Global Navigation Satellite System Methodology for Groundwater Resource Assessment, Gateway Wellfield, Hermanus Andiswa Mlisa, Chris Hartnady and Sheila Imrie Umvoto Africa (Pty) Ltd, Muizenberg, South Africa 5 th IGCP 565 Workshop, 29 – 30 October 2012, Johannesburg

Background & Outline Overstrand Municipality investigating groundwater potential of Greater Hermanus Area, for augmentation to supply Outline Outline: 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 Area Previously used schemes abandoned Overstrand Water Supply

Groundwater Investigation Phase A – Inception (2001/2002) Phase B – Detailed Design & Implementation (2002 – 2006) B1 – Wellfield Development at Gateway B2 – Monitoring programme B3 – Hydrogeological Reconnaissance B4 – License Application Phase C – Conjunctive Water Resource Planning (2006 onwards)

Global Navigational Satellite Systems (GNSS) in South Africa

TrigNet station distribution HNUS 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 system TrigNet 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) hydrogeology at the Gateway wellfield, Hermanus

Hermanus Geological Setting

Gateway wellfield and HNUS SANSA 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 wellfield

Monitoring Components 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 Concept (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 Gateway

Gateway and HNUS cGPS

cGPS 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 HGW3HGW1

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

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

HGW1 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 Pumping

HGW1 Vertical Displacement vs Water Level

HGW1 Vertical Displacement vs Rainfall

HGW1 Vertical Displacement vs Pressure

All Stations Relative to HNUS Vertical Displacement HGW1 trends upwards relative to HNUS, HGW2 seems to trend downwards, and HGW3 remains fairly stable

Summary 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 analysis HGW1 & HGW2 HGW3

HGW3 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 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)?

Recommendations 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” concept

Capacity 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

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