Subsurface water drips from pore to pore until it reaches the water table. It then moves on through water-saturated pores, in response to the distribution of pressures from areas of infiltration to zones where the groundwater debouches through springs, or through the beds of rivers, lakes or the ocean.
The frictional resistance to water movement from one pore to the next is enormous: where bed friction can moderate river flows by an order of magnitude, in groundwater flow systems the frictional resistances vary over 13 orders of magnitude! Not surprisingly, therefore, where we quantify river water velocities in metres per second, groundwater velocities are more typically millimetres per day or even less.
Although there are exceptions to this, notably in limestone caves and in similar man-made features such as abandoned mines, the vast majority of groundwater flow systems can be adequately analysed by combining mass conservation calculations with the determination of velocities using an empirical formula known as Darcy’s law. This simply states that the flow rate of groundwater is directly proportional to the change in groundwater ‘head’ (i.e. the sum of elevation and water pressure) along a flow path of a given length, with the coefficient of proportionality (with dimensions of L/ T) being a description of the frictional resistance to flow through the rock pores. This coefficient is usually called the hydraulic conductivity, or, more loosely, the coefficient of permeability, and it varies from values below 10-5 m/ d in mudstones to more than 104 m/ d in coarse gravels.
One peculiarity of groundwater flow is that most groundwater systems end up ‘pushing water uphill, wherever it approaches a zone of discharge to surface, such as a spring or permeable river bed. Many so-called artesian wells are simply due to boreholes intersecting zones of higher driving head below river valleys.
Truly artesian conditions occur where a permeable layer of water-bearing rock (an aquifer) containing water under high pressure underlies a less permeable layer (or aquitard), so that a well penetrating the aquitard will encounter water that rises naturally up the well under pressure.
The endless surging of the sea
Circulation of water in the oceans is predominantly driven by four forces:
The first one is the gravitational attraction of the Moon, which accounts for tides
Second is the wind, which drives most waves and gives rise to many currents
Third is heat, as the warming of the sea in tropical zones gives rise to vast convection currents
And fourth one is the salinity, where large inputs of freshwater from major rivers and melting ice sheets create vast gradients of density (and viscosity), leading to currents driven by imbalances in buoyancy
However, you don’t have to worry about the salinity of the sea water or the factors affecting its quality. Because you are a smart customer who has already got it water supply registered with Northumbrian water and in case you feel that there is a problem with the quality of water supply in your home, you can directly get in touch with their dedicated team at Northumbrian Water Contact Number.