Investigation of Groundwater Resources and Airborne-Geophysical Investigation of Selected Mineral Targets in Namibia

Transcription

1 NAMIBIA Department of Water Affairs DWA Windhoek FEDERAL REPUBLIC OF GERMANY Federal Institute for Geosciences and Natural Resources BGR Hannover TECHNICAL COOPERATION PROJECT NO.: Investigation of Groundwater Resources and Airborne-Geophysical Investigation of Selected Mineral Targets in Namibia Volume IV.GW.3.3 Groundwater Investigations in the Eiseb Windhoek December 2004

2 Volume IV.GW.3.3 Authors: Dr. Armin Margane (BGR), Dr. Jens Wrabel (DWA) Commissioned by: Federal Ministry for Economic Cooperation and Development (Bundesministerium für wirtschaftliche Zusammenarbeit und Entwicklung, BMZ) Project: Investigation of Groundwater Resources and Airborne- Geophysical Investigation of Selected Mineral Targets in Namibia BMZ-No.: BGR-Archive No.: Date of issuance: December 2004 No. of pages: 26 Page I

3 Table of Contents FOREWORD LOCATION OF BOREHOLES SUMMARY OF RESULTS STEP TEST RESULTS CONSTANT DISCHARGE TEST RESULTS PUMPING TEST AND ANALYSIS PROCEDURES BOREHOLE WW STEP TEST CONSTANT DISCHARGE TEST BOREHOLE WW STEP TEST CONSTANT DISCHARGE TEST REFERENCES ANNEX 1: EVALUATION OF STEP TEST AT WW Page II

4 List of Figures Figure 1: Location of DWA-BGR Boreholes drilled in the Eiseb Graben Area in Figure 2: Recommended Yields (m³/h) of Boreholes drilled by DWA/BGR and those drilled by INTERCONSULT (1996) in the Eiseb Graben Aquifer System... 5 Figure 3: Transmissivities (m²/d) of Boreholes drilled by DWA/BGR... 6 Figure 4: Time-Drawdown Curve to Step Test of Borehole WW Figure 5: Time-Electric Conductivity and Water Temperature Curve to Step Test of Borehole WW Figure 6: Time-Yield Curve to Step Test of Borehole WW Figure 7: Well Loss Graph to Step Test of Borehole WW Figure 8: Well Efficiency Graph to Step Test of Borehole WW Figure 9: Time-Drawdown Curve to Step Test of Borehole WW Figure 10: Time-Drawdown Curve to Step Test of Borehole WW41023 (lin-log) Figure 11: Time-Electric Conductivity and Water Temperature Curve to Constant Discharge Test of Borehole WW Figure 12: Time-Yield Curve to Constant Discharge Test of Borehole WW Figure 13: Evaluation of Aquifer Parameters for Recovery Phase of Constant Discharge Test of Borehole WW41023 using the Theis Recovery Method Figure 14: Evaluation of Aquifer Parameters for Drawdown Phase of Constant Discharge Test of Borehole WW41023 using the Stallman Method Figure 15: Time-Drawdown Curve to Step Test of Borehole WW Figure 16: Time-Electric Conductivity and Water Temperature Curve to Step Test of Borehole WW Figure 17: Time-Yield Curve to Step Test of Borehole WW Figure 18: Well Loss Graph to Step Test of Borehole WW Figure 19: Well Efficiency Graph to Step Test of Borehole WW Figure 20: Time-Drawdown Curve to Constant Discharge Test of Borehole WW Figure 21: Time-Electric Conductivity and Water Temperature Curve to Constant Discharge Test of Borehole WW Figure 22: Time-Yield Curve to Constant Discharge Test of Borehole WW Figure 23: Evaluation of Aquifer Parameters for Recovery Phase of Constant Discharge Test of Borehole WW Figure 24: Time-Drawdown Curve to Step Test of Borehole WW Figure 25: Time-Electric Conductivity and Water Temperature Curve to Step Test of Borehole WW Figure 26: Time-Yield Curve to Step Test of Borehole WW Figure 27: Well Loss Graph to Step Test of Borehole WW Figure 28: Well Efficiency Graph to Step Test of Borehole WW Page III

