Coastal Plain Aquifer Information

The following information is derived from a compilation of the hydrogeologic framework of Maryland’s Coastal Plain (MGS Open-File Report No. 12-02-20: Maryland Coastal Plain Aquifer Information System: Hydrogeologic Framework) assembled as part of a long-term multi-phase investigation, conducted by the Maryland Geological Survey and the U.S. Geological Survey, with funding support from the Maryland Department of the Environment. It was initiated in response to recommendations of the 2004 Maryland Advisory Committee on the Management and Protection of the State’s Water Resources. Geophysical logs, published reports, and file data from 901 boreholes were used to define the surface altitudes of the tops of 16 aquifers (or aquifer systems) and 14 confining units, ranging in age from Lower Cretaceous to Holocene. The altitude of the top of pre-Cretaceous basement rock was also mapped. The Waste Gate aquifer system, occurring below the Patuxent aquifer and likely containing salt water, is not included in the list of aquifers below. Gridded arrays of the surface altitudes, aquifer extents, and outcrop/subcrop areas were created. In addition, hydraulic properties (transmissivity, hydraulic conductivity, and storage coefficient) were compiled from the literature for 296 wells, and properties for 307 wells were analyzed from file aquifer-test data. All data were input into a GIS-based aquifer information system.

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Surficial aquifer

The water-table aquifer is referred to as the Surficial aquifer on the Eastern Shore of Maryland. The Surficial aquifer is a major source of water supply. The largest use is for seasonal irrigation (agriculture); however, some municipalities (most notably the Town of Salisbury in Wicomico County), as well as many domestic users, also utilize the Surficial aquifer. In earlier reports, the Surficial aquifer was referred to as the Columbia aquifer; and, local to the Wicomico County area, as the Salisbury Formation (aquifer), which included deposits of the Salisbury Paleochannel.

GEOLOGY

Quaternary alluvium, Pleistocene-age Parsonsburg, Sinepuxent, Ironshire, and Omar Formations, the Pliocene(?)-age Beaverdam Sand, and the Miocene(?)-age Pensauken Formation

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GEOMETRY AND EXTENT

The aquifer is present over most of the Delmarva Peninsula in areas of higher topographic relief. In low-lying areas of the Eastern Shore along Chesapeake Bay, the aquifer is thin and clayey or absent altogether; however, channels in-filled with Quaternary sands may occur locally. The base of the Surficial aquifer is generally inclined towards the south, with altitudes in Maryland ranging from more than 70 ft above sea level in southern Cecil County to 199 ft below sea level near Salisbury. The basal surface is typically irregular with numerous depressions (paleochannels). Thickness of the Surficial aquifer varies greatly, ranging from about 10 ft in central and northern Eastern Shore to over 230 ft in central Wicomico County.

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HYDRAULIC PROPERTIES

Predominantly an unconfined, water-table aquifer throughout its extent. However, reduced vertical permeability caused by localized silt and clay layers may result in confined or semi-confined conditions in some areas. Transmissivity of the Surficial aquifer ranges from 510 feet squared per day (ft2/d) in Kent County to 53,500 ft2/d in paleochannel deposits in Wicomico County

Surficial Upland aquifer

The water-table aquifer on the western shore in southern Maryland is referred to as the Surficial Upland aquifer. It is a relatively minor aquifer used sporadically for domestic and farm supply.

GEOLOGY

Upland sand and gravel deposits of Pliocene(?) age

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GEOMETRY AND EXTENT

The Surficial Upland aquifer occurs over much of southern Maryland, although it is highly incised by stream channels, thus resulting in a very irregular pattern, with the thickest portions capping the higher elevation interfluvial divides. The base of the aquifer is hummocky, but overall dips gently to the southeast, with altitudes ranging from 250 ft above sea level in southern Prince George’s County to 28 ft above sea level in southern St. Mary’s County. Thickness of the Surficial Upland aquifer varies significantly due to topography, but can reach greater than 80 ft at its thickest occurrence in south-central St. Mary’s County.

