Box 2 Springs
In the most general terms, a spring forms where concentrated groundwater discharges at the land surface. The Groundwater Project book, “Groundwater in Our Water Cycle: Getting to Know Earth’s Most Important Fresh Water Source” (Poeter et al., 2020) discusses the connection of groundwater with springs. Springs have been utilized throughout history as water supplies for homes, cities, sources of agricultural water, and have provided the underpinning of unique ecological systems since the dawn of civilization. Springs can be present at the discharge areas of local, intermediate, and regional flow systems (Figure 11). When springs are associated with large scale groundwater systems the discharge, water temperatures, and quality remain fairly constant because the long flow paths and residence time of the groundwater moderates its condition before reaching the spring. In contrast, spring discharge, temperature, and quality are more variable when springs are connected to local flow systems with short flow paths and residence time. Understanding the hydrogeologic conditions that account for springs provides the hydrogeologist with insight needed to answer questions such as: If a large mine is being dewatered will nearby springs be impacted? Will a spring provide water needed to open a water bottling plant? Can impacted springs associated with the formation a wetland be remediated to enhance the discharge to the wetland? The answers to these questions are not directly provided in this material; however, conceptual models of the occurrence of springs in a variety of hydrogeologic settings are described. Springs occur under a number of hydrogeologic settings and general types include: depression springs, contact springs, joint and fracture springs, fault and shear zone springs, and karst springs (Figure Box 2-1). Sources of springs originate from unconfined and confined groundwater systems. Bryan (1919) provides a classification of springs and textbooks such as Fetter (2001) and Todd and Mays (2005) describe how springs fit into the hydrogeological landscape.
Depression springs are found in topographic lows where the water table intersects the land surface (Figure Box 2-1a). When springs occur on slopes, they are associated with a zone of discharging groundwater that forms a seepage face, a wet area above the spring pool where groundwater is seeping from the surface. The source of water at depression springs are usually local flow systems so they typically have variable discharge rates in response to local recharge, however, they may occur in regional topographic lows where large regional groundwater systems, both confined and unconfined, terminate.
Contact springs occur when water percolating through rocks or sediments encounters a lower permeable material that underlies a more permeable formation (Figure Box 2-1b). If recharge is sufficient, a saturated zone builds up above the contact between the two formations. The water table in the permeable formation can be part of a completely saturated system or part of a perched system. A spring and seepage face occurs on the slope face between where the water table intersects the land surface and the geologic contact. These springs are typically generated by local flow systems.
Springs can also occur in saturated portions of jointed and fractured rocks (Figure Box 2-1c). Spring discharge in some fractured and jointed geological material occurs relatively near the surface as fractures receive local recharge, become saturated, and discharge is focused where features intersect the land surface. However, in some cases large units of jointed and fractured rocks act as equivalent porous media and larger groundwater flow systems develop and form springs in areas where joints or springs intersect the land surface.
Springs are also associated with faults and complex shear zones (Figure Box 2-1 d and e). Water table springs are formed when faulting results in juxtaposition of permeable material with less permeable material so that flow discharges as a spring at the fault interface where the water table is at land surface (Figure Box 2-1d). Faults and shear zones can be either less permeable, more permeable than the surrounding material or provide a combination of permeabilities (Figure Box 2-1e). When fault gouge is created, the fault has low permeability and inhibits groundwater flow. In some cases, the fault and its surrounding area are highly fractured, and the permeability is enhanced. These conditions tend to concentrate groundwater flow and discharge. When these fractured zones extend to depth, confined groundwater systems are often intercepted and when the zone is permeable, it provides an avenue for deeper groundwater to discharge at springs on the land surface. When the potentiometric surface is above the land surface, springs can occur along the fault trace. The water in springs receiving discharge from deep groundwater systems often has a fairly constant discharge and chemical composition, and water is warmer than local groundwater (hot springs occur when water temperatures are above 37°C).
When terrains are composed of soluble formations such as limestones and dolomites, karst springs can form where the water table or potentiometric surface exceeds the land surface elevation, creating depression springs (Figure Box 2-1f). When water levels in solutioned cavities and collapse features such as sinkholes occur in water table depressions, sinkhole springs are formed. Large karst springs can also occur at sinkholes or solution cavities that are connected to networks of saturated conduits in which heads are greater than the land surface elevation.
The water quality of springs varies with the character of the groundwater source. Springs supplied by short groundwater flow paths, like those of local flow systems, are impacted by seasonal changes in recharge, and water typically contains a low quantity of total dissolved solids. Springs receiving intermediate and regional flow have higher total dissolved solids and their quality is more consistent. Spring quality can also be impacted by the composition and solubility of the components of the earth materials that the discharging groundwater passed through. In some settings local flow systems can form springs with high total dissolved solids even though the flow path and water residence times are short.