2012 Meeting Reports

An Evening Devoted To African Water Resources

Meeting at Loughborough University on 19 January 2012

Report by Geoffrey Jago

The introduction used by the Two Ronnies of “and in a packed programme tonight” came to mind at our meeting at Loughborough University which featured no less than three expert speakers, all preceded by our Group’s Annual General Meeting.

Dr. Michael Smith of Loughborough University, before introducing the speakers, briefly described the work of the University’s Water, Engineering and Development Centre (WEDC) that is one of the world's leading education and research institutes for developing knowledge and capacity in water and sanitation for low- and middle-income countries. Its web address is: http://wedc.lboro.ac.uk/.

In the context of increasing population and water-using infrastructure in Africa, our theme for the meeting was the burgeoning need for increased clean water supplies.

Water Well Drilling in Ethiopia

Our first presenter, MSc student Addise Amado Dube, spoke of his experience of the water drilling industry in Ethiopia.

Many drilling organisations work in the country, subject to conditions controlling their efficiency in the context of social, technical, geographic and economic constraints. The operators are well versed in well design, drilling technology, ground water assessments, construction quality and logistics and it is planned to sink many thousands of new boreholes to increase the water cover in the country.

The Influence of Landscape Evolution and Hydrology on Alluvial Units in the Katonga Valley, Uganda

Graham Bradley, from University College London, next described his PhD work in the Katonga Valley in southwest Uganda. The broad, open, flat, 200 km valley joins the vast Lake Victoria in the east to the much smaller Lake George to the west. A river in the westernmost 40 km flows westwards to Lake George while Lake Victoria receives the rest of the valley water. Bedrocks are various gneisses and granite overlain by alluvium and, to the east, fluviolacustrine deposits.

Uganda’s population and productive work doubled and redoubled in the twentieth century intensifying the need for water. A deeper understanding of the genesis and nature of water-bearing rocks is the key to better skills in planning water supply. Here, the best geophysical study method was found to be electrical resistivity, in the basic method of which four electrode spikes are pushed into the ground in a line and direct current (regularly reversing to avoid unwanted polarisation effects) is pumped into the outer electrodes. Then voltage is read across the inner electrodes. A graph is drawn of a series of readings with increasing electrode spacing which provides a vertical section based on the electrical resistance of variable rock layers. Nowadays a multi-electrode device uses 72 electrodes spanning 160 metres which are used in a profusion of patterns, automatically producing tomographic resistivity profiles both as vertical sections and in area coverage.

Of the valley’s geological history, three broad cycles of erosion and deposition have been identified: The original ancient valley with hardened glacial strata, the remains of river and lake deposits of Neocene (upper Tertiary) age and the late Quaternary channel with recent wetland deposits. But this is on the western side of Great Rift Valley country and Lake Victoria has been heaved up and down over the ages, leaving river and lake deposits at varying levels. Fluctuating climate trends have been added to the mix, further to exercise geoscientists’ interpretative skills.

A number of cross-sections were shown of the sandy and silty deposits stretching along the long valley, and their significance was explained. Fortunately the water yield from boreholes in this countryside is nearly always sufficient for local use and in some places would supply a town.

Groundwater and Climate Change in Africa

Our third speaker, Dr. Alan McDonald, Principal Hydrogeologist at British Geological Survey, Keyworth, began by pointing out that 300 million Africans, having no easy access to water, were forced to carry it, often for several kilometres. Infant mortality is higher where water is scarce and the heavy work of water handling wastes time and energy as well as keeping children from schoolwork. Consequently, providing clean water is one of the best services that can be offered to underdeveloped countries.

Lack of water inhibits crop yields while, on the other side of the coin, multinationals sometimes aggravate the problem by growing thirsty crops such as maize which overuse available water.

The need must be met by groundwater but the important unknown factor is the resilience of this supply to climate change. To develop a base of groundwater maps from all existing sources of information is vital work. These data would include groundwater storage which, although varying widely from place to place, is commonly quite high at twenty times the annual use by handpump.

