|Hydrogeology & Hydrology Program|
|Queen Anne's and Talbot Counties||
contact: David Drummond (firstname.lastname@example.org)
The aquifer system beneath Queen Anne's and Talbot Counties supplies most of the water needs of the residents of the two counties. Although several aquifers are used in the area, the Aquia aquifer supplies the majority of water, and in many respects, is the most important. The presence of brackish water in the Aquia aquifer on the Chesapeake Bay shore of Kent Island, and the decline in water levels in the Aquia aquifer has led to concern that increased pumpage may induce the landward movement of brackish water. Declining water levels due to population increases and increased irrigation may also cause problems with wells going dry, and drawdowns exceeding state-mandated limits.
The major aquifers used for water supply in Queen Anne's and Talbot Counties include (from shallow to deep) the Columbia, several Miocene aquifers, the Piney Point, Aquia, Matawan, Magothy, Upper Patapsco, and Lower Patapsco aquifers. The Middle Patapsco and Patuxent aquifers may also be productive aquifers, but are not presently being developed. Bedrock, which underlies the Coastal Plain aquifers is not used for water supply, and is not considered a feasible water source.
The Columbia aquifer is a surficial, unconfined aquifer which extends throughout the entire study area, and supplies water for irrigation and for a few older farms and homes. The Miocene aquifers, which include the Calvert, Frederica, Federalsburg, and Cheswold aquifers, are shallow and moderately productive aquifers which are used primarily in the southeastern part of the study area. The Piney Point aquifer is confined and very productive in parts of Talbot County, but absent in most of Queen Anne's County.
The Aquia aquifer is a very productive, confined aquifer that is used extensively throughout most of the study area, but is absent in southeastern Talbot County. The top of the Aquia aquifer ranges in depth from about sea level in northeastern Queen Anne's County to about 650 ft below sea level in southern Talbot County. At Love Point and along parts of the Chester River, the Aquia aquifer subcrops beneath the Columbia aquifer, but elsewhere is separated from the overlying Miocene and Piney Point aquifers by the Nanjemoy confining unit, or by clayey units in the Chesapeake Group. The Aquia is separated from the underlying Matawan aquifer by the Monmouth confining unit.
A synoptic water-level measurement conducted in the fall of 1997 indicates that heads in the Aquia aquifer range from about 20 ft above sea level in northern Queen Anne's County to about 65 ft below sea level near Easton, and heads are below sea level throughout most of the study area. Head gradients indicate that ground water is moving eastward from the Chesapeake Bay, and southward from northern Queen Anne's County toward a regional cone-of-depression centered at Easton. Long-term hydrographs from wells at Chester and Prospect indicate that heads in the Aquia aquifer are decreasing at a rate of about 0.5 ft/y. Short-term hydrographs indicate that heads in the Aquia aquifer fluctuate seasonally by as much as 35 ft, due to irrigation pumpage and increased evapotranspiration during the summer months.
Water quality in the Aquia aquifer is good throughout most of the study area, except for a narrow strip along the Chesapeake Bay shore of Kent Island, where brackish-water intrusion has degraded water quality, and rendered the water unfit for drinking. Hydrochemical facies for the Aquia aquifer include calcium bicarbonate, sodium bicarbonate, sodium chloride, and calcium chloride types.
The Matawan aquifer provides modest quantities of water for domestic supplies on parts of Kent Island and the Queenstown Golf Course. Elsewhere in the study area its presence and production capacity are uncertain. The Magothy aquifer is used for water supply on Kent Island and at Easton, but in some places is not a productive aquifer. It is difficult to distinguish the Magothy and Matawan aquifers in drillers' logs, and they may be hydraulically connected in places. High iron and manganese concentrations (as high as 34 and 0.4 mg/L, respectively) in the Kent Island area render water from the Magothy aquifer unfit for most purposes without treatment.
The Upper Patapsco aquifer is used extensively for water supply on Kent Island and at Easton. It is lithologically similar to the Magothy aquifer, and difficult to distinguish from the Magothy in drillers' logs. Although the Upper Patapsco is a very productive aquifer, high iron and manganese concentrations (as high as 28 and 0.4 mg/L, respectively) require treatment for most purposes. The Lower Patapsco aquifer has supplied the public water system at Stevensville since September, 1999, but is not used elsewhere on the Eastern Shore of Maryland south of Cecil County. Iron and manganese concentrations, although above the SMCL's, are significantly lower in the Lower Patapsco aquifer (3.2 and 0.2 mg/L, respectively, at the Stevensville well) than in the Upper Patapsco or Magothy aquifers. Moderate iron and manganese concentrations, coupled with the very high production capability and large available drawdown, make the Lower Patapsco aquifer an attractive source for public supplies on Kent Island, and possibly elsewhere in the study area.
