Isotopic and geochemical evidence of anthropogenic recharge to the Thiaroye Urban aquifer of Dakar, Senegal S.CISSE FAYE, C.B. GAYE, R.G. TAYLOR, M.T. DIAW, A. POUYE [email protected]
INTERNATIONAL ATOMIC ENERGY AGENCY International Symposium on Isotope Hydrology : Advancing the Understanding of Water Cycle Processes IAEA Headquarters Vienna, Austria 20 -24 May 2019 GW and UN SDGs in Urban Africa
. Rapid, unplanned urban growth and inadequacies in urban planning constrain provision of universal access to safe water and sanitation by 2030 – United Nations’ Sustainable Development Goal (UN SDG) 6
. Urban groundwater represents a substantial, strategic freshwater resource to meet rising demand under accelerating rates of urbanisation and reduced river-intake security due to pollution and climate change (Foster et al., 2018)
. Risk of pollution from on-site and centralised sanitation systems as well as uncontained faecal waste remains key constraint to the sustainability of urban groundwater use Thiaroye suburb Quickbird view (2005) 2019 IAEA_IHS : Advancing the Understanding of Water Cycle Processes Vienna, Austria 20 -24 May 2019 Natural and anthropogenic recharge in the Thiaroye Urban aquifer
• understanding groundwater recharge and flow systems in Dakar is vital to assessing the sustainability of groundwater use and the contribution of groundwater to flooding
• in the Thiaroye aquifer of Dakar, evidence from piezometry, stable- isotope ratios of O and H, and tritium
Density of septic tank/ 3 2 ( H) shows that shallow groundwaters to pit latrine per 10000 m 01 - 10 be modern and from monsoonal rainfall 11 - 16 17 - 23 24 - 30 31 - 37 38 - 44 Malika 45 - 50 51 - 55
56 - 61 Sanghalkm Wakhinane Nimzatt • urban recharge is supplemented by 62 - 70 Ndiareme Limamoulaye Yeumbeul Nord Keur Massar effectively diffuse flows from on-site Sam Notoire camberene Medina Gounass Golf Sud Yeumbeul Sud sanitation facilities (i.e. 234 000 septic Djidah Thiaroye Kaw Parcelles Assainies pikine Nord Pikine Est Thiaroye Gare tanks draining to the Thiaroye aquifer) Pikine Ouest Guinaw Rail Nord Diamaguene Sicap Mbao Patte d'Oie Thiaroye Diaksao Mbao Guinaw Rail Sud
Dalifort Thiaroye Sur Mer Rufisque Ouest
Hann-Bel Air 3 0 3 Rufisque Nord Km AFRIWATSAN Í 2019 IAEA_IHS : Advancing the Understanding of Water Cycle Processes Vienna, Austria 20 -24 May 2019 Dakar Research: pan-African study AfriWatSan 1. to develop scientific evidence required to inform policies and practices that sustain the quantity and quality of low-cost, urban water supply and sanitation systems exploiting the sub-surface in Sub-Saharan Africa; and
2. to strengthen the capacity of individuals and institutions to conduct this vital research.
www.afriwatsan.org Urban Thiaroye Aquifer of Dakar
Geology / Geomorphology
Low-lying area between westward peninsula with an uplift of the sedimentary deposits (105m) and eastern cliff of Ndiass (127m)
Hydrogeology / Hydrology
shallow, unconfined quaternary sand aquifer occupying a low-lying area of ~300 km2 that is prone to flooding
Dakar and its suburb represent 54% of the total urban population of Senegal
2019 IAEA_IHS : Advancing the Understanding of Water Cycle Processes Vienna, Austria 20 -24 May 2019 AUTOMATED DATALOGGERS FOR GW LEVELS AND SALINITY MONITORING 2. Seasonal monitoring: 1. Installation of probes for continuous high-frequency a) Tracing groundwater movement from piezometry, hand- monitoring pumped wells, and production boreholes that enable sampling of δ18O, δ2H and residence-time indicators
b) Impact of on-site sanitation systems on groundwater quality using chemical tracers and microbiological parameters (Escherichia coli, TTCs)
Sanitary block
3. Research includes: a) Inter-disciplinary evaluations of the characteristics of water users and on-site sanitation systems on human health; and b) Improved awareness and understanding of linkages among leaky septic tank sanitation and health through the establishment of monitoring stations on the grounds of educational institutions. RESULTS – TRACING SOURCES OF GROUNDWATER
Rainwater LMWL Peri urban and Niayes zones Evaporation trend 0 Southeastern and Costal zones SW
-10 Precipitation weighted mean:
-20 (ẟ18 = -5.8‰; ẟ2H = -38‰)
