Borehole Yield and Quality Testing at Croydon Vineyard Estate, Somerset West. Western Cape.

Transcription

1 Borehole Yield and Quality Testing at Croydon Vineyard Estate, Somerset West. Western Cape. REPORT: GEOSS Report No: 2017/12-13 PREPARED FOR: Tobie Esterhuyzen Estate Manager Croydon Vineyard Estate c/o R102 & Kramat Rd Somerset West PREPARED BY: Cameron Brand GEOSS - Geohydrological and Spatial Solutions International (Pty) Ltd Unit 12, Technostell Building, 9 Quantum Street, Technopark Stellenbosch 7600 Tel: (021) info@geoss.co.za ( 06 December 2017

2 EXECUTIVE SUMMARY GEOSS Geohydrological and Spatial Solutions International (Pty) Ltd was appointed by Tobie Esterhuyzen of Croydon Vineyard Estate, to conduct yield and quality testing of two boreholes at Croydon Vineyard Estate near Somerset West, Western Cape. The boreholes on this property were drilled some time ago and are currently equipped with submersible borehole pumps which supply the estate with water. The yield testing was undertaken by GEOSS during the month of November The abstraction recommendations for the boreholes tested are presented in the table below: Site Pump depth (mbgl) Yield (L/s) Abstraction duration (hours) Rest duration (hours) Maximum water level (mbgl) Possible volume abstracted per day (Litres) CE_BH ,880 CE_BH ,000 A water sample was collected at the end of the test and submitted for inorganic chemical analysis. From the results it is clear that the groundwater from this borehole is good in terms of dissolved mineral concentrations. The level of chloride is slightly elevated and treatment may be required depending on the intended use. It is suggested that long term borehole water level monitoring is conducted. Monitoring can be done automatically using a water level logger, or done manually. The monitoring should be done at least on a monthly basis. A rest water level can be manually taken using a dip meter, along with a flow meter measurement. To facilitate monitoring and informed management of the borehole, it is recommended that the borehole be equipped with monitoring infrastructure: Installation of a 40 mm OD (class 10) observation pipe from the pump depth to the surface, closed at the bottom and slotted for the bottom 5 10 m. This enables manual or automated water level monitoring. Installation of a sampling tap. Installation of a flow volume meter. Correct registration / licensing procedures must be followed as required by the Department of Water Affairs prior to use of the groundwater. Ooo ooo ooo

3 TABLE OF CONTENTS 1. INTRODUCTION METHODOLOGY YIELD TESTING CE_BH CE_BH WATER QUALITY ANALYSIS RECOMMENDATIONS REFRENCES APPENDIX A: YIELD TEST DATA APPENDIX B: WATER QUALITY ANALYSIS LIST OF FIGURES Figure 1: Borehole Positions... 3 Figure 2: Step Test drawdown data for CE_BH Figure 3: Time vs Drawdown of CE_BH01 (CDT (first) at 0.2 L/s)... 5 Figure 4: Time vs Drawdown of CE_BH01 (CDT (second) at 0.2 L/s)... 6 Figure 5: Log - log plot of the time-series drawdown and derivatives during the CDT (CE_BH01) (0.2 L/s)... 7 Figure 6: Water level recovery graph after the second CDT (CE_BH01)... 7 Figure 7: Step Test drawdown data for CE_BH Figure 8: Drawdown vs Time for CE_BH02 (CDT (first) at 2.0 L/s)... 9 Figure 9: Log - log plot of the time-series drawdown and derivatives during the CDT (CE_BH02) (2.0 L/s)... 9 Figure 10: Time-series drawdown CE_BH02 (CDT(second) at 1.5 L/s) Figure 11: Log - log plot of the time-series drawdown and derivatives during the CDT (CE_BH02) (1.5 L/s) Figure 12: Water level recovery graph after the CDT (CE_BH02) LIST OF TABLES Table 1: Borehole Details... 3 Table 2: CE_BH02 Yield Determination Table 3: Borehole Abstraction Recommendations i