Brackish-water intrusion is a potential threat to water quality in the Aquia aquifer in the Kent Island area of Queen Anne's County. Brackish water (chloride concentration greater than 1,000 mg/L) is present in the lower part of the Aquia aquifer within about a quarter mile of the entire bay shore of Kent Island. Water with elevated chloride concentrations (10 to 1,000 mg/L) is present in the upper part of the Aquia aquifer along the bay shore, and the northern and southern sections of Kent Island. Sampling of 18 wells in western Talbot County showed elevated chloride concentrations in a few wells screened in the Aquia aquifer, but do not indicate a widespread problem in that area. If, however, brackish water was present in the lower part of the Aquia aquifer, this sampling program would not have detected it.
Monitoring of 49 wells screened in the Aquia aquifer on western Kent Island from 1982 to 1999 does not show a clear, consistent trend in chloride concentrations. Concentrations in some wells have increased, and some have decreased, and almost all wells have shown considerable variation. Some trends, however, have been identified which generally explain the variations. In the central area of the island, concentrations in the upper part of the Aquia aquifer are generally elevated and increasing in a narrow strip (within a quarter mile of the shore), but farther inland, the entire section is fresh. In the northern and southern areas of the island, extending to the Chester River and Eastern Bay, concentrations in the upper part of the Aquia are elevated, but show a slight decreasing trend or no trend. At the northern tip of Kent Island, the entire section of the Aquia is brackish, and concentrations show no discernable trend.
Variations in chloride concentrations are explained by several factors:
Although the projected pumpage conditions used in the 1988 solute-transport model have not occurred, heads used for boundary conditions in that model are close to present-day heads, and evaluation of the solute-transport model results is useful. Trends in heads and chloride concentrations predicted by the model are generally consistent with measured trends, but are not entirely accurate.
A ground-water flow model was used to estimate heads and drawdowns in the Aquia aquifer in response to various future pumping scenarios. The model was also used to estimate ground-water flow across the brackish-water interface on Kent Island, which in turn was used to estimate the relative impact of the future pumpage scenarios on brackish-water intrusion. Although the major focus of flow modeling was the Aquia aquifer, the Coastal Plain aquifer system from the Columbia aquifer down to the Upper Patapsco aquifer was simulated. The model area included southern Kent County, Caroline County, and northern Dorchester County to minimize boundary effects.
Model results indicate that pumpage increases caused by projected population growth between 1997 and 2020 will cause water levels in the Aquia aquifer to decrease by about 2 ft on eastern Kent Island and 16 ft at Easton. Water levels on western Kent Island did not change appreciably because the Chesapeake Bay acts as a recharge boundary, keeping water levels near sea level. Increasing projected pumpage throughout the model area by 20 percent caused about twice as much drawdown as the projected pumpage simulation, and decreasing projected pumpage by 20 percent caused water levels to recover somewhat from 1997 levels.
Doubling projected pumpage in the model area caused drawdowns in the Aquia aquifer of 90 ft at Easton, 70 ft at Oxford, and 15 ft on eastern Kent Island. Drawdown on western Kent Island ranged from about 1 ft at Love Point, and 6 ft at Kent Point for this simulation. A simulation in which all major users pumped at their maximum yearly allowable rates in addition to projected population increases caused drawdowns of about 40 ft in northern Talbot County, 35 ft at Easton, and 30 ft in northern Queen Anne's County.
A simulation in which two hypothetical production wells were added in western Kent Island to supplement the public-water-supply system caused drawdowns of 10 ft in the vicinity of the hypothetical wells, and about 1 ft on western Kent Island. A simulation in which 1 MGD of pumpage from the Aquia aquifer was added in Management Area B (fig. 1) indicated drawdowns of 25 ft at Grasonville, and about 1 ft on western Kent Island. Shifting pumpage from the Aquia aquifer to the Upper Patapsco aquifer at six major public-supply well fields caused recoveries of about 20 ft at Easton and Oxford.
A simulation in which irrigation pumpage in Queen Anne's and Talbot Counties was doubled caused drawdowns of 35 ft in northeastern Talbot County and 30 ft in southeastern Queen Anne's County. Quadrupling irrigation pumpage (an increase of 300 percent) caused drawdowns of 90 ft in northeastern Talbot County, 80 ft in southeastern Queen Anne's County, and 2 to 4 ft along the bay shore of Kent Island. A 1-year simulation of the irrigation cycle indicated drawdowns in the Aquia aquifer of 35 ft in southern Queen Anne's County, and 25 ft in central Queen Anne's County.
Simulated flux values and flow velocities across the brackish-water interface were used to estimate the relative impact of various pumpage simulations on movement of the interface. The flow model does not simulate solute transport or density-dependent flow, and cannot calculate rate of movement of the brackish-water interface. Simulated flow velocities ranged from -2.6 ft/yr (negative velocities indicate westward flow toward the bay) for prepumping conditions to 29.6 ft/yr when all pumpage in the model area was doubled. All future simulations produced landward flow, but reducing pumpage by 20 percent, and shifting pumpage from the Aquia aquifer to deeper aquifers at six major public-supply facilities reduced flow velocities appreciably. Increasing irrigation pumpage by 300 percent in Queen Anne's and Talbot Counties caused an increase in flow velocity of 68 percent across the brackish-water interface.