Aquifers

Aquifer Anatomy

Aquifers can be confined or unconfined, depending on the layers above them. Water exists in most places under the earth's surface. Plants use the water nearest ground level. The water table begin below that. Aquifers form where material - especially rock - is permeable. You might have a layer of impermeable material (rock or clay) between the aquifer and ground, that's a confined aquifer. If there is no impermeable layer, that's an unconfined aquifer.

Importance of Aquifers

• We can only use about 1 percent of earth's surface water
• 99 percent of all usable water is actually groundwater
• Aquifers provide 99 percent of all groundwater
• 50 percent of all potable (suitable for drinking) water comes from aquifers.

Aquifers in Zimbabwe

The Zimbabwe National Water Authority (ZINWA) drilled and rehabilitated 20 boreholes at the Nyamandlovu Aquifer to augment water supply in Bulawayo and other interventions, meant to make the city regain its status as the country’s industrial hub. Pumping water from Nyamandlovu aquifer boreholes is expected to ease Bulawayo’s water crisis as the city will have an additional 10 megalitres (ML) a day. In September 2020, Treasury released $205 million for the rehabilitation of the boreholes.^[1]

Nyamandhlovu Forest Sandstone aquifers (Karoo)

The Forest Sandstone is widely developed in the Hwange – Zambezi Basin and constitutes an important regional aquifer, with high groundwater development potential. It is mainly confined, being unconfined only close to outcrop. Boreholes are typically 30 – 100 m deep, with water levels sometimes less than 10 m depth but more commonly greater than 20 m depth. Yields range from 0.1 to 5.9 l/s.

Recharge estimates indicate an annual recharge of 105.5 mm with 38.4%, 52.1% and 9.5% accounting respectively for direct recharge, water mains and sewer leakages.

Water Quality Issues Water quality is generally good with total dissolved solids (TDS) concentration below 1000 mg/l. There is no recognised fluoride threat, although it may pose a mild encrustation hazard.

Save Alluvial Aquifer, Umzingwane Alluvial aquifer, Grootvlei/Limpopo (transboundary)

Alluvial deposits are only locally developed within Zimbabwe, with the largest occurrences found in the Save (Sabi)- Limpopo river system and major tributaries; along the Zambezi River; and along the Munyati and Sessami rivers in the northwest. The alluvial deposits vary in thickness, being generally less than 25 m in most areas, but as much as 45 m in the Zambezi Valley and up to 70 m in the Sabi Valley. Where thick and permeable enough, they can form unconfined aquifers with high groundwater development potential.

Aquifer properties are extremely variable and may range from locally low to locally high average hydraulic conductivity. The water table ranges from about 5 - 40 m deep, but is generally over 20 m deep.

Boreholes in the alluvial deposits are typically 20 – 70 m deep, and provide yields of 100-5000 m³/d.

Water Quality Issues Water quality is generally good. Lenses of brackish water occur in the alluvium of the Sabi river, which may be fossil water and/or water recharging via the adjacent Karoo strata.

Kalahari Sands

The Kalahari Sands aquifer is mainly unconsolidated, but also include the consolidated Pipe Sandstone. It forms an unconfined aquifer with high groundwater development potential. Boreholes in the Kalahari Sands are commonly 70 – 100 m deep, and provide yields of 100-1000 m³/d. The aquifer properties of the Kalahari Sands are extremely variable and may range from locally low to locally high average hydraulic conditions. The water table is generally over 20 m deep.

Igneous – Karoo Basalt

Aquifers in the Karoo volcanic rocks are limited to discrete zones of weathering, fracturing and jointing, and contact zones with underlying Forest Sandstone and interbedded sandstone. Wide sheets of basalt occur in the Victoria Falls and Nyamamdhlovu, and Beitbridge and Chiredzi areas with numerous smaller remnants forming high lying cappings in the Gokwe area. Where suitably weathered, they have moderate groundwater development potential. The aquifers are unconfined, and have variable transmissivity from about 9 to 90 m2/d, and specific capacity values of the order of 1 to 10 m3/d/m.

The water table is usually less than 15 m depth, and borehole depths average 50 m, varying from about 40 to 60 m. Borehole yields vary from 10 to 250 m3/d.

Water Quality Issues The quality of groundwater in the basalts is good, with total dissolved solids (TDS) concentrations normally below 1000 mg/l. There is no identified fluoride hazard, although it may pose a mild encrustation hazard in places.

Cretaceous Sedimentary

The aquifer properties of these sedimentary rocks are controlled by primary porosity and permeability. The more favourable horizons are the conglomerates and cleaner sandstones, particularly in the vicinity of rivers. However, overall these aquifers have low groundwater development potential.

Water tables are typically less than 15 m deep, and boreholes are usually drilled to 70 – 100 m depth. Transmissivity is usually less than 1.5 m2/d, and specific capacity below 10 m3/d/m.

Water Quality Issues Water quality is moderate to good with total dissolved solids (TDS) concentrations ranging from less than 1000 mg/l to 2000 mg/l. The water poses a potential encrustation hazard.

