Hydrologic Setting of the Area
Valley and ridge aquifer system extends from Southwest region of Pennsylvania to Northeast region of Virginia. The aquifer system is the major water supply for the regions. Its hydrologic settings are formed mainly due to limestone that is exposed in the core of anticline of valley and ridge aquifer. These regions receive heavy rainfall of as high as 43 inches on average annually, which provides 13 inches of water to the aquifer system, and through a proper mechanism, the rainfall can result in the recharge of the aquifer. Valley and ridge aquifer system is composed of layered Paleozoic sedimentary strata that are folded and deformed by a series of tectonic collisions during about 200 million years ago. The collisions cause fracture that has high permeability and contains holes, which allows the rapid recharge of aquifer system by rainwater. The water runoffs to less permeable regions such as shales and sandstone and is collected as groundwater above the limestone at the valley edge (Swain, Mesko, & Hollyday, 2004). The water collected in the aquifer is closely linked to surface water bodies, such as lake, river, and wetlands. This relationship is, however, affected by the position of surface water bodies in relative to the aquifer system and its geological characteristics and the climate of the region (Winter, 1999). Overall, the valley and ridge aquifer system is formed by the permeable rocks folded and layered between each other, allowing the recharge by rainwater, making it a rich source of drinking water.
Hydrologic and Geologic Properties of the Aquifer
Hydrologic and geologic properties of the valley and ridge aquifer system provide information on the types and properties of rocks that constitute the aquifer. Hydrological properties of the aquifer indicate that it is an unconfined aquifer because during its formation, the pervious layers of rocks were at the top surface, while the impervious layers were at the bottom. Therefore, water that enters at the top of the aquifer percolates downward to the impervious layers and forms a saturated zone. Total thickness of valley and ridge aquifer is about 10,000 feet. Other properties of the aquifer include: areal transmissivity (T) – the rate of flow under a unit hydraulic gradient through a unit width of aquifer of a given saturated thickness; storage coefficient (S) – the volume of water released from storage per unit declined hydraulic head of the aquifer; areal diffusivity (T/S) – the ratio of T and S; and recession index (K) – time taken by groundwater discharge to decline by one log cycle after the recession curve becomes nearly linear on the semi-log hydrograph. These parameters are the fundamental hydraulic parameters used to analyze groundwater potential of the aquifer (Rutledge & Mesko, 1996). The values of T, S, T/S, and K for the valley and ridge aquifer system are 140 ft2/day, 0.01, 1400 ft2/day, and 7 days, respectively (Hoos, 1990). On the other hand, the geologic properties of the valley and ridge aquifer system reveals that the aquifer is formed primarily by carbonate rocks that are in naturally metamorphic, igneous and sedimentary. These rocks consist of inter-granular porosity, fractures and voids, which give them the permeability, generated during the formation process of the valley and ridges. Moreover, to explain the chemical nature of the groundwater of valley and ridge aquifer, the concentration of dissolved solid, pH value, and dissolved ions should be determined. The concentration of dissolved solid of the aquifer is 226 mg/L, which is considerably high due to that a large amount of solid particles from the sediment is dissolved in groundwater. The aquifer also contains groundwater that is slightly basic due to large concentrations of calcium, magnesium, sulfate, bicarbonate, nitrate, and dissolved iron in the groundwater with a pH of 7.3 and hardness caused by dissolved CaCO3 of 149 mg/L. Lastly, the rocks contain large amounts of either pyrite or gypsum and sulfate which make it highly anionic (Chambers & Donahue, 1996). Thus, the valley and ridge aquifer has diverse hydrologic and geologic properties.
The Aquifer as a Water Resource
The aquifer is one of the largest sources of drinking water in most of the areas, which it covers. Around 115 million people rely on the ground water from this aquifer to use for various purposes, such as household, agricultural, and industrial activities. Water from the aquifer is rich in mineral, which makes it useful for such activities. However, excessive exploitation of water from the aquifer causes it to become contaminated. Contamination of ground water of aquifer naturally occurs due to the dissolution of natural rocks, which causes the increase in several harmful elements, such as calcium, magnesium, silica and fluoride. Higher levels of such elements can make the water to be unsafe for drinking as they may cause threat to human health. High level of sulfide has been found to produce an extreme odor in the valley and ridge aquifer. In addition, excessive bacterial growth causes contamination of groundwater of the aquifer. The contamination of water in aquifer also occurs due to human activities, such as the use of pesticides and fertilizers in farming. These activities cause these substances to leach into the ground and mix with groundwater of the aquifer. Pollutants emitted by factories, which can include bacteria from septic tanks, petroleum products, and hazardous chemicals, has also bee found released into the water of the aquifer. The source of contamination can be close to or far away from the aquifer as the contaminants can bleach into ground water, thereby access the water reservoir of the aquifer (Lindsey et al., 2014). Therefore, aquifers are used as the water resource in all areas where it extends to for various activities. However, it is prone to contamination by such activities.
Potential or Actual Ground-Water Contamination
Valley and ridge aquifer is contaminated. Various reports have shown that different pesticides were found in water obtained from the aquifer. The rate of contamination is 40% in rural areas while that is 80% in urban areas, and more than five types of pesticides were found. The impact of these pesticides on human health remains unknown. In addition, the aquifer was contaminated by nitrate and phosphorous, which are harmful when ingested by humans as they can cause serious health problems. These compounds cannot be detected easily; however, they do not have any impact if the water is boiled. Thus, various steps have been taken by the government to protect the aquifer that is located near the agricultural and industrial areas. The Ground Water Site Inventory and the National Water Information System have been formed to collect important information about the aquifer and maintain the database. Other problems related to the valley and ridge aquifer are controlled under the U.S. Department of Geological Survey (Swain et al., 2004).
Chambers, M., & Donahue, J. (1996). Ground-Water Quality in Georgia for 1992 [PDF file]. Retrieved from https://epd.georgia.gov/sites/epd.georgia.gov/files/related_files/ site_page/C-12I.pdf
Hoos, A. (1990). Recharge rates and aquifer characteristics for selected drainage basins in Middle and East Tennessee. Water-Resources Investigations Report. https://www.researchgate.net/publication/272790112_Recharge_Rates_and_Aquifer_Characteristics_for_Selected_Drainage_Basins_in_Middle_and_East_Tennessee
Lindsey, B. D., Zimmerman, T. M., Chapman, M. J., Cravotta, C. A., & Szabo, Z. (2014). Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993-2009 [PDF file]. Retrieved from https://pubs.usgs.gov/circ/1354/pdf/circ1354optimized.pdf
Rutledge, A. T., & Mesko, T. O. (1996). Estimated hydrologic characteristics of shallow aquifer systems in the Valley and Ridge, the Blue Ridge, and the Piedmont Physiographic Provinces based on analysis of streamflow recession and base flow [PDF file]. Retrieved from https://pubs.usgs.gov/pp/1422b/report.pdf
Swain, L. A., Mesko, T. O., & Hollyday, E. F. (2004). Summary of the Hydrogeology of the Valley and Ridge, Blue Ridge, and Piedmont Physiographic Provinces in the Eastern United States [PDF file]. Retrieved from https://pubs.usgs.gov/pp/pp1422A/PDF/pp1422 Apart1.pdf
Winter, T. C. (1999). Relation of streams, lakes, and wetlands to groundwater flow systems. Hydrogeology Journal, 7(1), 28-45. https://doi.org/10.1007/s100400050178