5 List of Tables Table 1: Main Borehole Data of DWA-BGR Boreholes drilled in the Eiseb Graben in Table 2: Design of Step Tests... 4 Table 3: Results of Step Tests... 4 Table 4: Design of Constant Discharge Tests... 5 Table 5: Results of Constant Discharge Tests... 6 Table 6: List of Pumping Tests conducted in the Eiseb Graben, Pump Setting and Pumps used... 7 Table 7: Test Characteristics of Step Test at WW Table 8: Test Characteristics of Constant Discharge Test at WW Table 9: Test Characteristics of Step Test at WW Table 10: Test Characteristics of Constant Discharge Test at WW Table 11: Test Characteristics of Step Test at WW Page IV

6 Abbreviations asl above (mean) sea level bgl below ground level B Linear well-loss coefficient (h/m 2 ) C Non-linear well-loss coefficient (h 2 /m 5 ) CD Constant discharge E Well efficiency (%) K hydraulic conductivity (m/d) L Leakage factor (m) Lat Latitude Long Longitude RWL Rest water level (m below datum) P Exponent in non-linear well loss term or P Factor in Stallman s evaluation method describing the distance to a boundary PID Pump intake depth (m bgl) PWL Pumped water level (m below datum) Q Pumping rate (m³/h) Q n Pumping (abstraction) rate during the n-th step Q rec Recommended maximum abstraction (m³/h) S Storage coefficient s drawdown (observed) (m) s n Total (steady state) drawdown s n during the n-th step (m) s max Maximum drawdown during pumping test (m) s res Residual drawdown after recovery (m) SRTM Shuttle Radar Topography Mission T Transmissivity (m²/d) or T Temperature TD Total depth UTM Universal Transverse Mercator Page V

7 Foreword This report is part of a series of Technical Reports published by the Technical Cooperation Project Investigation of Groundwater Resources and Airborne- Geophysical Investigation, which is being implemented by the Federal Institute of Geosciences and Natural Resources (BGR) of Germany, the Department of Water Affairs (DWA) and the Geological Survey of Namibia (GSN). This project started in October 2002 and ends with its first phase in March This report documents the evaluation of pumping tests which were carried out between April and June 2004 in the Eiseb Graben as part of the drilling campaign. Altogether four boreholes were drilled to depths between 230 and 378 m in order to verify the availability of groundwater resources in the Eiseb Graben, to delineate the aquifer system and to more precisely define the hydraulic characteristics of the aquifer. Annex 1 documents the result of a step test conducted at borehole WW35432 which was drilled by INTERCONSULT in 1996 but at that time had not been properly pump tested. The basic data to all boreholes drilled by the DWA-BGR project, such as among others the pumping test data, are documented in Volume IV.GW.3.2: Groundwater Investigations in the Eiseb Graben Documentation Compendium on the DWA-BGR drilling campaign. The overall results and conclusions of the investigation are discussed in the main hydrogeological report, Volume IV.GW.3.1: Groundwater Investigations in the Eiseb Graben Main Hydrogeological Report. Page 1

8 1. Location of Boreholes Figure 1 shows the location of the 4 boreholes drilled by the German-Namibian technical cooperation project in the Eiseb Graben. Table 1 lists the main basic data to those boreholes. Elevations are based on interpolated data from the Shuttle Radar Topography Mission (SRTM). Table 1: Main Borehole Data of DWA-BGR Boreholes drilled in the Eiseb Graben in 2004 WW-No Geophysical_ Sounding UTM-E UTM-S Lat Long Elevation_ SRTM TD Started Completed m m m m WW _ WW _ WW _ WW _ Top of Bottom of Top of Bottom of Screen WW-No Screen1 Screen1 Screen2 Screen2 length RWL_CDT RWL_asl Collar_height Development m m m m m m m h WW WW WW WW41026 not installed Page 2

9 Figure 1: Location of DWA-BGR Boreholes drilled in the Eiseb Graben Area in 2004 Page 3

10 2. Summary of Results 2.1 Step Test Results Table 2: Design of Step Tests Pumped Well Aquifer Date No. of steps Duration of steps [h] Duration of recovery [h] Range of pumping rate Q n [m 3 /h] Pump test crew WW41023 Kalahari DWA WW41024 Kalahari Metzger WW not tested yield too low - WW not tested yield too low - Table 3: Results of Step Tests Pumped Well RWL [m] Max. PWL [m] s max [m] s res [m] B [h/m 2 ] C [h 2 /m 5 ] P [-] Q rec [m 3 /h] Evaluation Method WW E Jacob (1947) WW E Jacob (1947) WW < WW < Determination of drawdown s n Observed steady state drawdown Hantush- Bierschenk (1964) Page 4

