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Groundwater and the Water Cycle (Pennsylvania Example)

Potable water is the most valuable and under-appreciated resource of our planet.

Why? Because in many locations, the "aquifer" is hundreds of feet below ground and extends over a vast area that includes multiple municipal and state boundaries. Over 70 percent of the earth's surface is covered with water, but < 0.5 % of this water is a usable freshwater resource. All the remainder of the water is salt water. The water on this planet moves through a cycle that is the ultimate solar power system and the cycle is in dynamic equilibrium. Water is constantly changing position, phase, and form, see Figure 1.

Diagram of the water cycle
Figure 1. Water Cycle

This dynamic cycle causes the water to be in motion which aids in the remediation/purification process, influences the movement of critical ions and elements through the ecosystem, and supports critical nutrient and energy cycles. One of the most misunderstood and poorly characterized components of this cycle is known as groundwater. This misunderstanding and lack of awareness is because groundwater can not be readily or easily seen or visualized and there are multiple biochemical processes that cause the water to transform. For information on the water cycle, please view this Power Point Image

Table 1 is a breakdown of the number and percent of homes that are serviced by a private water system in Pennsylvania. In most cases the private water system is a groundwater well.

Table 1 | PA Census Data

Number of Homes Served by Private Water Systems

County 1980 1990 2000 Avg. Change in homes served by private water systems per year % of all homes served by public water % of all homes served by private water system
Bradford 13,443 16,865 20,287 342 37 63
Carbon 6,594 12,235 17,876 564 55 45
Lackawanna 9,952 12,745 15,538 279 86 14
Luzerne 19,994 24,662 29,330 467 82 18
Monroe 21,129 37,246 53,363 1612 32 68
Pike 9,441 16,875 24,309 743 45 55
Sullivan 2,147 4,727 7,307 258 13 87
Susquehanna 9,423 15,212 21,001 579 25 75
Tioga 9,126 11,888 14,650 276 35 65
Wayne 9,913 19,097 28,281 918 33 67
Wyoming 7,236 8,657 10,078 142 27 73
Region Total 118,398 180,209 242,020 562 43 57

(Source: PA Census Data)

The primary sources of usable water in Pennsylvania include rainwater, stream inflow from other states, surface water (stored in lakes, streams, and ponds), and groundwater. In 2012 it was estimated that Pennsylvanians used approximately 54.18 billion gallons of water per day and there is approximately 80 trillion gallons of groundwater and only 2.5 trillion gallons of surface water. Below the freshwater aquifer, the bedrock contains salt or brine water. This brine water is likely water that was trapped in the formation or material during deposition.

Because of the rural nature of Pennsylvania, groundwater provides approximately 85 percent of the water used for human consumption, but because it is difficult to see how water moves through the soil, unconsolidated material (sand and gravel), and bedrock, it has mystified individuals. Some homeowners believe that the groundwater comes from a vast underground lake or from underground streams that come from Canada, Virginia, Vermont, or even Maine. Even though there is a large database of information on groundwater in Pennsylvania, it still is difficult to really document the total available water resource and its actual movement and quantity without implementing a very elaborate system of monitoring wells, observation points, and background water quality data.

Hydrologic Cycle

For Pennsylvania, the annual precipitation ranges from 30 to 60 inches per year with a mean rainfall of approximately 41 inches. Approximately 55 to 60 percent of the precipitation occurs during the warmer months. Of this about 20 inches or more is returned to the atmosphere via evapotranspiration (ET) or evaporation, 12 to 15 inches infiltrates into the groundwater system, and direct runoff accounts for approximately 6 to 12 inches of water. Groundwater storage in Pennsylvania is equivalent to approximately 100 inches of water, but a more conservative estimate is 47 trillion gallons (60 inches), of which, 9 to 12 trillion is naturally discharged to springs, seeps, streams, and lakes, see Figure 2. Therefore, groundwater is not only used for drinking water, but the discharge of groundwater to the surface and near-surface provides the necessary baseflow to support the aquatic habitats in Pennsylvania.

Diagram of the average water budget for Pennsylvania

Figure 2 | Average Water Budget for Pennsylvania (Source: PSU, 2007).

The hydrologic cycle describes the constant movement of water above, on, and below the earth's surface. As part of this cycle, water is transformed between liquid, solid, and a gas state. Condensation, evaporation, and freezing of water occur in the cycle in response to the earth's climatic conditions. Figure 2 is a representation of the general hydrologic cycle that directly affects Pennsylvania.

