salty sand dunes
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Dr Abi Stone, University of Oxford, UK --- Salty sand dunes: calculating recharge rates to the Stampriet Basin, Namibia, using the chloride mass balance approachTRANSCRIPT
Salty sand dunes:
Calculating recharge rates to the Stampriet Basin, Namibia, using the chloride mass balance approach.
Abi Stone and Mike Edmunds [email protected]
Landscape Dynamics Research Group and Oxford Luminescence Dating Laboratory, School of Geography and the Environment, University of Oxford, South Parks Road, Oxford, OX1 3QY, UK
Estimating groundwater recharge rates in the Stampriet Basin
underlies any assessment of resource sustainability.
This research investigates the direct (diffuse) recharge
pathway through the southern Kalahari linear dunefield
(overlying ~80% of basin) using the chloride mass balance
approach.
We report results of pilot study from four unsaturated zone
profiles from two regions of the basin.
Recharge rates are between 7 and 46 mm/y through profiles
that represent 10 to 30 years of infiltration.
This is the first time that substantial direct (diffuse) recharge
has been quantified for the basin.
Overview
𝑅𝑑 = 𝑃 𝐶𝑝 +𝐶𝑑
𝐶𝑠
Where Rd is the direct recharge, P is the long-term average precipitation of the region,
Cp is the mean concentration of chloride in rainfall, Cd is the is the mean chloride from
dry deposition (Cp and Cd can be considered together as total deposition) and Cs is the
mean concentration of chloride in the pore waters in the USZ (Allison & Hughes, 1978).
This is facilitated by the fact that Cl is an inert tracer and is based on an assumption of
transmission of water via diffuse (piston) flow and steady-state conditions.
Location and Hydrogeology
The Stampriet Basin (Fig.1) is an important transboundary
resource within SE Namibia, Botswana and South Africa.
Mean rainfall is ~240 mm/y at Leonardville, ~190 mm/y at
Stampriet and ~175 mm/y at Koes, but demonstrates high
inter- and intra-annual variability.
Geological and corresponding hydrogeological classification
(Fig.2) and Table 1 reveals that there are 3 major water-
containing units (members, Mmb)
Nossob Mmb (artesian)
Auob Mmb (artesian)
Kalahari Beds (unconfined)
Complex stratigraphy, with erosional contacts, faults and
dolerite intrusions, means there is differing levels of potential
hydraulic contact across the basin.
Figure 1 (a) The Stampriet Basin showing study sites (hexagons) triangles show boreholes considered by Tredoux et al. (2002) (b) Geomorphology of the region (adapted from JICA, 2002).
Results and discussion Vertical Profiles Fig.3
Profiles are relatively homogenous sands, (uni-
modal, moderately sorted , medium sand).
Moisture contents are low (from 0.9 to
4.7%). In KAL11/1 & 11/2 moisture is high
and variable near the surface (above ZFP) and
in 11/3 & 11/4 it broadly increases with depth,
until base of 11/3. Chloride concentrations
vary from 1.7 mg/L to 240 mg/L.
Chloride profiles show considerable differences
in shape & concentration (a common finding in
semi-arid profiles, see Scanlon et al., 2006).
Most likely peaks and troughs of chloride
relate to changing chloride inputs from top of
profile (above ZFP) into lower profile, and that
this reflects fluctuating climatic conditions.
Sample sites and methods
Conclusions and further work
References and acknowledgements
This poster is based on a paper under revision in Water SA for a special issue following the GSSA/IAH International Conference on Groundwater: Our source of security in an uncertain future. Pretoria, South Africa 19-21 Sept 2011. Stone, A. E. C., Edmunds, W. M. (under revision) Sand, salt and water in the Stampriet Basin: Calculating recharge
through the unsaturated zone (Kalahari dunefield) using the chloride mass balance approach. Water SA Allison, G. B., Hughes, M. W. (1978) The use of environmental tritium and chloride to estimate total rainfall to an
unconfined aquifer. Australian Journal of Soil Science 16, 181-195. de Vries, J. J., Selaolo, E. T., Beekman, H. E. (2000) Groundwater recharge in the Kalahari, with reference to palaeo-
hydrological conditions. Journal of Hydrology 238, 110-123. Edmunds, W. M., Gaye, C. B. (1994) Estimating the variability of groundwater recharge in the Sahel using chloride.
