Abstract
The Cretaceous aquifers underlying the central part of the North Carolina Coastal Plain are used extensively as the preferred source of water for municipalities, industries, and agriculture. Regional decline in head in the middle parts of the system (the Upper Cape Fear and Black Creek aquifers) began in the late 1950s to early 1960s. The trends in head during the 1960s, together with data on land subsidence, were analyzed to demonstrate the value of using early data, in conjunction with conceptual models, before collecting additional data. The sources of withdrawals from the Cretaceous aquifers identified in the conceptual model are: (1) movement of water (recharge) across confining beds, (2) reduction in natural discharge, (3) release of water from aquifer storage, (4) drainage of water from fine-grained lenses in aquifers and confining beds, and (5) release of water resulting from advance of the salt-water front. Estimates of the amounts derived from each of these sources resulted in a reduction of the estimated safe yield of the Cretaceous aquifers in the 2,486-mi2 (6,439-km2) area of the central Coastal Plain from 75 mgd (million gallons per day) (284,000 m3/day) to less than 5 mgd (19,000 m3/day).
Résumé
Les aquifères crétacés de la partie centrale de la plaine côtière de Caroline du Nord sont largement utilisés comme source d'eau préférée pour les collectivités, les industries et l'agriculture. La diminution régionale des charges dans les parties médianes du système (les aquifères de Cape Fear supérieur et de Black Creek) a débuté entre la fin des années 1950 et le début des années 1960. Les tendances des charges au cours des années 1960, associées aux données sur la subsidence des terrains, ont été analysées pour démontrer l'intérêt d'utiliser des données précoces, en association avec des modèles conceptuels, avant de récolter des données supplémentaires. Les termes d'écoulement dans l'aquifère crétacé identifiés dans le modèle conceptuel sont les suivants: (1) la drainance (recharge) au travers des niveaux de confinement, (2) la réduction de la décharge naturelle, (3) le déstockage de l'eau de l'aquifère, (4) le drainage de l'eau des lentilles à faible granulométrie dans les aquifères et les niveaux de confinement, et (5) la libération d'eau sous l'effet de la propagation du biseau salé. Les estimations des quantités provenant de chacune de ces sources ont conduit à réviser à la baisse l'estimation du débit d'exploitation optimal des aquifères crétacés, couvrant une surface de 6.439 km2 dans la partie centrale de la plaine côtière, de 284.000 m3/j à moins de 19.000 m3/j.
Resumen
Los acuíferos Cretácicos situados bajo la zona central del Llano Costero de Carolina del Norte (Estados Unidos de América) son utilizados ampliamente como suministro de agua para municipios, industrias y agricultura. El descenso regional del nivel pizométrico en las zonas centrales del sistema—acuífero superior de Cape Fear y de Black Creek—comenzó entre finales de la década de los cincuenta (1950) y principios de la década de los sesenta (1960). Se ha analizado las tendencias de los niveles durante los años sesenta y los datos de subsidencia para demostrar el valor de los datos iniciales, junto con los modelos conceptuales, antes de proceder a la adquisición de más datos. Los tipos de extracciones identificados en los acuíferos Cretácicos son: (1) el movimiento de agua (recarga) a través de capas confinantes; (2) la disminución de la recarga natural; (3) la obtención de agua del almacenamiento en el acuífero; (4) el drenaje de lentejones de grano fino en acuíferos y capas confinantes; y (5) la liberación de agua como consecuencia del avance del frente de aguas salinas. Se estima que las extracciones asociadas a estos conceptos significan una reducción del valor de garantía del almacenamiento en los acuíferos Cretácicos, que pasarían de valer 284.000 m3/d (75 mgd) a menos de 19.000 m3/d (5 mgd) en los 6.439 km2 (2.486 mi2) que conforman la llanura central costera.
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Acknowledgments
The authors appreciate reviews provided by Harry LeGrand, Ted Mew, Charles Daniel, Perry Olcott, Joe Rosenshein, Frank Trainer, Warren Wood, and one anonymous reviewer.
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Heath, R.C., Spruill, R.K. Cretaceous aquifers in North Carolina: analysis of safe yield based on historical data. Hydrogeology Journal 11, 249–258 (2003). https://doi.org/10.1007/s10040-002-0242-4
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DOI: https://doi.org/10.1007/s10040-002-0242-4