The velocity is very small in the laminar range—of the order of 1 cm/sec—and the Reynolds number (Re) for ground water flow varies from 1 to 10 and is given by- These maps define the potentiometric surface, which is much like a topographic contour map but defines the distribution of potential energy in the groundwater system. Although the rate of decline of head decreases continuously as the area of influence expands, but this results in the variation of the cone of depression with time and therefore unsteady or transient flow exists within the aquifer. If the water meets the water table (below which the soil is saturated), it can move both vertically and horizontally. In order to define groundwater flow directions and rates through aquifers, individual measurements of hydraulic head are combined to generate contour maps of water level – or potential energy (Figure 29). Some of the precipitation that falls onto the land infiltrates into the ground to become groundwater. Groundwater is stored in and moves slowly (compared to surface runoff in temperate conditions and watercourses) through layers or zones of soil, sand and rocks: aquifers. The rate of groundwater flow depends on the permeability (the size of the spaces in the soil or rocks and how well the spaces are connected) and the hydraulic head (water pressure). In order to analyse the unsteady flow towards wells the following differential equation has been derived. Ground water moves from levels of higher energy to levels of lower energy, its energy being essentially the result of elevation and pressure, the velocity heads being neglected since the flow is essentially laminar. Groundwater flow rate can then be calculated using Darcy’s law, which says that the flow rate is linearly proportional to the hydraulic gradient:  q = − ρ g k μ (∇ h) where q is the Darcy flux, or flow rate per unit surface area, and μ is fluid viscosity. in which . groundwater flow. depths, where the rate of flow is extremely slow, groundwater is saline, with concentrations ranging up to ten times the salinity of the sea. Seepage problems concern the percolation of water through dams and into excavations, and the movement of water into and through the soil from bodies of surface water such as canals, streams, or lakes. Before the industrial revolution, the main risk to groundwater quality was from bacteria and viruses. Academia.edu is a platform for academics to share research papers.