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Pocomoke aquifer

The Pocomoke aquifer is an important source of water in Wicomico, Worcester, and Somerset Counties for larger users as well as domestic supply.

GEOLOGY

The Pocomoke aquifer is the shallower of two aquifers in the Chesapeake Group that overlie the St. Mary’s Formation. The aquifer correlates with an upper sand layer within the Eastover(?) Formation, which consists of gray, fine to medium, fossiliferous (shelly) sand, as well as glauconitic, fine sandy silts and brown to green clays.

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GEOMETRY AND EXTENT

The Pocomoke aquifer is present in the southeastern two-thirds of Somerset County and most of Worcester County. The aquifer pinches out updip in northeastern Worcester County. The altitude of the top of the Pocomoke aquifer decreases from its subcrop area to about 200 ft below sea level beneath Ocean City in Worcester County, Maryland. The Pocomoke aquifer is composed of individual sands 10 to 20 ft thick, which cumulatively reach a maximum thickness of over 100 ft at Ocean City.

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HYDRAULIC PROPERTIES

Transmissivity of the Pocomoke aquifer calculated at three sites in Worcester County ranges from 1,070 ft2/d at Pocomoke City to 9,170 ft2/d near Ocean City. A belt of above-average transmissivity values extends northeastward from Newark, Maryland to Isle of Wight Bay, near Ocean City. Storage coefficient ranges from 0.0002 to 0.003 in two wells tested at Pocomoke City. No aquifer-test data are available for the Pocomoke aquifer in Somerset County.

Ocean City aquifer

The Ocean City aquifer is an important source of water supply at Ocean City, Maryland.

GEOLOGY

The aquifer consists of the sandy portions of the Upper Miocene-age Ocean City beds, an informal name used to describe the sandy beds and associated clayey silts that lie above the Manokin Formation. The aquifer is characterized by fine to coarse, orange to tan sands with greenish-gray, glauconitic clayey silts and fine sands interbedded with silty clay. The aquifer contains lignite and shell material in places.

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GEOMETRY AND EXTENT

The Ocean City aquifer is present in Maryland in the eastern half of Worcester County and the easternmost portion of Wicomico County. The altitude of the top of the Ocean City aquifer ranges from about 150 ft below sea level in northern Worcester County near the Wicomico County boundary, to 254 ft below sea level south of Ocean City. The aquifer pinches out updip in eastern Wicomico County. The aquifer ranges from about 30 to 110 ft thick and dips at about 10 ft/mi. The aquifer is thickest at the Town of Ocean City.

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HYDRAULIC PROPERTIES

Transmissivity of the Ocean City aquifer calculated at eight sites in Worcester County ranges from 670 to 5,500 ft2/d. The most transmissive portion of the aquifer occurs in the fine to coarse sands that dominate the section in the southern portion of the Town of Ocean City.

Manokin aquifer

The Manokin aquifer is an important aquifer in Worcester, Wicomico, and Somerset Counties.

GEOLOGY

The Manokin aquifer consists of the sand layer immediately overlying the St. Mary’s confining unit. It is Miocene in age and part of the Chesapeake Group. This unit, described in earlier reports using hydrogeologic names, was referred to as the Manokin aquifer and “Manokin bed”. The Manokin was raised to formation rank in Delaware, which was later superseded by the Cat Hill Formation in part to avoid confusion between the lithostratigraphic unit and the aquifer unit. In some locations the aquifer may include contiguous sand beds of the St. Mary’s Formation. The aquifer is composed of gray, medium- to coarse-grained sand in Wicomico and Worcester Counties. Manokin aquifer sands in Somerset County are predominantly fine- to very-fine grained. Shelly beds are common in central Worcester County and eastward, but are rarely seen in the western parts of its extent such as Wicomico County.

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GEOMETRY AND EXTENT

The Manokin aquifer is present in Maryland in Wicomico, Worcester, and Somerset Counties. The altitude of the top of the aquifer decreases from its subcrop area in the western portion of Wicomico County to approximately 370 ft below sea level at Ocean City and southeastern Worcester County. Individual sands within the Manokin aquifer average 10 to 20 ft thick, with the greatest cumulative thickness reaching 195 ft in Worcester County. The aquifer generally dips to the southeast at about 5 to 10 ft/mi.

