Reports
Hydrogeology of the Triassic rocks of Maryland
1975, Nutter, L.J.
Report of Investigations 26
Abstract
The rocks of the Newark Group of Late Triassic age occur in the northern and western parts of the Frederick Valley in Frederick and Carroll Counties and in the western part of Montgomery County. These consolidated bedrock aquifers consist of a complex interbedded sequence of arkosic sandstone, shale, siltstone, and quartz-pebble and limestone-pebble conglomerate.
Water is stored in and transmitted through joints, faults, and bedding-plane partings in the rock and, to a limited extent, in interstitial pore space in the weathered rock mantle; the primary pore space in the rock has been almost eliminated by compaction and cementation. The Triassic-rock aquifers are a reliable source of water for domestic, farm, and small commercial use; only 6.5 percent of the wells inventoried yield less than 3 gallons per minute (0.2 litre per second). Ground-water supplies sufficient for most commercial and light industrial use can be developed under favorable hydrogeologic conditions.
Several factors affect well yields, the most important of which include: geologic structure (occurrence of joints and faults), topographic position, lithology, and well depth. Topography is to a large extent controlled by the geologic structure because the stream network tends to be aligned along major joints and faults; therefore, wells drilled in valleys or draws are more likely to intersect water-bearing fractures than are wells drilled on hilltops. The lithology is also an important factor influencing the yield of wells. The limestone-pebble conglomerate contains many of the highest yielding wells in the study area because solution along joints and bedding-plane openings has substantially increased the permeability. Sandstone and conglomerate beds are likely to yield more water to wells than are shale and siltstone beds, mainly because joints tend to be more closely spaced in sandstone and conglomerate. Water-bearing zones in the Triassic-rock aquifers can occur at depths of 500 feet (150 metres) or more, so it is important to drill wells at least 300 or 400 feet (90 or 120 metres) deep when maximum yields are sought.
The water in the Triassic-rock aquifers in generally of good chemical quality; the most common water-quality problems are excessive concentrations of iron, manganese, and nitrate, and low pH. The nitrate concentration in ground water underlying some older communities having small lots and fairly shallow wells constitutes a potential health hazard, particularly if wells are inadequately constructed.
