Cape Cross Salt Works PO Box 81307, Windhoek, Namibia GROUNDWATER EXPLORATION FOR SUPPLY TO THE CAPE CROSS SALT WORKS FINAL REPORT rev1 NHN170 NOVEMBER 2016 Namib Hydrosearch, PO Box 11546 Windhoek Namibia, 1iganta@geonamibia.net
Contents 1 INTRODUCTION...1 2 DRILLING...1 3 TEST PUMPING...5 3.1 INTERPRETATION OF CONSTANT RATE TEST DATA BOREHOLE WBH-5... 5 3.2 TEST PUMPING OF THE HAND-DUG WELL... 6 3.3 WATER QUALITY... 7 4 CONCLUSION...7 5 REFERENCES...7 Table of Figures Figure 1: Location of boreholes and well/ pit, Gecko Salt, Cape Cross Salt works, Namibia.... 1 Figure 2: Borehole Log of borehole number WBH-4.... 2 Figure 3: Borehole Log of borehole WBH-5.... 3 Figure 4: Borehole log of borehole WBH-6... 4 Figure 5: Estimate of transmissivity using the Cooper-Jacob method.... 5 Figure 7: Time drawdown plot of hand-dug well test pumping... 6 List of Tables Table 1: Borehole Information... 1 Table 2: Summary of the test pumping done on the hand-dug well.... 6 Table 3: Water quality data from Borehole WBH5... 8 NAMIB HYDROSEARCH i
1 INTRODUCTION Gecko Salt appointed Namib Hydrosearch to carry out a groundwater exploration for supply to their Salt Works in Cape Cross. It is known that the groundwater in this location is saline and the water will undergo desalination. A drilled borehole and a hand-dug well were test pumped and the interpretation is given. The borehole and well locations are shown in Figure 1. Figure 1: Location of boreholes and well/ pit, Gecko Salt, Cape Cross Salt works, Namibia. 2 Drilling Three boreholes were drilled, Table 1 below summarises the borehole information on these boreholes. Table 1: Borehole Information Borehole Latitude Longitude Water Strike (m) Blowout Yield (m 3 /h) Water Level (m) WBH-4-21.77561 14.06764 61, 66 2.2 13 WBH-5-21.77373 14.07403 11, 38 2.9 10.53 WBH-6-21.77312 14.07612 19, 63 0.1 10.27 Figure 2 to Figure 4 below show the borehole logs of the three boreholes drilled. lithologies, water strikes and water levels are shown. NAMIB HYDROSEARCH 1
PROJECT: CLIENT: COORDINATES: Latitude: GEOLOGIST: Cape Cross Salt Works -Water Exploration GECKO SALT P. O. BOX 81307, Windhoek, Namibia Longitude: G. KALUMBU -21.775607 14.067639 BOREHOLE NUMBER: Water Level (m) STATIC WATER LEVEL 13 BOREHOLE NAME: WBH-4 NH PROJECT NO: NHN170 Date 8/16/2016 DEPTH (m) DESCRIPTION WATER LEVEL WATER STRIKE LITHOLOGY BOREHOLE CONSTRUCTION 0.0 Gravel: Coarse to very coarse gravel with brown top soil, abundant Stickup Standpipe 10.0 Dolerite: Fine grained dolerite, abundant pyroxene, weakly magnetic, minor disseminated pyrite 20.0 Dolerite: Fine grained dolerite, abundant pyroxene, moderately magnetic, minor disseminated pyrite 30.0 Dolerite: Mixture of dark-grey dolerite and reddish-brown granite 40.0 Dolerite: Dark grey dolerite, greenish tint, strongly magnetic, minor Dolerite: Mixture of dark-grey dolerite and light-brown granite 50.0 Dolerite: Fine grained dolerite, abundant pyroxene/olivine?, minor red granite on the last meter upvc Casing 60.0 Granite: Medium to coarse grained granite, abundant k-feldspar, minor 70.0 Granite: Red-brown granite, water strike at 61m (fracture?), estimated yield: 1-2 m³/h 80.0 Dolerite: Mixture of dolerite and granite (70/30%), much water from Granite: K-feldspar rich granite, accessory biotite, much water; 90.0 100.0 Dolerite: Dolerite, strongly magnetic, minor light-brown granite, lightgrey veins, cross cutting on the chips, dry samples obtained from 97m till EOH. Page 1 of 1 Figure 2: Borehole Log of borehole number WBH-4. NAMIB HYDROSEARCH 2
