If the grass in your yard and neighborhood is anything like ours, it has seen some major changes over the last few months as we have gone from a period of drought back into our normal summer afternoon rain showers. These fluctuations are all-too common, and we thought it appropriate to provide some general information on groundwater and hydrogeology in North Carolina. Where is groundwater stored? How does it vary across the state? What exactly are the differences between the water table, fractured rock aquifers, sand aquifers, confined and unconfined systems? This month we provide a general overview of these terms, and groundwater behavior within our state.
What is an aquifer?
The United States Geological Survey (USGS) defines an aquifer as “a formation, group of formations, or part of a formation that contains sufficient saturated, permeable material to yield significant quantities of water to wells and springs.” A confined aquifer is overlain and underlain by layers of impermeable material, causing it to be under pressure. In contrast, an unconfined aquifer is an aquifer whose upper surface is at atmospheric pressure, lacking the impermeable layer associated with a confined system. How is the water table different? The water table is simply the top of an unconfined aquifer, the depth at which the soil or rock moves from being dry/unsaturated to wet/saturated. The water table depth can fluctuate throughout the seasons, moving deeper during dry periods and raising during periods of increased precipitation. The aquifer itself is the entire volume of water stored beneath the water table, or within a confined system.
North Carolina has the fourth highest number of private water well users in the country, with more than 50% of its residents obtaining their drinking water from private wells. These wells tap directly into the local aquifer systems, and understanding how they work and vary across the state is crucial for water conservation and well design.
How does groundwater and the aquifer system vary in North Carolina?
As we know from previous Pyramid newsletters, North Carolina’s geologic regions are generally separated into the western mountain/Blue Ridge zone, the central Piedmont zone, and the eastern Coastal Plain zone. The aquifer system in the state is directly linked to these geologic regions. The aquifers in the western 60 percent of the state (spanning the Piedmont and Blue Ridge zones) are typically zones of complex fractured rocks of the Triassic Basin overlain by regolith (soil, alluvium, rock). The aquifers in the eastern 40 percent of the state are typically coastal sediment (sand) and limestone aquifers associated with a wedge of sedimentary rock layers dipping to the southeast in the Coastal Plain zone. (USGS)
Fractured Rock Aquifers in Western & Central NC
The aquifers of the Piedmont and Mountain regions are localized, complex fractured rock aquifers where the igneous, metamorphic and sedimentary rock units are overlain by porous regolith (see above). In most cases, the producing aquifer is located within the shallow porous regolith above the crystalline rock units; however, fractures deeper within the bedrock can contain producing volumes of groundwater. The groundwater depths, volumes, and lateral distribution are extremely variable throughout the Piedmont and Mountain regions due to the complex geology, thereby resulting in the “localized” aquifer hydrogeologic characteristics.
Coastal Plain Aquifers
In contrast to the localized characteristics of aquifers in the western portion of the state, the Coastal Plain aquifer system is more regional in its scale and behavior. These aquifers are typically porous sand and limestone aquifers. Generally speaking, an unconfined surficial aquifer spans the region, underlain by a series of confined aquifers separated by impermeable layers. Most residential wells obtain their water from the lower-yielding surficial aquifer, while larger municipal and industrial wells drill deeper into one of the upper confined aquifers to produce larger volumes of water.
The water table depth measured in water wells across the state can fluctuate dramatically on a seasonal and longer-term basis. These fluctuations are due to period of increased and decreased recharge to the aquifer system. Recharge typically comes from periods of snow melt or rainfall which percolates through the soil and into the aquifer system. Pyramid has performed hundreds of site assessments and long-term studies associated with remediation projects in NC that have allowed us to observe these types of water level fluctuations. The chart below shows an example of water level data in a monitor well in central NC collected from 2001-2013. Fluctuations of as much as 20 feet in the calculated groundwater depth are observed over this 12 year period. These fluctuations help to show the importance of designing wells properly, and locating them in the proper area and depth to mitigate natural water level variance over time.
Groundwater is one of the most important resources to the residents of North Carolina, and understanding its behavior is crucial to maintaining this resource properly for future generations. Please click through the links in this article to learn more about groundwater in NC, and contact us today with any questions!