11 < Eiseb Graben Botswana boreholes drilled by DWA/BGR boreholes drilled by Interconsult boreholes in DWA database boreholes retrieved during hydrocensus 2004 TEM soundings of DWA/BGR lineaments assumed limit of Eiseb Graben aquifer system < UTM zone 34S Figure 2: Recommended Yields (m³/h) of Boreholes drilled by DWA/BGR and those drilled by INTERCONSULT (1996) in the Eiseb Graben Aquifer System (values as reported by INTERCONSULT; compare Annex 1) 2.2 Constant Discharge Test Results Table 4: Design of Constant Discharge Tests Pumped Well Aquifer Observation Well (s) Date Duration of pumping [h] Duration of recovery [h] Pumping rate Q [m 3 /h] Pump test crew WW41023 Kalahari DWA WW41024 Kalahari Metzger WW not tested - - yield too - low WW not tested - - yield too low - Page 5

12 Table 5: Results of Constant Discharge Tests Pumped Well RWL [m] Max. PWL [m] s max [m] s res [m] EC [ms/m] T [m 2 /d] Evaluation Method Comments WW Theis (1935) boundary effects observed WW Theis (1935) boundary effects likely test too short WW yield too low WW yield too low boreholes drilled by DWA/BGR boreholes drilled by Interconsult Eiseb Graben Botswana boreholes in DWA database boreholes retrieved during hydrocensus 2004 TEM soundings of DWA/BGR lineaments assumed limit of Eiseb Graben aquifer system UTM zone 34S Figure 3: Transmissivities (m²/d) of Boreholes drilled by DWA/BGR Page 6

13 3. Pumping Test and Analysis Procedures Pumping tests were carried out using submersible pumps manufactured by Grundfos. Since different pumping characteristics concerning pumping lift and yield were required, different pumps had to be used: Table 6: List of Pumping Tests conducted in the Eiseb Graben, Pump Setting and Pumps used Borehole WW-No. WW41023 WW41024 Test Step Test Constant Discharge Test Step Test Constant Discharge Test WW41025 yield too low - - WW41026 yield too low - - Conducted by DWA DWA Metzger Metzger Date PID (m bgl) Pump Grundfos SP Grundfos SP 8A Usually step test were conducted in order to evaluate the well characteristics, such as well losses and maximum recommended yields, whereas constant discharge tests were used to obtain the main hydraulic parameters of the aquifer, such as transmissivity, horizontal/vertical hydraulic conductivities, and leakage factor. The interpretation of constant discharge tests also allowed determining boundary conditions and leakage effects. The storage coefficients could not be determined because there were no observation wells nearby. The duration of pumping tests varied largely, depending on how fast a steady-state condition was reached. During step testing the individual tests lasted mostly between 60 and 120 minutes. In some cases the pumping duration of individual steps was apparently not sufficiently long since the time-drawdown curves indicate that no steady-state conditions were reached (e.g. WW40932). The recovery phase commonly lasted at least as long as the pumping duration and not less than until an acceptable residual drawdown was reached. Most constant discharge tests lasted 24 hours, except WW40931 which has a relatively low yield. In order to obtain the correct yield, flowmeter as well as manual volumetric measurements using sufficiently large volumes (either 20 l canisters or 100/200 l drums) were conducted. Yield measurements, as well as EC measurements were supposed to be carried out every 10 minutes, however, in practise this was not always achieved. If there were considerable differences between the two measurements, manual volumetric measurements were preferred. In some cases the yield seems not to have been sufficiently stable, especially at the beginning of the test. This is mainly due to the fact that sometimes either the outflow valve was regulated even until several minutes after the start of the test in order to Page 7