4 ABBREVIATIONS DWA Department of Water Affairs ( ) DWAF Department of Water Affairs and Forestry (pre- 2010) DWS Department of Water and Sanitation ( current) ha hectare L/s litres per second m metres mamsl metres above mean sea level MAP Mean Annual Precipitation mbch metres below collar height mbgl metres below ground level WGS84 Since the 1st January 1999, the official co-ordinate system for South Africa is based on the World Geodetic System 1984 ellipsoid, commonly known as WGS84. Suggested reference for this report: GEOSS (2017). Borehole Yield and Quality Testing at Croydon Vineyard Estate, Somerset West. Western Cape. GEOSS Report Number: 2017/ GEOSS - Geohydrological & Spatial Solutions International (Pty) Ltd. Stellenbosch, South Africa. Cover photo: CE_BH02 GEOSS project number: 2017_ Review: Julian Conrad (6 December 2017). ii

5 1. INTRODUCTION GEOSS Geohydrological and Spatial Solutions International (Pty) Ltd was appointed by Tobie Esterhuyzen of Croydon Vineyard Estate, to conduct yield and quality testing of two boreholes at Croydon Vineyard Estate near Somerset West, Western Cape. The boreholes on this property were drilled some time ago, drilling information and was not made known to GEOSS. Both boreholes are currently equipped with submersible borehole pumps which supply the estate with water. The borehole details are presented in Table 1 below, and spatially in Figure 1. Table 1: Borehole Details Site Latitude Longitude Depth (m) CE_BH CE_BH Figure 1: Borehole Positions 3

6 2. METHODOLOGY The yield testing was undertaken by GEOSS and carried out according to the National Standard (SANS :2003, Part 4 Test pumping of water boreholes). This included a Step Test, Constant Discharge Test and recovery monitoring. For the Step Test, the borehole is pumped at a constant rate for one hour intervals and the flow rates are incrementally increased for each step. This test is followed by a Constant Discharge Test (CDT) where the borehole is pumped at a constant rate for an extended period of time, followed by recovery monitoring. The water level drawdown is monitored at predetermined intervals during these tests (drawdown refers to the difference in water level from the rest water level (RWL) measured before commencement of the yield test). All raw data and measurements taken during the actual yield test is presented in Appendix A. The yield test data was analysed using the FC Software developed by the IGS (Institute for Groundwater Studies) in Bloemfontein. This method evaluates fractal pumping tests and well performance, and makes use of derivatives, boundary information and error propagation to evaluate the sustainable yield of a borehole. A water sample was collected at the end of the yield test and submitted for inorganic chemical analysis. 3. YIELD TESTING 3.1 CE_BH01 The yield testing of CE_BH01 was conducted between the 27 th and the 30 th of November The existing pump is installed at a depth of 66 meters - this pump was removed for the yield test and reinstalled after the test was completed. The borehole was measured at a depth of 83.2 meters. The test pump was initially installed at a similar depth to the existing pump but then lowered to the bottom of the borehole for the second Constant Discharge Test (CDT). The rest water level (RWL) at the start of the test was 0.99 meters below ground level. During the Step Test, the water level was drawn down to the pump inlet during the second step conducted at a rate of 0.5 L/second. Figure 7 shows the time-series drawdown for the Step Test. 4

7 Dewatering of water bearing fracture zone Figure 2: Step Test drawdown data for CE_BH01. Based on the results of the Step Test, the Constant Discharge Test (CDT) was conducted at a rate of 0.2 L/second. A CDT was initially conducted with the pump installed at a depth of 60 meters: Once the water bearing fracture zone at approximately 10 meters below ground level is dewatered, there is a steady but rapid decrease in the water level causing the water level to be drawn down to the pump after approximately 2.5 hours of pumping. The drawdown for the first CDT is presented in Figure 3. Dewatering of water bearing fracture zone Figure 3: Time vs Drawdown of CE_BH01 (CDT (first) at 0.2 L/s) 5

8 Once the water level had recovered, the test pump was lowered to the bottom of the borehole. A second CDT was then conducted at the same rate (0.2 L/second). This caused the water level to be drawn down to the pump after 3.25 hours of pumping, following a similar trend to the first CDT. The drawdown for the second CDT is presented in Figure 4. Dewatering of water bearing fracture zone Figure 4: Time vs Drawdown of CE_BH01 (CDT (second) at 0.2 L/s) The log-log plot of the drawdown with time for the second CDT is shown in Figure 5. The derivative plots are useful for indicating where changes in the rate of drawdown occur. An increase in the rate of drawdown is visible after approximately 30 minutes of pumping as the water bearing fracture zone is dewatered (indicated by the increased downward slope of the first derivative). 6