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HYDRAULIC PROPERTIES

Transmissivity of the Manokin aquifer ranges from 480 to 14,800 ft2/d. At Salisbury, transmissivity is as high as 7,440 ft2/d. Storage coefficient ranges from 2 x 10-4 to 1 x 10-3.

Choptank aquifer

The Choptank aquifer is an important source of water in Caroline, Dorchester, and Talbot Counties. Use of the aquifer is constrained in portions of the lower Eastern Shore by the presence of brackish or salt water.

GEOLOGY

The Choptank aquifer is composed of sands of the Choptank Formation of Miocene age. The Choptank aquifer on the lower Eastern Shore consists of gray medium- to fine-grained sands. On the western shore, the Choptank Formation thins and is undifferentiated from the underlying Calvert aquifer system. The Choptank Formation contains intermittent layers of coarse sand and small gravel as well as thin lenses of brown or blue clay. Shell marl and foraminifera microfossils are also present.

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GEOMETRY AND EXTENT

The Choptank aquifer is present in southern Caroline and Talbot Counties, central and eastern Dorchester County, and Wicomico, Worcester, and Somerset Counties. The unit outcrops along the Choptank River west of Cambridge and in stream cuts in central Talbot County. Southeast of the outcrop and subcrop areas the altitude of the top of the unit decreases to 725 ft below sea level at Ocean City. The aquifer ranges in thickness from about 12 ft to 170 ft in northeastern Worcester County.

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HYDRAULIC PROPERTIES

Transmissivity of the Choptank aquifer in Maryland is reported to range from about 500 to 1,400 ft2/d

Calvert aquifer system

The Calvert aquifer system is an important water supply in the central Eastern Shore of Maryland (Caroline, Dorchester, and Talbot Counties). The unit is a minor aquifer in southern Maryland. Use of the aquifer is constrained in portions of the lower Eastern Shore by the presence of brackish or salt water.

GEOLOGY

The Calvert aquifer system on the Eastern Shore of Maryland consists of the sandy portions of the Miocene-age Calvert Formation, but may also contain silty-sand layers of the St. Mary’s and Choptank Formation on Maryland’s western shore. Productive intervals within the system are composed of gray, fine- to medium-grained sands, often containing fossil shells. Sand intervals within the formation are grouped on the Eastern Shore of Maryland into an aquifer system due to the difficulty of regionally correlating the sands and also to their probable hydraulic connection. In Delaware, three discrete hydrologic units (from shallowest to deepest, the Frederica, Federalsburg, and Cheswold aquifers) are correlative to the Calvert aquifer system. In southern Maryland, the Calvert aquifer system is predominantly clay-rich, functioning only as a minor aquifer.

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GEOMETRY AND EXTENT

The Calvert aquifer system is present as a confined system over much of central and lower Eastern Shore in Maryland. The unit subcrops beneath the Surficial aquifer on the Eastern Shore in a band extending from northern Talbot County through northern Caroline County and southern and eastern Queen Anne’s County. Southeast of the subcrop area on the Eastern Shore, the altitude of the top of the unit decreases from approximately sea level to approximately 900 ft below sea level at Ocean City. Cumulative sand thickness within the Calvert aquifer system, determined from geophysical logs, averages about 90 ft, but can vary greatly among locations.

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HYDRAULIC PROPERTIES

Transmissivity of the Calvert aquifer system at three sites tested on the Eastern Shore of Maryland ranges from about 30 to 467 ft2/d. A storage coefficient of 1 x 10-4 was reported Talbot County.

Piney Point aquifer

The Piney Point aquifer is an important source of water supply in Calvert, Caroline, Dorchester, St. Mary’s, and Talbot Counties. It is also used to a limited extent in Queen Anne’s and Somerset Counties.