Journal of Hydrology 156, 47-59.
KAL11/1
KAL11/3 KAL11/4
Geological Stratigraphy Hydrogeological
classification
Kalahari Beds
Tertiary-Quaternary
Linear dunes Unsaturated zone
Sands, gravels and calcretes overlying
calcrete-cemented conglomerate Kalahari Aquifer
Ka
roo
Kalkrand Basalt (in NW of basin)
Jurassic to Triassic
Rietmond Mmb
Permian
Sandstone, shale (and in east Whitehill
black shale and limestone)
Shale (yellow and grey) Impermeable layer
Auob Mmb
Permian
U Sandstone
Auob Aquifer
U Coal and black shale
M Sandstone
L Coal and black shale
L Sandstone
Mukorob Mmb
Permian
Sandstone
Mukarob Impermeable layer
Nossob Mmb
Permian
U Sandstone Nossob Aquifer
U Siltstone-shale
L Sandstone
L Siltstone-shale Impermeable layer
Dwyka Mmb
Carboniferous
Mudstone
Tillite
Pre Karoo
Cambrian
U Nama red sandstone, shale
L Nama Grey shale, sandstones
The author gratefully acknowledges: The Royal Geographical Society for Oman-Thesiger International fellowship for funding this research and to the British Society for Geomorphology for an small grant. The Oxford Fell Fund are also thanked for funding that will facilitate one of the next steps of this research.
Figure 2 Geological and hydrogeological cross sections (from JICA 2002).
Table 1 Stratigraphy (after Miller, 2000) and corresponding hydrogeological classification (modified from JICA, 2002).
Variation in Cl signature down profile stores information about previous moisture availability near the surface (since which time the water has infiltrated to sampled depth).
It is assumed moisture moves vertically via piston flow
Figure 3 Vertical soil moisture, chloride concentration and textural characteristics.
Linear dunes (the uppermost unsaturated zone)
were sampled in two regions (Fig.1)
Hand augured cores taken from bare patches of sand.
The dunes are virtually carbonate-free, and stand 15 to 25 m
above the interdune surface.
Moisture content and sedimentology measured using standard
procedures.
Steady-state chloride mass balance approach to calculate
direct (diffuse) recharge.
Recharge rates Table 2 and Fig. 4
Using steady-state average chloride concentrations forwhole
profiles (excluding elevated near-surface zones above ZFP)
recharge rates are between 7±2 & 46±14 mm/y (Table 2).
Variations are not unusual within a dunefield, or smaller region
(e.g. 0.5 km plot in Senegal (Edmunds & Gaye, 1994)).
Moisture residence times can be calculated for and depth, and
the base of these profiles this is ~13 to ~33 years (Table 2).
During this time mean precipitation values have changed (wetter
phase 1971-75, drier phase, 1976-85 & 2nd wetter phase late
1990s-2010 using 5-y moving rainfall average). Fig.5
When main control over chloride depth profiles is climatic (wetter
phases = lower Cl concentrations, drier phases = higher Cl
concentrations) we would expect to see this reflected in the data.
KAL11/2
Profile Depth Mean
rainfall
(P)
(mm/y)
Profile
steady
state Cl
(Cs)
(mg/L)
Surface
peak Cl
(mg/L)
Mean
annual
recharge
(Rd)
(mm/y)
Depth to
water table
(from core
base)
(m)
Time interval
of profile
(t)
(years)
KAL11/1 11.3 175 27.59 - 9 ± 3 ~40 25 (+10, -5)
KAL11/2 9.2 175 37.43 - 7 ± 2 ~ 40 33 (+13, -7)
KAL11/3 11.8 190 6.58 - 35 ± 11 3 - 8 13 (+10, -5)
KAL11/4 11.6 190 4.98 204 46 ± 14 3 - 8 15 (+6, -4)
Tredoux, G., Kirchner, J., Miller, R McG, Yamasaki, Y., Christelis, G.M., Wirenga, A. (2002) Redefining the recharge behaviour of the Stampriet Artesian Basin, Namibia. Proceedings of the IAH Conference ‘Balancing the Groundwater Budget’ Darwin, Australia, May 2002.
