Using Pressure Sensors for Groundwater Level Monitoring in Sierra Leone

Transcription

1 Using Pressure Sensors for Groundwater Level Monitoring in Sierra Leone 1

2 Sierra - Geology I RRG BC Unit Basement complex (BC) - weathered zone - fractured bedrock % of Land Area (Approx) 78% I Rokel River Group (RRG) consolidated rocks - weathered zone? - fractured bedrock 9% BG Bullom Group (BG) unconsolidated sediments 12% Other crystalline intrusives (I) 1% 2

3 Groundwater Sources in Sierra Leone Apart from in Freetown and a few other urban centres, there are very few distributed water systems.

4 Groundwater a Vital Resource 18,000 hand dug wells, 2,000 boreholes Waterpoint survey 2012 Distribution of non-seasonal sources (49% of sources) Source: MEWR and UNICEF Sierra Leone Waterpoint Report, 26 June

5 Water Security Project Pilot Monitoring Network: Rokel Seli River Basin 28 rain gauges. 18 wells and boreholes recording groundwater levels. 4 V notch weirs recording stream flow 1 river flow at Bumbuna Dam Weir 5

6 Rokel-Seli Pilot Monitoring Network 34 Sites (23 projects sites / 9 others)

7 Pressure Sensors in Wells Installation 2.5 litre plastic container Disinfected (chlorine-based bleach) Drilled with holes ~1/3 rd filled with gravel Logger secured inside Lowered to rest on base of well

8 Programming and Data Collection Logging every 15 mins / 2 to 3 months between visits 8

9 Groundwater levels using loggers measurements validated against manual readings Rain gauges sited near to wells daily readings. Site Monitoring 9

10 Magburaka Boys Secondary School 90 days 18 August 4.3m A shallow (9.9m) hand-dug well with a hand pump constructed in weathered basement. It is pumped throughout the dry season, but without ever drying up. It responds quickly to rain from early June onwards, peaking 18 August. Water level range is 4.3 m 10

11 One week in March Depth to Water Level (m below datum) 1 Week Period 12th to 19th March

12 One day in April

13 Any problems?

14 Total Pressure Sensors Barometric Compensation Barometric sensor inc logger & battery Suspension or Direct Read Cable Transducer inc logger memory & battery Pressure Sensor (Measurement position) Non-Vented loggers use a separate barometric device At least 1 Barometric Logger is usually needed per site Closer proximity improves accuracy May not be needed Short duration use (seconds or minutes, sometimes hours e.g. pump tests) In stable pressure conditions

15 Annual Atmospheric Pressure Variation Sierra Leone Annual variation is < 15 mbar (~ 15 cm water)

16 Diurnal Pressure Variation Sierra Leone Diurnal variation is 3 to 9 mbar 11 baro units at 9 sites Elevations vary from 61 to 402 masl

17 Pressure and Altitude Sierra Leone Sea Level pressure = 1013 mbar Reduction with altitude at rate of 1.17mbar / 10m 1.19 cm / 10m Averages based on 1 years data (2013) at 15 minute intervals from (most) sites Conform to global norms (1.21 cm / 10m) => instruments working well => good data!

18 Processing Data Baro logger issues Barologgers submerged (water collecting in tubes ) 3 Baro units were installed in tubes 1 m below ground level. Caps were left off tubes during periods of rainfall resulting in these collecting water. Data for these periods has to be removed from the data record unusable for compensation purposes.

19 Any other problems?

20 Validation with manual measurements Logger adjusted to manual measurement 2.1 cm 20

21 Validation with manual measurements Which is correct? Logger or manual measurement? 7.5 cm 21

22 Validation with manual measurements Whole record 22

23 Processing Data Datum shift: logger started before being submerged Logger moved these shifts also need correcting Not only was the logger installed after starting, but it has been moved several times due to pump maintenance / curiosity of well users.

24 Processing Data Practical issues Gap in record Logger stolen! Well is pumped most days for water supply. Water levels are falling below the logger during pumping. Logger is not deep enough

25 The real problems 51% of wells are seasonal. Why so many?

26 The real problems Standpipes in urban areas rapid population growth / unreliable supply

27 Sensors for water level measurements Wish list Simpler sensors / other technologies? Are there other (better & cheaper?) technologies that could be used as an alternative to pressure sensors? Improved software Better, more affordable (free?!), post-processing software with validation, data management, graphics, reporting routines without the need to export to 3 rd party software this is particularly important for small project teams or consultants who don t need or want to use a large overcomplex Data Management system 27

28 Using Pressure Sensors for Groundwater Level Monitoring in Sierra Leone Kabala Kambia Makeni Port Loko Freetow n Waterlo o Mayamba Magburaka B o Koidu Kailahun Kenema Thank you! Bonth e Pujehun 28