GEOLOGY

The Piney Point aquifer consists chiefly of the sandy portion of the Late Eocene-age Piney Point Formation. In southern Maryland, the Piney Point aquifer may include sands of the upper portion of the Nanjemoy Formation of Early Eocene age, the basal strata of the Calvert Formation of Miocene age, and a thin section of upper Oligocene(?) strata. The Piney Point Formation consists of medium to coarse, slightly glauconitic quartz sand. Colors range from olive green to greenish gray.

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GEOMETRY AND EXTENT

The Piney Point aquifer is present across the central portion of Maryland’s Eastern Shore and in Southern Maryland. Downdip on the lower Eastern Shore, the unit becomes progressively clayey. The aquifer does not crop out, but is found only in the subsurface. The altitude of the top of the Piney Point aquifer ranges from near sea level along its updip truncation with the overlying Calvert confining unit to approximately 1,220 ft below sea level near Berlin. The Piney Point aquifer in Maryland ranges in thickness from zero at its point of truncation to a maximum of 155 ft at Cambridge in Dorchester County. The aquifer likely contains brackish or salt water (chloride greater than 1,000 mg/L) east of a line trending from western Worcester County through eastern Wicomico County.

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HYDRAULIC PROPERTIES

Transmissivity of the Piney Point aquifer on Maryland’s Eastern Shore ranges from 100 ft2/d in Caroline County to 6,350 ft2/d in Dorchester County. The highest transmissivity values in Maryland occur in the vicinity of Cambridge and Secretary in Dorchester County. The Piney Point aquifer is generally less transmissive on Maryland’s western shore, with values ranging from 260 to 1,340 ft2/d. Storage coefficient of the aquifer in Maryland ranges from 1.6 x 10-4 to 3.8 x 10-4.

Aquia aquifer

The Aquia aquifer is an important source of supply in Anne Arundel, Calvert, Queen Anne’s, Kent, St. Mary’s and Talbot Counties. It is also used to a lesser extent in Caroline, Cecil, Charles, Dorchester, and Prince George’s Counties.

GEOLOGY

The Aquia aquifer consists chiefly of the Paleocene-age Aquia Formation. On the western shore in Anne Arundel County, the underlying Severn Formation, a thin, silty-sand layer, functions locally as part of the Aquia aquifer. On Maryland’s Eastern Shore, in Kent, Queen Anne’s and Talbot Counties, the Aquia aquifer includes the Aquia and Hornerstown Formations. The Aquia aquifer typically consists of fine- to coarse-grained, quartzose and glauconitic sands interbedded with layers of cemented sandstone and shell. The unit is commonly greenish-brown (and has historically been called a “greensand”) due to the occurrence of the minerals glauconite and goethite.

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GEOMETRY AND EXTENT

The Aquia aquifer extends from Virginia through southern Maryland and into the central portion of the Eastern Shore of Maryland. In Delaware, the Rancocas aquifer (equivalent to the Aquia aquifer) is present in a relatively narrow band extending along the boundary of New Castle and Kent Counties. The altitude of the top of the Aquia aquifer ranges from approximately 140 ft above sea level near its outcrop area in Prince George’s County to as much as about 680 ft below sea level in west-central Dorchester County. The Aquia aquifer ranges in thickness from zero ft at its downdip facies change to 289 ft at its maximum in Queen Anne’s County. On the western shore, the aquifer is relatively thick in Anne Arundel, Calvert, and Prince George’s Counties, and then thins towards southern St. Mary’s County. On the Eastern Shore, the aquifer tends to thicken northward from Dorchester and Talbot Counties into Queen Anne’s County.

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HYDRAULIC PROPERTIES

Transmissivity of the Aquia aquifer in Maryland ranges from 180 ft2/d in Queen Anne's County to 8,090 ft2/d in Kent County. Transmissivity is highest in a band extending from southern Anne Arundel and northern Calvert Counties to northern Queen Anne’s and southeastern Kent Counties. Storage coefficient ranges from 0.005 to 0.00013.

