HYDROGEOLOGICAL CONCEPTUAL MODEL OF A COMPLEX AQUIFER SYSTEM CENTRAL KALAHARI BASIN (BOTSWANA) DATE: 27 OCTOBER 2016 VENUE: GICC MR. M. LEKULA DR. M.W. LUBCZYNSKI PROF. E.M. SHEMANG
PRESENTATION OUTLINE INTRODUCTION STUDY AREA AND DATA DESCRIPTION METHODOLOGY RESULTS AND DISCUSSION CONCLUSIONS 2
INTRODUCTION Successful groundwater resources evaluation and management using distributed numerical GW flow models. done Their reliability largely determined by realistic hydrogeological conceptual models developed from surface and subsurface data. Surface Data Hydrostratigraphic units Sub-Surface Data (Geology) Hydrogeological conceptual model Hydrogeological Parameters Hydrogeological Boundary conditions Groundwater flow patterns 3
INTRODUCTION CONT. Conceptual hydrogeological models set through geological and hydrogeological synthesizing using GIS and/or 3-D geological models. 3-D geological models evaluates the structural geology complexity and hydrogeological heterogeneity, which is the basis of a conceptual model. 3-D geological models provide a check on the logic of a hydrogeological conceptualization. 4
MOTIVATION No regional hydrogeological conceptual model of the Central Kalahari Basin (CKB) yet, utilizing 3-D geological model. Integration of localized Hydrogeological studied which has never been done before. Mostly regional studies have been on the CKB s potential on the occurrences of oil and coal bedded methane gas. 5
OBJECTIVES Development a regional hydrogeological conceptual model of the CKB to improve understanding on: Spatial distribution of the CKB hydrostratigraphic units Effects of regional faults and basin morphology on hydrostratigraphic units, spatial aquifer parameter distribution, hydraulic heads and groundwater flow pathways. 6
STUDY AREA AND DATA LOCATION & PHYSOGRAPHY CKB with ~181,000 Km 2 in Botswana and ~14,000 km 2 in Namibia. Large-scale hydrogeological basin. Currently nearly flat due to accumulation of eolian sand. ~90% being desert. Semi-arid to arid with convectional rainfall Rainfall: 250 550 mm yr -1 (Sep April) Winter temperature range between 6 o c 25 o c and summer has average Max 35 o c 7
STUDY AREA AND DATA CONT GEOLOGY Karoo Basin occurring in majority of CKB. Basin type filled with sedimentary and volcanic rocks from Paleazoic to Mesozoic. Kalahari Karoo Basin divided into: SW Botswana Central Kalahari NE Botswana NW Botswana Karoo Supergroup Formation, in which GW resources in CKB occur, Subdivided into: Stormberg Basalt Group Lebung Group Beaufort Group Ecca group Dwyka Group Johnson et al., 1996 8
STUDY AREA AND DATA CONT HYDROGEOLOGY Principal aquifers in CKB are: Ntane and Nakalatlou Sandstones (Lebung group). Ecca group Ghanzi Group Regional GW flow has radial concentric pattern towards outflow. Recharge is incidental, restricted to very wet years/seasons, occurring every 5-10 yrs. Long term mean recharge range from 5-10 mm/year in the fringes to less t < 1 mm/year in CKB central. Groundwater abstraction mainly in the fringes of the CKB. DATA 293 boreholes information Unpublished Government reports 90 m SRTM DEM 9 Geological shapefiles
MATERIALS AND METHODS Hydrostratigraphic layers AGE DESCRIPTION PERIOD Karoo Division Groups South West Botswana Sub-Basin (i) Kweneng area & Western Central Kalahari Sub-basin (ii and v) Mmamabula area (vi) South East Central Kalahari Sub-Basin (iv) North East & Northern belt Central Kalahari Sub-Basins (iii) Hydrostratigraphy CENOZOIC MESOZOIC Quaternary Jurassic Triassic Upper Permian Post- Karoo Upper Karoo Kalahari Stormberg Basalts Lebung Nakalatlou Sst. Dondong Fm. Beaufort Kule Fm. Kwetla Fm. Kalahari Group Stormberg Lava Group Ntane Sandstone Formation Mosolotsane Fm. Thabala Formation Ngwasha Fm. (North East Only) Pandamatenga Fm. Kalahari Group (Layer 1) Stormberg Basalt (Layer 2) Lebung Aquifer (layer 3) Aquitard (Layer 4) Otshe Fm. Boritse Fm. Korotlo Fm. Serowe Fm. Tlapana Fm. PALEOZOIC Lower Permian Lower Karoo Ecca Mmamabula Fm. Morupule Fm. Kweneng Fm. Mosomane Fm. Kamotaka Fm Mea Arkose Fm Ecca Aquifer (Layer 5) Kobe Fm. Bori Fm. Bori Fm. Makoro Fm. Tswane Fm. PROTEROZOIC Meso- Proterozoic Pre- Karoo Ghanzi Group Ghanzi Aquifer (Layer 6 ARCHAEN Basement 10
MATERIALS AND METHODS CONT METHODS Hydrostratigraphic Unit thickness + Major faults Hydrogeological parameters + Hydraulic heads Inverse distance power 2 interpolation XYZ thickness maps x-sections maps CKB HYDROGEOLOGICAL CONCEPTUAL MODEL 11