Escarpment Grit

The Escarpment Grit forms a confined aquifer in the Hwange and Save-Limpopo basin, with a high groundwater development potential. Boreholes are typically 30 – 100 m deep, with water levels about 10 – 15 m and greater than 20 m depth respectively. Yields range from 1.2 to 3.5 l/s.

Madumabisa Mudstone

The Madumabisa Mudstone is associated with shallow weathering, but has low groundwater development potential. Boreholes are typically 30 – 100 m deep, with water levels about 10 – 15 m and greater than 20 m depth respectively. Yields are relatively low in the Madumabisa Mudstone (0.1 - 0.6 l/s); successful boreholes are usually sited in the vicinity of rivers.

Upper and Lower Hwange (Wankie) Sandstone

The Upper and Lower Hwange Sandstone is widespread in the Karoo basin and has high groundwater development potential. It is found at depth and the aquifer is always confined. It was previously known as the Wankie Sandstone. Boreholes are usually 100 – 150 m deep in the Upper and Lower Hwange Sandstones. Yields range from 1.2 to 5.8 l/s.

Precambrian Metasediments - Lomagundi Dolomite and Tengwe River Formation

Some water-bearing horizons exist within the Precambrian metasediments. Aquifers exist primarily due to karst features in the calcareous rocks of the Tengwe River and Lomagundi Dolomite formations (massive dolomites and limestones). Karst features are thought to be developed to an average depth of approximately 60 -70 m. Weathering of shaley horizons in the limestones also increases the potential for groundwater storage and flow.

The Lomagundi Dolomite has high groundwater development potential. Average specific capacity of boreholes is 505 m3/d/m. Typical borehole depths are 60 to 80 m . The water level varies from ground level at spring occurrences to 50 m depth, depending on land use (commercial or subsistence farming).

The Tengwe River Formation has moderate groundwater development potential. Borehole specific capacity is 4 to 120 m3/d/m. Typical borehole depths are 50 – 70 m. Water levels tend to be generally shallow (about 10 m) in the area of Tengwe limestone/shaley limestone.

Yields of 500 - >2000 m3/d are possible.

Water Quality Issues Water quality in the Lomagundi Dolomite is generally very good with a slightly hard calcium/magnesium character. Water quality in the Tengwe River Formation is generally very good with a slightly hard calcium/magnesium character.

Precambrian Basement Complex

The gneissose rocks and intrusive granites of the Basement Complex are devoid of any primary porosity, so the aquifer properties of these basement rocks are controlled by the degree of secondary porosity and permeability, often associated with fracturing, jointing, schistosity planes and weathering. Two hydrogeological sub-units are recognized, each with characteristic groundwater occurrence:

• the granite and gneiss below the African erosion surface; and
• the granite and gneiss below the Post-African and Pliocene Quaternary erosion surfaces.

These aquifers have low groundwater development potential. The highest groundwater development potential is found in those areas possessing the deepest and most aerially extensive weathering as in the larger African surface. Chemical weathering along faults, shear zones and dyke contacts may produce equally important water bearing structures.

Incorrectly sited boreholes may fail during the dry season.

Granite and gneiss below the African erosion surface

Rocks beneath the African erosion surface have relatively well developed hydraulic properties resulting from extensive weathering, which generally exceeds 30 - 35 m in depth. Aquifer thickness is about 30 to 50 metres; boreholes are usually drilled to 40 – 50 m depth. Sustainable borehole yields are in the region of 50 - 100 m3/d. Here, transmissivity is low to moderate (<10 m2/d) and specific capacity is moderate (30-50 m3/d/m).

Water Quality Issues Groundwater in the aquifers beneath the African erosion surface is generally of good quality with a total dissolved solids (TDS) concentration below 1000 mg/l and no recorded fluoride hazard.

Granite and gneiss below the Post-African and Pliocene Quaternary erosion surfaces

The hydraulic properties of the rocks beneath the Post-African and Pliocene-Quaternary erosion surfaces are considerably less well developed. Areas of weathering tend to be patchy and shallow (10 - 30 m). Aquifer thickness is about 10 to 30 m, often less than 15 m. Boreholes are typically 30 – 40 m depth. The transmissivity of these rocks is low (1 – 10 m2/d), while boreholes have low to moderately low specific capacity of the order 2 – 20 m3/d/m.

Areas of granite and gneiss pavement, very shallow bedrock and inselbergs and other features producing positive relief common in the Post-African and Pliocene/Quaternary surfaces are associated with marginal to nil groundwater resources.

Water Quality Issues In aquifers associated with the Post-African and Pliocene/Quaternary surface the groundwater quality is generally good. TDS is usually below 1000 mg/l, however it is higher in the Beitbridge and Nuanesti area (TDS of 1000 to 2000 mg/l) where there is also a fluoride hazard.

Greenstone belts, including the Bulawayan Supergroup

The rocks of the Greenstone belts generally have very low permeability, but groundwater occurs where fracturing and weathering have created adequate porosity and permeability, and consequently this aquifer has high groundwater development potential. The Bulawayan Supergroup can support yields of 1.2 - 2.9 l/s (Interconsult 1986). The average borehole depth is in the range 30 - 50 m, and the water level is typically from 5 to greater than 20 m below ground level.^[2]

References