Global Navigation Satellite System Global Navigation Satellite System Methodology for Groundwater Resource Methodology for Groundwater Resource

Assessment, Gateway Wellfield, Assessment, Gateway Wellfield, HermanusHermanus

Andiswa Mlisa, Chris Hartnady and Sheila Imrie

Umvoto Africa (Pty) Ltd, Muizenberg, South Africa

www.umvoto.com

5th IGCP 565 Workshop, 29 – 30 October 2012, Johannesburg

Background & OutlineBackground & OutlineOverstrand Municipality investigating groundwater potential of Greater

Hermanus Area, for augmentation to supply

OutlineOutline:

Global Navigational Satellite Systems (GNSS) in South Africa

Table Mountain Group (TMG) hydrogeology at the Gateway wellfield, Hermanus

Geodetic Monitoring at Gateway wellfield, Hermanus

Until 1976 single schemes in each town:Surface water use (dams) in Fisherhaven and Voelklip

Groundwater use (boreholes or small wellfields) in Hawston, Onrus, Hermanus

Since 1976 DeBos Dam as source for all towns in Greater Hermanus AreaPreviously used schemes abandoned

Overstrand Water SupplyOverstrand Water Supply

Groundwater InvestigationGroundwater InvestigationPhase A – Inception (2001/2002)Phase B – Detailed Design &

Implementation (2002 – 2006)B1 – Wellfield Development at GatewayB2 – Monitoring programmeB3 – Hydrogeological ReconnaissanceB4 – License Application

Phase C – Conjunctive Water Resource Planning (2006 onwards)

Global Navigational Satellite Global Navigational Satellite Systems (GNSS) in South AfricaSystems (GNSS) in South Africa

TrigNet station distributionTrigNet station distribution

HNUSHNUS

Network of permanent continuously operating GPS (cGPS) base stations

Distributed throughout South Africa

All stations record 1-sec epoch data on both GPS frequencies (L1 and L2)

South African TrigNet systemSouth African TrigNet systemTrigNet system was developed as a national control

survey network used for land reform projects with the following as spinoff applications: Serve as a baseline geodetic datum; Track crustal movements to millimeter per year

precision; Contribute to the understanding of plate tectonics and

earthquake hazards; Provides a convenient platform for developing a new

space and ground-based system for monitoring the seasonal fluctuations in aquifer storage through detection of associated small deformations; and

Has applications in ionospheric physics, meteorology and atmospheric profiling

Table Mountain Group (TMG) Table Mountain Group (TMG) hydrogeology at the Gateway hydrogeology at the Gateway wellfield, Hermanuswellfield, Hermanus

Hermanus Geological SettingHermanus Geological Setting

Hermanus Geological SettingHermanus Geological Setting

Gateway wellfield and HNUSGateway wellfield and HNUS

SANSA Space ScienceSANSA Space Science

Peninsula aquifer has high confining pressure, artesian conditions and fracture dominated flows

Wellfield pumping rates at 10l/s – 30l/s

Licensed for 1.5Mm3/a

Gateway Gateway wellfieldwellfield

Monitoring Monitoring ComponentsComponents Water-level in fractured rock

aquifer Water-level in primary alluvium

aquifer Water quality in fractured rock

aquifer Spring and surface-water flow

rate and quality Rainfall, atmospheric temperature

and air-pressure Record of abstraction rates and

volumes

Effective Stress Effective Stress ConceptConcept

(1) The initial position of the piezometric surface in the confined fractured aquifer;

(2) The change in the piezometric surface in the confined aquifer due to the cone of depression (drawdown) around the pumping well;

(3) The resultant effect of skeletal compression on the aquifer material; and

(4) The elastic rebound of the aquifer when pumping stops and water levels recover.

Geodetic Monitoring at Geodetic Monitoring at GatewayGateway

Gateway and HNUS cGPSGateway and HNUS cGPS

cGPS at Gateway wellfieldcGPS at Gateway wellfield Monument and antenna installation at wellheads (Oct-Nov 2008) for

measurement of surface subsidence during groundwater abstraction Precise positions; 30 second dual frequency data Relative to IGS stations

HGW3 HGW1

HNUS Horizontal DisplacementHNUS Horizontal Displacement

An average motion of 19.6 mm/yr Northwards and 16.2 mm/yr Eastwards.

The NU-ITRF2005 solution indicates a model NU velocity at the HNUS site of 18.8 mm/yr Northwards and 16.7 mm/yr Eastwards, corresponding to motion of 25.2 mm/yr towards azimuth (Altamimi et al., 2007).

HGW1 Horizontal DisplacementHGW1 Horizontal Displacement

an average movement of 19.3 mm/yr Northwards and 16.2 mm/yr Eastwards

HNUS Vertical DisplacementHNUS Vertical Displacement

WMRS error 11 mm compared to 2 mm in the horizontal displacement downward motion of ~3.0 mm/yr

HGW1 Vertical DisplacementHGW1 Vertical Displacement

Upward motion of ~4.5 mm/yr for HGW1 Apparent vertical motion is roughly equal to that at HNUS, but

opposite in direction – due to fault location?

HGW1 Vertical Displacement vs HGW1 Vertical Displacement vs PumpingPumping

HGW1 Vertical Displacement vs Water HGW1 Vertical Displacement vs Water LevelLevel

HGW1 Vertical Displacement vs RainfallHGW1 Vertical Displacement vs Rainfall

HGW1 Vertical Displacement vs HGW1 Vertical Displacement vs PressurePressure

All Stations Relative to HNUS Vertical All Stations Relative to HNUS Vertical DisplacementDisplacement

HGW1 trends upwards relative to HNUS, HGW2 seems to trend downwards, and HGW3 remains fairly stable

Summary ResultsSummary Results

horizontal displacements at Hermanus GPS stations are closely aligned with calculated horizontal velocity components of Nubia plate relative to ITRF;

vertical displacements showed varying long-term trends (upwards at HGW1, downwards at HGW2 and flat at HGW3) with high frequency of peaks and troughs that correlate between stations

vertical displacement showed no clear link to pumping or atmospheric phenomenon at daily (one session per day) time-scale;

Way forward: short-term scale Way forward: short-term scale analysisanalysis

HGW1 & HGW2HGW3

HGW3 to HNUS Horizontal DisplacementHGW3 to HNUS Horizontal Displacement

WRMS values on the order of 1 mm – 2 mm, compares with relative to global stations and daily processing

end pumping

HGW3 to HNUS Vertical DisplacementHGW3 to HNUS Vertical Displacement

WRMS value of 3 mm compared to 11 mm on daily & relative to global station

clear downward movement followed by an upward movement in response to a pump switch off - Noordbergum effect (reverse water-level fluctuation)?

RecommendationsRecommendations

Further processing of shorter time steps Another site on same fault block as HNUS would be

potentially useful An experimental system using a combination of land-

based observations (viz., GPS, and high-precision microgravity measurements) and complementary satellite gravity (e.g., GRACE) and satellite radar (e.g., InSAR)

Use of analysis results from study to update hydrogeological numerical models of Hermanus area.

Wider “Natural Laboratory” Wider “Natural Laboratory” conceptconcept

Capacity BuildingCapacity Building Workshops

2009, SANSA Space Science, Hermanus 2010, African Institute for Mathematical Sciences, Cape Town 2011, University of Johannesburg, co-located with CAG 23

Conference presentations

Thank YouThank You