The hydrologic cycle can begin with water evaporation from the earth's soil, plant, and water surfaces to form water vapor. The energy required to evaporate water is supplied by the sun – therefore the system is Solar-Powered, see Figure 3. Most of the evaporation occurs near the equator in the open ocean. It is estimated that 39 inches of water annually evaporate from each acre of ocean. Water vapor is drawn into the atmosphere and can be transported over hundreds of miles by large air masses (atmospheric advection). When water vapor cools, it condenses to form clouds. As water condenses within clouds, water droplets increase in size until they fall to the earth's surface as precipitation such as rain, hail, sleet, or snow.

Approximately 50 to 90 percent of the water that falls to the earth's surface enters (infiltrates) into the soil. This water can become groundwater, but most of it evaporates from the soil surface or is returned to the atmosphere by vegetation via evapotranspiration (ET). Some of the soil water may move laterally and emerge at the surface as springs or become part of the base flow of streams. Water that passes through the root zone may continue to move downward to become part of the groundwater. In soils with fragipans, claypans, or other low-permeable strata, this water may create a seasonal high or perched water table. The vertical distance water has to travel to reach groundwater table can range from a few feet to hundreds of feet. Water movement toward groundwater may take hours or years, depending on the depth to the aquifer and the characteristics of the unsaturated zone.

Diagram of the solar powered water cycle

Figure 3 | The Solar Powered Water Cycle - Cross-Section of Water Cycle System (Source: League of Women Voters- "Groundwater: A Primer for Pennsylvanians").

Diagram of the water cycle within a community

Figure 4 | The Water Cycle and Community
(Source: League of Women Voters- "Groundwater: A Primer for Pennsylvanians").

Occurrence of Groundwater

Groundwater is stored in the voids, spaces, and cracks between particles of soil, sand, gravel, rock, or other materials. These cracks or spaces can include fractures, faults, bedding planes, solution channels (limestone formations), dissolution channels associated with more easily-weathered material, or other structural features such as bed planes or deformation in the bedrock due to folding. These materials form what is sometimes called the groundwater aquifer or reservoir. In most areas of the world, and specifically in Pennsylvania, water does not flow in and is not stored in large underground lakes or rivers. The only exception to this might be the dissolution channels and caverns associated with limestone formations (karst), abandoned mining sites, and mine shafts and mine pools associated with underground mining operations.

The types of aquifers in Pennsylvania include: unconsolidated (sand and gravel deposits), sandstone, carbonate, and crystalline rock, see Figure 5. From a review of Figure 5, the major water-bearing aquifers in Pennsylvania are associated with sandstone and shale or sedimentary rock units.

Types of aquifers in Pennsylvania

Figure 5 | Types and Distribution of Aquifers in Pennsylvania
(Source: League of Women Voters- "Groundwater: A Primer for Pennsylvanians").

Near the surface the material can be divided into an unsaturated and a saturated zone. Recently, the unsaturated zone has been renamed the vadose zone to make it clear that this zone may at times be saturated. Water in the vadose zone can move under both saturated and unsaturated conditions. Under saturated conditions, gravity is the driving force, but under unsaturated conditions osmotic and matric forces are major influences, see Figure 6. Figure 7 depicts the potential relationship between a recharge area and a discharge zone and the influence of an aquitard (water can move through it but slowly and with difficulty). Figure 7 shows that at some point in the landscape the aquifer is exposed near the surface. Recharge enters that aquifer, but in some cases an aquitard, i.e., a formation with a permeability that is at least 10 times lower than the aquifer, acts as a confining layer. This confining layer causes the water to be directed downslope and causes pressure to "build-up" in the confined aquifer. If there is a fracture or weakness in the confining layer, the water will move up from the deeper groundwater zone and discharge to the surface or shallow groundwater aquifer (look at the arrows in Figure 7). Figure 7 also depicts the difference between an unconfined and a confined aquifer.

Diagram of the water table showing flow of water in a saturated zone

Figure 6 | The Water Table - Which way is water flowing? (Look at the Arrows).Which way is the Water Flowing in the Saturated Zone? In This Image - there is no horizontal water flow only vertical.
(Source: League of Women Voters- "Groundwater: A Primer for Pennsylvanians").

Diagram of water table showing confined and unconfined aquifiers and direction flow

Figure 7 | Confined and Unconfined Aquifers and Direction of Flow. Which way is water moving? (Down, Up, to the Left) - Water Under Saturated Conditions - Always Moves from an Area of High Head (pressure) to Areas of Low Head
(Source: League of Women Voters- "Groundwater: A Primer for Pennsylvanians").