14 obtain the requested yield value as precisely as possible or that some pumps had no foot valve. Conducting step tests allows determining the combined effects of aquifer (linear) and well losses (linear and non-linear). Depending on the well design, non-linear head losses could be significant due to friction losses at the well screen and turbulent flow. These losses determine the efficiency (E) of a well and thus the energy consumption for pumping lifts: E = B Q s n n Step tests were analyzed using the Jacob method, the Rorabaugh method or the Hantush-Bierschenk method: Jacob s method (1947, in Krusemann & deridder, 1994): s n = B Q + C Q n 2 n Hantush-Bierschenk method (1964, in Krusemann & deridder, 1994): s = B Q + C Q 2 w( n) n n where s w(n) drawdown during the n-th step after elapsed time t Rorabaugh method (1953, in Krusemann & deridder, 1994): p sn = B Qn + C Qn where s n Total (steady state) drawdown s n during the n-th step Q n Pumping (abstraction) rate during the n-th step B linear well-loss coefficient C non-linear well-loss coefficient P exponent in non-linear well loss term (between 1.5 and 3.5) Constant discharge tests were analyzed depending on the characteristics of the drawdown curve. If boundary conditions were recognizable, the Stallman method (1962, in Krusemann & deridder, 1994) was used to determine the transmissivity. This is clearly the case in well WW However, commonly tests were analyzed using the Theis method (1935; in Krusemann & deridder, 1994) for recovery data. The software Aquifer Test Pro 3.5 (WHI) was used for evaluation of the pumping tests. Page 8

15 4. Borehole WW Step Test Table 7: Test Characteristics of Step Test at WW41023 STEP-TEST Pumped Well Observation Well(s) WW41023 Start of Test 24-Aug-04 07:00:00 Start of Recovery 24-Aug-04 12:40:00 Duration of Test [h] 5.7 Duration of Recovery [h] 2.3 RWL [m] Residual Drawdown [m] 0.51 Step Yield (m³/h) Time 06:00 08:00 10:00 12:00 14:00 16: RWL [m] WW Figure 4: Time-Drawdown Curve to Step Test of Borehole WW41023 Page 9

16 EC [ms/m] Time since test start [h] EC [ms/m] T [ C] Figure 5: Time-Electric Conductivity and Water Temperature Curve to Step Test of Borehole WW41023 Q [m 3 /h] WW Time since test start [h] Figure 6: Time-Yield Curve to Step Test of Borehole WW41023 WW observed calculated Linear (observed) 12 sn [m] y = x 4 sn = 0.129*Qn + 1.1e-04*Qn Qn [m3/h] Figure 7: Well Loss Graph to Step Test of Borehole WW41023 Page 10

17 WW % 100.0% 98.0% Well efficiency 96.0% 94.0% 92.0% 90.0% Qn [m3/h] Figure 8: Well Efficiency Graph to Step Test of Borehole WW41023 Interpretation of Result The five steps performed at well WW41023 show that well losses are mainly linear. Overall well efficiencies are rather high. Using an acceptable limit of the well efficiency of 90%, the maximum yield of the well should not exceed 123 m³/h. With this, drawdown would amount to around 16.5 m. This, however, may not be the longterm sustainable yield, since the constant discharge test revealed that the well is affected by boundary conditions resulting from a nearby fault that possibly delimits the northern extent of the aquifer. Page 11

18 4.2 Constant Discharge Test Table 8: Test Characteristics of Constant Discharge Test at WW41023 STEP-TEST Pumped Well Observation Well(s) WW41023 Start of Test 25-Aug-04 07:00:00 Start of Recovery 28-Aug-04 07:00:00 Duration of Test [h] 72.0 Duration of Recovery [h] 72.0 RWL [m] 150 Residual Drawdown [m] 0.12 Yield (m³/h) 49.7 Time : : : :00 WW : : : : RWL [m] Figure 9: Time-Drawdown Curve to Step Test of Borehole WW41023 Page 12

19 WW41023 CD Test Time since test start [min] WW Drawdown [m] Figure 10: Time-Drawdown Curve to Step Test of Borehole WW41023 (lin-log) EC [ms/m] Time since test start [h] EC [ms/m] T [ C] Figure 11: Time-Electric Conductivity and Water Temperature Curve to Constant Discharge Test of Borehole WW41023 Q [m 3 /h] 55 Q RPM [m3/h] 53 Q Flow [m3/h] 51 Q Drum [m3/h] Time since test start [h] Figure 12: Time-Yield Curve to Constant Discharge Test of Borehole WW41023 Page 13