Monmouth aquifer

The Monmouth aquifer is an important source of water in the northern portion of the Eastern Shore of Maryland, particularly in Kent County. The Monmouth is also used for domestic supply in parts of Kent County, and utilized to a limited extent, mainly for domestic use, in southern Cecil County.

GEOLOGY

The Monmouth aquifer consists predominantly of sandy portions of the Upper Cretaceous-age Mount Laurel Formation (lower member of the Monmouth Group). In Kent and northern Queen Anne’s Counties, the Monmouth aquifer may include locally-developed sandy zones of the upper part of the Matawan Formation. The aquifer typically consists of dark gray, micaceous, clayey, glauconitic and quartzose fine sand. In places it contains marine fossils and siderite concentrations.

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GEOMETRY AND EXTENT

The Monmouth aquifer is present in Kent, Queen Anne’s, and Talbot Counties, as well as the north-central portion of Dorchester County. The altitude of the top of the aquifer ranges from about sea level near the outcrop area to approximately 1,700 ft below sea level at Ocean City. The aquifer ranges in thickness from 10 ft in Dorchester and Talbot Counties to 120 ft in Worcester County.

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HYDRAULIC PROPERTIES

Data is very limited on the hydraulic properties of the Monmouth aquifer. Transmissivity of the Monmouth at two sites tested in Kent County, Maryland, range from 220 to 340 ft2/d. A storage coefficient of 0.0012 was reported in Kent County.

Matawan aquifer

The Matawan aquifer is a localized unit occurring in the central part of the Eastern Shore of Maryland.

GEOLOGY

The Matawan aquifer consists of the sandy portion of the Upper Cretaceous-age Matawan Formation or Englishtown Formation (part of the Matawan Group). The Matawan Formation typically consists of dark gray to green, fine- to medium-grained, glauconitic and quartzose sand, interbedded with lenses of gray clay. It is characteristically glauconitic and micaceous, which helps distinguish it from the underlying Magothy aquifer. The Englishtown Formation consists of light-gray to white, micaceous, slightly silty to silty, fine-grained, slightly glauconitic quartz sand.

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GEOMETRY AND EXTENT

The Matawan aquifer is present from central Queen Anne’s County to southern Talbot County. The altitude of the top of the aquifer ranges from about 300 ft below sea level on northern Kent Island to more than 1,400 ft below sea level in east-central Wicomico County. The thickness of the aquifer ranges from 15 ft in Talbot County to 70 ft in Queen Anne's County.

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HYDRAULIC PROPERTIES

Data is very limited on the hydraulic properties of the Matawan aquifer. Transmissivity of the aquifer was calculated for two sites. In Talbot County the transmissivity is 410 ft2/d, and in Queen Anne’s County the transmissivity is 931 ft2/d.

Magothy aquifer

The Magothy aquifer is an important source of water for both public and domestic supply on Maryland’s western shore in Anne Arundel and Charles Counties, and in the central and northern portions of the Eastern Shore of Maryland in Cecil, Dorchester, Kent, Queen Anne’s, and Talbot Counties.

GEOLOGY

The Magothy aquifer consists chiefly of the sandy portions of the Magothy Formation. The aquifer may also include portions of the Patapsco Formation in locations where there is a sand-on-sand contact with the underlying Patapsco Formation. Examples of this occur in east-central Anne Arundel County, and parts of Kent County, Maryland. On the Eastern Shore, the aquifer may include sand of the Lower Cretaceous-age Raritan(?) Formation. The Magothy aquifer is composed of medium- to coarse-grained, light gray to white quartzose sands and gravels, interbedded with layers of white, gray and black clay. The sands are often described as being “sugary” in texture. Pyrite and lignite are common accessory constituents.

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GEOMETRY AND EXTENT

The Magothy aquifer is present in the central portion of Maryland's western shore. On the Eastern Shore, the unit is present from northern Dorchester, Wicomico, and Worcester Counties to southern Cecil County. In southern Maryland and the lower Eastern Shore, the Magothy aquifer pinches out on the southern flank of the Salisbury embayment The altitude of the top of the aquifer ranges from approximately 70 ft above sea level near its outcrop in Anne Arundel County to approximately 2,350 ft below sea level near Ocean City. Thickness ranges from zero ft at its pinchout to 214 ft in Anne Arundel County.