Comparison of recharge rates
Other CMB recharge estimates using unsaturated zone
Applied further east in the Kalahari (central & eastern
Botswana) for ten profiles indicates recharge value range of
0.8 to 33.8 mm/y of recharge (Selaolo, 1998).
A subset of these profiles were used to conclude recharge was
only ~1 mm/y in the central Kalahari and ~5 mm/y in the
eastern fringes (de Vries et al., 2000).
The Botswana profiles were complicated by calcrete layers and
preferential flow, whilst the profiles sampled here (Namibia)
didn’t encounter calcrete layers or textural variation (Fig.3).
Other recharge estimates from alternative methods
Other estimates for the Stampriet Basin come from CMB work
applied to the saturated zone (JICA, 2002) (but this
represents a mixed signal from infiltrated rainfall above and
from horizontal groundwater flow path) (0.1 mm/y in south,
1-2 mm/y in north (with patches of 2 to 5 mm/y).
Stable oxygen and hydrogen in groundwater compared to a
local meteoric water line gives a crude estimate of 2 to 7
mm/y (JICA, 2002). However, groundwater isotopic are not
straightforward (kinetic effects & non-equilibrium enrichment).
Best estimate for Cl input at top of
profile [P (Cp) + Cd)] = 1.4 ± 0.8 mg/L
JICA Study Team (2002) The study on the groundwater potential evaluation and management plan in the Southeast Kalahari (Stampriet) Artesian Basin in the Republic of Namibia. Final Report to the Department of Water Affairs, Ministry of Agriculture, Water and Rural Development, Republic of Namibia, submitted by Pacific Consultants International, in association with Sanyu Consultants Inc., Tokyo.
Miller, R. McG . (2000) The geology of the Stampriet Artesian Basin. Report submitted to Pacific Consultants International. Windhoek, Namibia.
Miller, R. McG. (2008) The Geology of Namibia volume 3. Geological Survey Namibia. Scanlon, B. R., Keese, K. E., Flint, A. L., Flint, L. E., Gaye, C. B., Edmunds, W. M., Simmers, I. (2006) Global synthesis
of groundwater recharge in semi-arid and arid regions. Hydrological Processes 20, 3335-3370. Selaolo, E. T. (1998) Tracer studies and groundwater recharge assessment in the eastern fringers of the Botswana
Kalahari: The Lethlankeng-Botlhapatlou Area. PhD thesis Vrije Unversitat Amsterdam, The Netherlands. pp229.
Chloride concentration (from rainfall & atmosphere) increases in surface zone as water is lost to evaporation and transpiration (giving Cl signature).
Chloride bulge (~3 m) may
relate to changing levels of
enrichment through time in the
near-surface zone before
percolation,
OR a deeper root zone.
Moisture moves up and down above the zero-flux plane (ZFP) and dominantly downwards below it..
Surface peak s relate to removal of
water via evaporation and transpiration,
leaving increased concentration of Cl
Basal Cl peaks may relate to capillary fringe
(close to groundwater table) with groundwater
enriched in Cl leaving an elevated zone.
Table 2 Unsaturated zone profiles and recharge estimates
Figure 4 Cumulative chloride against cumulative moisture plots to show recharge variations with depth.
Figure 5 Five-point moving rainfall averages.
Direct (diffuse) through the Kalahari dune unsaturated zone
has been estimated for the Stampriet Basin for first time.
Recharge is between 7 & 46 mm/y over last 13 to 33 years.
Lower rates in south east region and higher in central region.
Further work is needed to better constrain the spatial
variability of (and the potential influence of geomorphological
conditions, and vegetation cover, on) recharge rates and to
calculate a regional average for the basin.
Investigations into the direct recharge mechanism, such as
this, are vital part of understanding water balance in this basin
and being able to assess resource sustainability.