Hydro-geological Survey

A hydro-geological survey is an investigation of the hydro-logic and geologic parameters at the subsurface level in a particular area. Hydro-geological maps may be formulated with the data gathered during such a study. It involves the detailed evaluation of the water-bearing levels of rocks and their capability for filtration. Moreover, the intrinsic ability of these rocks to either store or resist water is also assessed. The pressure, type and quality of the underground water is noted, delving into the intensity of the water flow through pores or fractures. Find some meanings of the common terms used in geology in Glossary.

Glossary

A. Aquifer: A subsurface rock or sediment unit that is porous and permeable and contains water. In an aquifer these characteristics are highly developed: useful quantities of water are stored and transmitted

B. Confined: An aquifer that is bounded above and below by impermeable rock or layers of sediments. There may or may not be enough pressure in the aquifer to make it an ‘artesian aquifer (piezometric level above ground level)

C. Perched: Usually, an unconfined aquifer that is resting on an impermeable layer of limited extent surrounded by permeable formations or surmounting another unconfined aquifer.

D. Unconfined: The water in this aquifer is under atmospheric pressure. This kind of aquifer is replenished by rainfall in the area of its watershed or by infiltration from a river

E. Bedrock: Solid rock present beneath any soil, sediment or other surface cover. In some locations it may be exposed on the surface of the Earth

F. Formation: A laterally continuous rock unit with a distinctive set of characteristics that make it possible to recognize and map from one outcrop or well to another. The basic rock unit of stratigraphy.

G. Host: The rock formation containing the water. The rock and the water together form an aquifer.

H. Fracture: Any local separation or discontinuity plane in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. Fractures are commonly caused by mechanical stress exceeding the rock strength

TYPES OF GEOLOGICAL FORMATION

A) Perched aquifers

A perched aquifer is normally limited in size and lies on an impervious layer higher than the area’s general water table.

B) Shallow unconfined aquifers

The term ‘unconfined’ refers to an aquifer within which the water is open to atmospheric pressure: the so called piezometric surface (pressure head level) is the same as the static water level (SWL) in the borehole.

C) Confined aquifers

A ‘confined’ aquifer may hold groundwater under greater pressure, so that when punctured by a borehole, the SWL rises to the higher piezometric level. If the piezometric surface happens to be above ground level (which is not common), water will flow out of the borehole by itself: this is known as ‘artesian water.’

D) Fracture zone

Fracture zones develop during geological times as a result of the severe mechanical stress, caused by tectonic movements, that is exerted on non-plastic formations.

E) Hydro-geological basement

In this situation, it is a waste of time and money to extend a deep borehole into the metamorphic basement, which is generally known as the ‘hydro-geological basement’ or ‘bedrock.’ The bedrock marks the level below which groundwater is not likely to be found.

The Importance of Hydro-geological Surveying

A hydro-geological survey is done to determine the underground water level. Hydro-geological surveying is conducted prior to drilling a borewell in order to ascertain the quality and quantity of water available at a particular location. It has been well established that underground water is a better source of drinking water than surface water. This is due to the following reasons:

Lower chances of pollution, especially if it is at least twenty meters away from the sanitary work.
Water becomes naturally purified as it flows through sand and stone.
Borewells remain shut and therefore have a reduced chance of contamination.
Water temperature at the underground level is always at desirable levels in spite of seasonal changes.
Greater chances of water availability in summer as the water sources are situated at a deeper level. Typically, the hydro-geological assessment may be basic or complete. A basic survey comprises of the following:
1. Establish the needs of the end-user regarding the location and quantity of groundwater required
2. Nearby boreholes
3. Evaluation of the quality of groundwater
4. Appraisal of the catchment area
5. Borehole construction and design recommendations
6. Assessment of risk to health or environment
7. The amount of groundwater that is anticipated from the given location
A full hydro-geological assessment consists of a few additional features such as determining the availability of aquifers risk assessment of the catchment and analysis of the water source especially for possible pathways of contamination. The hydraulic properties of the aquifers, the direction of flow of groundwater and the rate of flow are also verified.

There are many companies and contractors in Kenya dealing with Borehole submersible pumps, however one has to be very careful while choosing for Borehole submersible pumps.

Aren’t Pumps sized by Kilowatt/Horsepower?

Don’t fall into the trap of thinking you need a certain horsepower of pump. In the pump industry we typically designate pumps by both the flow rate and horsepower required. Another, more precise, way to designate a pump is by what is called the “design point” of the pump. The design point includes the flow rate needed and the amount of pressure (head) that the pump needs to produce. When specifying the correct pump, it is more important to specify the amount of pressure and flow required than the horsepower required for the motor. Some pumps may be extremely efficient and be able to provide the flow and pressure with less power and thus require a lower horsepower motor than their cheaper competitors. While a more efficient pump may cost more, remember that it is working for several hours each day.

Basic Design Point Calculation.

As we just mentioned, the design point for a submersible borehole pump is the combination of how much pressure the pump needs to provide and the quantity of water the pump is expected to supply at that pressure. It is important to understand the difference between flow and pressure because they are distinctly different aspects of fluid dynamics.

Flow has to do with the amount, or quantity of water that flows in a given period of time. In the US this is typically measured in Gallons per Minute (GPM). If your borehole can produce 1.000 gallons per minute, that simply means that 1,000 gallons of water are available for extraction from your borehole every 60 seconds. That is 1,000*3.785=3785 Liters in a minute, meaning the borehole has 45.48 cubic meter per hour.

Pressure has to do with how much force the water exerts because of the effects of gravity and is measured in units of pressure. Your water pressure, just like tire pressure, is measured in Pounds per Square Inch (PSI) in the US. When working with water in our industry we often convert the pressure to a unit call ‘head’. Water head pressure simply means that we are talking about a unit of pressure that has units that are measured in feet of vertical water column. The higher the column of water, the more pressure there will be! 1 PSI = 2.31 ft of head. This means that a pipe, tank or any other container of water that has 2.31 ft of water in it will exert a pressure of 1 PSI at the bottom of that pipe, tank, etc… Believe it or not, it doesn’t make any difference how big around the pipe, tank or container are; the only factor that influenced the amount of pressure is the height of the water in the pipe, tank or container!

The following are steps in choosing The best Borehole submersible pumps in Kenya:

Have a test pumbing report for the borehole
Get a quotation from the Company/Contractor which includes the following:
1. Pumping head for the borehole including the height of the tank
2. The yield of the borehole I.E The yield given in the quotation ought to be 75% of that in the test pumbing report
3. The size of the draw pipes in the quotation ought to merge with the size outlet of the pump in order to avoid use of unnecessary reducers.
All the named steps in step two above should be done by an expert

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