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HYDRAULIC PROPERTIES

Transmissivity of the Magothy aquifer ranges from 445 to 24,000 ft2/d. The highest values typically occur in Anne Arundel County. Storage coefficient ranges from 3.0 x 10-5 in Prince George’s County to 3.0 x 10-4 in Kent County.

Upper Patapsco aquifer system

The Upper Patapsco aquifer system is an important source of water supply on Maryland’s western shore in Anne Arundel, Calvert, Charles, Prince George’s and St. Mary’s Counties. It is also utilized in Cecil, Dorchester, Kent, Queen Anne’s, Somerset, and Talbot Counties.

GEOLOGY

The Upper Patapsco aquifer system consists of the sandy portions of the upper part of the Lower Cretaceous-age Patapsco Formation (part of the Potomac Group in Maryland). On the Eastern Shore, the aquifer may include sand of the Lower Cretaceous(?)-age Elk Neck Beds. The Upper Patapsco aquifer system typically consists of medium- to coarse-grained feldspathic and quartzose sands and gravels, interbedded with layers of red, gray, and mottled clay.

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GEOMETRY AND EXTENT

The Upper Patapsco aquifer system occurs throughout the Maryland Coastal Plain. The altitude of the top of the aquifer system ranges from 100 ft above sea level near its outcrop to more than 2,400 ft below sea level near Ocean City. The total thickness of the aquifer system along a line trending approximately parallel to strike from southern Maryland to the upper Eastern Shore ranges from about 125 to 390 ft. Analysis of geophysical logs indicates that sand percentage of the entire Upper Patapsco aquifer system ranges from approximately 80 percent in the Baltimore region to approximately 30 percent in the downdip facies of Worcester County.

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HYDRAULIC PROPERTIES

Transmissivity of the Upper Patapsco aquifer system ranges from 20 ft2/d in Charles County to 9,990 ft2/d in Anne Arundel County. The highest values typically occur in Anne Arundel County and decrease both to the north and to the south. Storage coefficient ranges from 8.4 x 10-5 to 0.0096

Lower Patapsco aquifer system

The Lower Patapsco aquifer is an important source of water supply on Maryland’s western shore in Anne Arundel, Calvert, Charles, and Prince George’s Counties, as well as in Cecil and Queen Anne’s Counties on the Eastern Shore.

GEOLOGY

The Lower Patapsco aquifer system consists of the sandy portions of the lower part of the Lower Cretaceous-age Patapsco Formation. The aquifer system is composed of white to yellow, fine- to medium-grained feldspathic and quartzose sands and gravels interbedded with layers of red, gray, and mottled silty clay.

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GEOMETRY AND EXTENT

The Lower Patapsco aquifer system is present throughout the Maryland Coastal Plain; however, correlation of the unit on the Delmarva Peninsula is problematic given the sparse borehole control. The altitude of the top of the aquifer system ranges from about 100 ft above sea level near its outcrop to more than 2,900 ft below sea level near Ocean City. The total thickness of the Lower Patapsco aquifer system along a line trending approximately parallel to strike from southern Maryland to the upper Eastern Shore ranges from about 250 to 350 ft.

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HYDRAULIC PROPERTIES

Transmissivity of the Lower Patapsco aquifer system ranges from 40 ft2/d in Prince Georges County to 11,900 ft2/d in Anne Arundel County. The highest values typically occur in Anne Arundel County and decrease both to the north and to the south. Storage coefficient ranges from 8.6 x 10-5 to 0.025

Patuxent aquifer system

The Patuxent aquifer system is an important source of supply on Maryland’s western shore in Anne Arundel, Charles, and Prince George’s Counties, as well as in Cecil County.

