Geological Origin of Springs
All spring water ultimately originates from precipitation (Figure 2). Rain and melted snow seep into the ground where they collect as groundwater in porous rock material (aquifer) lying above a relatively impervious layer of rock (confining bed). The top of this layer of groundwater is called the water table. By gravity, pressure, and (or) other forces, the groundwater flows along paths of least resistance, often emerging as springs wherever the water table intersects the land surface. A spring may emanate from water saturated soil or porous rock, from a fracture, fissure or cave, or along a contact zone between two rock types of different permeability, often at the bottom of or along steep slopes of the land surface (including hills and mountains). In general, aquifers gain water (are recharged) by precipitation and may lose water (are discharged) by spring flow, though there is a lag time between these two processes because of underground water storage. For example, after a rain storm, a spring brook may show little immediate change in flow rate, whereas a nearby runoff stream will quickly (within a few hours or less) become engorged with rapidly flowing water (Figure 3). Springs associated with small, shallow aquifers or those having poor storage capacities (as in shale-siltstone areas) tend to show greater variability of flow than those associated with large, deep aquifers or those having high storage capacities and long water residence times (as in some karst terrains containing numerous underground pockets and passage ways in carbonate rocks such as limestone). Over relatively long periods of time (weeks, months or more), the amount of springwater flow (discharge) varies with aquifer (recharge) size and the local balance between precipitation and evapotranspiration (loss of water to the atmosphere by evaporation from vegetation, the landscape, and bodies of water). Some karst springs show daily or annual rhythms of outflow rate, the former being the result of the filling and draining of an internal siphon, and the latter the result of seasonal changes in regional water balance (precipitation minus evapotranspiration). Others show irregular changes in discharge rates. Because of variable storage times for groundwater, spring water varies greatly in age from less than a day to over 10 000 (and possibly over a million) years old.