Part of the study is concerned with recharge from rainfall. Generally the African continent contains large reserves which are sufficient for handpumps even if access is sometimes difficult, but yields higher than that pumped by hand may not be sustainable.

To avoid conflicts between ever thirstier communities, the hydrologist’s contribution becomes increasingly significant. The following web address provides further reading: www.bgs.ac.uk/gwresilience/.

To complete this interesting evening Dr. Smith’s final speech included thanks to our three speakers.

Geochemical Mapping in the United Kingdom

Meeting at the University of Nottingham on 7 February 2012

Report by Geoffrey Jago

Our country’s rocks and soils contain many chemicals. Some comprise a valuable resource, many are contaminants and some present problems because of past industrial activity. Hence, a register of where they lie and in what concentration is clearly a valuable asset.

Nottingham University was host when Andreas Scheib, Geochemist with the British Geological Survey, described his work with the Geochemical Baseline Survey of the Environment (G-BASE) project. Soil, stream sediment and water have all been comprehensively sampled and studied by BGS experts to provide a geochemical baseline of the UK. While this information has widespread value and interest in our crowded island, where it becomes increasingly necessary to bring old industrial sites back into beneficial use, this information has particular relevance to developers, researchers and those who study the environment.

Background and Baseline

Beginning some 40 years ago with mineral exploration as its centre, G-BASE has evolved into a high-resolution survey with available information on over 50 elements and its methods have been adopted internationally. This knowledge falls into two major classes: Background, which describes the amounts of elements which occur naturally, and Baseline, which also includes the changes made by man. Study areas fall into the two classes of Regional and Urban.

Where relevant, the elements As, Ba, Ca, Cu, Fe, K, La, Mg, Mn, Ni, Pb, Se, Sr, U, V, Zn and Zr all feature in the analyses of samples as does acidity (pH) and other parameters. We were shown a number of maps of the United Kingdom each devoted to specific elements or groups of them.

In conclusion Dr. Scheib recapped by stressing that Geochemistry is necessary to establish baselines, legislative initiatives, archives, contributions to domestic and world-wide initiatives, for example in training, improvements to the quality of life and safer and healthier environments.

Further reading is available at www.bgs.ac.uk/gbase.

A speech of thanks was given by Group Chairman John Black.

Hydrogeology and Contaminated Land

or the ‘dark art’ of understanding head

Meeting at the British Geological Survey, Keyworth on 27 March 2012

Report by Geoffrey Jago with thanks to the speaker.

Those at the back grew wan at best:
They feared the master’s voice.
“Today you’re going to have a test
“A simple multi-choice.”

The bard’s enduring words came to mind in the new De la Beche theatre at British Geological Survey, Keyworth (the site of the old De la Beche having suffered acute surface erosion prior to a change of use) when our Group Chairman, John Black, Director of In Site Hydro, spoke on the increasingly important work of assessing the water regime where land contaminated by past industry is to be found a new role, often with public access.

M. Darcy’s Law

John’s wide knowledge and straightforward approach to the way water behaves underground places him in the best position to explain the complexities to layman and specialist alike. Modern science centres around the law by which in 1855 M. Henry Darcy defined the way water flows through any permeable rock. A head (or pressure) of water across a given earthly medium produces across it a gradient of head. A generation earlier Herr George Ohm had enacted his very similar law of electricity. But botheration and geology banish all simple solutions. We were led into the mysterious world of how a head of water sneakily deceives the investigator.

A Typical Example

The example site was a suburban site in Durham with a bedrock of Namurian Carboniferous and Magnesian Limestone over which the ice sheets had laid down a glacial till of silty sand with boulder clay on top. Some of the latter had been made into bricks, taken from a pit covering half the site area, and the void had then been filled with refuse. Four boreholes together with a number of trial pits and window samples provided subsurface information. The hydrogeologist’s task was firstly to assess and interpret the groundwater regime and then to report on the suitability of the site for its intended future use as a public park, in this case bearing in mind any hazards of undue flooding or any perils traceable to the deposited refuse.