PROJECT: CLIENT: COORDINATES: Latitude: GEOLOGIST: Cape Cross Salt Works -Water Exploration GECKO SALT P. O. BOX 81307, Windhoek, Namibia Longitude: G. KALUMBU -21.77373 14.074034 BOREHOLE NUMBER: Water Level (m) STATIC WATER LEVEL 10.53 BOREHOLE NAME: WBH-5 NH PROJECT NO: NHN170 Date 8/21/2016 DEPTH (m) DESCRIPTION WATER LEVEL WATER STRIKE LITHOLOGY BOREHOLE CONSTRUCTION 0.0 Gravel: Mixture of coarse to very coarse gravel and coarse sand, abundant quartz and dolerite fragments, pebble sizes: 0.2-10mm in Stickup Standpipe 10.0 Granite: Coarse grained granite, abundant k-feldspar, minor disseminated black mica (biotite). 20.0 30.0 Dolerite: Fine grained dolerite, weakly magnetic, minor weathered red 40.0 Dolerite: Greyish dolerite, water strike at 11m (estimated yield: 0.5-1.2m³/h), V notch reading is 20mm; fracture. 50.0 60.0 Dolerite: Grey dolerite, moderately magnetic, minor red granite, V notch discharge = 30mm (estimated yield: 1-1.5m³/h), less moisture/no water from 27m. upvc Casing 70.0 Dolerite: Mixture of grey dolerite and red-brown granite (55/45%), dry 80.0 Granite: Mixture of red granite & dark grey dolerite,, dry samples up to 37m, water strike at 38m (second fracture?), estimated yield 2-3m³/h, V 90.0 Gneiss: Light-grey, medium to coarse grained rock (biotite rich gneiss/syenite??), moderately magnetic, light-grey to green veins x- cutting the chips, V notch discharge 30mm on average, strong water 100.0 Gneiss: Mixture of light-grey gneiss and red-brown granite, V notch discharge: 40-50mm. 110.0 Gneiss: Fresh gneiss/syenite, greenish tint, abundant dark-grey mica 120.0 Gneiss: Weathered gneiss/syenite, strong amount; estimated yield > Gneiss: Fresh gneiss/syenite, greenish tint, abundant dark-grey mica (biotite?), moderately magnetic, V notch discharge = 40mm. Page 1 of 1 Figure 3: Borehole Log of borehole WBH-5. NAMIB HYDROSEARCH 3
PROJECT: CLIENT: COORDINATES: Latitude: GEOLOGIST: Cape Cross Salt Works -Water Exploration GECKO SALT P. O. BOX 81307, Windhoek, Namibia Longitude: G. KALUMBU -21.773117 14.076119 BOREHOLE NUMBER: Water Level (m) STATIC WATER LEVEL 10.27 BOREHOLE NAME: WBH-6 NH PROJECT NO: NHN170 Date 8/24/2016 DEPTH (m) DESCRIPTION WATER LEVEL WATER STRIKE LITHOLOGY BOREHOLE CONSTRUCTION 0.0 Gravel: Gravel with brownish op soil, mixture of quartz, dolerite 7 Stickup Standpipe 10.0 Gravel: Mixture of gravel and fine grained basalt. 20.0 30.0 40.0 Granite: Weathered granite, minor gneiss at top meter, moisture from 17m, water strike at 19m (fracture), little amount of water obtained; estimated yield < 0.5m³/h. Gneiss: Moderately weathered gneiss, greenish tint, less moisture, dry 50.0 Granite: Fresh to moderately weathered granite, minor disseminated 60.0 Basalt: Fresh basalt, weakly magnetic, minor biotite, dry samples. 70.0 Gneiss: Biotite gneiss, dry samples up to 61m, moisture observed from 62m, water strike at 63m (second fracture); estimated yield of < Gneiss: Mixture of fresh gneiss and moderately weathered granite, more 80.0 Basalt: Fresh mafic rock, V notch : 20mm on average. 90.0 Gneiss: Mixture of light-grey gneiss and red-brown granite, decrease in 100.0 Gneiss: Fresh biotite rich gneiss, light-greenish rock; fine grained Basalt: Fine grained mafic rock, slightly weathered, V notch discharge : 110.0 Basalt: Fresh mafic rock, V notch : 20mm on average. 120.0 Basalt: Mixture of dark-grey basalt and red, slightly weathered granite, Page 1 of 1 Figure 4: Borehole log of borehole WBH-6 NAMIB HYDROSEARCH 4
