Salty sand dunes:

Calculating recharge rates to the Stampriet Basin, Namibia, using the chloride mass balance approach.

Abi Stone and Mike Edmunds abigail.stone@ouce.ox.ac.uk

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.