20 WW41023_CD [Theis Recovery] t/t' WW T = 5.53E+2 m²/d 0.66 s' [m] Figure 13: Evaluation of Aquifer Parameters for Recovery Phase of Constant Discharge Test of Borehole WW41023 using the Theis Recovery Method WW41023_CD [Stallman (1962) Barrier Boundary - Forw ard Solution] Time [min] WW Drawdown [m] Figure 14: Evaluation of Aquifer Parameters for Drawdown Phase of Constant Discharge Test of Borehole WW41023 using the Stallman Method Interpretation of Result The time-drawdown curve clearly shows boundary effects which set in at about 600 minutes after start of the test. However, evaluation of the boundary effect is difficult because no measurements at an observation borehole are available. Transmissivity Page 14

21 was therefore evaluated using the Theis recovery method as 553 m²/d. The combined saturated screened and open section of the well is m long, so that the hydraulic conductivity of the aquifer is around 2.49 m/d (2.9E-5 m/s). The saturated screened section can therefore be characterized as a good aquifer. Applying the Stallman method, despite the fact that it should in principle only be applied for observation wells, the best approximation is reached for: T = 334 m²/d S = 1E-5 P = 1E-6 (i.e. boundary at approx. 10 km distance) In order to obtain a better approximation of the transmissivity value and the distance to the boundary it is recommended to drill an observation well close enough to WW41023 and conduct another pumping test with a pumping duration of at least 72 hrs. Page 15

22 5. Borehole WW Step Test Table 9: Test Characteristics of Step Test at WW41024 STEP-TEST Pumped Well Observation Well(s) WW41024 Step Yield (m³/h) Aug-04 Start of Test 11:10:00 13-Aug-04 Start of Recovery 17:10:00 Duration of Test [h] 6.0 Duration of Recovery [h] 6.0 RWL [m] Residual Drawdown [m] Time 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00: RWL [m] WW41024 Figure 15: Time-Drawdown Curve to Step Test of Borehole WW41024 Page 16

23 EC [ms/m] Time since test start [h] EC [ms/m] T [ C] Figure 16: Time-Electric Conductivity and Water Temperature Curve to Step Test of Borehole WW41024 Q [m 3 /h] WW Time since test start [h] Figure 17: Time-Yield Curve to Step Test of Borehole WW41024 sn [m] WW41024 observed calculated Linear (observed) y = x sn = 0.7*Qn + 1.1e-01*Qn Qn [m3/h] Figure 18: Well Loss Graph to Step Test of Borehole WW41024 Page 17

24 Well efficiency 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% WW Qn [m3/h] Figure 19: Well Efficiency Graph to Step Test of Borehole WW41024 Interpretation of Result No steady-state condition was reached during the last step. Well efficiencies are altogether rather low. At a well efficiency of 70% the maximum recommended well yield would be 2.5 m³/h. This would result in a drawdown of approximately 2.8 m. 5.2 Constant Discharge Test Table 10: Test Characteristics of Constant Discharge Test at WW41024 CD-TEST Pumped Well WW41024 Start of Test 15-Aug-04 11:00:00 Start of Recovery 16-Aug-04 23:00:00 Duration of Test [h] 36.0 Duration of Recovery [h] 12.0 RWL [m] Residual Drawdown [m] 0.33 Yield (m³/h) 5.4 Page 18

25 Time 08:00 16:00 00:00 08:00 16:00 00:00 08:00 16: WW RWL [m] Figure 20: Time-Drawdown Curve to Constant Discharge Test of Borehole WW41024 EC [ms/m] WW41024 T [ C] Time since test start [h] Figure 21: Time-Electric Conductivity and Water Temperature Curve to Constant Discharge Test of Borehole WW Q [m 3 /h] Time since test start [h] WW41024 Figure 22: Time-Yield Curve to Constant Discharge Test of Borehole WW41024 Page 19