3 Test pumping One borehole, WBH-5, was tested and WBH-6 was used as an observation borehole. WBH6 did not respond to pumping of WBH-5. WBH-4 could not be tested because an obstruction was encountered as the pump was being installed. A hand-dug well was pumped both the pumping and recovery water level data was collected. 3.1 Interpretation of constant rate test data Borehole WBH-5 Borehole WBH-5 was test pumped in November 2016 at a pumping rate of 1.3 m 3 /h for 12 hours and recovery of water level was monitored for the same period of time. A radial flow plot was used to identify the time when wellbore storage effect ceases during early time of the test. This is the influence of water stored in the borehole casing and the effect of wellbore storage is seen to last the first three (3) minutes of pumping. Displacement (m) 10 2 10 1 Dewatering of first water strike Dewatering of second water strike Obs. Wells WBH-5 Aquifer Model Confined Solution Dougherty-Babu Parameters T = 0.2998 m 2 /day S = 0.005996 Kz/Kr = 0.1 Sw = 0.525 r(w) = 1.25 m r(c) = 0.07186 m 10 0 10 0 10 1 10 2 10 3 10 4 Time (min) Figure 5: Estimate of transmissivity using the Doherty-Babu's method. Decrease in drawdown rate in early time of the time-drawdown curve is due to dewatering of the shallow low yielding water strike. Aquifer response is seen and radial acting flow is identified from 15 to 180 minutes and a straight fit is (Doherty-Babu's solution) shown in Figure 5. Transmissivity (T) is estimated from the fit (0.33 m 2 /day) assumed a storativity (S) value of 0.003. Storativity is assumed as there was no observation borehole. The closest borehole available for monitoring was WBH-6 at distances of 226 m did not respond. NAMIB HYDROSEARCH 5
CAPE CROSS SALTWORKS GROUNDWATER EXPLORATION At late time, dewatering of the water strike at a depth 38 m is noted. The drawdown rate increased from 180 minutes of pumping when the pumping water level was at 39 m, 1 m below the recorded second water strike. Dewatering of the water strike caused the aquifer condition to change from confined to unconfined causing drawdown rate to decrease temporarily (van Tonder et al., 2002). A lower rate of drawdown is seen from 290 to 540 minutes following which drawdown rate increased till the test was terminated at 720 minutes. The excessive drawdown at a relatively small discharge rate indicates an aquifer of limited extent. The groundwater potential of the aquifer is limited and not likely to meet the water demand of the project. 3.2 Test pumping of the hand-dug well A hand-dug well with a total surface area of 54 m2 and average depth of 1.2 m was pumped at 15.4 m3/h for approximately 4 hours (241 minutes). The recovery of water level was monitored for approximately 48 hours (2,910.3 minutes). A total drawdown of the 0.68 m was recorded at the end of the pumping period (Figure 6) and a residual drawdown of 0.1 m at the end of the recovery period. A summary of the pumping and recovery data obtained from the test is given in Table 2. 0.800 0.700 Dawdown (m) 0.600 0.500 0.400 0.300 0.200 0.100 0.000 0.00 260.00 520.00 780.00 1,040.00 1,300.00 1,560.00 1,820.00 2,080.00 2,340.00 2,600.00 2,860.00 3,120.00 3,380.00 Time (min) Figure 6: Time drawdown plot of hand-dug well test pumping The approximate inflow rate to the well is 0.68 m3/h (16.3 m3/d) and can t be sustainably pumped at the rate as required for supply (10m³/h). Table 2: Summary of the test pumping done on the hand-dug well. TEST Pumping Recovery NAMIB HYDROSEARCH Time (min) Drawdown (m) Q m³/h Outflow m³/d Inflow m³/d 241.50 0.68 15.35 368.38 2860.00 0.10 0.00 16.31 6
