
long time before there is a reduction in the contamination of the streams and the
lagoon.
Groundwater Scoping Report For Rarotonga, Cook Islands 16
Groundwater vulnerability to pollution is best understood in relation to travel
time, which is the approximate time that elapses when water reaches the aquifer or
reaches a specific target such as a spring. This is also often referred to as
residence time. Several techniques can be used to estimate groundwater travel
times in an aquifer including use of natural or human made tracers and radioactive
and stable isotopes (see Tracer methods in section Second Phase). Most of these
methods are expensive, require a strict sampling procedure and are not always
applicable. Groundwater movement through the aquifer however, can be
expressed as its average linear velocity through aquifer material. This method uses
the physical properties of the aquifer system and has been used to estimate
groundwater travel times in many hydrogeologic projects. Since some
contaminants are assumed to travel at the same rate as groundwater (i.e., nitrate,
chloride), the estimated travel times can be used to assess the time required to see
the effects of wastewater and/or agricultural management methods upon
downstream groundwater and lagoon water quality.
The factors controlling the groundwater movement are expressed in an equation
known as Darcy’s Law: Q = KiA; where Q is discharge; K is hydraulic
conductivity; i is hydraulic gradient; and A is cross-sectional area. Darcy’s Law
can be rearranged and used to calculate groundwater velocity (v) in the aquifer.
Since groundwater moves through aquifer materials that impede groundwater
velocity, effective porosity is used to better represent the water flow through the
aquifer. The linear average velocity is expressed as: v = Ki/ne, where ne is the
effective porosity (see Freeze and Cherry for representative porosity values for
various substrates).
6.2.3 Groundwater Discharge (baseflow) Estimate
The volume of groundwater that discharges in the catchment is directly related to
the volume of water that recharges the aquifer. In other words, on along-term
basis, the system reaches a steady state where all the water that is infiltrated is
discharged through evapotraspiration and baseflow. To understand the impact of
groundwater on the streams and lagoon quality we must quantify the volume of
groundwater that discharges in the catchment as baseflow.
A simple water balance method can be used to approximate groundwater
discharge.
Groundwater Scoping Report For Rarotonga, Cook Islands 17
Streamflow at any given point in time could be considered to be the sum of
surface runoff and groundwater components. During dry periods with no
precipitation or surface runoff, the stream discharge is maintained by groundwater
discharge (baseflow). On a long-term basis, this baseflow must balance recharge.
The streamflow measurement integrates all baseflows along the river upstream of
the gauging station, and therefore provides a regional estimate of groundwater
recharge for the catchment. A baseflow recession/separation analysis (e.g., Todd
and Mays, 2005; Rasmussen and Andreasen, 1959) using a continuous record of
groundwater level and streamflow discharge (i.e., concurrent weekly
measurements of both parameters) can be used to convert complete cycles of
water table drop into volumes of water equivalent to groundwater discharge.
Local seepage from the stream channel to the shallow aquifer can also be
estimated using gauging observations. Repeated streamflow gauging along the
length of a stream can convey some information about the gaining and losing
segments of the stream and provide an upper and lower bound for recharge and
discharge. For example, the difference in discharge between two points measured
during lowflow (baseflow) and stormflow can provide spatial and temporal
estimates of recharge (losing segment) and discharge (gaining segment) along the
stream length. These values can then be used to calculate and map contaminant
loading to and from the aquifer system in the catchment. The success of this
method is highly dependent on the accuracy of flow measurements and sufficient
distance between points of measurements. Insignificant flow differences may be
due to inaccurate flow measurements and/or short stream stretches.
6.3 Groundwater Sampling
The objective of the sampling programme should be to collect representative
samples of groundwater in the catchment. The groundwater sampling method
depends on the site-specific conditions and may require equipment such as hand-
operated or motor-driven pumps, peristaltic pumps and bailers. Dedicated
sampling systems are greatly preferred since they avoid the need for
decontamination of equipment. Sampling devices (pumps, bailers, tubing) should
be constructed of inert materials (i.e., PVC, stainless steel, glass, etc.).
Groundwater Scoping Report For Rarotonga, Cook Islands 18
Prior to sampling, wells must be purged three to five times their volume or purged
dry until water-quality indicator parameters (i.e., specific electrical conductance,
temperature, pH) stabilize. Purging should be followed by sample collection to
assure samples are representative of groundwater in the aquifer and not stagnant
water left inside the monitoring well. Although the appropriate sampling
frequency is difficult to ascertain without any knowledge of prior fluctuations, it
is initially recommended that monitoring wells be sampled monthly.