Heads Down Look In

And so to the exams which could have been predicted by each of us having been given a card to display. They showed A on one side and B on the other. Not difficult so far. The audience was divided into three groups: professionals dealing in contaminated land, hydrogeologists and the rest.

Maps and a cross-section gave the picture: gently sloping eastwards with layers of glacials on the bedrock of Namurian Carboniferous and, further east, Magnesian Limestone. We were shown three geological cross-sections with the observed water levels in the test holes followed by three (differing) interpretations of the site’s water regime by three consultants.

Audience participation was next sought in the form of a test on our understanding of M. Darcy’s work in 1855. He had set up a sloping column of typically permeable rocks, applied a head of water to the top, measured the head at points along the column and then how fast the water ran out at the base.

Now for the first tests. No talking at the back. We were shown alternative instances of vertical water flow, one where the lower end was sealed (no flow) and the other not (gravel and small flow). We were asked to choose in each case between displayed correct and incorrect consultant’s interpretations of how head varied with depth. A and B cards were duly waved. Memories were sparked of school when the success of those who smirked was ascribed to chance by the rest of us.

Two conclusions were drawn: the water table is where the ground water pressure equals atmospheric, and gravel will saturate to the top when recharge rate equals saturated permeability.

Heads and Flows

Further diagrams demonstrated what head of water could be expected in varying rocks and conditions of water flow. For example, in silt with small flow, head reduces as it flows downward.

After having been shown mock Darcy experiments by John, we were given a second look at the three cross-sections with observed water levels and again asked to vote. John pointed out that such information could indicate either fully saturated strata or perched water tables.

An important point is that a regime of perched water tables is a high permeability system, whereas saturated vertical flow occurs in an average to low permeability system.

All formations (not just gravels) saturate to their tops when faced with recharge equal to their saturated hydraulic conductivity.

Example Site Summary

John gave his summary of the hydrogeology of the example site:

The park is underlain by Glacial Till with a water table very close to the surface except in the old landfill where it is lower down.

All recharging water in the immediate area is leaking downwards into the glacial sands and gravels that underlie the till.

The downwards-leaking water meets upwards-leaking water from the Magnesian Limestone and both flow within the glacial sands and gravel southwards towards the local river (which they augment).
Any historical contamination from the landfill is either in the glacial sands to the south of the site or has already migrated into the local river.

No gas pathway is likely (contrary to conclusion of consultant’s report).

Waste Disposal System, Drigg, Cumbria - A Contrast

By comparison an example was given of a site where the density of test holes was much greater than at the previous example. The waste disposal system at Drigg is on a low-lying area near the coast where drift deposits overlie Triassic Sherwood sandstone. This site was perceived as a perched water table system above a regional flow system and the site presented many problems. One was to distinguish between the water regimes of drift and bedrock where “headroom” was low owing to the flat topography, and a specific technique was described.

Finally Some Dos and Don’ts

• Don’t use the term ‘lack of hydraulic continuity’ and be suspicious of anybody who does use it
• Don’t use water table and piezometric level interchangeably
• Don’t use Allen et al., 1997 as a ‘bible’
• Convert all water levels and pressures into heads
• Don’t use terms like aquiclude, aquitard, and semi-confined to describe formations under natural conditions since the terms are derived from pump testing and only apply to the formation’s response to the peculiar stress of a pump test
• Bear in mind that a borehole in a low permeability formation could take a year or more to fill up to an equilibrium level – ‘dry’ doesn’t necessarily mean dry.
• Don’t believe driller’s ‘first strike’ water levels

And Some Simple Rules of Thumb

• Perched water tables are unusual in UK (most common in high dry deserts)
• Only occur in high or steep places (not valley bottoms)
• Use simple basic information like relief (water flows downhill) and amazing how often surface water indicates conditions
• Maximum recharge that any formation will take is its own value of saturated hydraulic conductivity
• Take notice of geological formation names: i.e. Boulder Clay, Lacustrine Fluvial/Pebbly Clay
• A perched water system requires low K layers in a high K system not high K lenses in a low K system
• Perched water systems require more headroom than you think

David Bailey gave the speech of thanks for an interesting and stimulating evening