Reports

The Columbia aquifer of the Eastern Shore of Maryland

1984, Bachman, L.J. and Wilson, J.M.

Report of Investigations 40

Abstract

The potential for water-supply development of the Columbia aquifer in the Maryland part of the Delmarva Peninsula is evaluated. For this study, 96 test wells were drilled, more than 250 observation wells were inventoried, and water samples were collected from 30 wells for chemical analysis.

The Columbia aquifer consists of a blanket of surficial sediments that truncates older, southeastwardly dipping, Coastal Plain sediments. The surficial deposits consist of unconsolidated continental and nearshore marine sediments. Continental sediments are fluvial in origin and tend to be feldspar-bearing quartz sands. Nearshore coastal deposits consist of sandy barrier beach and dune complexes and clayey and silty lagoon complexes.

In general, the Columbia aquifer is less than 50 ft thick in Queen Annes, Kent, and Cecil Counties. In these counties and north of the town of Denton in Caroline County, the deposits often lack enough saturated thickness to be a major aquifer except where they are underlain by a subcropping sand or where a paleochannel exists. In Dorchester and Wicomico Counties the Columbia aquifer is appreciably thicker, often exceeding 100 ft, except in lowland areas, along the tidal rivers and Chesapeake Bay. Between Salisbury and Ocean City, Maryland, the aquifer becomes mostly confined due to increasing occurrences of clayey beds.

Previous studies revealed thick sediment-filled channels (paleochannels) near Salisbury and Hurlock, Md. Test drilling during this study revealed the existence of two more paleochannels near the towns of Harmony and Ridgely in Caroline County, and a possible third paleochannel near the village of Barclay in Queen Annes County. The newly discovered paleochannels, about 1/4 to 112 mi wide, consist of deposits of sand and gravel with a maximum thickness of about 80 feet.

The ground-water flow systems in the Columbia aquifer are generally local in nature with recharge areas occurring within a few miles of discharge areas. Potentiometric-surface data indicate that flow systems tend to be deeper and longer in the southeastern part of the study area.

The generally short, shallow flow systems and the low solubility of the minerals of the Columbia aquifer result in ground water with low dissolved solids (14-425 milligrams per liter) and low pH (3.8-6.7). The predominant anions are bicarbonate and chloride, and the main cations are sodium, potassium, and calcium. The higher concentration of dissolved solids, which are commonly accompanied by nitrate concentrations of more than 3 mg/L, may be caused by infiltration of contaminants into the shallow, unconfined part of the aquifer.

The Columbia aquifer is a major water-supply source in northeastern Dorchester, Wicomico, and Worcester Counties. It is locally important in Caroline, Talbot, and Queen Annes Counties, particularly in areas where paleochannels are present. The water quality is good, but there is a very real danger of groundwater contamination in areas where the aquifer is unconfined.