Collecting a groundwater sample from an uncontaminated part of the aquifer
system, more than likely found in the upper catchment where no prior land use
has been identified, can provide information on background (pristine)
groundwater quality. Seasonal sampling is recommended.
Sampling equipment typically includes:
• Water level sensor – used to measure depth-to-water
• Measuring tape and weight – used to measure total depth of well
• Foot valves and plastic tubing – used for developing, purging and sampling
of monitoring wells
• Multi-parameter meter with flow-through cell – used to measure water
quality parameters (i.e., pH, specific electrical conductance, dissolved
oxygen, oxidation-reduction potential (Eh) and temperature)
• Decontamination supplies
• Sample bottles, sample preservation
Springs are natural groundwater discharge points. Their location in the catchment
needs to be identified and their quality monitored monthly. To estimate the
pollutant mass stored in the unsaturated zone, soil cores can be extracted below
root zone at several locations across the catchment to establish vertical profiles of
pore water concentrations. It is important to differentiate the recharge rates to
different substrates (i.e., volcanic – less permeable vs. sand and gravel – more
permeable) in the catchment since best management practices can be targeted to
land uses with shortest travel times and largest nutrient loads.
As previously mentioned, establishment of a wastewater treatment initiative can
change groundwater quality. It is therefore necessary to establish a groundwater
quality baseline (current state) in the catchment. By doing so, it becomes possible
to measure any change in groundwater quality (improvement or degradation) over
time.
A brief description on nutrient parameters and load calculation is provided below.
Groundwater Scoping Report For Rarotonga, Cook Islands 19
6.3.1 Sample Parameters and Load Calculation
Livestock and inadequate septic systems in the catchment are known to be the
primary causes of deteriorated water quality, it is therefore suggested that
groundwater samples be analyzed for their nutrient and microbiological content.
A one-time analysis of major and minor ionic constituents (Ca, Mg++, Na+, K+, Cl,
SO4-, NO3
-, total alkalinity) is also recommended to obtain an overall picture of
the groundwater system.
Quantifying nutrient concentrations, nitrate nitrogen (NO3-N) and dissolved
reactive phosphorus (DRP), in groundwater is recommended to evaluate the
effectiveness of alternative wastewater treatment and/or agricultural management
practices. Nitrates do not adsorb to soil particles and can easily move in the
aquifer system resulting in high groundwater nitrate concentrations. Permeable
and sandy deposits provide favorable conditions for vertical leaching of nitrates to
the aquifer system.
In addition to NO3-N, water samples can also be analyzed for ammonia (NH4) and
total Kjeldahl nitrogen (TKN). TKN is a measure of organic nitrogen plus
ammonia and its analysis is recommended because the TKN fraction can be added
to nitrate concentration to yield total nitrogen concentration in water. This value
along with the total nitrogen loading from surface runoff is essential for
determining the total catchment nitrogen load to the lagoon. Time series statistics
can be used to analyze and relate nitrogen and DRP concentrations to
wastewater/agricultural management and weather conditions.
Microbiological quality of groundwater samples can be determined for
enterococci (currently monitored in streams and lagoon), fecal coliforms, and
E. coli. The microbial groundwater quality can provide indicator levels to certain
catchment features and management characteristics which are likely to affect
water quality.
Baseflow loading can be calculated using stream water samples collected during
dry weather (3-5 day dry antecedent conditions) according to: L = QC, where L is
the loading (kg), Q is the discharge and C is the concentration.
Groundwater Scoping Report For Rarotonga, Cook Islands 20
Recommended Equipment
• YSI Professional Plus handheld multiparameter meter with flow-through
cell
• Wattera foot valves and tubing
• Inline disposable 0.45 micron polyethersulphone filters
• Solinst temperature-level-conductivity meter (can be used for depth-to
water measurements and conductivity and/or temperature profiling in wells)
• Pressure transducers (Aqua Troll 200) – measures and logs water
level/pressure, conductivity and temperature.
Potential supplier in New Zealand: Envco –Environmental Equipment (www.envcoglobal.com)
6.4 Recommendations for a groundwater sampling programme (summary):
The basis of the proposed groundwater sampling programme is to obtain
representative groundwater samples from established sampling points to reflect
the average properties of the system - including:
- A one-time analysis of major/minor ionic constituents (Ca, Mg++, Na+, K+,
Cl, SO4-, NO3
-, total alkalinity).
- Monthly measurement of NO3-N, DRP, NH4, and TKN.
- Monthly field determinations of pH, Eh, temperature, conductance and
alkalinity.
- Monthly microbiological quality of groundwater samples for enterococci,
fecal coliforms, and E. coli.
Following the collection, interpretation, and analysis of results in phase one, water
resource managers may find the recommendations below to be useful in
developing goals and objectives for a second phase. The recommendations below
are purely preliminary in nature and may be initiated or expanded upon as more
data become available during phase one.