26 0 WW [Theis Recovery] t/t' WW T = 5.21E+1 m²/d s' [m] Figure 23: Evaluation of Aquifer Parameters for Recovery Phase of Constant Discharge Test of Borehole WW41024 Interpretation of Result The time-drawdown curve shows the effect of the missing foot valve at the beginning and at the end of the pumping period. Steady-state conditions were reached after around 18 hrs of pumping. However, after around 30 hrs drawdown seems to increase, which is possibly related to boundary effects. However, the pumping test is altogether too short to determine whether boundary effects really occur. Transmissivity was evaluated using the Theis recovery method as 52.1 m²/d. The saturated screened section of the well is 81.6 m long, so that the hydraulic conductivity is around 0.64 m/d (7.4E-6 m/s). The screened section can be characterized as a moderate aquifer. For a proper determination of the transmissivity and in order to be certain whether a boundary effect exists, it is recommended to conduct another pumping test at this site for a pumping duration of at least 72 hrs. In order to properly determine the transmissivity in a bounded aquifer, water level measurements in an observation well are required. This means that an observation well would have to be drilled near well WW Page 20

27 FEDERAL REPUBLIC OF GERMANY Federal Institute for Geosciences and Natural Resources BGR Hannover NAMIBIA Department of Water Affairs DWA Windhoek Windhoek, December 14, 2004 Authors: Dr. Armin Margane (BGR) Dr. Jens Wrabel (DWA) >>>>>>>>>>>>>>><<<<<<<<<<<<<<< Approved by: Dr. Frieder Schildknecht (BGR Project Team Leader) Greg Christelis (Deputy-Director of Geohydrology Division of DWA) Page 21

28 6. References Cooper, H.H. and C.E. Jacob, A generalized graphical method for evaluating formation constants and summarizing well field history, Am. Geophys. Union Trans., vol. 27, pp Hantush, M.S., 1961a. Drawdown around a partially penetrating well, Jour. of the Hyd. Div., Proc. of the Am. Soc. of Civil Eng., vol. 87, no. HY4, pp Hantush, M.S., 1961b. Aquifer tests on partially penetrating wells, Jour. of the Hyd. Div., Proc. of the Am. Soc. of Civil Eng., vol. 87, no. HY5, pp Jacob, C.E., Drawdown test to determine effective radius of artesian well, Trans. Amer. Soc. of Civil Engrs., vol. 112, paper 2321, pp Kruseman, G.P. & N.A. de Ridder (1994): Analysis and evaluation of pumping test data.- Int. Institute for Land Reclamation and Improvement Publ. no. 47, 3rd ed., 377 pp.; Wageningen. Rorabaugh, M.J., Graphical and theoretical analysis of step-drawdown test of artesian well, Proc. Amer. Soc. Civil Engrs., vol. 79, separate no. 362, 23 pp. Theis, C.V., The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using groundwater storage, Am. Geophys. Union. Page 22

29 Annex 1: Evaluation of Step Test at WW35432 Table 11: Test Characteristics of Step Test at WW35432 STEP-TEST Pumped Well Observation Well(s) WW41024 Step Yield (m³/h) Start of Test 9:57:00 08-Sep-04 Start of Recovery 13:58:00 Duration of Test [h] 4.0 Duration of Recovery [h] 0.3 RWL [m] Residual Drawdown [m] Time since test start [h] Drawdown [m] WW Figure 24: Time-Drawdown Curve to Step Test of Borehole WW35432 Page 23

30 EC [ms/m] Time since test start [h] EC [ms/m] T [ C] T [ C] Figure 25: Time-Electric Conductivity and Water Temperature Curve to Step Test of Borehole WW35432 Q [m3/h] Q [m 3 /h] Q [m3/h] Time since test start [h] Figure 26: Time-Yield Curve to Step Test of Borehole WW35432 Page 24

31 25 20 observed calculated sn [m] Qn [m3/h] Figure 27: Well Loss Graph to Step Test of Borehole WW % 95% 90% 85% Well efficiency 80% 75% 70% 65% 60% 55% 50% Qn [m3/h] Figure 28: Well Efficiency Graph to Step Test of Borehole WW35432 Page 25

32 Interpretation of Result The borehole was drilled to a total depth of 161 m in 1996 by INTERCONSULT. The position of the screens is unknown. Since the rest water level during the step test was at m, the maximum drawdown should not exceed 10 m. During step testing well efficiencies were in the acceptable range. At a well efficiency of 78% the maximum recommended well yield would be 11 m³/h. This would result in a drawdown of approximately 10 m. The step test result shows that if the well would have been drilled deeper, i.e. to about 200 m, the recommended yield might have been even a lot higher (around 20 m³/h). Page 26