9 Figure 5: Log - log plot of the time-series drawdown and derivatives during the CDT (CE_BH01) (0.2 L/s) The recovery of the water level was monitored after the CDT and is presented in Figure 6. The recovery of the water level is moderate; attaining 95 % recovery in approximately 11 hours. Figure 6: Water level recovery graph after the second CDT (CE_BH01) It is recommended that the borehole is pumped a rate of 0.2 L/second for 2 hours per day followed by a 10 hour recovery period. 7

10 3.2 CE_BH02 The yield testing at CE_BH02 was conducted between the 22 nd and the 27 th of November The existing pump is installed at a depth of meters below ground level - this pump was removed for the yield test and reinstalled after the test was completed. The borehole was measured at a depth of meters. The test pump was installed at a depth of 18.5 meters below ground level. The rest water level (RWL) at the start of the test was 2.5 meters below ground level. During the Step Test, the water level was drawn down to the pump inlet during the 5 th step conducted at a rate of 3.5 L/second. Figure 7 shows the time-series drawdown for the Step Test. Dewatering of water bearing fracture zone Figure 7: Step Test drawdown data for CE_BH02. Based on the results of the Step Test, the Constant Discharge Test (CDT) was conducted at a rate of 2.0 L/second. Once the water bearing fracture zone (approximately 10 meters below ground level) is dewatered, there is rapid increase in the rate of water level drawdown causing the water level to be drawn down to the pump after approximately 10 hours of pumping. The semi-log plot of the drawdown is presented in Figure 8. 8

11 Dewatering of water bearing fracture zone Figure 8: Drawdown vs Time for CE_BH02 (CDT (first) at 2.0 L/s) The log-log plot of the drawdown with time for the CDT is shown in Figure 9. The derivative plots are useful for indicating where changes in the rate of drawdown occur. A increase in the rate of drawdown is visible after approximately 550 minutes of pumping as the water bearing fracture zone is dewatered (indicated by the sudden downward slope of the first derivative). Dewatering of water bearing fracture zone Figure 9: Log - log plot of the time-series drawdown and derivatives during the CDT (CE_BH02) (2.0 L/s) 9

12 Once the water level had recovered, a second CDT was restarted at a lower rate of 1.5 L/second. The drawdown of the water level is very similar to the first CDT, remaining stable until the dewatering of the water bearing fracture zone (approximately 10 meters below ground level). Once the water bearing fracture zone has been dewatered, there is rapid increase in the rate water level drawdown causing the water level to be drawn down to the pump after approximately 14.4 hours. The semi-log plot of the drawdown is presented in Figure 10. Dewatering of water bearing fracture zone Figure 10: Time-series drawdown CE_BH02 (CDT(second) at 1.5 L/s) The log-log plot of the drawdown with time for the CDT is shown in Figure 11. The derivative plots are useful for indicating where changes in the rate of drawdown occur. An increase in the rate of drawdown is visible after approximately 750 minutes of pumping as the water bearing fracture zone is dewatered (indicated by the sudden downward slope of the first derivative). 10

13 Dewatering of water bearing fracture zone Figure 11: Log - log plot of the time-series drawdown and derivatives during the CDT (CE_BH02) (1.5 L/s) The recovery of the water level was monitored after the CDT and is presented in Figure 12. The recovery of the water level is slow; attaining 95 % recovery in approximately 50 hours. Figure 12: Water level recovery graph after the CDT (CE_BH02) 11

14 Several methods were used to assess the yield test data and a summary is presented in Table 2. It is recommended that the borehole is pumped a rate of 1.5 L/second for 10 hours per day followed by a 14 hour recovery period. Table 2: CE_BH02 Yield Determination Applicable Method Sustainable yield (l/s) Std. Dev Early T (m 2 /d) Late T (m 2 /d) S AD used FALSE Basic FC E FALSE Advanced FC E FALSE FC inflection point TRUE Cooper-Jacob E TRUE FC Non-Linear E TRUE Barker K f = 222 S s = 1.00E Average Q_sust (l/s) b = 0.10 Fractal dimension n = 1.95 Recommended abstraction rate (L/s) 1.01 for 24 hours per day Hours per day of pumping L/s for 10 hours per day 4. WATER QUALITY ANALYSIS Lab results not yet received 12