H‰] 2
[δ -30
-40
-50
-60 -8 -7 -6 -5 -4 -3 -2 -1 0 1 δ18 O [‰]
• GWs regress along a slope of 5.2 consistent with evaporative 0 0,00 enrichment in the heavy isotope of O and H 5 10 0,50 15 • GWs intercept LMWL at (δ18O = -6.3, depleted relative to 1,00 20 weighted mean composition of rainfall (δ18O: -5.8) suggesting 25 1,50
30 bias to heavy, isotopically depleted rainfalls Water table table (m) Water 35 2,00
40 (mm) Daily rainfall 45 Rainfall 2,50 • Diffuse recharge to a shallow water table, indicated Daily water table fluctuations and rainfall from May by piezometry, reveals the vulnerability of shallow, to December 2017 unconfined aquifer and risk of groundwater flooding
2019 IAEA_IHS : Advancing the Understanding of Water Cycle Processes Vienna, Austria 20 -24 May 2019 RESULTS – SOURCE OF GW Groundwater samples from suburban and “Niayes” zones enriched in stable isotopes 900 Peri urban andNiayes zones δ18O and δ2H compared to the rest of the Southeastern and Costal zones 800 system
700
600 Variation in isotopes suggests contribution of recharge traced to leakage from septic tanks 500
to shallow groundwater NO3 (mg/l) NO3 400
300 15N/14N ratios in NO - range from + 9 to 200 3 +22‰ , indicative of faecal sources of N 100 found in 87% of sampled groundwaters (Re 0 et al., 2010; Diedhou et al., 2012) -6 -5 -4 -3 -2 -1 0 ẟ18O (‰ vs SMOW)
δ18O vs. Nitrate in groundwater
Density of on-site sanitation directly correlated to concentration of faecal chemical pollutants (nitrate) Thiaroye aquifer
2019 IAEA_IHS : Advancing the Understanding of Water Cycle Processes Vienna, Austria 20 -24 May 2019 RECENT OBSERVATIONS
T EC Mean chemistry of GW in the peri-urban Thiaroye (°C) pH (µS/cm) DOC Cl NO3 P K Na Br Na/K Cl/Br aquifer; all concentrations are given in mg/L
26.7 7.1 1930 18.5 237 327 1.7 25 199 0.5 7.8 480
3300 0
3100 10
2900 20
2700 30 Rainfall stabilises trend 2500 40 toward increase salinity 2300 50 measured by EC
2100 (mm) rainfall Daily
60 ElectricalConductivity (µs/cm) 1900 70 Rainfall (mm) Electrical Conductivity (µs/cm) 1700
EC of GW and daily rainfall from May 2017 to February 2019
extent of leaching surface and near-surface contamination from uncontained and contained faecal waste (on-site sanitation) is the subject of on-going research
2019 IAEA_IHS : Advancing the Understanding of Water Cycle Processes Vienna, Austria 20 -24 May 2019 NEW OBSERVATORIES
Constructed to inform:
• degree of access, in hydraulic terms provided for penetration of contaminants and;
• assess attenuation capacity of the unconsolidated sand aquifer • quantify recharge • assess viability of urban groundwater abstraction to irrigate peri-urban Urban groundwater observatories monitor effluent migration from septic agriculture tank to shallow groundwater
2019 IAEA_IHS : Advancing the Understanding of Water Cycle Processes Vienna, Austria 20 -24 May 2019 SUMMARY - CONCLUSIONS & PERSPECTIVES
• combined use of isotopic and bacteriological tracers provides new insight into sources of recharge to the Thiaroye aquifer of Dakar;
• groundwater derives from diffuse recharge and faecal effluent from a vast network of septic tanks (on-site sanitation);
• pathways by which faecal effluent causing nitrate concentrations exceeding 500 mg/L examined using newly constructed field observatories to monitor pollutant migration through thin unsaturated zone from septic tanks to shallow groundwater;
• seasonal rain-fed recharge and perennial anthropogenic recharge via septic tank effluent exacerbate frequency and duration of groundwater flooding in Dakar.
2019 IAEA_IHS : Advancing the Understanding of Water Cycle Processes Vienna, Austria 20 -24 May 2019 SUMMARY - CONCLUSIONS & PERSPECTIVES
Outstanding uncertainties in tracing the hydrology & hydrogeology of the Cap Vert Peninsula in Senegal using stable-isotope ratios of O and H include:
• inadequate resolution of seasonal variability in and controls on δ18O and δ2H in rainfall; and
• lack of measurements on δ18O and δ2H in perennial and seasonal surface waters (Niayes).
=> need to resurrect & sustain observatories.
2019 IAEA_IHS : Advancing the Understanding of Water Cycle Processes Vienna, Austria 20 -24 May 2019 Acknowledgments to :
Africa Capacity Building Initiative