by Mariajose Romero-Segura, Freelance Project Hydrogeologist and Technical Translator
Groundwater, the big unknown
Groundwater is an invisible natural resource. It is available in different proportions, in different rock types, and at different depths, in the Earth’s surface layer. I cannot remember how many times I have heard people saying that there is water everywhere. This is not true. Eighty percent of the water found on the continents is on the surface. The remaining 20% is underground or in the form of atmospheric water vapor.
For the ground to hold water, it has to be either porous (made of clay, silt, sand, gravel) or broken up (with fractures or voids). Does clay hold water? Yes, it does; against the general idea, clay ground can contain a fair amount of water, but clay particles and water molecules join together with electrical forces that are difficult to break. Clay ground can hold a lot of water, but the point is that it does not release all this water very easily.
How do drillers and geologists know if the water is fresh? By testing it, of course, in the field, either with smart portable devices that we often carry when conducting a drilling program, or in some cases licking it. Yes, it is funny how drillers and hydrogeologists don’t mind tasting a drop of water, but don’t worry, we do not swallow it, of course not, we spit it out.
And how often have I been challenged to say where water is ‘because I am a hydrogeologist’? Hang on there, if for finding minerals we need to use technology, to find water we have to as well.
Another funny thing is how often people have asked me if I use a divining rod; Oops! Nope, at university we are not taught that technique. So, how do we find water? Using similar tools as when looking for minerals, that is geological maps, geophysics, drilling.
Hydrogeology exploration starts at the desk. The investigation begins with the collection, analysis, and interpretation of hydrogeology of existing topographic maps, aerial photographs, geological maps and records, and other related documents. Geological mapping often forms the starting point of exploration and should identify potential aquifers. Aerial photographs are a good way for identifying major faults. The reason for looking for regional structures is that if they hold water, there is a better chance of finding good amounts.
If available, geophysical data provide indirect indicators to identify potential aquifers. Again, the aim is to find lithological contacts, deeper sections of palaeochannels (ancient rivers), etc. Among geophysical methods, electrical (geoelectrics) and electromagnetic methods (transient electromagnetics and airborne electromagnetics) are most commonly used to explore groundwater conditions because aquifers, aquitards and bedrock often differ in the measured resistivity or electrical conductivity.
The desktop study should be supplemented, when possible, by geologic field reconnaissance and by evaluation of available hydrogeologic data on well yields, groundwater recharge, discharge, groundwater levels, and water quality. It is very important to understand the objectives as it is not the same looking for dewatering to keep the mine dry as it is to find good quality water as a supply.
A next step is to design a drilling program. This part is much more expensive than the desktop study. In order to minimize cost, whenever possible, it is very important to gather field knowledge from the mineral exploration geologists that have conducted drilling in the area, and searching in the geological database for some evidence like ‘holes abandoned’, ‘wet samples’, or ‘no sample returns’.
Because during mineral exploration drilling the water table is sometimes intersected, it is of great advantage to record those intersections. For example, above the water table, the rock chip samples come to surface very loose and dusty; once the water table is intersected, they start being less dusty and the color darkens very slightly until they become clearly damp and then wet. The subtle changes in color are too difficult to record in the geology database, but there could be a dry/damp/wet field, and when that field exists in the database, exploration geologists should be encouraged to fill it in.
During drilling, the interaction of the hydrogeologist with the driller is essential as he is the expert in the matter and he knows what is happening at depth even before the samples arrive at surface. This happens more often when drilling hard rock as he can feel the fractures before the geologist identifies them by the size of the rockchips, or the stain in the surfaces, etc.
There are several different types of drilling methods and choosing one depends on factors like ground type, expected water level and common practice in each country. A good practice is to propose a drilling type but to supply the drilling company with details of the expected geology, and let them have the last decision. In Australia, water exploration in hard ground is usually drilled using RC rigs while soft, crumbly ground is more appropriate for Mud Rotary.
Essentially, an assessment of groundwater resources involves the location of potential aquifers within economic drilling depths. Whether or not an aquifer will be able to supply the required amount or quality of water depends on its thickness and spatial distribution, its porosity and permeability, whether it is fully or partially saturated, and whether or not the quality of the water is acceptable.
And while on the subject of water quality, something that I’ve heard often and is a misconception is that fresh water is safe to drink. Water is fresh when its amount of dissolved salts is below a certain threshold and it is based on taste, so according to the Australian Drinking Water Guidelines water is of good taste when its TDS (total dissolved salts) does not exceed 500 mg/L. Still, water of up to 1000 mg/L TDS is acceptable to many.
But water is the greatest solvent in the world; usually, it’s called the ‘universal solvent’. Water dissolves more substances than any other liquid and this is due to its molecular structure. So salts should not be our only concern when considering drinking non-treated water. Water quality refers to the suitability of water for different uses according to its physical, chemical, biological and organoleptic (taste-related) properties.
Costs of groundwater exploration
I have been part of teams working on the pre-feasibility of a deposit where the cost of bringing water to a dry deposit or getting rid of excess water has killed the project. That issue worsens when the quality of the water makes it unusable. And it can get even worse when the mine site sits in an environmentally sensitive place. Or when management does not consider water matters early enough in project work.
Costs of water supply and dewatering
This is something that is often underestimated. A mine site requires a great amount of water, firstly during the construction phase and later on during operations. And if the groundwater is at a depth that will be reached during excavation, dewatering is a must. In that case, there is a need to drill and equip as many bores as it is necessary regardless of the cost. After the initial expense of drilling and equipping bores (pumps, pipes and gensets), there is a regular cost of diesel, maintenance and personnel.
Not always can all the dewatering be managed via bores, so in-pit sumps and trenches have to be dug and kept clean. There is a need to have sump pumps and it is important to have spare pumps and parts because they fail sometimes.
Water in mining
Water is needed during each phase of the mining project in greater or lesser amounts. During exploration, water is required for the camp (drinking, cooking, showers) and sometimes for drilling.
During construction, a great amount of water is essential for dust suppression, making of concrete, etc, and again for the camp. According to the Australian Bureau of Statistics (ABS), in 2019-20, the Australian mining sector extracted 1.100 gigaliters of water – 1 100 000 000 000 liters. A mine water truck uses more water in one trip than each of us consumes in a year, just for dust suppression.
Mining operations need water to process ore and run camp operations. Once the mining reaches the production phase, water is required for the camp, tea for the hydrogeologists in the offices, dust suppression, mineral processing, slurry transport, fire control, flotation, grinding, screening, dust scrubbing, wash-down water, pump gland seal water, reagent mixing, cooling of equipment, and to meet the water needs of employees.
Ideally, reasonably good quality water is used at the mine site for dust suppression, which helps with managing excess groundwater. The amount of dissolved salts acceptable for dust suppression is set by the environmental department and it is closely related to the surrounding flora and fauna.
I don’t want to finish my story without recalling that after a long day of drilling, drillers and geologists enjoy a good drink of cold beer and, by the way, water accounts for 85 to 92% of the beer’s composition. Or, if we prefer to drink Coca-Cola, to produce one liter of Coca-Cola, 200 liters of water are needed. A hard-working driller needs a lot of shower water to spruce himself up before going to dinner.
For more information, get in touch with Mariajose Romero-Segura on LinkedIn