15 5. RECOMMENDATIONS Based on the information obtained from the yield test, the abstraction recommendation for the borehole is presented in Table 3. Site Table 3: Borehole Abstraction Recommendations Pump Abstraction Rest Maximum Yield depth duration duration water level (L/s) (mbgl) (hours) (hours) (mbgl) Possible volume abstracted per day (Liters) CE_BH ,880 CE_BH ,000 It is recommended that CE_BH01 is pumped at a rate of 0.2 L/second for 2 hours per day followed by a 10 hour recovery period (twice per day if necessary if so monitoring is crucial). Once the water level drops below the water bearing fracture zone (approximately 10 meters below ground level), the rate of drawdown increases. Best practise is to not let the water level be drawn down past the fracture zone. This causes a loss in the hydrostatic pressure (within the fractures) which can cause the fractures to collapse - and in turn, can reduce the yield from the borehole. However as this borehole is very low yielding, to abstract the maximum volumes, the borehole will need to be pumped until the water level is just above the pump. It is recommended that the pump is fitted with an automatic cut off switch to prevent the water level dropping to the pump depth and causing the pump to burn out. Through long term monitoring data the abstraction volumes can be optimised by adjusting the pumping duration. It is recommended that CE_BH02 is pumped at a rate of 1.5 L/second for 10 hours per day followed by a 14 hour recovery period. The water level should not exceed 10 meters below ground level - just above the point at which the fracture zone is dewatered. As this borehole has limited storage it is recommended that the pumping duration is only optimised through long term monitoring data. Monitoring can be done automatically using a water level logger, or done manually. The monitoring should be done at least on a monthly basis. A rest water level can be manually taken using a dip meter, along with a flow meter measurement. To facilitate monitoring and informed management of the borehole, it is highly recommended that the borehole be equipped with monitoring infrastructure: Installation of a 40 mm OD (class 10) observation pipe from the pump depth to the surface, closed at the bottom and slotted for the bottom 5 10 m. This enables manual or automated water level monitoring. Installation of a sampling tap. Installation of a flow volume meter. The legal compliance with regard to use of the groundwater also needs to be addressed with the Department of Water and Sanitation 13

16 6. REFRENCES DWAF (1998). Quality of domestic water supplies, Volume 1: Assessment guide. Department of Water Affairs and Forestry, Department of Health, Water Research Commission, National Water Act (1998). The National Water Act, No 36. Department of Water Affair and Forestry. Pretoria. SANS 241-1:2015. Drinking water Part 1: Microbiological, physical, aesthetic and chemical determinants. SANS :2003. South African National Standard. Development, maintenance and management of groundwater resources. Part 4: Test-pumping of water boreholes. ISBN

17 7. APPENDIX A: YIELD TEST DATA 15

18 CE_BH01 Borehole Yield Test Results Project Name Project Number Borehole Name Ian_de_Jager_Croydon_Estate 2481_A CE_BH01 Site Details Test date Province Area Farm/Site Name Western Cape Somerset West Croydon Estate 27-Nov-17 - Step test details 00-Jan-00 Observer Maneul/Nunens Date Step no Length Flow rate (set) Comments Pump type WILO QFN Nov hr 0.2 L/s Complete Lat 27-Nov min 0.5 L/s Pump inlet Co-ordinates Long BoreholeStatus Borehole depth (Before test) Open 83.2 m Borehole depth (After test) 83.2 m Borehole diameter (OD,ID) 200, 120 mm Dummy pump test Complete Constant discharge test details Casing depth PVC entire m Start Date Length (h) Flow rate (set) Comments Casing height 0.34 m 29-Nov l/s Pump inlet Datum level above ground 0.49 m Test pump depth 80 m Observation pipe depth 75 m Recovery details Logger depth m Start Date Data Capture Available Drawdown m 29-Nov-17 Solinst logger 1.48 mbch Rest water level before test 0.99 mbgl Monitoring boreholes Outlet distance 100 meters Borehole: Data: Distance from: CE_BH01 Water sample taken & type Inorganic Comments: Low yielding, slight obstruction at 30 meters. \ 16

