Types and Classifications of Springs

2.9.1 Types and Classifications of Springs

In general, a spring is any location at the land surface where groundwater discharges from an aquifer, creating a visible flow. When the flow is not visible, but the land surface is wet compared to the surrounding area, such a discharge of groundwater to the surface is called seep. A seepage spring is a term often used to indicate the discharge of water through numerous small intergranular openings of unconsolidated sediments (e.g., sand and gravel). They are usually marked by abundant vegetation and commonly occur where valleys are cut downward into the zone of saturation of a uniform water-bearing deposit.

A fracture (or fissure) spring refers to a discharge of water along bedding planes, joints, cleavage, faults, and other breaks in the consolidated (hard) rock. Geysers are springs in which at more or less regular intervals hot water and steam are ejected with force

GroundwaterSystem

Magnitude Discharge First

100 ft 3 /s or more

100 gal/min to 1 ft 3 /s

Fifth 10 to 100 gal/min Sixth

1 to 10 gal/min Seventh

1 pint per minute to 1 gal/min Eight

less than 1 pint per minute From Meinzer, 1923.

T ABLE 2.5 Classification of Springs Based on Average Discharge

from considerable depth. Geyser springs generally emerge from tubular conduits that are lined with silica, deposited by the water, and end at the surface in a cone of similar material.

There have been various proposed classifications of springs, based on different char- acteristics, of which the following are the most common:

r Discharge rate and uniformity r Character of the hydraulic head (pressure) creating the discharge r Geologic structure controlling the discharge r Water quality and temperature

Meinzer’s classification of springs based on the average discharge expressed in U.S. units is still widely used in the United States (Table 2.5). However, the classification based solely on average spring discharge, without specifying other discharge parameters, is not very useful when evaluating the potential for spring utilization. For example, a spring may have a very high average discharge but it may be dry or just trickling most of the year. It is therefore essential that a spring is evaluated based on the minimum discharge recorded over a long period, typically longer than several hydrologic years (hydrologic year is defined as spanning all wet and dry seasons in a full annual cycle). When evaluating the availability of spring water, it is important to include a measure of spring discharge variability, which should also be based on periods of record longer than one hydrologic year. The simplest measure of variability is the ratio of the maximum and minimum discharge called the index of variability (I v ):

Springs with the index of variability greater than 10 are considered highly variable, and those with I v ≤ 2 are sometimes called constant or steady springs. Meinzer (1923)

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proposed the following measure of variability expressed in percentage: Q max −Q min

where Q max ,Q min , and Q av are maximum, minimum, and average discharge respectively. Based on this equation, a constant spring would have variability less than 25 percent, and a variable spring would have variability greater than 100 percent.

Intermittent springs discharge only for a period of time, while at other times they are dry, reflecting directly the aquifer recharge pattern. Ebb-and-flow springs, or periodic springs, are usually found in limestone (karst) terrain and are explained by the existence of a siphon in the rock mass behind the spring that fills up and empties with certain regularity, regardless of the recharge (rainfall) pattern. Periodic springs can be permanent or intermittent. Estavelle has a dual function: it acts as a spring during high hydraulic heads in the aquifer, and as a surface water sink during periods when the hydraulic head in the aquifer is lower than in the body of surface water (estavelles are located within or adjacent to surface water features). Secondary springs issue from locations located away from the primary location of spring discharge, which is covered by colluvium or other debris and therefore not visible.

Springs are usually divided into two main groups based on the nature of the hydraulic head in the underlying aquifers that forces them to discharge to the land surface:

r Gravity springs emerge under unconfined conditions where the water table in- tersects land surface. They are also called descending springs. r Artesian springs discharge under pressure due to confined conditions in the un- derlying aquifer, and are also called ascending or rising springs.

Geomorphology and geologic fabric (rock type and tectonic features such as folds and faults) play a key role in the emergence of springs. When site-specific conditions are rather complicated, springs of formally different types may actually appear next to each other causing confusion. For example, a lateral impermeable barrier in fracture rock, caused by faulting, may force groundwater from a greater depth to ascend and discharge at the surface. This water may have high temperature due to the normal geothermal gradient in the earth’s crust—such springs are called thermal springs. At the same time, groundwater of normal temperature may issue at a spring located very close to the thermal spring. Yet a third spring may be present with water temperature varying between “hot” and “cold.” All three springs are caused by the same lateral contact between the aquifer and the impermeable barrier, and can all be called barrier springs, although the hydraulic mechanism of groundwater discharge is quite different.

Figure 2.104 shows several common spring types. In general, when the contact be- tween the water-bearing porous medium and the impermeable medium is sloping to- ward the spring, in the direction of groundwater flow, and the aquifer is above the impermeable contact, the spring is called a contact spring of descending type (Fig. 1.104a). When the impermeable contact slopes away from the spring, in the direction opposite of groundwater flow, the spring is called overflowing (Fig. 1.104b). Depression springs are formed in unconfined aquifers when topography intersects the water table, usually due to surface stream incision (Fig. 1.104c). Possible contact between the aquifer and the

GroundwaterSystem

F IGURE 2.104 Different spring types based on the hydraulic head and geologic controls. (From Kresic, 2007a; copyright Taylor & Francis Group, LLC, printed with permission.)

underlying low-permeable formation is not the reason for spring emergence (this contact may or may not be known). Figure 1.104d–f shows some examples of barrier springs, the term generally referring to springs at steep (vertical) or hanging lateral contacts between the aquifer and the impermeable rock. When such contact forces groundwater to ascend under hydrostatic pressure, i.e., because the hydraulic head in the aquifer is higher than the land surface elevation at the spring location, the spring is called ascending or artesian. Artesian springs are usually caused by tectonic structures (faults, fractures, and folds) and often have steady temperatures and discharge, because they are not directly exposed to the atmosphere and recharge from precipitation. Thermal springs are almost always ascending. Figure 2.104g shows both ascending and descending springs in fractured rock aquifers.

Meinzer (1940) gives this account of large springs in the United States: According to a study completed about 10 years ago, there are in the United States 65 springs of the

first magnitude. Of these springs, 38 rise in volcanic rock or in gravel associated with volcanic rock, 24 in limestone, and 3 in sandstone. Of the springs in volcanic rock or associated gravel 16 are in Oregon,

15 in Idaho, and 7 in California. Of the springs in limestone, 9 rise in limestone of Paleozoic age, 8 of them in the Ozark area of Missouri and Arkansas; 4 are in Lower Cretaceous limestone in the Balcones fault belt in Texas; and 11 are in Tertiary limestone in Florida. The 3 springs that issue from sandstone are in Montana. The great discharge of these springs is believed to be due to faults or to other special

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features. With the additional data now available, some revision of these figures could be made but it would be of minor character.

Since this account by Meinzer and the USGS, the numbers have changed due to more precise flow measurements and contributions of other agencies and investigators across the country. In Florida alone there are 33 documented first-magnitude springs and nearly 700 other significant springs (Scott et al., 2004). Florida represents perhaps the largest concentration of freshwater springs on the earth. Other regions of the world with large springs are also located in karst areas such as the Dinarides (the Balkans), the Alps in Europe, France, Mediterranean countries, Turkey and the Middle East, and China (Kresic and Stevanovic, in preparation).