Figure 8 demonstrates the creation of a perched water table and the relationship between a stream/wetland and the groundwater system. In this figure, the flow from the stream is supported by a discharge from a perched water table (at the surface this could appear as a spring or seepage) and a discharge from the unconfined aquifer. Because the confining layer was competent (good at stopping water flow), the confined aquifer does not discharge to this stream. If the confining layer was weak just below the stream location, it would be possible for water to "leak" up out of the confined aquifer to discharge at this stream. In this figure make sure to note the location of the watershed divides. Figure 9 depicts the relationship of the various water flow paths in a vertical section of the groundwater aquifer. It is important to note that the age of the groundwater discharging from the freshwater aquifer in some of our major waterways could be counted in centuries to millennia and that below the freshwater zone, the groundwater formation can contain salt or brine water.

Diagram showing groundwater and its relationship with surface water.

Figure 8 | Groundwater and Its Relationship with Surface Water (Source: League of Women Voters- "Groundwater: A Primer for Pennsylvanians").

Groundwater flow paths

Figure 9  | How Old is this Water I am Drinking? There is Salt Water in Pennsylvania.
(Source: DCNR, Education Series 3 - The Geology of Pennsylvania's Groundwater)

To understand how we can remove groundwater using wells we must understand how groundwater moves. Some people attempt to associate the flow of water on the earth's surface with groundwater movement. Surface water typically flows in rivers or streams at velocities of 2-8 miles per hour. Pennsylvania's groundwater moves through the spaces between particles of a saturated material at rates between 0.1 foot per day to 3 feet per day. That translates into a movement of 35 to 1,100 feet per year.

Groundwater moves only if sufficient pressure, or head, is available to force water through the spaces between porous aquifer materials. The rate of movement is determined by the hydraulic gradient, permeability, and porosity of the material. The hydraulic gradient, or slope of the water surface between two points in an aquifer, and the aquifer material determine how rapidly water moves from one location to another.

Groundwater moves from high water-surface-elevations (high pressure or head) to low water-surface-elevations (low pressure or head). In general, the water flows more rapidly where large differences exist in water-surface-elevations (steep hydraulic gradients), but this is not always the case. A large variation in the hydraulic gradient could also mean a lower-permeability formation. Groundwater may move toward or away from streams or lakes, depending on the hydraulic gradient. As groundwater moves it may be brought to the surface by a pumping well, or it may be discharged to the earth's surface as a spring, a lake, or stream. Groundwater supplies are recharged by precipitation or from rivers and lakes. Groundwater removed by wells or discharged by springs may have been underground for thousands of years, or may have entered the aquifer quite recently.

Factors Affecting Groundwater Declines

Under natural conditions, a balance exists between the volume of water entering an aquifer and the volume of water being discharged from an aquifer. Under natural conditions the water is discharged from the aquifer through evapotranspiration, seepages, streamflow, and direct discharge to bays/oceans. With the development of water wells, the natural balance between recharge rates and discharge rates is disrupted and an artificial groundwater discharge zone is created when water is extracted from the ground. As long as the artificial discharge is balanced by enhanced recharge at the surface, such as the use of on-site well and septic systems, facilitated or induced stormwater recharge, or high-volume treated-effluent recharge systems, the water cycle stays near balanced. If these additional man-made or influenced recharge systems are not established, the result of over-pumping or over-withdrawing water from the aquifer could cause lower base flows in streams, warmer streams, less aquatic habitat, high storm or peak flows in streams because of more runoff, and potential failure of the groundwater system because of settling of an unconsolidated formation, or induced contamination because of over-pumping.

Diagram of groundwater elevation, baseflow and recharge

Figure 10 | Groundwater Elevation, Baseflow, and Recharge.

(Source: DCNR, Education Series 3 - The Geology of Pennsylvania's Groundwater)

Just like streams, the water level in the groundwater aquifer changes throughout the year and from year to year. The groundwater elevation and amount of baseflow is directly influenced by the amount of precipitation and recharge. From Figure 10, it is apparent that, as the amount of recharge increases, so does the baseflow for the stream, but as the groundwater recharge rate decreases so does the baseflow for the stream.

Note: Baseflow is that part of streamflow derived from groundwater flowing into a stream. It usually makes up most of the flow of a stream most of the time. In comparison, Quickflow is part of a storm rainfall that moves quickly to a stream channel via surface runoff or interflow and forms a flood wave in the channel.

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