HYDROGEOLOGY OF THE COASTAL PLAIN AQUIFER SYSTEM IN QUEEN
ANNE'S AND TALBOT COUNTIES, MARYLAND, WITH EMPHASIS ON WATER-SUPPLY POTENTIAL
AND BRACKISH-WATER INTRUSION IN THE AQUIA AQUIFER
by
David D. Drummond
KEY RESULTS
Coastal Plain aquifers supply the majority of water needs in Queen Anne's
and Talbot Counties (about 77 percent in 1997). Of these, the Aquia aquifer
is perhaps the most important because of its wide extent, good water-bearing
properties, and generally excellent water quality. However, because the Aquia
aquifer is shallow in the vicinity of the Chesapeake Bay, and water levels have
declined below sea level, brackish-water intrusion poses a threat to water quality
in the Aquia aquifer.
Eight major aquifers are used for water supply in Queen Anne's and Talbot
Counties:
- The Columbia aquifer is a surficial aquifer that extends over most of the
study area. The Columbia aquifer supplies some older homes and farms, and
is used for irrigation, but because it is shallow, it is vulnerable to contamination
from surface sources, and to going dry during droughts.
- The Miocene aquifers underlie the Columbia aquifer in southeastern Queen
Anne's and Talbot Counties, and are used for domestic, commercial, and irrigation
supplies in that area.
- The Piney Point aquifer underlies the Miocene sediments in the southeastern
part of the study area, but is absent in the northwest, and is a poor aquifer
in some parts of the study area. It is used for domestic and commercial supplies
where it is present, and for municipal supplies in neighboring Caroline and
Dorchester Counties.
- The Aquia aquifer underlies the Piney Point and Columbia aquifers, and
is used extensively throughout the study area, except for the southeastern
part of Talbot County. Brackish water is present in the Aquia aquifer in a
narrow strip along the Chesapeake Bay shore of Kent Island. Water levels in
the Aquia aquifer have declined at a rate of about one-half foot per year
since 1980, and may continue to decline as the region's population increases,
and demand for irrigation water increases.
- The Matawan aquifer underlies the Aquia aquifer in western Queen Anne's
County and possibly elsewhere. It is used for small domestic supplies in parts
of Kent Island where it provides an alternative water source to the Aquia
aquifer and deeper Cretaceous aquifers that have severe iron problems.
- The Magothy aquifer underlies the Matawan aquifer and may be hydraulically
connected to it in places. It supplies water for domestic and commercial uses
on Kent Island but water from the Magothy is very high in iron, and must be
treated before use. The Magothy aquifer is also used for much of the municipal
water supply at Easton, where iron concentrations do not pose a problem.
- The Upper Patapsco aquifer underlies the Magothy aquifer and supplies water
for domestic, commercial, and municipal uses on Kent Island and eastward to
Grasonville. Water from the Upper Patapsco aquifer also has a severe iron
problem in the Kent Island area but becomes less severe to the east and south.
The Upper Patapsco aquifer is also used for the municipal supply at Easton
where iron concentrations do not pose a treatment problem.
- The Lower Patapsco aquifer underlies the Upper Patapsco aquifer on Kent
Island, and probably elsewhere in the study area. It has been used for part
of the public supply system on Kent Island since late 1999, but nowhere else
on the Eastern Shore of Maryland south of Cecil County. Although water from
the Lower Patapsco aquifer requires treatment for iron, concentrations are
much lower than in the Magothy and Upper Patapsco aquifers. Aquifer tests
have shown that the Lower Patapsco aquifer is very productive, and provides
an excellent alternative to shallower aquifers, in spite of its great depth
(1,445 feet below sea level at Stevensville).
- The Middle Patapsco and Patuxent aquifers are potential ground-water sources,
but are not currently used for water supply in Queen Anne's and Talbot Counties,
and have not been tested thoroughly.
- Bedrock underlying the Coastal Plain sediments is not considered a potential
water supply.
Brackish-water intrusion poses a threat to water quality in the Aquia aquifer
on Kent Island.
- Brackish water is present in the lower part of the Aquia aquifer in a narrow
strip (about a quarter-mile wide) along the entire bay shore of Kent Island.
- Ground water with elevated chloride concentrations is present in the upper
part of the Aquia aquifer on northern and southern Kent Island, and a narrow
strip along the Chesapeake Bay on the central part of the island. At the northern
tip of Kent Island, the entire section of the Aquia aquifer contains brackish
water.
- Monitoring ground water in a network of wells on Kent Island since 1984
does not indicate an overall, consistent trend in chloride concentrations,
but does identify an area where concentrations are generally increasing.
- Variations in water chemistry caused by sporadic but widespread pumping,
fresh-water leakage from overlying aquifers, and prepumping invasion of brackish
water from the Chester River and Eastern Bay may obscure an overall increase
in chloride concentrations.
- Water with elevated chloride concentrations was detected in the Aquia aquifer
in western Talbot County, but a widespread problem is not indicated. However,
due to the lack of wells screened in the lower part of the Aquia aquifer in
western Talbot County, it is uncertain if brackish water is present in the
lower part of the aquifer, as it is on Kent Island.
Projected and hypothetical pumpage scenarios simulated with a ground-water
flow model indicate water levels will decline in the Aquia aquifer as population
and irrigation requirements increase.
- Water levels could decrease by as much as 90 feet in parts of the study
area as a result of increased pumpage demands.
- Increasing irrigation pumpage by 300 percent in Queen Anne's and Talbot
Counties could greatly increase the potential for brackish-water intrusion
on Kent Island.
- Adding 0.5 MGD pumpage from the Aquia aquifer on easternmost Kent Island,
increasing pumpage from the Aquia aquifer by 1 million gallons per day in
the Grasonville area, and doubling irrigation pumpage in Queen Anne's and
Talbot Counties would moderately increase the potential for brackish-water
intrusion on Kent Island.
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updated
3/4/05