3.3 Water Quality A water sample was taken for a standard water quality test and taken to the lab. Table 3 below shows the test report of the sample taken. The water is of Na-Cl type, these two components and others such as Mg, N, SO 4 2- and Ca are classified as group D. The sample falls in Group D and is high risk water and cannot be used for human consumption or livestock watering. 4 CONCLUSION The following are concluded from the study. Of the three boreholes drilled two had sufficient yield to be test pumped. One of the boreholes (WBH-5) could be test pumped as the WBS6 had an obstruction and the pump could not be installed. During the test water strikes were dewatered and the tested yield of 1.3 m³/h could not be sustained. The aquifer has low potential and further tests were stopped. Water quality from borehole WBS-5 is of high salinity and Na-Cl type. Water is classified as Group D and is high risk water not suitable for human or animal consumption. A hand-dug well in the Cape Cross Salt Pan was tested by pumping and monitoring recovery of water levels to estimate the inflow rate to the well. Limited water resource is available from this source. 5 References van Tonder G, Bardenhagen I, Riemann K, van Bosch J, Dzanga P, Xu Y (2002) Manual on pumping test analysis in fractured rock aquifers. Report no. 1116/1/02, Water Research Commission, Pretoria, South Africa NAMIB HYDROSEARCH 7
Table 3: Water quality data from Borehole WBH5 To: Namib Hydrosearch cc P.O.Box 11546 Windhoek Date received: 4-Jul-17 0 Date required: 0-Jan-00 0 Date completed: 11-Jul-17 Attn: Mr D Sarma e-mail: diganta@geonamibia.net Sample details water sample Location of sampling point Cape Cross Description of sampling point WBH5 Date of sampling 2017/07/02 Time of sampling 16:08 Test item number I171243/1 Your Reference: Cape Cross Lab Reference: I171243 Recommended maximum limits Human consumption Livestock Parameter Value Units Classification Group A Group B Group C watering p H 7.5 A 6-9 5.5-9.5 4-11 Electrical Conductivity 4380 ms/m D 150 300 400 Turbidity 0.10 NTU A 1 5 10 Total Dissolved Solids (calc.) 29346 mg/l 6000 P-Alkalinity as CaCO 3 0 mg/l Total Alkalinity as CaCO 3 83 mg/l Total Hardness as CaCO 3 4572 mg/l D 300 650 1300 Ca-Hardness as CaCO 3 2554 mg/l D 375 500 1000 2500 Mg-Hardness as CaCO 3 2018 mg/l D 290 420 840 2057 Chloride as Cl - 15438 mg/l D 250 600 1200 1500-3000 Fluoride as F - 1.1 mg/l A 1.5 2.0 3.0 2.0-6.0 2- Sulphate as SO 4 3859 mg/l D 200 600 1200 1000 Nitrate as N 123 mg/l D 10 20 40 100 Nitrite as N 0.02 mg/l 10 Sodium as Na 9660 mg/l D 100 400 800 2000 Potassium as K 257 mg/l B 200 400 800 Magnesium as Mg 490 mg/l D 70 100 200 500 Calcium as Ca 1023 mg/l D 150 200 400 1000 Manganese as Mn <0.05 mg/l A 0.05 1.0 2.0 10 Iron as Fe 0.06 mg/l A 0.1 1.0 2.0 10 Stability ph, at 25 C 6.7 Langelier Index 0.8 scaling >0=scaling, <0=corrosive, 0=stable Ryznar Index 5.9 scaling <6.5=scaling, >7,5=corrosive, >6.5 and <7.5=stable Corrosivity ratio 311.0 increasing corrosive tendency Applies to w ater in the ph range 7-8 Remark: TEST REPORT w hich also contains dissolved oxygen ratios <0.2 no corrosive properties ratios >0.2 increasing corrosive tendency Overall classification of water, considering only constituents that have been tested for: Group D, high risk water. Interpretation based on guidelines for the evaluation of drinking w ater for human consumption, DWA, Namibia, July 1991 NAMIB HYDROSEARCH 8