Geophysical Surveys for Groundwater Modelling of Coastal Golf Courses

Transcription

1 1 Geophysical Surveys for Groundwater Modelling of Coastal Golf Courses C. RICHARD BATES and RUTH ROBINSON Sedimentary Systems Research Group, University of St. Andrews, St. Andrews, Scotland Abstract The majority of golf courses in the world require extensive irrigation systems and large water supplies, even in wet climates. The links golf courses around Scotland s coastline are typically located on vulnerable coastal dune systems and many utilise groundwater to sustain their water requirements. A geophysical survey and hydrological modelling exercise was conducted on a local links course to determine the extent of groundwater supply and the groundwater vulnerability to contamination by saltwater intrusion. The geophysical survey successfully mapped the freshwater-saltwater boundary and modelling results confirmed existing extraction strategies based on current climatic conditions and irrigation demands. A number of pumping scenarios were constructed based on the results of the survey for future water use planning during times of drought and heavy extraction. The methodologies employed in this study can be built into course management strategies where decisions on sighting new irrigation wells and defining future pumping regimes requires predictive capabilities of the aquifer and the subsurface geology. The techniques used in this study readily apply to any golf course or recreational area that relies on groundwater. Background The Fife courses are located on the east coast of Scotland between the Tay and Forth estuaries. The drift sequences above solid bedrock, including soil horizons, were mostly deposited since the last ice sheet retreated and include till, pebbly and sandy clays and low permeability marine clays. Bedrock consists of decametre thick sandstone beds interbedded with siltstone, mudstone, thin coals and thin carbonate layers of the Carboniferous Strathclyde Group (Forsyth and Chisholm, 1977). Geophysical Methods Electrical and electro-magnetic geophysical method were chosen for investigating the subsurface geology and hydrogeology over the course following similar investigations in coastal margins elsewhere (Mills et al.,1988; Hoekstra and Blohm, 1990; Reynolds, 1997). These methods were chosen as the contrast in electrical properties between the target formations of unconsolidated sands and consolidated siltstones, coals and sandstones were anticipated to be high. Furthermore, the electrical contrast between freshwater saturated ground and saltwater saturated ground is high. The combined electrical methods were needed to analyse from the surface to depths in excess of m. For near surface measurements down to 8m depth, direct current resistivity (DC-Resistivity) methods were used, while for deeper measurements a time domain electromagnetic method (TDEM) was used. The DC-resistivity techniques used a combination of both Wenner and Schlumberger arrays, with a maximum a-spacing of 60m, to produce onedimensional soundings that were then combined into two-dimensional geo-electric crosssections. The TDEM survey used central loop soundings with transmitter loops of 30m and a receiver coil in vertical dipole mode. A number of sounding locations were made along traverses

2 2 across the course from the abstraction boreholes to the open ocean and also from the boreholes to a nearby estuary. Results The results of all soundings were combined and plotted as geo-electric cross-sections. Two examples are given in figure 1. The surface wind-blown, unconsolidated sand is characterised by resistivity values between -0ohm-m. Across most of the course, the bedrock surface is shown as a decrease in resistivity (down to 12- ohm-m) followed by zones of increased resistivity (greater than 500ohm-m) in the consolidated, freshwater saturated sandstones. Towards the shore (north) on line 1 (figure 1A), a conductive zone was mapped at around the bedrock surface. This is interpreted as a small saltwater wedge intruding along the bedrock surface or along less permeable clay on the bedrock surface for m inland. A similar wedge was seen along line 2 (figure 1B) N S BH shoreward Sand Sandstone Sandstone/Mudstone Mudstone Saltwater intrusion m 0 10 N 1 2 Resistivity in Ωms S BH Figure 1A and 1B - Results of DC resistivity and TDEM along lines 1 and 2. Numbers represent resistivity in ohm-m