19 Step Test 27-Nov _A CE_BH01 Ian_de_Jager_Croydon_Estate Step 1 Step l/s 0.5 l/s Time interval (min) Water level drawdown (m) Time interval (min) Water level drawdown (m) Pump inlet

20 Constant Discharge Test (CDT 1) - Raw data Ian_de_Jager_Croydon_Estate 2481_A CE_BH01 28-Nov-17 Absatraction rate (L/s) 0.2 Test Duration: 3.25 hours Hours Time interval (min) Water level drawdown (m) Pump inlet 18

21 Constant Discharge Test (CDT 2) - Raw data Ian_de_Jager_Croydon_Estate 2481_A CE_BH01 29-Nov-17 Absatraction rate (L/s) 0.2 Test Duration: 3.25 hours Hours Time interval (min) Water level drawdown (m) Pump inlet 19

22 Post CDT Recovery Ian_de_Jager_Croydon_Estate 2481_A CE_BH01 29-Nov-17 Hours Time interval (min) Water level drawdown (m)

23 CE_BH02 Borehole Yield Test Results Project Name Project Number Borehole Name Ian_de_Jager_Croydon_Estate 2481_A CE_BH02 Site Details Test date Province Area Farm/Site Name Western Cape Somerset West Croydon Estate 22-Nov-17 - Step test details 25-Nov-17 Observer Maneul/Nunens Date Step no Length Flow rate (set) Comments Pump type WILO QFN Nov hr 0.3 L/s Complete Lat 22-Nov hr 0.8 L/s Complete Co-ordinates Long 22-Nov hr 1.5 L/s Complete BoreholeStatus Open 22-Nov hr 2.2 L/s Complete Borehole depth (Before test) m 22-Nov mins 3.5 L/s Pump inlet Borehole depth (After test) m Borehole diameter (OD,ID) 125 mm Dummy pump test Complete Constant discharge test details Casing depth PVC entire m Start Date Length (h) Flow rate (set) Comments Casing height 0.41 m 23-Nov l/s Pump inlet Datum level above ground 0.65 m 23-Nov l/s Pump inlet Test pump depth 18.8 m Observation pipe depth 15.8 m Recovery details Logger depth m Start Date Data Capture Available Drawdown m 24-Nov-17 Solinst logger 3.15 mbch Rest water level before test 2.5 mbgl Monitoring boreholes Outlet distance 100 m to neighboring dam Borehole: Data: Distance from: CE_BH02 Water sample taken & type Inorganic CE_BH01 Hand 717 meters Comments: 21

24 Step Test 22-Nov _A CE_BH02 Ian_de_Jager_Croydon_Estate Step 1 Step l/s 0.8 l/s Time interval (min) Water level drawdown (m) Time interval (min) Water level drawdown (m) Step 3 Step l/s 2.2 l/s

25 Step Test 22-Nov _A CE_BH02 Ian_de_Jager_Croydon_Estate Step l/s Time interval (min) Water level drawdown (m) Pump inlet 23

26 Constant Discharge Test (CDT 1) - Raw data Ian_de_Jager_Croydon_Estate 2481_A CE_BH02 22-Nov-17 Absatraction rate (L/s) 2.0 Test Duration: 10.1 Hours Time interval (min) Water level drawdown (m) Pump inlet 24

27 Constant Discharge Test (CDT 2) - Raw data Ian_de_Jager_Croydon_Estate 2481_A CE_BH02 23-Nov-17 Absatraction rate (L/s) 1.5 Test Duration: Hours Time interval (min) Water level drawdown (m) Pump inlet 25

28 Post CDT Recovery Ian_de_Jager_Croydon_Estate 2481_A CE_BH02 24-Nov-17 Hours Time interval (min) Water level drawdown (m)

29 8. APPENDIX B: WATER QUALITY ANALYSIS 27

30 Outstanding (Last page) 28