GEOLOGY

The Patuxent aquifer system consists of the sandy portions of the Lower Cretaceous-age Patuxent Formation (next to lowest member of the Potomac Group). The aquifer system is typically composed of medium- to coarse-grained, feldspathic and quartzose sands and gravels interbedded with layers of red, mottled, and gray clay. Patuxent sands are white or light gray to orange brown, angular and moderately sorted, and commonly contain significant amounts of interstitial clay. Gravels, often containing angular to rounded clasts of gray clay, and coarse ferruginous conglomerates occur commonly in the lowest portions of the unit.

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GEOMETRY AND EXTENT

The Patuxent aquifer system is present throughout the Maryland Coastal Plain; however, correlation of the unit on Maryland’s Eastern Shore is problematic given the sparse borehole control. The altitude of the top of the aquifer system ranges from about 170 ft above sea level near its outcrop to as much as 4,200 ft below sea level near Ocean City. The total thickness of the Patuxent aquifer system along a line trending approximately parallel to strike from southern Maryland to the upper Eastern Shore ranges from about 125 to 525 ft.

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HYDRAULIC PROPERTIES

Transmissivity of the Patuxent aquifer system ranges from 20 ft2/d in Charles and Harford Counties to 21,950 ft2/d in Baltimore County. Values are typically highest northeast of Washington, D.C., and decrease significantly in Charles and southern Prince George’s Counties. Storage coefficient ranges from 3.4 x 10-5 to 0.0012.

Waste Gate aquifer system

The Waste Gate aquifer system is a subsurface unit consisting of early Cretaceous-age deposits underlying the Patuxent aquifer system. The unit, composed of unconsolidated to moderately-lithified fluvial sands and gravels interbedded with drab to mottled silty clay, is present in the lower Eastern Shore, and may possibly extend westward into southern St. Mary’s and Calvert Counties, and northeast to New Jersey. The sandstones often contain a clayey or calcareous matrix. The altitude of the top of the Waste Gate aquifer system on the Eastern Shore of Maryland ranges from approximately 3,900 ft below sea level near Crisfield in southwestern Somerset County to approximately 5,600 ft below sea level near Ocean City. The Waste Gate is extended into southern St. Mary’s County to reflect the presence of relatively thin strata, likely older than the Patuxent Formation, which is possibly correlative with the Waste Gate Formation on the Eastern Shore. The Waste Gate aquifer system pinches out and does not outcrop. The unit attains its maximum known thickness of over 1,500 ft in Worcester County. It is underlain by pre-Cretaceous basement rocks and is overlapped by the younger Patuxent Formation. Resistivity logs suggest that the aquifer contains brackish to briny water. Water samples collected at Crisfield contain chloride concentrations more than twice that of ocean water. Relatively low permeabilities have been calculated using porosity values estimated from compensated formation-density logs. While the Waste Gate aquifer system is not a potable drinking-water source, it may have future utility for waste storage or geothermal production . Additionally, it may be a potential repository for carbon-dioxide sequestration.

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Pre-Cretaceous Basement Rock

The Maryland Coastal Plain is underlain by a basement complex consisting of Precambrian to Paleozoic crystalline rocks, and Mesozoic (Upper Triassic[?] to Lower Jurassic[?]) sedimentary and volcanic rocks. The contact with the overlying Cretaceous-age Potomac Group sediments is often marked by the presence of a saprolitic layer of weathered rock. The basement rock dips to the east-southeast, attaining a depth of approximately 7,200 ft below sea level at Ocean City, Maryland. The basement rocks dip relatively steeply near the Fall Line at about 70 ft/mi, flatten in southern Maryland and the central Delmarva Peninsula to about 30 ft/mi, and then increase in the eastern Delmarva Peninsula to about 85 ft/mi. At a local scale, the surface of the basement rock appears very irregular and undulating. A prominent curvature in the strike of the basement rocks marks the axis of the Salisbury Embayment, a broad, structural depression extending from New Jersey to Virginia. Faulting of the basement rock has been described near Brandywine in southern Prince George’s County and Waldorf in northern Charles County, but little is known of the structural geology of the basement complex beneath other areas of the Coastal Plain of Maryland.

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