3 3 A large saltwater wedge was mapped along line 1 from m to over 90m depth (figure 1A). A borehole positioned at approximately sounding location 17 encountered saltwater at 70m confirming the geophysics results. Along line 2, this deeper saltwater wedge is less pronounced near to the shore and was mapped at depths greater than 90m (figure 1B). Groundwater Modelling The geophysical results described above were combined with previously logged borehole data to construct a five layer baseline groundwater model with hydraulic characteristics obtained from engineering pump testing. The model grid, consisting of 4 boreholes and five layers, is shown in figure 2; all layers are partially unconfined except the top layer which is fully unconfined and the aquifer is the third layer. The United States Geological Survey MODFLOW-96 (Harbaugh and McDonald, 1996) was used to model the flowpaths and was calibrated with the pump test data as boundary conditions. Initial hydraulic heads were estimated from boreholes and were assumed to linearly decrease from west to east between wells and towards the shoreline (from south to north). Hydraulic heads, conductivities and porosities were adjusted until a "best fit" match was found. The initial results showed a drawdown that was too low in the model borehole compared to the pump tests and the recovery was delayed. A density-driven flow model (Schaars and Gerven, 1997) was then incorporated using water densities which were estimated from the electrical conductivity results and a calibration of Stuyfzand (1993). Saline boundary 10 (42 cells) 1600 (59 cells) 5 consntant head boundary consntant head boundary North no flow boundary Figure 2 - Model setup with position of four wells and west and east constant head boundaries. The maximum defined pumping rates for the boreholes were then used to investigate the location of the salt water intrusion after 3 continuous days of pumping, although typical pumping durations for irrigation last only several hours. The former scenario might, however, be realistic in 1) extreme drought conditions; 2) during pump failure; and/or 3) because of contamination of one or more boreholes. Transient simulations were used to model the flow field for each time step and each given density distribution after the solute transport (density distribution) for the

4 4 previous flow field had been calculated. Figure 3 illustrates the initial and final position of the salt water wedge for layer 2. Figure 3 - Initial and final density distribution in layer 2 for 3 days of pumping at the maximum values A. Initial values estimated from conductivity results. B. Salt water intrusion in layer 2 after 3 days pumping. B Contours of the 1.016g/cm3 value superimposed on Line 1 cross-section for Day 1, Day 2 and Day3. Conclusion This geophysical methodology can be cost effectively implemented to provide a wealth of data that not only can define the subsurface architecture of an aquifer and any aquitard layers beneath

5 5 coastal golf courses, but also the position of saltwater wedges and the resistive characteristics of the subsurface geology. The method is easy to install and employ and could be used as a monitoring technique during high demand periods over summer months. References Forsyth, S. S. D. and Chisholm, J. I. (1977). The Geology of East Fife. Memoir of the Geological Society of Great Britain, Sheet 49 (Scotland). Mills, Theodore, Hoekstra, P., Blohm, M. and Evans, L. (1988). Time domain electromagnetic soundings for mapping sea-water intrusion in Monterey County, California. Ground Water, 26, Harbaugh, A.W., and McDonald, M.G. (1996). Users documentation for MODFLOW-96, an update to the US. Geological Survey modular finite-difference ground-water flow model (MODFLOW). U.S.G.S Open-File Report , 56p. Hoekstra, P. and Blohm, M. (1991). Case histories of time-domain electromagnetic soundings in environmental geophysics. In ed. Ward. S. (Ed.), Geotechnical and environmental geophysics; Volume 2, Environmental and groundwater geophysics (pp. 1-15), Society of Exploration Geophysicists, Tulsa, OK, USA. Reynolds, J. (1997). An introduction to applied and environmental geophysics. Wiley. Schaars, F.W. and van Gerven, M.W. (1997). Simulation of density driven flow in MODFLOW. Kiwa NV, Research and Consultancy, Vewin, p. 21. Stuyfzand, P.J. (1993). Hydrochemistry and hydrology of the coastal dune area of the Western Netherlands. Academisch proefschrift. Kiwa NV, Research and Consultancy, Vewin, p. 83.