7. Second Phase
The following objectives are recommended for initiation during phase two.
They include:
• Installation of deeper monitoring wells to improve the understanding of the
deeper hydrostratigraphic units and determine the nature of vertical
hydraulic gradients, physical parameters and nutrient concentrations at key
locations in the catchment.
Groundwater Scoping Report For Rarotonga, Cook Islands 21
• Hydrograph separation (see earlier section on Groundwater Recharge
Estimate) – used to isolate the groundwater component of streamflow and
calculate loadings. It ideally requires a continuous record of streamflow and
groundwater levels and water quality (i.e., nitrate, chloride) for a minimum
period of one year.
• Water balance study – used to quantify groundwater recharge and
discharge.
• Numerical modeling – used to simulate groundwater flow, surface water –
groundwater interaction, and contaminant transport. Integration of the
various datasets collected in phase one will form the basis for the
construction of a three-dimensional flow and transport model. The model
will represent an integrative view of the system and can be used to make
predictions of the magnitude and timing of the influence of land use and
wastewater management practice changes on groundwater resources. Model
calibration to steady state and transient conditions is required. A transport
model can be added to simulate contaminant transport and travel times.
Numerical modeling requires considerable expertise and extensive
knowledge of groundwater flow systems.
• Tracer methods – Environmental tracers are dissolved substances
introduced into the hydrologic cycle either by nature or anthropogenic
addition. They are able to trace water movements in unsaturated and
saturated zones over long time periods. Artificially applied tracers (i.e.,
bromide, chloride, etc) can be sued to trace waters over small spatial and
temporal scales. Tracers that undergo radioactive decay (i.e., tritium,
carbon-14) can be used to determine water ages. Stable isotopes of water
(deuterium and oxygen-18) can be used to track water movement in the
unsaturated and saturated zones. Their concentrations are affected over the
catchment area by temperature and altitude and can be used an indicators
for mixing ratios of waters from different regions. The tracer methods are
expensive, require a strict sampling procedure and are not always
applicable.
Groundwater Scoping Report For Rarotonga, Cook Islands 22
8. References
Binnie & Partners. 1984. Water resources and water supply of Rarotonga.
Burke, E. and Ricci, G. 1997. Comment on the groundwater resources surface
water resources and water supply of Rarotonga, Cook Islands. SOPAC
Technical Report 248, 44p.
Clement, H. and Bourguet, L., 1992. Outline scheme for water development and
management. Island of Rarotonga, Cook Islands Water Department.
R.1236/A.3077-192904. 53p.
Freeze, R.A. and Cherry, J.A. 1979. Groundwater. Prentice-Hall, Inc.,
Englewood Cliffs, New Jersey.
Parakoti, B. and Davie, T. 2007. Sustainable integrated water resources and
wastewater management in pacific island countries. National integrated water
resource management diagnostic report. Cook Islands. SOPAC Miscellaneous
Report 635. 47p.
Ramussen, W.C. and Anderasen, G.E. 1959. Hydrogeologic budget of the
Beaverdam Creek Basin in Maryland. US Geological Survey Water-Supply
Paper 1472: 106p.
Ricci, G. and Scott, D. 1998. Groundwater potential assessment of Rarotonga
coastal plain. SPAC Technical Report 259, 76p.
Todd, D.K. and Mays, L.W. 2005. Groundwater Hydrology, 3rd ed. John Wiley
&Sons, Inc., Hoboken, NJ, 636pp.
WMI & BURGEAP. 1992. Outline scheme for water development and
management.
Waterhouse, B.C. and Petty, D.R. 1986. Hydrogeology of the southern Cook
Islands, South Pacific. New Zealand geological Survey Bulletin 98. 93p.
Wood, B.L. and Hay, R.F., 1970. Geology of the Cook Islands. New Zealand
Geological Survey. 103p.
Groundwater Scoping Report For Rarotonga, Cook Islands 23
APPENDIX - photos
A pre-1980 installed septic tank. Note the depressed ground around the tank area.
A stream channel exists beyond the trees in the background. In the case of older
septic systems with limited or no drain field, the effluent from the septic tank can
reach the water table and be discharged into the stream just a short distance away
with minimal, if any, treatment.
Groundwater Scoping Report For Rarotonga, Cook Islands 24
A septic system covered by a concrete pad on top and an access port. No
installation date available.
Groundwater Scoping Report For Rarotonga, Cook Islands 25
A more recent (1990s) three-tank septic system.
Groundwater Scoping Report For Rarotonga, Cook Islands 26
Livestock often utilise streams.