RESULTS & DISCUSSION BH DISTRIBUTION AND CROSS-SECTION LOCATION 12
RESULTS & DISCUSSION CONT WEST-EAST Sections 13
RESULTS & DISCUSSION CONT Hydrostratigraphy, aquifer spatial distribution & aquifer parameters 1: Kalahari Group Unsaturated Zone. Continuous spatial extent. Thickness variable from 6 m (West) to >100 m in central CKB. Localized perched aquifers exist in the Kalahari Group. Groundwater flow show a concentric radial pattern from the W, E, and S to the N Makgadikgadi GW discharge area. 14
RESULTS & DISCUSSION CONT Hydrostratigraphy, aquifer spatial distribution & aquifer parameters 2: Stormberg Basalt Confining layer to aquifers beneath. Thickness highly variable up to > 200 m due to block faulting and basin morphology. 15
RESULTS & DISCUSSION CONT Hydrostratigraphy, aquifer spatial distribution & aquifer parameters 3: Lebung aquifer One of the main aquifers in CKB. Thickness variable from 0 m (NW and S Zoetfontein) to ~230 m in NE and SW CKB. Depth to top of Lebung Variable due to regional faulting and basin deepening towards the center. Confined where Stormberg Basalt are present. K range from 0.09 17.6 m/d, T range from 1 100 m 2 /d Groundwater flow show a concentric radial pattern from the W, E, and S to the N Makgadikgadi GW discharge area. GW flow parallel to regional faults Heads above top of the aquifer where confining Layer is present even until base of Kalahari in some places 16
RESULTS & DISCUSSION CONT Hydrostratigraphy, aquifer spatial distribution & aquifer parameters 4: Central Kalahari Basin Aquitard (CKB Aquitard) Spatially variable permeability. Thickness highly variable up to > 250 m between regional faults and S CKB. Depth to top of CKB Aquitard Variable due to regional faulting and basin deepening towards the center. Its low permeability ensure very low to no vertical water exchange between the overlying Lebung and underlying Ecca Aquifers where present. 17
RESULTS & DISCUSSION CONT Hydrostratigraphy, aquifer spatial distribution & aquifer parameters 5: Ecca aquifer Other main aquifers in CKB. Thickness variable from 0 m (NW and S Zoetfontein) to ~290 m in S CKB. Depth to top of Ecca Variable due to regional faulting and basin deepening towards the center. Confined where CKB aquitard Stormberg is present. K range from 0.16 22.9 m/d, T range from 1 1000 m 2 /d Groundwater flow show a concentric radial pattern from the W, E, and S to the N Makgadikgadi GW discharge area. GW flow parallel to regional faults Heads above top of the aquifer where confining Layer is present even until base of Kalahari in some places 18
RESULTS & DISCUSSION CONT Hydrostratigraphy, aquifer spatial distribution & aquifer parameters 6: Ghanzi aquifer Other main aquifers in CKB but not part of the Karoo system. Only present in NW CKB. Thickness variable from 0 m to ~230 m. Depth to top of Ecca Variable due to basin deepening towards the center. K range from 0.05 6.5 m/d, T range from 1 100 m 2 /d Groundwater flow towards the Makgadikgadi GW discharge area. 19
RESULTS & DISCUSSION CONT Schematization of CKB hydrogeological conceptual model Recharge occur at the CKB fringes where Kalahari is thin especially in the Namibian side where depth to GW is as shallow as 1m in the Ecca. CKB hydrostratigraphic units thickening towards Makgadikgadi GW outflow. Thick Kalahari in central CKB makes recharge impossible. Infiltrated rainwater create perched aquifers in Kalahari and some utilized by vegetation water uptake and direct evaporation. Regional faults are important controls on GW flow paths laterally and vertically. Groundwater flow towards the Makgadikgadi GW discharge area. 20
CONCLUSIONS Developed regional hydrogeological conceptual model showed that: Recharge in the CKB largely originate from the Namibian side. Possible Ecca and Ntane vertical interaction where hydraulically in contact. Regional faults act as flow barriers. Lebung and Ecca confined in the majority of the CKB. 3-D geological model approach very important in development of hydrogeological conceptual models because: It integrate all available data tests plus knowledge to develop meaningful model. It is flexible to edit layers back and forth between conceptual and numerical modelling. 21
THANK YOU FOR YOUR ATTENTION KEA LEBOGA 22